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Tom Vogt

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Papers by Tom Vogt

Phase transition inBaBi2Nb2O9:Implications for layered ferroelectrics

Physical Review B, 2002

Synchrotron powder diffraction methods have been used to determine the structure of the layered f... more Synchrotron powder diffraction methods have been used to determine the structure of the layered ferroelectric oxide BaBi 2 Nb 2 O 9 ͑BBN͒ above and below its Curie temperature. Although BBN is found to be tetragonal above and below the ferroelectric-paraelectric transition, there is clear evidence for a structural phase transition. The low-temperature ferroelectric structure is found to be well described in space group I4mm, while the high-temperature structure is in the more common I4/mmm space group. The identification of the lowtemperature tetragonal structure resolves a long-standing question as to the origin of ferroelectric behavior in BBN and related oxides.

Mixed Iron−Manganese Oxide Nanoparticles

The Journal of Physical Chemistry B, 2004

Designing nanoparticles for practical applications requires knowledge and control of how their de... more Designing nanoparticles for practical applications requires knowledge and control of how their desired properties relate to their composition and structure. Here, we present a detailed systematic study of mixed iron-manganese oxide nanoparticles, showing that ultrasonication provides the high-energy reaction conditions required for complete atomic level mixing of Fe(III) and Mn(III) when amorphous Fe 2 O 3 nanoparticles are irradiated in the presence of Mn 2 (CO) 10 in ambient atmosphere. X-ray diffraction (XRD) results reveal that the crystal structure of manganese iron mixed oxide nanoparticles changes from spinel to bixbyite with increasing of Mn(III) content. The results of room-temperature magnetization curves are consistent with the XRD patterns and spin density from electron paramagnetic resonance measurements, showing samples converting from superparamagnetic to antiferromagnetic, when the crystal structures of these samples transform from spinel to bixbyite.

$Research paper thumbnail of The Determination of Brønsted Acid Sites in Zeolite ERS-7 by Neutron and X-ray Powder Diffraction$

The Determination of Brønsted Acid Sites in Zeolite ERS-7 by Neutron and X-ray Powder Diffraction

The Journal of Physical Chemistry B, 2001

The deuterated acid form of zeolite ERS-7 (Si/Al) 8.4) has been prepared by repeated cycles of D ... more The deuterated acid form of zeolite ERS-7 (Si/Al) 8.4) has been prepared by repeated cycles of D 2 O exposure and calcination. 27 Al NMR data show that the relative proportions of tetrahedral and octahedral Al in the resulting material are 70 and 30%, respectively. Three Brønsted sites were identified by a combined Rietveld refinement that simultaneously employed synchrotron X-ray powder diffraction data and neutron powder diffraction data. Final lattice parameters were a) 9.7843(1) Å, b) 12.3676(1) Å, and c) 22.8336(2) Å, the combined R wp factor was 6.3%, and 2 (reduced) was 1.48. Acid deuterons were located approximately 1 Å from oxygens O3, O5, and O7, with occupancy factors of 0.16(2), 0.16(3), and 0.18(2), respectively. These correpsond to 1.28(16), 0.64(12), and 1.44(16) deuterons per unit cell, respectively, for a total of 3.36(26) acid sites per unit cell, which is consistent with the tetrahedral Al content. Only deuteron D5 lies on a special position. TO bond lengths suggest a site preference for Al at T4, T1, and possibly T2, each of which is bonded to at least one of the observed acid-site oxygens. A bridging framework oxygen site was identified in both the PND and PXD difference Fourier maps that appears to be disordered over at least two distinct sites; this corresponds to different local arrangements of Al at the neighboring T-sites. The out-of-plane framework hydroxyl tilt angles at each acid site were comparable to those of H + SSZ-13 and D + RHO. Acid site D7 demonstrated a significant in-plane hydroxyl tilt angle (7.5°), consistent with ab initio quantum mechanical calculations. This tilt clearly indicates an Al preference for site T4 over T6.

Magnetic spin glass properties of the bi-layer hydrate Na0.3NiO2·1.3H2O

Solid State Communications, 2006

The magnetic properties of the new metal oxide bi-layer hydrate Na 0.3 NiO 2 $1.3H 2 O were inves... more The magnetic properties of the new metal oxide bi-layer hydrate Na 0.3 NiO 2 $1.3H 2 O were investigated. DC and AC magnetic susceptibility measurements revealed the compound to be a spin glass at low temperatures with a freezing point of T f w3.5 K. Using the Vogel-Fulcher relationship an activation energy of E a /kw71.6 K, a Vogel-Fulcher temperature of T o w1.9 K and a frequency dependence of the freezing temperature consistent with a frustrated magnetic spin-glass rather than a RKKY spin glass were determined. Ni K-edge X-ray absorption spectroscopy (XAS) show an absorption edge shift toward higher energies in Na 0.3 NiO 2 $1.3H 2 O compared to Na 0.7 NiO 2 and suggests that Ni occurs in two possible electronic configurations: a combination of 3d 7 (SZ1/2) and 3d 6 (SZ0) or 3d 7 L states, with L representing a hole in the oxygen p-states.

