Stability and Lattice Dynamics of Ruddlesden–Popper Tetragonal Sr2TiO4

We report a combined experimental and theoretical lattice dynamics study of the Ruddlesden–Popper layered compound Sr2TiO4. From inelastic neutron scattering experiments, we derive the generalized phonon density of states of Sr2TiO4. We also report its heat capacity, thermal expansion, and thermodynamic Gruneisen parameters using the calculated bulk modulus and find a large value of about 2. Using Raman scattering experiments under pressure, we discuss a potential structural distortion of the tetragonal structure above 11 GPa, which could be due to nonhydrostatic compression. The mode Gruneisen parameters of the four Raman-active modes are determined and shown to be in reasonable agreement with those obtained by density functional perturbation theory (DFPT) calculations. The temperature behavior of the Raman-active modes was studied, allowing us to determine the implicit volume and explicit anharmonic contributions. Above 400 K, the implicit volume contribution dominates the temperature-induced variation of the four Raman-active modes, whereas, below this temperature, the explicit anharmonic contribution is the dominant contributor to the highest energy mode. Our results underline the importance of anharmonicity in vibration-related properties of Sr2TiO4.