Department of Physics

The low thermal expansion ceramic system, Ca1-xSr{x}Zr4P6O24, for the compositions with x = 0, 0.25, 0.50, 0.75 and 1 was synthesized by solid-state reaction. The sintering characteristics were ascertained by bulk density measurements. The fracture surface microstructure examined by scanning electron microscopy showed the average grain size of 2.47 μm for all the compositions. The thermal expansion data for these ceramic systems over the temperature range 25–800°C is reported. The sinterability of various solid solutions and the hysteresis in dilatometric behaviour are shown to be related to the crystallographic thermal expansion anisotropy. A steady increase in the amount of porosity and critical grain size with increase in x is suggested to explain the observed decrease in the hysteresis.

The ferroelectric Pb(Zr0.53Ti0.47)O3 (PZT) and SrBi2Ta2O9 (SBT) thin films were prepared by laser ablation technique. The dielectric analysis, capacitance–voltage, ferroelectric hysteresis and DC leakage current measurements were performed before and after 50 MeV Li3+ ion irradiation. In both thin films, the irradiation produced some amount of amorphisation, considerable degradation in the ferroelectric properties and change in DC conductivity. On irradiation of these thin films, the phase transition temperature [Tc] of PZT decreased considerably from 628 to 508 K, while SBT exhibited a broad and diffuse transition with its Tc decreased from 573 to 548 K. The capacitance–voltage curve at 100 kHz showed a double butterfly loop with a large decrease in the capacitance and switching voltage. There was decrease in the ferroelectric hysteresis loop, remanant polarisation and coercive field. After annealing at a temperature of 673 K for 10 min while PZT partially regained the ferroelectric properties, while SBT did not. The DC conductivity measurements showed a shift in the onset of non-linear conduction region in irradiated SBT. The degradation of ferroelectric properties of the irradiated thin films is attributed to the irradiation-induced partial amorphization and the pinning of the ferroelectric domains by trapped charges. The regaining of properties after annealing is attributed to the thermal annealing of the defects generated during the irradiation.

Pb(Zr0.53Ti0.47)O3 (PZT) and SrBi2Ta2O9 (SBT) ferroelectric thin films were prepared on Pt coated Si substrates by pulsed laser ablation technique and their responses to the high energy Lithium ion irradiation were studied in terms of their ac conductivity, dielectric studies, polarization hysteresis and structural properties. The PZT and SBT thin films were fabricated in Metal–Insulator–Metal capacitor configuration and were irradiated with the 50 MeV Lithium ions with varying fluence from 1×1012 to 1×1014 ions cm−2. The ferroelectric properties were observed to be degraded by the irradiation. The ac conductivity of the films was explained by the charge carrier hopping model. Irradiated PZT thin films exhibited a significant variation on the frequency dependent exponent(s) while the irradiated SBT thin films had a negligible influence. The activation energy calculated from the Arrhenius plots was found to increase from 1.05 to 1.55 eV for PZT and from 0.85 to 0.91 eV for SBT thin film after irradiation. These energies were attributed to the oxygen vacancies and the deep trap carriers induced by the irradiation.

A novel solid-state, single-step calcination route is adopted to synthesize pure perovskite phase of PMN1−x–PTx (x = 0.0, 0.2 and 0.35) ceramics by utilizing the information obtained from dilatometry in association with thermal analysis and XRD studies. Using nitrate- and carbonate-based starting materials fine particles of reaction intermediates (Pb3Nb2O8 type pyrochlores) are obtained in the temperature range of 600–700 °C. Higher calcination temperatures lead to the formation of B-site rich pyrochlores (Pb3Nb4O13 type pyrochlores), which is found to be detrimental for perovskite formation. The pyrochlore phase formed in-between 600 and 700 °C helps in achieving pure perovskite phase below 850 °C and bring down the sintering temperature to ∼1000 °C. In this method the material is exposed to >900 °C, only once during the entire processing, which is found to be beneficial in achieving better lead stoichiometry and the enhancement of the electronic properties. Ceramics of up to 93% theoretical density are investigated for their microstructure, dielectric and piezoelectric properties.