Anisotropic compression of edingtonite and thomsonite to 6 GPa at room temperature

Physics and Chemistry of Minerals, 2004

Polycrystalline samples of natural edingtonite (New Brunswick, Canada) and thomsonite (Oregon, US... more Polycrystalline samples of natural edingtonite (New Brunswick, Canada) and thomsonite (Oregon, USA) were studied up to 6 GPa using monochromatic synchrotron X-ray powder diffraction and a diamondanvil cell with a methanol:ethanol:water mixture as a penetrating pressure-transmitting fluid. Unlike natrolite, previously studied under the same conditions, edingtonite and thomsonite do not show any apparent pressure-induced hydration (PIH) or phase transitions. All these fibrous zeolites are characterized by their anisotropic compressibilities, with the linear compressibilities of the fibrous chains (c-axis) being as small as one third of those perpendicular to the chains (a-, b-axes); for edingtonite, b a 0 ¼ 0.0050(3) GPa)1 , b b 0 ¼ 0.0054(2) GPa)1 , b c 0 ¼ 0.0034(1) GPa)1 ; for thomsonite, b a 0 ¼ 0.0080(2) GPa)1 , b b 0 ¼ 0.0084(2) GPa)1 , b c 0 ¼ 0.0032(1) GPa)1. The pressure-volume data were fitted to a secondorder Birch-Murnaghan equation of state using a fixed pressure derivative of 4. As a result of the 0000-type connectivity of the chains, the bulk modulus of edingtonite is found to be about 40% larger than that of thomsonite; K EDI 0 ¼ 73(3) GPa, K THO 0 ¼ 52(1) GPa. Distance least-squares refinements were used to model the expected framework, following the observed linear compression behaviors. The chain-bridging TO -T angle is proposed to be correlated with the different compressibilities across the chains in each framework type.

$Research paper thumbnail of Pressure-induced hydration and cation migration in a Cs+ exchanged gallosilicate zeolite LTL: Synchrotron X-ray powder diffraction study at ambient and high pressures$

Pressure-induced hydration and cation migration in a Cs+ exchanged gallosilicate zeolite LTL: Synchrotron X-ray powder diffraction study at ambient and high pressures

Microporous and Mesoporous Materials, 2010

The ambient and high pressure structures of a partially cesium-exchanged K-gallosilicate with a z... more The ambient and high pressure structures of a partially cesium-exchanged K-gallosilicate with a zeolite LTL framework topology (Cs0.65K0.35–GaSi–LTL) was studied using synchrotron X-ray powder diffraction. In contrast to the cation distribution present in the aluminosilicate analogues, the larger cesium cations replace part of the potassium cations in the narrow 8-ring channel as well as in the main 12-ring channel of the gallosilicate LTL. Under hydrostatic pressures mediated by a pore-penetrating alcohol and water mixture, anomalous compression behavior is observed with a slight increase in the unit cell volume upon initial compression, i.e., 0.15% expansion at 0.44GPa. This is the result of the continuous increase of the a-axis length up to 2GPa and is found to be related to the gradual pressure-induced hydration (PIH) occurring inside the main 12-ring channel, where the water content increases from 15.9(1) H2O at ambient conditions to 26.7(1) H2O per formula unit at 2.83GPa. During PIH, part of the cesium cations along the main 12-ring channel migrate into the narrow 8-ring channel. Compared to the structural changes observed in K–GaSi–LTL, the degree of pressure-induced hydration and the accompanying cation migration is found to be diminished in Cs0.65K0.35–GaSi–LTL.

$Research paper thumbnail of Synchrotron X-ray Powder Diffraction and Computational Investigation of Purely Siliceous Zeolite Y under Pressure$

Synchrotron X-ray Powder Diffraction and Computational Investigation of Purely Siliceous Zeolite Y under Pressure

Journal of the American Chemical Society, 2004

Doping γ-Fe2O3 Nanoparticles with Mn(III) Suppresses the Transition to the α-Fe2O3 Structure

Journal of the American Chemical Society, 2003

Here we report on a mixed oxide system, gamma-Fe2O3 nanoparticles doped with Mn(III), where the t... more Here we report on a mixed oxide system, gamma-Fe2O3 nanoparticles doped with Mn(III), where the transition from the cubic to the more stable hexagonal alpha-Fe2O3 structure is suppressed. When amorphous Fe2O3 is heated at 300 degrees C for 3 h, ferrimagnetic gamma-Fe2O3 is observed as the sole product. On the other hand, when the temperature is raised to 500 degrees C, one observes only antiferromagnetic alpha-Fe2O3 as the product. However, upon doping with 8.5 wt % Mn(III), the amorphous nanoparticles crystallized to mainly the gamma-Fe2O3 matrix after heating at 500 degrees C for 3 h, and need to be heated to &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;650 degrees C for the complete transition to the alpha-Fe2O3 structure to take place.

Pressure-induced phase transition and octahedral tilt system change of Ba2BiSbO6

Journal of Solid State Chemistry, 2006

High-resolution X-ray synchrotron powder diffraction studies under high-pressure conditions are r... more High-resolution X-ray synchrotron powder diffraction studies under high-pressure conditions are reported for the ordered double perovskite Ba2BiSbO6. Near 4 GPa, the oxide undergoes a pressure-induced phase transition. The symmetry of the material changes during the phase transition from space group R3¯ to space group I2/m, which is consistent with a change in the octahedral tilting distortion from an a-a-a- type to a0b-b- type using the Glazer notation. A fit of the volume pressure data using the Birch Murnagaham equation of state yielded a bulk modulus of 144(8) GPa for the rhombohedral phase.

