Kanoknan Sarasamak

Elastic properties of perovskite ATiO3 (A = Be, Mg, Ca, Sr, and Ba) and PbBO3 (B = Ti, Zr, and Hf): First principles calculations

The mechanical properties of perovskite oxides depend on two metal oxide lattices that are intercalated. This provides an opportunity for separate tuning of hardness, Poissons ratio (transverse expansion in response to the compression), and shear strength. The elastic constants of series of perovskite oxides were studied by first principles approach. Both A-site and B-site cations were systematically varied in order to see their effects on the elastic parameters. To study the effects of A-site cations, we studied the elastic properties of perovskite ATiO3 for A being Be, Mg, Ca, Sr, or Ba, one at a time. Similarly, for B-site cations, we studied the elastic properties of PbBO3 for B being Ti, Zr, or Hf, one at a time. The density functional first principles calculations with local density approximation (LDA) and generalized gradient approximation (GGA) were employed. It is found that the maximum C11 elastic constant is achieved when the atomic size of the cations at A-site and B-site are comparable. We a...

High Pressure Phase of LiAlO2: A First Principles Study

The homogeneous structural phase transition between the natural and high-pressure forms of LiAlO2, i.e., γ-LAO and δ-LAO, were investigated by using first principles calculations. For the exchange-correlation functional, the generalized-gradient approximation (GGA) was used in comparison with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. The projector augmented wave (PAW) method was used to allow a relatively low-energy cutoff plane wave expansion outside the core region while maintaining the complicated description of the wave function near the core region. Our calculations showed that both HSE and GGA calculations provided comparable energetic properties. For the electronic properties, the HSE calculations do not suffer from band gaps underestimations but require much more computation demand. The HSE calculations showed that, under hydrostatic pressure, the natural phase γ-LAO is in energetic equilibrium with the high-pressure phase δ-LAO at 2.3 GPa. The calculated equilibrium phase pressures are in a reasonable agreement with the experimental transformation pressure (2 GPa) obtained by an anvil cell technique [J. Solid State Chem. 188, 6 (2008)]. However, the transformation pressure obtained by a shock recovery technique [J. Solid State Chem. 177, 5 (2004)] is much higher (9 GPa). The large difference in the transformation pressure obtained by different experimental techniques could be attributed to the energetic transformation barrier between the two phases. Based on HSE calculations, the enthalpy barrier for the homogeneous transformation between the two phases at the phase equilibrium pressures is 1.8 eV. The band structures and the partial density of states of both γ-LAO and δ-LAO at the ambient pressure are also presented.

First-principles Study of Antisite Defects in Orthorhombic PbZrO3

First-principles calculations based on density functional theory (DFT) within local density approximation were employed to investigate the antisite defects, including PbZr and ZrPb, in orthorhombic PbZrO3 by determining their defect formation energies. The formation energies of antisite defects were then compared with those of other dominant defects, i.e., lead Pb, zirconium Zr, and oxygen O vacancies to examine the likelihood of their existence. Our results revealed that PbZr defect in neutral charge state is the most dominant defect under O-rich or oxidizing condition in agreement with the previous work. In addition, there is a little structural relaxation when the Zr atom is replaced by Pb atom to form PbZr defect in neutral charge state. In opposite, under O-poor or reducing condition, the formation energies of antisite defects are quite high and higher than those of vacancy defects. This suggests that antisite defects are unlikely to form under reducing condition.

First Principles Calculations on Crystal and Electronic Structure of Co2P4O12

Crystal and electronic structure of violet-pink Co2P4O12 have been investigated using first principles calculations based on density functional theory. Its theoretical X-ray diffraction and X-ray absorption fine structure spectra were calculated and compared with their experimental spectra to verify its monophasic. The calculated spectra are in good agreement with the experimental data giving parameters of a = 11.993 Å, b = 8.328 Å, c = 10.150 Å and β = 118.51°. Our calculations on band structure and density of states of Co2P4O12 showed that its major electronic transition is associated with internal Co-3d. The calculations indicated that Co2P4O12 is a half metal ferromagnetic material which disagreed with the experimental knowledge.

First-Principles Investigation on Structural and Electronic Properties of Ferromagnetic Fe2P4O12

Structural and electronic properties of Fe2P4O12 have been investigated using first-principles calculation technique. The results indicated that the Fe2P4O12 structure is monoclinic of C2/c with lattice parameters of a = 12.228 Å, b = 8.530 Å, c = 9.835 Å and β = 118.67°. Two nonequivalent octahedral FeO6 from the calculation have an average Fe−O distance of 2.143 Å. Both FeO6 are dominated by covalent interactions assigned to Fe3d and O2p at the valent electronic states. The DOS calculation gives well explanation on its half-metallic ferromagnetic property. These results are in very good agreement with the previous experimental reports.

First-Principles Investigation on Elastic Constants of TiN under High Pressure

Elastic constants of NaCl-type TiN under pressure were investigated by first-principles calculations within both local density approximation (LDA) and Perdew-Burke-Ernzerhof generalized-gradient approximation (PBE-GGA). At ambient pressure, the calculated lattice parameter, bulk modulus, and elastic constants of NaCl-type TiN are in well agreement with other available values. Under pressure, all elastic constants, C11, C12, and C44, are found to increase with pressure. C11, which is related to the longitudinal distortion, increases rapidly with pressure while C12 and C44 which are related to the transverse and shear distortion, respectively, are much less sensitive to pressure.