Comment on "First-principles study of the influence of (110)-oriented strain on the ferroelectric properties of rutile TiO2"

• In a recent Brief Report, Grunebohm et al. [Phys. Rev. B 84, 132105 (2011)] report that they fail to reproduce the A(2u) ferroelectric instability of TiO2 in the rutile structure calculated with density functional theory within the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) by Montanari et al. [Chem. Phys. Lett. 364, 528 (2002)]. We demonstrate that this disagreement arises from an erroneous treatment of Ti 3s and 3p semicore electrons as core in their calculations. Fortuitously the effect of the frozen-semicore pseudopotential cancels the phonon instability of the PBE exchange correlation, and the combination yields phonon frequencies similar to the local density approximation harmonic values. Grunebohm et al. also attempted and failed to reproduce the soft acoustic phonon mode instability under (110) strain reported by Mitev et al. [Phys. Rev. B 81, 134303 (2010)]. For this mode the combination of PBE-GGA and frozen semicore yields a small imaginary frequency of 9.8i. The failure of Grunebohm et al. to find this mode probably arose from numerical limitations of the geometry optimization approach in the presence of a shallow double well potential; the optimization method is not suitable for locating such instabilities.

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NATURVETENSKAP  -- Kemi -- Teoretisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Theoretical Chemistry (hsv//eng)

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