Compression mechanisms of symmetric and Jahn–Teller distorted octahedra in double perovskites: A2CuWO6 (A=Sr, Ba), Sr2CoMoO6, and La2LiRuO6

Journal of Solid State Chemistry, 2006

The Energy Citations Database (ECD) provides access to historical and current research (1948 to t... more

Structure of Sn(ND3)2F4—A Molecular Precursor for the Synthesis of Nitride Fluorides

Journal of Solid State Chemistry, 1998

The molecular and crystal structure of Sn(ND 3) 2 F 4 has been determined at room temperature usi... more The molecular and crystal structure of Sn(ND 3) 2 F 4 has been determined at room temperature using high resolution neutron powder diffraction. The compound crystallizes in a monoclinic unit cell containing two formula units with a ‫؍‬ 7.7673(7) A s , b ‫؍‬ 6.2765(5) A s , c ‫؍‬ 5.1708(4) A s , ‫؍‬ 102.871(5)°, and spacegroup P2 1 /n. The structure consists of isolated tin-centered octahedra with ammonia groups in trans configuration. The intramolecular stability of this compound has been investigated using Hu¨ckel molecular orbital calculations. Compared to Si(NH 3) 2 F 4 , the metal-nitrogen bonding interaction is found to be stronger, while the metal-fluorine bond is weaker in Sn(ND 3) 2 F 4. Due to a different hydrogen bonding network the intermolecular orientation and packing of the octahedral M(NH 3) 2 F 4 molecules in Sn(ND 3) 2 F 4 is different from the arrangement observed in Si(NH 3) 2 F 4. The implications for the use of these compounds as molecular precursors for the synthesis of SiNF and SnNF are discussed.

Crystal structures and phase transitions in Sr doped Ba2InTaO6 perovskites

Journal of Solid State Chemistry, 2013

Laboratory and synchrotron X-ray powder di4raction and neutron powder di4raction have been used t... more Laboratory and synchrotron X-ray powder di4raction and neutron powder di4raction have been used to determine crystal structures in the perovskites SrTi x Zr 1؊x O 3. Superlattice re6ections in the neutron di4raction patterns proved to be the most reliable indicators of lower symmetry structures. The room temperature structures are orthorhombic Pbnm in the range of 04x40.4, tetragonal I4/mcm in the range 0.4(x40.95, and cubic Pm3 m for 0.95(x41. Additional measurements were made on the room-temperature tetragonal samples at x ‫؍‬ 0.5, 0.75, and 0.9. The 5rst two were examined at elevated temperatures, using neutron di4raction methods, where the transition tetragonal to cubic was noted. Measurements at low temperatures using synchrotron X-ray powder di4raction are reported.

Low temperature structural studies on PrAlO3

Journal of Physics: Condensed Matter, 2001

... E-mail: [email protected] (Sandra M Moussa), [email protected] (Brendan J Ken... more

Low-Temperature Structure and Magnetic Properties of the Spinel LiMn2O4: A Frustrated Antiferromagnet and Cathode Material

Chemistry of Materials, 1999

Powder neutron diffraction has been used to study the nature of the structural transition away fr... more Powder neutron diffraction has been used to study the nature of the structural transition away from the Fd3m cubic structure upon cooling below ∼285 K in the spinel LiMn 2 O 4. We report powder data taken between 10 K and 333 K and propose a large cell tetragonal structure in space group I4 1 /amd for the material at 100 K. While complete segregation of the Mn 3+ and Mn 4+ ions is not possible in this space group, bond-valence analysis indicates that the distribution of Mn 3+ and Mn 4+ ions is not random and that there is a degree of charge segregation. Further, LiMn 2 O 4 is also of interest because it is an example of a geometrically frustrated antiferromagnet. Direct current magnetic susceptibility measurements show field-cooled, zero-field-cooled irreversibility at ∼65 K and a maximum in zerofield-cooled data at ∼40 K. Neutron diffraction shows magnetic scattering in the form of a broad peak assigned to short-range order which develops above 100 K. Upon cooling to 60 K additional Bragg peaks are seen, signaling long-range magnetic order. The Bragg peaks grow at the expense of the diffuse feature on cooling to 10 K but the latter persists even at the lowest temperature studied which indicates that a significant fraction of the spins still remain disordered. The magnetic Bragg peaks index on a tetragonal cell which is 2a, 2b, and 4c with respect to the low-temperature tetragonal cell and contains 1152 spins. The large size and implied complexity of the magnetic structure is consistent with both charge segregation and significant further neighbor exchange interactions.

Influence of Cation Size on the Structural Features of Ln1/2A1/2MnO3 Perovskites at Room Temperature

Chemistry of Materials, 1998

Sm, and Y 0.5 Sm 0.5) have been prepared, and structure determinations have been carried out at r... more Sm, and Y 0.5 Sm 0.5) have been prepared, and structure determinations have been carried out at room temperature using high-resolution synchrotron X-ray powder diffraction data. The octahedral tilting distortion increases as the average ionic radius of the Ln/A cations, 〈r A 〉, decreases. The two crystallographically distinct Mn-O-Mn bonds [Mn-O(eq)-Mn and Mn-O(ax)-Mn] are almost identical for Ln 0.5 Ca 0.5 MnO 3 compounds, with the exception of La 0.5 Ca 0.5 MnO 3. The La 0.5 Ca 0.5 MnO 3 compound and the entire Ln 0.5 Sr 0.5 MnO 3 series adopt structures where the Mn-O(eq)-Mn bond angle is consistently and significantly larger (2-6°) than the Mn-O(ax)-Mn bond angle. All of the Ln 0.5 Ca 0.5 MnO 3 compounds have Pnma symmetry, whereas across the Ln 0.5 Sr 0.5 MnO 3 series with increasing 〈r A 〉, an evolution from Pnma (tilt system ab + a-) over Imma (tilt system ab 0 a-) to I4/mcm (tilt system a 0 a 0 c-) symmetry is observed. It appears that the latter two tilt systems are stabilized with respect to the rhombohedral (R3 hc) aaatilt system, by short-range layered ordering of A-site cations. Changes in the octahedral tilt system at room temperature are linked to changes in the low-temperature magnetic structure. In particular, the simultaneous onset of charge ordering and CE-type antiferromagnetism in the Ln 0.5 Sr 0.5 MnO 3 series appears to be closely associated with the Imma structure. The average Mn-O bond distance is relatively constant across the entire series, but individual Mn-O bond distances show the presence of a cooperative Jahn-Teller effect (orbital ordering) at room temperature in Sm 0.5 Ca 0.5 MnO 3 and Sm 0.25 Y 0.25 Ca 0.5 MnO 3 .