The effects of oxygen plasma treatment on epitaxial ZnO thin films grown by molecular beam epitaxy were studied. The Au-ZnO-In junction exhibiting ohmic behaviour before the treatment gradually changes to a Schottky junction with the increase in oxygen plasma treatment time. The crystallinity and the surface microstructure did not change to any great extent after the treatment. However, the x-ray photoelectron spectroscopy studies show the removal of conductive OH layer from the surface of ZnO films and the current-voltage characteristics of Au-ZnO-In junction exhibit the rectifying behaviour after oxygen plasma treatment. The fabricated Au-ZnO-Au ultraviolet (UV) detector was successfully tested and was observed to be sensitive to the two UV sources used. The photoresponsivities of the UV detector for the irradiation of two different power densities 350 (λ = 356 nm) and 420 µW cm −2 (λ = 254 nm) are 13.5 A W −1 , 15.3 A W −1 at 5 V and 128.9 A W −1 , 160 A W −1 at 10 V, respectively.

We present structural, electrical transport, magnetic, and electronic structure studies of Mg 0.95 Mn 0.05 Fe 2−2x Ti 2x O 4 ferrite using x-ray diffraction, dielectric spectroscopy, DC magnetization and near edge x-ray absorption fine structure (NEXAFS) measurements. The x-ray diffraction study shows a structural transition from cubic to tetragonal with Ti substitution. The dielectric constant and DC conductivity increase with Ti substitution up to x = 0.2. However, with further increase of substitution both the dielectric constant and DC conductivity decrease. This electrical behavior indicates that at low values of substitution, hopping between Fe 3+ and Fe 2+ increases whereas at higher concentrations the total content of Fe ions decreases. It is observed that all the samples exhibit ferrimagnetic behavior at 300 K and the saturation magnetization decreases with increase in Ti substitution. The NEXAFS measurements have been carried out at O K-, Fe L-, Fe K-, and Ti L-edges to investigate the chemical states and the electronic structure of the Mg 0.95 Mn 0.05 Fe 2−2x Ti 2x O 4 (0 x 0.5) system at room temperature. The O K-edge spectra indicate that the Fe 3d orbitals are considerably modified with the substitution of Ti ions. At x 0.3, a new spectral feature appears (∼532 eV) due to the transitions from oxygen 2p to Ti 3d orbitals which starts dominating the pre-edge spectra of the system. Both Fe L 3,2 -and Fe K-edge spectra indicate that iron Fe 3+ ions convert into Fe 2+ with the substitution of Ti ions. The Ti L 3,2 -edge NEXAFS spectra reveal that the Ti remains in the 4+ state for all the samples. The observed experimental results have been explained on the basis of dilution of the magnetic sublattice by Ti substitution, which provides a strong interplay between electrical and magnetic properties along with their electronic structure.

The influence of 50 MeV Li 3þ ion irradiation on the thermal expansion of the low thermal expansion ceramic Ca 1Àx Sr x Zr 4 P 6 O 24 (x ¼ 0:00, 0.25, 0.50, 0.75 and 1.00) belonging to the sodium zirconium phosphate (NZP) family of ceramics was studied in the temperature range 300-1100 K. The observed changes in XRD, microstructure and thermal expansion are reported. These changes are strongly dependent upon the strontium substitution at calcium sites in the basic structure. The XRD and scanning electron microscope (SEM) studies indicate that the ion irradiation causes amorphization, especially at the grain boundaries. The thermal expansion hysteresis reduces due to Sr substitution and is further reduced upon irradiation. It is suggested that the amorphization due to irradiation helped release of thermal stress and thereby lead to the reduction in the hysteresis in thermal expansion. The composition with x ¼ 0:50 having low coefficient of thermal expansion (CTE), low hysteresis and the least thermal expansion anisotropy is least affected by the Li 3þ irradiation and hence may qualify as the radiation resistant material. Ó

