Effect of M site cation ordering on the lattice dynamical properties of the tetragonal Sr2MO4 (M=Cr, Fe, Co, Mo, and Sn) crystals

Using normal coordinate analysis, the Raman and Infrared phonons are investigated for the tetragonal Ruddlesden-Popper series in phase I4/mmm and symmetry D4h17. Three stretching and six bending force constants have been included for the calculations of the zone centre vibrational modes. Theoretical assignments for the Raman and infrared wavenumbers for Sr2CrO4, Sr2MoO4, and Sr2SnO4 compounds have been reported for the first time. As per our knowledge, no theoretical assignment has been done for the infrared frequencies for Sr2FeO4 and Sr2CoO4 compounds in I4/mmm phase so far. A strong correlation of the M site has been observed for the various frequencies giving insight into the structure of the studied compounds. The force constants involving the M-site atom show an expected set trend and also indicate the changes in values whenever the atomic orbit changes. Also, the frequencies which are completely dominated by the M atoms show a specific nature as the atomic number changes indicating the strong effect of the size of the M atom in the vibrational properties. In the tetragonal structure of the Ruddlesden-Popper phase of Sr2MO4 (M=Cr, Fe, Co, Mo, and Sn) crystals for each normal mode, the potential energy distributions have been analysed and the contributions of bending and stretching force constants towards different frequencies have been determined.