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Pressure-induced phase transformations of LiGaO2: First principles study
(2017-08-01)
Sailuam, Wutthigrai
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Sarasamak, Kanoknan
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Polanco, Miguel Angel Mendez
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Limpijumnong, Sukit
Phase transformations of β-LiGaO<inf>2</inf> (Pna2<inf>1</inf>) under different pressure conditions are studied using first principles calculations. Under specific conditions, LGO can exist in various structures some of which have not been previously reported (oP16 and P4<inf>1</inf>2<inf>1</inf>2). Under sufficiently strong hydrostatic pressure, LGO can transform into a rocksalt-like structure. Under strong uniaxial stress LGO can transform into a tetragonal structure or an orthorhombic structure depending on the orientation of the applied stress. The stability of different crystal structures under different applied stresses and the phase transformation processes are studied by calculating the enthalpy surfaces and transformation barriers under different stresses.
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First Principles Calculations of Structural and Elastic Properties of Perovskite Crystals: The Case of SrTiO3
(2016-06-01)
Pandech, Narasak
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Sarasamak, Kanoknan
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Limpijumnong, Sukit
The structural and elastic properties of perovskite crystals can be independently calculated by using first principles calculations without bias. The detailed calculation of cubic perovskite (Pm3m) crystal by first-principles calculations both LDA and GGA approaches was shown, using SrTiO<inf>3</inf> as an illustration case. The equilibrium volumes and bulk modulus of SrTiO<inf>3</inf> were evaluated by fitting to Birch-Murnaghan equation of states. The C<inf>11</inf>, C<inf>12</inf>, and C<inf>44</inf> elastic constants were obtained from a polynomial fit to the calculated energy-strain relations. For SrTiO<inf>3</inf>, the calculated lattice constants, bulk modulus, and all three elastic constants by both LDA and GGA calculations are in good agreement with experimental values. Because LDA gives a slightly smaller lattice constant compared to that obtained from GGA, the bulk modulus and all three elastic constants obtained from LDA are slightly larger than those values obtained from GGA. The calculation steps illustrated here can be used as a guideline to study elastic properties of other novel materials.
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First-principles study of Bi and Al in orthorhombic PbZrO3
(2016-04-01)
Chotsawat, Maneerat
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Sarasamak, Kanoknan
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Thanomngam, Pitiporn
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Limpijumnong, Sukit
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T-Thienprasert, Jiraroj
Lead zirconate PbZrO<inf>3</inf> (PZO) is one of the most important ceramic materials due to its antiferroelectric property, which can be used in many technological applications. Due to the toxicity of Pb, there is an attempt to replace Pb with other non-toxic elements. It has been reported that doping orthorhombic-PZO with Bi and Al atoms could stabilize the antiferroelectric property in a wide temperature range and reduce the lead content in the material. In this work, we used first-principles calculations based on density functional theory to investigate the microscopic and electronic structures of Bi and Al defects in orthorhombic-PZO. Our calculated defect formation energies revealed that Bi atom can substitute on either Pb site (A-site) or Zr site (B-site); depending on the Fermi-level as well as the crystal growth condition. On the other hand, Al atom is likely to substitute only on the Zr site. In addition, our calculations revealed that there is only a small binding between Bi<inf>Pb</inf> and adjacent Al<inf>Zr</inf> or Bi<inf>Zr</inf> with the binding energies of ∼0.2 eV. This indicates that Bi<inf>Pb</inf> and Al<inf>Zr</inf> (or Bi<inf>Zr</inf>) are unlikely to form complexes.
