Raman measurement of materials with the K₂NiF₄ structure

The discovery of high temperature superconductivity in (La,Ba)2CuO4, and related systems which have the K2NiF4 structure, has renewed interest in these types of materials. We report measurements of the Raman active phonons in a transparent material, Sr2TiO4, that has this structure (as well as the related compounds Sr3Ti2O7 and Sr4Ti3O10, with similar structures). The group theoretical analysis is given, which sorts the normal modes in terms of the irreducible representatives of the 4/mmm-D4h point group. It is found that only the oxygen atoms in the long TiO bond (parallel to the c-axis) and the Sr atoms move in the four modes that are Raman active. The motion of the other atoms, as well as these atoms, is associated with the seven infrared active modes and the one silent mode. We observe all of the four allowed Raman active modes, and find that they are rather narrow and at low frequencies. This is understandable in terms of the structure. © 1987.