Raman scattering studies of the pressure- and temperature-dependent phases of the orbital ordering material KCuF$_{3}$*

We present a study of the temperature- and pressure-dependence of the prototypical orbital ordering system KCuF$_{3}$ using Raman scattering. Temperature-dependent measurements offer evidence for structural instabilities in KCuF$_{3}$ at temperatures well below the putative 800K orbital ordering temperature; this evidence includes the observation of anomalous softening of several phonon modes between 50K and 300K and an E$_{g}$ phonon mode splitting below 50K. The latter indicates a tetragonal-to-orthorhombic structural transition near 50K just preceding the 3D magnetic ordering temperature at T$_{N} \quad \approx $ 40K. Low-temperature, pressure-dependent Raman studies of KCuF$_{3}$ are also reported to clarify the pressure dependence of the low temperature structural phases in this material.