Epitaxial Thin-Film Deposition and Dielectric Properties of the Perovskite Oxynitride BaTaO2N

Chemistry of Materials, 2007

Pulsed-laser deposition was employed to grow epitaxial thin films of the oxynitride perovskite Ba... more Pulsed-laser deposition was employed to grow epitaxial thin films of the oxynitride perovskite BaTaO 2 N on a conducting SrRuO 3 buffer layer deposited on a 100-cut SrTiO 3 single-crystal substrate. Phase purity and epitaxy were optimized at a substrate temperature of 760°C in a mixed gas atmosphere of 100 mTorr N 2 /O 2 (∼20:1). The dielectric permittivity, κ, of the BaTaO 2 N film was large, exhibiting a slight frequency dependence ranging from about 200 to 240 over the frequency range 1-100 kHz. Furthermore, over the temperature range 4-300 K the permittivity showed minimal variation as a function of temperature. The temperature coefficient of the dielectric constant, τ κ , is estimated to be in the range of-50 to-100 ppm/K. The coexistence of high dielectric permittivity and weak temperature dependence is an unusual combination in a single-phase material.

Synthesis and characterization of Na0.3RhO2·0.6H2O—a semiconductor with a weak ferromagnetic component

Solid State Communications, 2005

We have prepared the oxyhydrate Na 0.3 RhO 2 ⋅0.6H 2 O by extracting Na + cations from NaRhO 2 an... more We have prepared the oxyhydrate Na 0.3 RhO 2 ⋅0.6H 2 O by extracting Na + cations from NaRhO 2 and intercalating water molecules using an aqueous solution of Na 2 S 2 O 8. Rietveld refinement, thermogravimetric analysis (TGA), and energy-dispersive x-ray analysis (EDX) reveal that a non-stoichiometric Na 0.3 (H 2 O) 0.6 network separates layers of edge-sharing RhO 6 octahedra containing Rh 3+ (4d 6 , S=0) and Rh 4+ (4d 5 , S=1/2). The resistivities of NaRhO 2 and Na 0.3 RhO 2 •0.6H 2 O (T < 300) reveal insulating and semi-conducting behavior with activation gaps of 134 meV and 7.8 meV, respectively. Both Na 0.3 RhO 2 ⋅0.6H 2 O and NaRhO 2 show paramagnetism at room temperature, however, the sodium-deficient sample exhibits simultaneously a weak but experimentally reproducible ferromagnetic component. Both samples exhibit a temperature-independent Pauli paramagnetism, for NaRhO 2 at T > 50 K and for Na 0.3 RhO 2 •0.6H 2 O at T > 25 K. The relative magnitudes of the temperatureindependent magnetic susceptibilities, that of the oxide sample being half that of the oxyhydrate, is consistent with a higher density of thermally accessible electron states at the Fermi level in the hydrated sample. At low temperatures the magnetic moments rise sharply, providing evidence of localized and weakl-ordered electronic spins with effective moment per formula unit values of 2.0 x 10-1 B µ for NaRhO 2 and 0.8 x 10-1 B µ for Na 0.3 RhO 2 •0.6H 2 O

Low to High Spin-State Transition Induced by Charge Ordering in AntiferromagneticYBaCo2O5

Physical Review Letters, 2000

The oxygen-deficient double perovskite YBaCo 2 O 5 , containing corner-linked CoO 5 square pyrami... more The oxygen-deficient double perovskite YBaCo 2 O 5 , containing corner-linked CoO 5 square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range Co 21 ͞Co 31 charge ordering occur as well as a change in the Co 21 spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering.

Magnetostriction in a simple trivalent manganese perovskite

Physical Review B, 2004

We found an unusual negative thermal expansion in the simple perovskite MnF 3 below the Neel poin... more We found an unusual negative thermal expansion in the simple perovskite MnF 3 below the Neel point, where the spins are ordered in an A-type magnetic structure. The structure of MnF 3 has been studied between 5 and 500 K using powder neutron and synchrotron diffraction. The structure remains monoclinic C2/c over the entire temperature range. We have found that the interplay of spin and orbital ordering is sufficient to result in a large positive magnetovolume effect without charge ordering. This expansion is reminiscent of the Invar effect observed in certain alloys.

Phase Transition of Zeolite RHO at High-Pressure

Journal of the American Chemical Society, 2001

rho, undergo a reversible phase transition from centric to acentric structures. In the case of th... more rho, undergo a reversible phase transition from centric to acentric structures. In the case of the NaCs-rho, the volume of the centric form expands upon pressure increase, probably due to the sorption of pressure media into the pores [2] (see Figure). In the Li-rho, the transition back to the centric structure on pressure release is sluggish and occurs over a period of weeks. The Cd-rho shows intermediate back-transition kinetics and compressibility compared to NaCs-and Li-rho, which indicates the pressure response of rho is dependent upon the initial cation distribution in the pores. Analysis on peak half-width indicates the existence of deviatoric stress in acentric phase, which is released upon transition back to the centric form.