We report the x-ray absorption studies on O K, Fe L 3,2 , Ni L 3,2 , and Pr M 5,4 edges in PrFe 1−x Ni x O 3 along with their structural, electrical transport, and magnetization characterizations. All the samples are in single phase having orthorhombic structure with space group Pnma for x ഛ 0.4. Ni doping at Fe site brings the system in the conducting regime, resistivity decreases from G⍀ cm to 260 m⍀ cm at room temperature, and the magnetic ordering is stabilized. The temperature dependent resistivity follows the semiconducting behavior and fits well with Greaves' variable range hopping model. The gap parameter is reduced from 2 to 0.118 eV. The materials are in weak ferromagnetic state and magnetization is gradually decreasing with the enhancement of Ni substitution, whereas magnetic anisotropy is reduced substantially. A new feature about 2.0 eV lower than the pre-edge of PrFeO 3 in O K edge is observed with Ni substitution at Fe site due to the 3d contraction effect and is growing with the increase of Ni substitution. From 2p edges ͑L 3,2 edges͒ of Fe and Ni, it is confirmed that both are in trivalent state and Ni is in mixed spin state with 3d 7 ͑t 2g 5 , e g 2 and t 2g 6 , e g 1 ͒ configuration in ground state. The trivalent state of Pr is confirmed by x-ray absorption spectroscopy of Pr at M 5,4 edges. These observations have been explained on the basis of charge carrier doping in PrFeO 3 with the Ni substitution. The disorder induced localization is found to control the conductivity and magnetism in the present materials. The controlled doping of carriers in semiconducting regime and ferromagnetism propose these materials as a promising candidate for the spintronic applications.

The relaxor ferroelectric compositions Pb(Fe 1=2 Nb 1=2 )O3 (PFN) and [Pb(Mg 1=3 Nb 2=3 )O3] 0:8 -[PbTiO3] 0:2 (PMN-PT) are studied for their radiation response to the high energy heavy ions (50 MeV Li 3+ , uence 1 × 10 13 -1 × 10 14 ions=cm 2 ) in terms of their structural, dielectric and piezoelectric properties. There was no change in the crystallinity of both the compositions after irradiation as seen from the XRD. The PFN composition did not show much change in the dielectric constant but the value of Tm decreased by 8 • C. The PMN-PT composition showed an increase in the dielectric constant with increase in the irradiation uence from 1 × 10 13 to 1 × 10 14 ions=cm 2 with no change in the value of Tm. The piezoelectric coe cient decreased in both the samples after irradiation. Among the compositions studied, PFN is observed to be more radiation resistant to changes in structural and dielectric properties than PMN-PT.

High quality gallium doped ZnO ͑Ga:ZnO͒ thin films were grown on c-Al 2 O 3 ͑1000͒ by plasma-assisted molecular beam epitaxy, and Ga concentration N Ga was controlled in the range of 1 ϫ 10 18 -2.5ϫ 10 20 /cm 3 by adjusting/changing the Ga cell temperature. From the low-temperature photoluminescence at 10 K, the donor bound exciton I 8 related to Ga impurity was clearly observed and confirmed by comparing the calculated activation energy of 16.8 meV of the emission peak intensity with the known localization energy, 16.1 meV. Observed asymmetric broadening with a long tail on the lower energy side in the photoluminescence ͑PL͒ emission line shape could be fitted by the Stark effect and the compensation ratio was approximately 14-17% at N Ga ജ 1 ϫ 10 20 /cm 3 . The measured broadening of photoluminescence PL emission is in good agreement with the total thermal broadening and potential fluctuations caused by random distribution of impurity at N Ga lower than the Mott critical density.

