REALIZATION OF A MARTENSITIC PHASE-TRANSITION IN KFEF4 UNDER HIGH-PRESSURE

The room-temperature Raman spectra of KFeF4 (phase II) were measured u nder high pressures up to 3.5 GPa. The pressure dependence of the freq uencies exhibits strong changes at 1.2 GPa and minor changes at 2.2 Gp a. The former is attributed to a martensitic phase transition leading to the so-called TlAlF4-type structure. A hysteresis of about 0.15 GPa is observed. A strong hardening of two modes under pressure suggests that the phase change is achieved because of octahedra rotations which weaken the clamping between successive octahedra sheets. The singular ity at 2.2 Gpa presumably arises from a quasi-reversible phase transit ion in the TlAlF4-type structural arrangement. The vibrational frequen cies are discussed in comparison with those obtained in isomorphous co mpounds.