Phase transition inBaBi2Nb2O9:Implications for layered ferroelectrics

Physical Review B, 2002

Synchrotron powder diffraction methods have been used to determine the structure of the layered f... more Synchrotron powder diffraction methods have been used to determine the structure of the layered ferroelectric oxide BaBi 2 Nb 2 O 9 ͑BBN͒ above and below its Curie temperature. Although BBN is found to be tetragonal above and below the ferroelectric-paraelectric transition, there is clear evidence for a structural phase transition. The low-temperature ferroelectric structure is found to be well described in space group I4mm, while the high-temperature structure is in the more common I4/mmm space group. The identification of the lowtemperature tetragonal structure resolves a long-standing question as to the origin of ferroelectric behavior in BBN and related oxides.

Mixed Iron−Manganese Oxide Nanoparticles

The Journal of Physical Chemistry B, 2004

Designing nanoparticles for practical applications requires knowledge and control of how their de... more Designing nanoparticles for practical applications requires knowledge and control of how their desired properties relate to their composition and structure. Here, we present a detailed systematic study of mixed iron-manganese oxide nanoparticles, showing that ultrasonication provides the high-energy reaction conditions required for complete atomic level mixing of Fe(III) and Mn(III) when amorphous Fe 2 O 3 nanoparticles are irradiated in the presence of Mn 2 (CO) 10 in ambient atmosphere. X-ray diffraction (XRD) results reveal that the crystal structure of manganese iron mixed oxide nanoparticles changes from spinel to bixbyite with increasing of Mn(III) content. The results of room-temperature magnetization curves are consistent with the XRD patterns and spin density from electron paramagnetic resonance measurements, showing samples converting from superparamagnetic to antiferromagnetic, when the crystal structures of these samples transform from spinel to bixbyite.

$Research paper thumbnail of The Determination of Brønsted Acid Sites in Zeolite ERS-7 by Neutron and X-ray Powder Diffraction$

The Determination of Brønsted Acid Sites in Zeolite ERS-7 by Neutron and X-ray Powder Diffraction

The Journal of Physical Chemistry B, 2001

The deuterated acid form of zeolite ERS-7 (Si/Al) 8.4) has been prepared by repeated cycles of D ... more The deuterated acid form of zeolite ERS-7 (Si/Al) 8.4) has been prepared by repeated cycles of D 2 O exposure and calcination. 27 Al NMR data show that the relative proportions of tetrahedral and octahedral Al in the resulting material are 70 and 30%, respectively. Three Brønsted sites were identified by a combined Rietveld refinement that simultaneously employed synchrotron X-ray powder diffraction data and neutron powder diffraction data. Final lattice parameters were a) 9.7843(1) Å, b) 12.3676(1) Å, and c) 22.8336(2) Å, the combined R wp factor was 6.3%, and 2 (reduced) was 1.48. Acid deuterons were located approximately 1 Å from oxygens O3, O5, and O7, with occupancy factors of 0.16(2), 0.16(3), and 0.18(2), respectively. These correpsond to 1.28(16), 0.64(12), and 1.44(16) deuterons per unit cell, respectively, for a total of 3.36(26) acid sites per unit cell, which is consistent with the tetrahedral Al content. Only deuteron D5 lies on a special position. TO bond lengths suggest a site preference for Al at T4, T1, and possibly T2, each of which is bonded to at least one of the observed acid-site oxygens. A bridging framework oxygen site was identified in both the PND and PXD difference Fourier maps that appears to be disordered over at least two distinct sites; this corresponds to different local arrangements of Al at the neighboring T-sites. The out-of-plane framework hydroxyl tilt angles at each acid site were comparable to those of H + SSZ-13 and D + RHO. Acid site D7 demonstrated a significant in-plane hydroxyl tilt angle (7.5°), consistent with ab initio quantum mechanical calculations. This tilt clearly indicates an Al preference for site T4 over T6.

Magnetic spin glass properties of the bi-layer hydrate Na0.3NiO2·1.3H2O

Solid State Communications, 2006

The magnetic properties of the new metal oxide bi-layer hydrate Na 0.3 NiO 2 $1.3H 2 O were inves... more The magnetic properties of the new metal oxide bi-layer hydrate Na 0.3 NiO 2 $1.3H 2 O were investigated. DC and AC magnetic susceptibility measurements revealed the compound to be a spin glass at low temperatures with a freezing point of T f w3.5 K. Using the Vogel-Fulcher relationship an activation energy of E a /kw71.6 K, a Vogel-Fulcher temperature of T o w1.9 K and a frequency dependence of the freezing temperature consistent with a frustrated magnetic spin-glass rather than a RKKY spin glass were determined. Ni K-edge X-ray absorption spectroscopy (XAS) show an absorption edge shift toward higher energies in Na 0.3 NiO 2 $1.3H 2 O compared to Na 0.7 NiO 2 and suggests that Ni occurs in two possible electronic configurations: a combination of 3d 7 (SZ1/2) and 3d 6 (SZ0) or 3d 7 L states, with L representing a hole in the oxygen p-states.