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First-Principles Investigation on Structural and Electronic Properties of Ferromagnetic Fe2P4O12
(2015-06-23)
Rerksompus, Pathompong
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Sarasamak, Kanoknan
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Boonchom, Banjong
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Thanomngam, Pitiporn
Structural and electronic properties of Fe<inf>2</inf>P<inf>4</inf>O<inf>12</inf> have been investigated using first-principles calculation technique. The results indicated that the Fe<inf>2</inf>P<inf>4</inf>O<inf>12</inf> structure is monoclinic of C<inf>2</inf>/c with lattice parameters of a = 12.228 Å, b = 8.530 Å, c = 9.835 Å and β = 118.67°. Two nonequivalent octahedral FeO<inf>6</inf> from the calculation have an average Fe-O distance of 2.143 Å. Both FeO<inf>6</inf> are dominated by covalent interactions assigned to Fe<inf>3d</inf> and O<inf>2p</inf> 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. © 2015
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Elastic properties of perovskite A TiO3 (A=Be, Mg, Ca, Sr, and Ba) and Pb B O3 (B=Ti, Zr, and Hf): First principles calculations
(2015-05-07)
Pandech, Narasak
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Sarasamak, Kanoknan
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Limpijumnong, Sukit
The mechanical properties of perovskite oxides depend on two metal oxide lattices that are intercalated. This provides an opportunity for separate tuning of hardness, Poisson's 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 ATiO<inf>3</inf> for A being Be, Mg, Ca, Sr, or Ba, one at a time. Similarly, for B-site cations, we studied the elastic properties of PbBO<inf>3</inf> 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 C<inf>11</inf> elastic constant is achieved when the atomic size of the cations at A-site and B-site are comparable. We also found that C<inf>12</inf> elastic constant is sensitive to B-site cations while C<inf>44</inf> elastic constant is more sensitive to A-site cations. Details and explanations for such dependencies are discussed.
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First principles calculations on crystal and electronic structure of Co2P4O12
(2014-09-02)
Rerksompus, Pathompong
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Sarasamak, Kanoknan
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Boonchom, Banjong
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Thanomngam, Pitiporn
Crystal and electronic structure of violet-pink Co<inf>2</inf>P <inf>4</inf>O<inf>12</inf> 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 Co<inf>2</inf>P<inf>4</inf>O<inf>12</inf> showed that its major electronic transition is associated with internal Co-3d. The calculations indicated that Co<inf>2</inf>P<inf>4</inf>O<inf>12</inf> is a half metal ferromagnetic material which disagreed with the experimental knowledge. © 2014 Taylor & Francis Group, LLC.
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First-principles study of antisite defects in orthorhombic PbZrO 3
(2014-09-02)
Chotsawat, Maneerat
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Sarasamak, Kanoknan
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Thanomngam, Pitiporn
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T-Thienprasert, Jiraroj
First-principles calculations based on density functional theory (DFT) within local density approximation were employed to investigate the antisite defects, including Pb<inf>Zr</inf> and Zr<inf>Pb</inf>, in orthorhombic PbZrO<inf>3</inf> 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 Pb<inf>Zr</inf> 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 Pb <inf>Zr</inf> 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. © 2014 Taylor & Francis Group, LLC.
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High pressure phase of LiAlO2: A first principles study
(2014-09-02)
Sailuam, Wutthigrai
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Sarasamak, Kanoknan
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Limpijumnong, Sukit
The homogeneous structural phase transition between the natural and high-pressure forms of LiAlO<inf>2</inf>, 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. © 2014 Taylor & Francis Group, LLC.
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First-principles investigation on elastic constants of TiN under high pressure
(2013-10-29)
Prasert, Kittiya
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Thanomngam, Pitiporn
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Sarasamak, Kanoknan
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, C<inf>11</inf>, C<inf>12</inf>, and C<inf>44</inf>, are found to increase with pressure. C<inf>11</inf>, which is related to the longitudinal distortion, increases rapidly with pressure while C<inf>12</inf> and C<inf>44</inf> which are related to the transverse and shear distortion, respectively, are much less sensitive to pressure. © (2013) Trans Tech Publications, Switzerland.
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Sound velocities and elastic properties of PbTiO3 and PbZrO 3 under pressure: First principles study
(2013-05-01)
Pandech, Narasak
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Sarasamak, Kanoknan
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Limpijumnong, Sukit
The elastic constants and sound velocities as a function of pressure for perovskite materials PbTiO<inf>3</inf> (PTO) and PbZrO<inf>3</inf> (PZO) were investigated by first principles calculations. Under ambient pressure, the calculated structural parameters were calculated and found to be in good agreement with known values. To study properties under pressure, PTO and PZO were calculated at several reduced volumes, each of which corresponds to the system under pressure. The C<inf>11</inf>, C<inf>12</inf> and C<inf>44</inf> elastic constants are all found to increase with pressure for the pressure range studied. Because the sound velocities are directly derived from the elastic constants, the relationships between the sound velocities and pressure also follow similar trends. The longitudinal modes are all larger than those of the transverse modes. © 2012 Elsevier Ltd and Techna Group S.r.l.

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