Papers by Kanoknan Sarasamak
Physical Review B, 2008
First principles calculations are carried to study the structural stability of SiC, GaN, InN, ZnO... more First principles calculations are carried to study the structural stability of SiC, GaN, InN, ZnO, and CdSe which are found to transform from a fourfold coordinated wurtzite ͑WZ͒ structure under ambient conditions to two different crystalline structures under loading of different triaxialities. Under hydrostatic compression, transformation into a sixfold coordinated rocksalt ͑RS͒ structure occurs, and under uniaxial compression along the ͓0001͔ direction and uniaxial tension along the ͓0110͔ crystalline direction ͑except SiC and GaN͒, transformation into a fivefold coordinated unbuckled wurtzite phase ͑HX͒ is observed. The lack of the WZ→ HX transformation for SiC and GaN under uniaxial tension along the ͓0110͔ direction is because for these two materials the tensile stress required for the enthalpy of HX to become lower than the enthalpy of WZ is higher than their corresponding ultimate tensile strength. Critical stress levels for the transformations are found to depend on the formation energies of the WZ, HX, and RS structures which in turn are related to the ionicity of each material. The transformations are a manifestation of the tension-compression response asymmetry of these materials.
Physical Review B, 2007
We report a previously unknown body-centered-tetragonal structure for ZnO. This structure results... more We report a previously unknown body-centered-tetragonal structure for ZnO. This structure results from a phase transformation from wurtzite in Í“0001Í”-oriented nanorods during uniaxial tensile loading and is the most stable phase for ZnO when stress is above 7 GPa. The stress-induced phase transformation has important implications for the electronic, piezoelectric, mechanical, and thermal responses of ZnO. The discovery of this polymorph brings about a more complete understanding of the extent and nature of polymorphism in ZnO. A crystalline structure-load triaxiality map is developed to summarize the relationship between structure and loading.
Physical Review Letters, 2006
We predict a previously unknown phase transformation from wurtzite to a graphitelike (P6 3 =mmc) ... more We predict a previously unknown phase transformation from wurtzite to a graphitelike (P6 3 =mmc) hexagonal structure in 01 10-oriented ZnO nanowires under uniaxial tensile loading. Molecular dynamics simulations and first principles calculations show that this structure corresponds to a distinct minimum on the enthalpy surfaces of ZnO for such loading conditions. This transformation is reversible with a low level of hysteretic dissipation of 0:16 J=m 3 and, along with elastic stretching, endows the nanowires with the ability to recover pseudoelastic strains up to 15%.
Physical Review B, 2010
Elastic constants and sound velocities calculated from first principles as function of pressure a... more Elastic constants and sound velocities calculated from first principles as function of pressure are presented for wurtzite SiC, GaN, InN, ZnO, and CdSe. The C 11 and C 33 elastic constants, which are involved in longitudinal sound waves along symmetry directions, are found to monotonically increase with pressure. The shear moduli C 44 and C 66 , which are involved in transverse sound waves along symmetry directions, either decrease with increasing pressure or initially increase from zero pressure but then turn over and start decreasing. Of special interest is the pressure at which the C 44 and C 66 elastic constants cross. At this pressure, the transverse acoustic waves in the basal plane, which are shown to be closely related to the symmetry breaking strain component that leads to the phase transition, become easier to excite than the ones with displacement along the c axis. It is found that this crossover pressure is an upper limit to the actual phase transition pressure. The average of the calculated equilibrium transition pressure and the crossover pressure is proposed as a good estimate for the actual transition pressure in cases where the transition is strongly kinetically hindered by an enthalpy barrier between the two phases. This occurs for SiC and GaN and is confirmed with literature data for AlN. For the remaining materials, all these pressures are close to each other. The trends of the elastic constants and sound velocities with the materials' Phillips scale ionicity are also reported.
Computational Materials Science, 2010
First principles density functional theory is employed to study the elastic constants and sound v... more First principles density functional theory is employed to study the elastic constants and sound velocities in wurtzite-SiC under pressure. The focus is on the behavior of the elastic constants (C ij ) and sound veloc-
Phase transformations of SiC, GaN, InN, ZnO, and CdSe from wurtzite (WZ) to three other different... more Phase transformations of SiC, GaN, InN, ZnO, and CdSe from wurtzite (WZ) to three other different crystalline structures under loading of different stress tensors are studied using first-principle calculations. The first transformation studied is well known and occurs under hydrostatic compression and leads to a six-fold coordinated rocksalt (RS) structure. The equilibrium pressures for this transformation of the materials are calculated and found to be proportional to the energy difference between the phases at zero stress and vary monotonically with the materials' ionicity. The second and third transformations studied occur under uniaxial stresses and lead to two new crystal structures previously unknown for these materials. Specifically, uniaxial compression along the [0001] direction or uniaxial tension along the [011 0] direction, causes a transformation to a five-fold coordinated unbuckled wurtzite structure which we named HX. On the other hand, uniaxial tension along the [0001] direction causes the materials to transform into a body-centered-tetragonal structure which we named BCT-4. The critical equilibrium transformation stresses for these transformations are obtained and their correlation with the ionicity of the materials is analyzed.
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Papers by Kanoknan Sarasamak