Anisotropic compression of edingtonite and thomsonite to 6 GPa at room temperature

Physics and Chemistry of Minerals, 2004

Polycrystalline samples of natural edingtonite (New Brunswick, Canada) and thomsonite (Oregon, US... more Polycrystalline samples of natural edingtonite (New Brunswick, Canada) and thomsonite (Oregon, USA) were studied up to 6 GPa using monochromatic synchrotron X-ray powder diffraction and a diamondanvil cell with a methanol:ethanol:water mixture as a penetrating pressure-transmitting fluid. Unlike natrolite, previously studied under the same conditions, edingtonite and thomsonite do not show any apparent pressure-induced hydration (PIH) or phase transitions. All these fibrous zeolites are characterized by their anisotropic compressibilities, with the linear compressibilities of the fibrous chains (c-axis) being as small as one third of those perpendicular to the chains (a-, b-axes); for edingtonite, b a 0 ¼ 0.0050(3) GPa)1 , b b 0 ¼ 0.0054(2) GPa)1 , b c 0 ¼ 0.0034(1) GPa)1 ; for thomsonite, b a 0 ¼ 0.0080(2) GPa)1 , b b 0 ¼ 0.0084(2) GPa)1 , b c 0 ¼ 0.0032(1) GPa)1. The pressure-volume data were fitted to a secondorder Birch-Murnaghan equation of state using a fixed pressure derivative of 4. As a result of the 0000-type connectivity of the chains, the bulk modulus of edingtonite is found to be about 40% larger than that of thomsonite; K EDI 0 ¼ 73(3) GPa, K THO 0 ¼ 52(1) GPa. Distance least-squares refinements were used to model the expected framework, following the observed linear compression behaviors. The chain-bridging TO -T angle is proposed to be correlated with the different compressibilities across the chains in each framework type.

$Research paper thumbnail of Pressure-induced hydration and cation migration in a Cs+ exchanged gallosilicate zeolite LTL: Synchrotron X-ray powder diffraction study at ambient and high pressures$

Pressure-induced hydration and cation migration in a Cs+ exchanged gallosilicate zeolite LTL: Synchrotron X-ray powder diffraction study at ambient and high pressures

Microporous and Mesoporous Materials, 2010

The ambient and high pressure structures of a partially cesium-exchanged K-gallosilicate with a z... more The ambient and high pressure structures of a partially cesium-exchanged K-gallosilicate with a zeolite LTL framework topology (Cs0.65K0.35–GaSi–LTL) was studied using synchrotron X-ray powder diffraction. In contrast to the cation distribution present in the aluminosilicate analogues, the larger cesium cations replace part of the potassium cations in the narrow 8-ring channel as well as in the main 12-ring channel of the gallosilicate LTL. Under hydrostatic pressures mediated by a pore-penetrating alcohol and water mixture, anomalous compression behavior is observed with a slight increase in the unit cell volume upon initial compression, i.e., 0.15% expansion at 0.44GPa. This is the result of the continuous increase of the a-axis length up to 2GPa and is found to be related to the gradual pressure-induced hydration (PIH) occurring inside the main 12-ring channel, where the water content increases from 15.9(1) H2O at ambient conditions to 26.7(1) H2O per formula unit at 2.83GPa. During PIH, part of the cesium cations along the main 12-ring channel migrate into the narrow 8-ring channel. Compared to the structural changes observed in K–GaSi–LTL, the degree of pressure-induced hydration and the accompanying cation migration is found to be diminished in Cs0.65K0.35–GaSi–LTL.

$Research paper thumbnail of Synchrotron X-ray Powder Diffraction and Computational Investigation of Purely Siliceous Zeolite Y under Pressure$

Synchrotron X-ray Powder Diffraction and Computational Investigation of Purely Siliceous Zeolite Y under Pressure

Journal of the American Chemical Society, 2004

Doping γ-Fe2O3 Nanoparticles with Mn(III) Suppresses the Transition to the α-Fe2O3 Structure

Journal of the American Chemical Society, 2003

Here we report on a mixed oxide system, gamma-Fe2O3 nanoparticles doped with Mn(III), where the t... more Here we report on a mixed oxide system, gamma-Fe2O3 nanoparticles doped with Mn(III), where the transition from the cubic to the more stable hexagonal alpha-Fe2O3 structure is suppressed. When amorphous Fe2O3 is heated at 300 degrees C for 3 h, ferrimagnetic gamma-Fe2O3 is observed as the sole product. On the other hand, when the temperature is raised to 500 degrees C, one observes only antiferromagnetic alpha-Fe2O3 as the product. However, upon doping with 8.5 wt % Mn(III), the amorphous nanoparticles crystallized to mainly the gamma-Fe2O3 matrix after heating at 500 degrees C for 3 h, and need to be heated to &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;650 degrees C for the complete transition to the alpha-Fe2O3 structure to take place.

Pressure-induced phase transition and octahedral tilt system change of Ba2BiSbO6

Journal of Solid State Chemistry, 2006

High-resolution X-ray synchrotron powder diffraction studies under high-pressure conditions are r... more High-resolution X-ray synchrotron powder diffraction studies under high-pressure conditions are reported for the ordered double perovskite Ba2BiSbO6. Near 4 GPa, the oxide undergoes a pressure-induced phase transition. The symmetry of the material changes during the phase transition from space group R3¯ to space group I2/m, which is consistent with a change in the octahedral tilting distortion from an a-a-a- type to a0b-b- type using the Glazer notation. A fit of the volume pressure data using the Birch Murnagaham equation of state yielded a bulk modulus of 144(8) GPa for the rhombohedral phase.