The lead based ferroelectric PbZr 0.53 Ti 0.47 O 3 (PZT), (Pb 0.90 La 0.10 )TiO 3 (PLT10) and (Pb 0.80 La 0.20 )TiO 3 (PLT20) thin films, prepared by pulsed laser ablation technique, were studied for their response to the 70 MeV oxygen ion irradiation. The dielectric analysis, capacitance-voltage (C-V) and DC leakage current measurements were performed before and after the irradiation to high-energy oxygen ions. The irradiation produced considerable changes in the dielectric, C-V, leakage characteristics and induced some amount of amorphization. The PZT films showed partial recrystallization after a thermal annealing at 400°C for 10 min. The phase transition temperature [T c ] of PLT20 increased from 115°C to 120°C. The DC conductivity measurements showed a shift in the onset of non-linear conduction region. The current density decreased by two orders of magnitude after irradiation. After annealing the irradiated films at a temperature of 400°C for 10 min, the films partially regained the dielectric and electrical properties. The results are discussed in terms of the irradiation-induced amorphization, the pinning of the ferroelectric domains by trapped charges and the thermal annealing of the defects generated during the irradiation.

The lead based ferroelectric PbZr 0.53 Ti 0.47 O 3 (PZT), (Pb 0.90 La 0.10 )TiO 3 (PLT10) and (Pb 0.80 La 0.20 )TiO 3 (PLT20) thin films, prepared by pulsed laser ablation technique, were studied for their response to the 70 MeV oxygen ion irradiation. The dielectric analysis, capacitance-voltage (C-V) and DC leakage current measurements were performed before and after the irradiation to high-energy oxygen ions. The irradiation produced considerable changes in the dielectric, C-V, leakage characteristics and induced some amount of amorphization. The PZT films showed partial recrystallization after a thermal annealing at 400°C for 10 min. The phase transition temperature [T c ] of PLT20 increased from 115°C to 120°C. The DC conductivity measurements showed a shift in the onset of non-linear conduction region. The current density decreased by two orders of magnitude after irradiation. After annealing the irradiated films at a temperature of 400°C for 10 min, the films partially regained the dielectric and electrical properties. The results are discussed in terms of the irradiation-induced amorphization, the pinning of the ferroelectric domains by trapped charges and the thermal annealing of the defects generated during the irradiation.

Negative thermal quenching (NTQ) was observed in bound exciton emission line in undoped ZnO and the donor-to-valence-band emission in heavily Ga-doped ZnO thin films grown on c-Al2O3 (1000) through low temperature photoluminescence spectra. In both cases, the enhanced feature of PL peak intensity occurred in the temperature range of 35–45 K corresponding to the energies of either excitation to the vibrational/rotational resonance states or the involvement of B-valence band considering the activation energy of about 5 meV.

We report on the synthesis, microstructure and thermal expansion studies on Ca 0.5+x/2 Sr 0.5+x/2 Zr 4 P 6-2x Si 2x O 24 ( x = 0.00 to 1.00 ) system belong to NZP family of low thermal expansion ceramics. The ceramics synthesized by co-precipitation method at lower calcination and the sintering temperatures were in pure NZP phase up to x = 0.37. For x  0.5, in addition to NZP phase, ZrSiO 4 and Ca 2 P 2 O 7 form as secondary phases after sintering. The bulk thermal expansion behavior of the members of this system was studied from 30 to 850 C. The thermal expansion coefficient increases from a negative value to a positive value with the silicon substitution in place of phosphorous and a near zero thermal expansion was observed at x = 0.75.

