Ab initio electronic and phonon simulations have been carried out within DFT/GGA-PBE/PAW for layered titanium-oxypnictides, BaTi2Pn2O (Pn = As, Sb, Bi) [K. Nakano, K. Hongo, and R. Maezono, Sci. Rep. 6, 29661 (2016)]. There has been a puzzle between experiments and theoretical predictions on the charge ordering of these compounds. Unconventional mechanisms to explain this discrepancy have been argued so far, even affecting the understanding of superconductivity on the compound. We provide a new prediction, by which the discrepancy itself is resolved without any complicated unconventional explanation. Phonon dispersions and changes of nesting vectors in Fermi surfaces are clarified to lead to the variety of superstructures even for the common crystal structures when without CDW, including orthorhombic 2x2x1 one for BaTi2As2O, which has not yet been explained successfully so far, being different from tetragonal √2x√2x1 for BaTi2Sb2O and BaTi2Bi2O. Lattice instabilities are detected by the imaginary phonon dispersions evaluated for undistorted lattice structures. Taking each of the imaginary phonon modes, the structural relaxations along the mode are evaluated with the structural symmetries fixed to Pbmm for BaTi2As2O, P4/mbm for BaTi2Sb2O and BaTi2Bi2O. To estimate Tc, we used Allen-Dynes formula with the effective Coulombic interaction. The superstructure predicted from the imaginary phonon modes for Pn = As is found to be consistent with the symmetry observed in recent experiments [B.A. Frandsen et al., Nat. Commun. 5 5761 (2014)]. We further optimized the structure along the predicted phonon mode so that the imaginary modes disappear. The resultant geometry gives the magnitude of distortion being fairly well coincident with the experimental values. The striking finding here is that we can account for the varieties of observed superstructures even starting with the common undistorted structure when without the charge ordering. Because of the common structure, many preceding theoretical works never expected such variety [D. Suetin et al., JETP Letter 97, 220 (2013); D. Singh et al., New J. Phys. 14, 123003 (2012); A. Subedi, Phys. Rev. B. 87, 054506 (2013)]. This new finding can naturally resolve a discrepancy between experiments and theoretical predictions on the charge ordering of the compounds without any relying on complicated unconventional mechanism proposed recently, which could also affect the understanding of superconductivity on the compounds.

Phonon-induced superstructures in layered titanium-oxypnictides superconductors

255th National Meeting (2018)

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