Compression mechanisms of symmetric and Jahn–Teller distorted octahedra in double perovskites: A2CuWO6 (A=Sr, Ba), Sr2CoMoO6, and La2LiRuO6

Journal of Solid State Chemistry, 2006

The Energy Citations Database (ECD) provides access to historical and current research (1948 to t... more

Structure of Sn(ND3)2F4—A Molecular Precursor for the Synthesis of Nitride Fluorides

Journal of Solid State Chemistry, 1998

The molecular and crystal structure of Sn(ND 3) 2 F 4 has been determined at room temperature usi... more The molecular and crystal structure of Sn(ND 3) 2 F 4 has been determined at room temperature using high resolution neutron powder diffraction. The compound crystallizes in a monoclinic unit cell containing two formula units with a ‫؍‬ 7.7673(7) A s , b ‫؍‬ 6.2765(5) A s , c ‫؍‬ 5.1708(4) A s , ‫؍‬ 102.871(5)°, and spacegroup P2 1 /n. The structure consists of isolated tin-centered octahedra with ammonia groups in trans configuration. The intramolecular stability of this compound has been investigated using Hu¨ckel molecular orbital calculations. Compared to Si(NH 3) 2 F 4 , the metal-nitrogen bonding interaction is found to be stronger, while the metal-fluorine bond is weaker in Sn(ND 3) 2 F 4. Due to a different hydrogen bonding network the intermolecular orientation and packing of the octahedral M(NH 3) 2 F 4 molecules in Sn(ND 3) 2 F 4 is different from the arrangement observed in Si(NH 3) 2 F 4. The implications for the use of these compounds as molecular precursors for the synthesis of SiNF and SnNF are discussed.

Crystal structures and phase transitions in Sr doped Ba2InTaO6 perovskites

Journal of Solid State Chemistry, 2013

Laboratory and synchrotron X-ray powder di4raction and neutron powder di4raction have been used t... more Laboratory and synchrotron X-ray powder di4raction and neutron powder di4raction have been used to determine crystal structures in the perovskites SrTi x Zr 1؊x O 3. Superlattice re6ections in the neutron di4raction patterns proved to be the most reliable indicators of lower symmetry structures. The room temperature structures are orthorhombic Pbnm in the range of 04x40.4, tetragonal I4/mcm in the range 0.4(x40.95, and cubic Pm3 m for 0.95(x41. Additional measurements were made on the room-temperature tetragonal samples at x ‫؍‬ 0.5, 0.75, and 0.9. The 5rst two were examined at elevated temperatures, using neutron di4raction methods, where the transition tetragonal to cubic was noted. Measurements at low temperatures using synchrotron X-ray powder di4raction are reported.

Low temperature structural studies on PrAlO3

Journal of Physics: Condensed Matter, 2001

... E-mail: [email protected] (Sandra M Moussa), [email protected] (Brendan J Ken... more

Low-Temperature Structure and Magnetic Properties of the Spinel LiMn2O4: A Frustrated Antiferromagnet and Cathode Material

Chemistry of Materials, 1999

Powder neutron diffraction has been used to study the nature of the structural transition away fr... more Powder neutron diffraction has been used to study the nature of the structural transition away from the Fd3m cubic structure upon cooling below ∼285 K in the spinel LiMn 2 O 4. We report powder data taken between 10 K and 333 K and propose a large cell tetragonal structure in space group I4 1 /amd for the material at 100 K. While complete segregation of the Mn 3+ and Mn 4+ ions is not possible in this space group, bond-valence analysis indicates that the distribution of Mn 3+ and Mn 4+ ions is not random and that there is a degree of charge segregation. Further, LiMn 2 O 4 is also of interest because it is an example of a geometrically frustrated antiferromagnet. Direct current magnetic susceptibility measurements show field-cooled, zero-field-cooled irreversibility at ∼65 K and a maximum in zerofield-cooled data at ∼40 K. Neutron diffraction shows magnetic scattering in the form of a broad peak assigned to short-range order which develops above 100 K. Upon cooling to 60 K additional Bragg peaks are seen, signaling long-range magnetic order. The Bragg peaks grow at the expense of the diffuse feature on cooling to 10 K but the latter persists even at the lowest temperature studied which indicates that a significant fraction of the spins still remain disordered. The magnetic Bragg peaks index on a tetragonal cell which is 2a, 2b, and 4c with respect to the low-temperature tetragonal cell and contains 1152 spins. The large size and implied complexity of the magnetic structure is consistent with both charge segregation and significant further neighbor exchange interactions.

Influence of Cation Size on the Structural Features of Ln1/2A1/2MnO3 Perovskites at Room Temperature

Chemistry of Materials, 1998

Sm, and Y 0.5 Sm 0.5) have been prepared, and structure determinations have been carried out at r... more Sm, and Y 0.5 Sm 0.5) have been prepared, and structure determinations have been carried out at room temperature using high-resolution synchrotron X-ray powder diffraction data. The octahedral tilting distortion increases as the average ionic radius of the Ln/A cations, 〈r A 〉, decreases. The two crystallographically distinct Mn-O-Mn bonds [Mn-O(eq)-Mn and Mn-O(ax)-Mn] are almost identical for Ln 0.5 Ca 0.5 MnO 3 compounds, with the exception of La 0.5 Ca 0.5 MnO 3. The La 0.5 Ca 0.5 MnO 3 compound and the entire Ln 0.5 Sr 0.5 MnO 3 series adopt structures where the Mn-O(eq)-Mn bond angle is consistently and significantly larger (2-6°) than the Mn-O(ax)-Mn bond angle. All of the Ln 0.5 Ca 0.5 MnO 3 compounds have Pnma symmetry, whereas across the Ln 0.5 Sr 0.5 MnO 3 series with increasing 〈r A 〉, an evolution from Pnma (tilt system ab + a-) over Imma (tilt system ab 0 a-) to I4/mcm (tilt system a 0 a 0 c-) symmetry is observed. It appears that the latter two tilt systems are stabilized with respect to the rhombohedral (R3 hc) aaatilt system, by short-range layered ordering of A-site cations. Changes in the octahedral tilt system at room temperature are linked to changes in the low-temperature magnetic structure. In particular, the simultaneous onset of charge ordering and CE-type antiferromagnetism in the Ln 0.5 Sr 0.5 MnO 3 series appears to be closely associated with the Imma structure. The average Mn-O bond distance is relatively constant across the entire series, but individual Mn-O bond distances show the presence of a cooperative Jahn-Teller effect (orbital ordering) at room temperature in Sm 0.5 Ca 0.5 MnO 3 and Sm 0.25 Y 0.25 Ca 0.5 MnO 3 .

