AN EXPERIMENTAL AND THEORETICAL INVESTIGATION OF PHONONS AND LATTICE INSTABILITIES IN METASTABLE DECOMPRESSED SRGEO3 PEROVSKITE

We report detailed Raman and IR spectroscopic measurements for the dec ompressed high-pressure perovskite phase of SrGeO3. The appearance of a first-order Raman spectrum and slight splittings in the infrared ban ds suggest that the symmetry of the recovered metastable perovskite ph ase is lowered from Pm3m. This interpretation is fully supported by fi rst-principles LDA calculations using the LAPW method, which indicate a small tetragonal distortion. The static lattice energy is lowered by 3.3 meV (per formula unit) by allowing rotational relaxation of the G eO6 octahedra. The calculations permit a reliable assignment of the zo ne centre phonon modes of SrGeO3 perovskite. The calculated pressure d ependence of the ferroic IR-active modes is in excellent agreement wit h our measured data and reveals an incipient soft-mode behaviour in th e tension regime. Further calculations of the GeO6 unit as a function of octahedral volume reveal instabilities to local off-centre Ge4+ dis placements as the octahedra are expanded. This provides a detailed und erstanding of the large dielectric constant (e(0) similar to 100) for this nontransition-metal-containing decompressed perovskite phase.