OBSERVATION OF PRESSURE-INDUCED STRUCTURAL TRANSITION IN K2CUF4 BY PHONON-RAMAN SCATTERING EXPERIMENTS - CHANGE OF THE ORBITAL ORDERING BY PRESSURE

Since the ferromagnetic intralayer exchange interaction in K2CuF4 is d ue to the antiferrodistortive (AFD) orbital ordering of the 3d holes o f Cu2+ ions, we expect that the ferromagnetism of this compound to dis appear when the AFD order of the distortion of CuF6 octahedra collapse s. Recent magnetic susceptibility measurements performed at high press ures on this compound actually confirmed the disappearance of the ferr omagnetism above approximately 9 GPa [M. Ishizuka ct al.: J. Phys. Sec . Jpn. 65 (1996) 1927]. To obtain evidence of the pressure-induced str uctural change which corresponds to the magnetic change mentioned abov e, are measured the pressure dependence of the phonon-Raman spectra of this compound. As a result, we found that the scattering lines releva nt to the vibrational modes of CuF6 octahedra ordered in the AFD fashi on disappeared above 9 similar to 10 GPa. This result is consistent wi th the structure above 9 GPa revealed by recent X-ray diffraction expe riments that we performed.