Epitaxial Thin-Film Deposition and Dielectric Properties of the Perovskite Oxynitride BaTaO2N

Chemistry of Materials, 2007

Pulsed-laser deposition was employed to grow epitaxial thin films of the oxynitride perovskite Ba... more Pulsed-laser deposition was employed to grow epitaxial thin films of the oxynitride perovskite BaTaO 2 N on a conducting SrRuO 3 buffer layer deposited on a 100-cut SrTiO 3 single-crystal substrate. Phase purity and epitaxy were optimized at a substrate temperature of 760°C in a mixed gas atmosphere of 100 mTorr N 2 /O 2 (∼20:1). The dielectric permittivity, κ, of the BaTaO 2 N film was large, exhibiting a slight frequency dependence ranging from about 200 to 240 over the frequency range 1-100 kHz. Furthermore, over the temperature range 4-300 K the permittivity showed minimal variation as a function of temperature. The temperature coefficient of the dielectric constant, τ κ , is estimated to be in the range of-50 to-100 ppm/K. The coexistence of high dielectric permittivity and weak temperature dependence is an unusual combination in a single-phase material.

Synthesis and characterization of Na0.3RhO2·0.6H2O—a semiconductor with a weak ferromagnetic component

Solid State Communications, 2005

We have prepared the oxyhydrate Na 0.3 RhO 2 ⋅0.6H 2 O by extracting Na + cations from NaRhO 2 an... more We have prepared the oxyhydrate Na 0.3 RhO 2 ⋅0.6H 2 O by extracting Na + cations from NaRhO 2 and intercalating water molecules using an aqueous solution of Na 2 S 2 O 8. Rietveld refinement, thermogravimetric analysis (TGA), and energy-dispersive x-ray analysis (EDX) reveal that a non-stoichiometric Na 0.3 (H 2 O) 0.6 network separates layers of edge-sharing RhO 6 octahedra containing Rh 3+ (4d 6 , S=0) and Rh 4+ (4d 5 , S=1/2). The resistivities of NaRhO 2 and Na 0.3 RhO 2 •0.6H 2 O (T < 300) reveal insulating and semi-conducting behavior with activation gaps of 134 meV and 7.8 meV, respectively. Both Na 0.3 RhO 2 ⋅0.6H 2 O and NaRhO 2 show paramagnetism at room temperature, however, the sodium-deficient sample exhibits simultaneously a weak but experimentally reproducible ferromagnetic component. Both samples exhibit a temperature-independent Pauli paramagnetism, for NaRhO 2 at T > 50 K and for Na 0.3 RhO 2 •0.6H 2 O at T > 25 K. The relative magnitudes of the temperatureindependent magnetic susceptibilities, that of the oxide sample being half that of the oxyhydrate, is consistent with a higher density of thermally accessible electron states at the Fermi level in the hydrated sample. At low temperatures the magnetic moments rise sharply, providing evidence of localized and weakl-ordered electronic spins with effective moment per formula unit values of 2.0 x 10-1 B µ for NaRhO 2 and 0.8 x 10-1 B µ for Na 0.3 RhO 2 •0.6H 2 O

Low to High Spin-State Transition Induced by Charge Ordering in AntiferromagneticYBaCo2O5

Physical Review Letters, 2000

The oxygen-deficient double perovskite YBaCo 2 O 5 , containing corner-linked CoO 5 square pyrami... more The oxygen-deficient double perovskite YBaCo 2 O 5 , containing corner-linked CoO 5 square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range Co 21 ͞Co 31 charge ordering occur as well as a change in the Co 21 spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering.

Magnetostriction in a simple trivalent manganese perovskite

Physical Review B, 2004

We found an unusual negative thermal expansion in the simple perovskite MnF 3 below the Neel poin... more We found an unusual negative thermal expansion in the simple perovskite MnF 3 below the Neel point, where the spins are ordered in an A-type magnetic structure. The structure of MnF 3 has been studied between 5 and 500 K using powder neutron and synchrotron diffraction. The structure remains monoclinic C2/c over the entire temperature range. We have found that the interplay of spin and orbital ordering is sufficient to result in a large positive magnetovolume effect without charge ordering. This expansion is reminiscent of the Invar effect observed in certain alloys.

Phase Transition of Zeolite RHO at High-Pressure

Journal of the American Chemical Society, 2001

rho, undergo a reversible phase transition from centric to acentric structures. In the case of th... more rho, undergo a reversible phase transition from centric to acentric structures. In the case of the NaCs-rho, the volume of the centric form expands upon pressure increase, probably due to the sorption of pressure media into the pores [2] (see Figure). In the Li-rho, the transition back to the centric structure on pressure release is sluggish and occurs over a period of weeks. The Cd-rho shows intermediate back-transition kinetics and compressibility compared to NaCs-and Li-rho, which indicates the pressure response of rho is dependent upon the initial cation distribution in the pores. Analysis on peak half-width indicates the existence of deviatoric stress in acentric phase, which is released upon transition back to the centric form.