The chemical bond and atomic displacements in SrTiO3 from x-ray diffraction analysis

The deformation electron-density (dynamic Fourier) maps and the anharmonicity of atomic displacements in strontium titanate, SrTiO3 (Gram-Charlier model), were studied by high-precision single-crystal X-ray diffraction analysis at 145(1) and 296(2)K. Space group <P(m)over bar 3m>,cubic, lambda(Mo K alpha) = 0.71069 Angstrom, Z = 1, F(000) = 84, T = 145 (1) K, a = 3.8996 (5) Angstrom, V = 59.30 (2) Angstrom(3), D-x = 5.138 (2) g cm(-3) mu = 26.728 mm(-1), R = 0.0063, wR = 0.0040, S = 1.05 for 131 unique reflections and T = 296 (2) K, a = 3.901(1) Angstrom, V = 59.36 (5)Angstrom(3), D-x = 5.133 (4)g cm(-3), mu = 26.700 mm(-1), R = 0.0071, wR = 0.0050, S = 1.40 for 109 unique reflections. Strong anharmonicity of the atomic displacements was observed for all atoms at 145 K and for Ti and O atoms at 296 K. These are explained by a model in which electronic instability in the TiO6 octahedron leads to a displacement of the Ti atom from the center of the octahedron, and the lattice instability resulting from the consequent stretching of the Sr-O bonds leads to a rotation of the octahedra. Both distortions show only short-range order at the temperatures studied, but show indications of freezing out as the temperature is lowered towards the rotational phase transition at 106 K. The experimental dynamic Fourier deformation electron-density maps and the Hirshfeld atomic charges were calculated.