The effect of strain on the permittivity of SrTiO3 from first-principles study

The frequencies of Gamma transverse-optical phonons in the cubic perovskite phase of SrTiO3 were investigated within the frozen-phonon scheme using the first-principles method with ultrasoft pseudopotentials. The calculated frequencies in the cubic phase showed good agreement with the experimental data. Strain was found to have a strong influence on the frequency of the lowest frequency mode. The atomic motion of the lowest frequency mode mainly consisted of the Slater mode, but the Last mode, in which the strontium atom shows an out-of-phase vibration relative to the rigid TiO3 cluster, was found to be very important. At T = 0, the reciprocal of the permittivity was approximately in proportion to strain x (the difference of the lattice constant from the theoretically determined lattice constant). The calculated permittivity epsilon strongly depends on strain x at finite temperatures T; epsilon = C(x)/[T - T-0(x)], where C(x) approximate to 9.0 x 10(4)/(1 - 23.7x) and T-0(x) approximate to 38.0 + 1.53x. (C) 1997 Elsevier Science Ltd.