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    Elastic properties of A2Ti6O13 ( A = H, Li, Na, K and Rb): a computational study
    (2023-09-21)
    Simalaotao, Kodchakorn
    ;
    Thanasarnsurapong, Thanasee
    ;
    Maluangnont, Tosapol
    ;
    Phacheerak, Kanoknan
    ;
    Boonchun, Adisak
    The elastic properties of the alkali hexatitanate family A <inf>2</inf>Ti<inf>6</inf>O<inf>13</inf> (A = H, Li, Na, K, and Rb) are investigated based on density functional theory within a generalized gradient approximation plus Hubbard U (GGA+U) approach. The results showed that all members of the family are wide-band semiconductors and the calculated lattice parameters are consistent with experimental values. In terms of mechanical stability, the results indicated that the alkali hexatitanates are highly incompressible to uniaxial stress, with the largest elastic constant C<inf>22</inf> reaching values as high as 265 GPa in K<inf>2</inf>Ti<inf>6</inf>O<inf>13</inf>. The obtained elastic constants, using the stress-strain method, were used to calculate bulk modulus, shear modulus, Young’s modulus, brittleness and ductility, elastic anisotropy, Vickers hardness, sound velocities, and the Debye temperature. It was found that the member of the family with the highest atomic number of the alkaline group, Rb<inf>2</inf>Ti<inf>6</inf>O<inf>13</inf>, had the highest values of bulk, shear, and Young’s modulus, as well as the lowest values of shear and compression anisotropy, and a high Vickers hardness.
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    First-principles study of antisite defects in orthorhombic PbZrO 3
    (2014-09-02)
    Chotsawat, Maneerat
    ;
    Sarasamak, Kanoknan
    ;
    Thanomngam, Pitiporn
    ;
    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
    ;
    Sarasamak, Kanoknan
    ;
    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.