In the present work, a systematic study has been carried out to understand the effect of In doping on the various properties of the ZnO nanocrystalline thin films. In-doped ZnO nanocrystalline thin films with different indium concentrations (1.98%, 4.03%, 6.74%, 8.62% and 10.48% In) have been synthesized by sole gel method. The grain size and surface roughness of the In-doped ZnO thin films are observed to be smaller than those of the ZnO thin films. 6.74% In-doped ZnO films with a low resistivity of 1.95 Â 10 À3 U cm and a high mobility of 2.19 cm 2 V À1 S À1 have been prepared under optimal deposition conditions. Inverted organic solar cells containing In-doped ZnO as an electron extraction layer with the structure indium tin oxide (ITO)/In-doped ZnO/poly[N-9 0 -heptadecanyl-2,7-carbazole-alt-5,5-(4 0 ,7 0 -di-2-thienyl-2 0 ,1 0 ,3 0 -benzothiadiazole)] (PCDTBT): [6,6]-phenyl C 71 -butyric acid methyl ester (PC 71 BM)/MoO 3 /Al have been fabricated. The inverted organic solar cell with 6.74% In-doped ZnO exhibited a power conversion efficiency of 5.58%, which is the best efficiency reported so far for these type of solar cells. The device performance has been optimized by varying the indium doping concentration. The results clearly demonstrate that significant improvement in power conversion efficiency can be obtained by incorporating In into the ZnO films.

We report on the single phase synthesis and room temperature structural characterization of PbFe 0.67 W 0.33 O 3 (PFW) multiferroic. The PFW was synthesized by low temperature sintering, Columbite method. Analysis of powder XRD pattern exhibits single phase formation of PFW with no traces of pyrochlore phase. Detailed analysis of room temperature neutron diffraction (ND) reveals cubic phase at room temperature, space group Pm-3m. The ND pattern clearly reveals magnetic Bragg peak at 2 = 18.51 o (Q = 1.36Å -1 ). The refinement of magnetic structure reveals G-type antiferromagnetic structure in PFW at room temperature. The dielectric constant and loss tangent decreases with increasing frequency. The room temperature P-E measurements shows a non-linear slim hysteresis, typical nature of relaxor multiferroics, with saturation and remnant polarizations of P s = 1.50 µC/cm 2 and P r = 0.40 µC/cm 2 , respectively.

Stability of MnB2 with AlB2-type structure revealed by first-principles calculations and experiments Appl. Phys. Lett. 102, 061906 (2013) Communication: Designed diamond ground state via optimized isotropic monotonic pair potentials J. Chem. Phys. 138, 061101 (2013) Search for long-range ferromagnetism: Charge-spin co-doped Ba1−x−yLax+yTi1−xMxO3 (M = Cr, Fe, and Co) J. Appl. Phys. 113, 063902 (2013) Structure and multiferroic properties of Bi(1-x)DyxFe0.90Mg0.05Ti0.05O3 solid solution J. Appl. Phys. 113, 054102 First-principles study of the structural, electronic, and optical properties of Y-doped SrSi2

We report the origin of room temperature (RT) ferromagnetic and ferroelectric properties of Pb(Fe 1/2 Nb 1/2 )O 3 (PFN) ceramic sample prepared by modified solid-state reaction synthesis by a single-step method, based on X-ray diffraction (XRD), neutron diffraction (ND), Mössbauer spectroscopy and electron paramagnetic resonance (EPR) spectroscopy results. Formation of singlephase monoclinic PFN ceramic with Cm space group was confirmed by XRD and ND at RT. The morphology studied by scanning electron microscopy (SEM) confirmed uniform microstructure of the sample with average grain size of ∼ 2 μm. The ND, Mössbauer spectroscopy, M-H loop and EPR studies were carried out to confirm the existence of weak ferromagnetism at RT. A clear opening of hysteresis (M-H ) loop is evidenced as the existence of weak ferromagnetism at RT. EPR spectrum clearly shows the ferromagnetism through a good resonance signal. The symmetric EPR line shape with g = 1.9895 observed in PFN sample was identified to be due to Fe 3+ ions. Mössbauer spectroscopy at RT shows superparamagnetic behaviour with presence of Fe in 3+ valence state. Ferroelectric P -E loops on PFN at RT confirm the existing ferroelectric ordering. Our observation is in agreement with literature, and it supports that the origin of ferromagnetism and ferroelectricity is isolated, i.e. from different regions in the sample. Our results do not support the multiferroic nature of PFN at RT.