PRESSURE EFFECT ON THE RAMAN-SPECTRA OF RBALF4 AND KALF4 - A PRESSURE-INDUCED MARTENSITIC PHASE-TRANSITION IN KALF4

The Raman spectra of RbAlF4 (phase II) and KAlF4 (Phase II and IV) at room temperature were measured under hydrostatic pressure up to 10 GPa and 4 GPa, respectively. While the frequencies of the former do not e xhibit any significant singularity against temperature, in the case of KAlF4 a strong frequency jump is observed at 0.21 GPa. This is attrib uted to the martensitic phase transition already observed as a functio n of temperature. The pressure dependence of the frequencies in the tw o compounds is discussed in relation to their structure and the struct ural phase transitions.