STRUCTURAL TRANSITIONS IN THE 2-DIMENSIONAL JAHN-TELLER SYSTEM K2CUXZN1-XF4 WITH THE VARIATION OF X - EXPERIMENTS BASED ON THE RAMAN-SCATTERING OF PHONONS

Measurements are reported of the Raman scattering of phonons in many crystals of K2CuxZn1-xF4 with different Cu concentrations x. With decreasing x, a structural change is confirmed at x approximately 0.8 besides the one at x approximately 0.4. Comparing the observed polarized spectra for 1 greater-than-or-equal-to x > 0.8 with those for 0.8 > x > 0.4, we find a transition at x approximately 0.8 from the D2h to an apparent D4h symmetry, which corresponds to a change of the long-range order of the cooperative Jahn-Teller distortion of CuF6 octahedra from three-dimensional to two-dimensional. Below x approximately 0.4, the long-range antiferrodistortive order in each c plane is destroyed as reported by Natsume and Yamada (NY) in 1985. The observed Stokes line including several lines which were missed in the report by NY for x > 0.8, 0.8 > x > 0.4 and 0.4 > x are well explained on the assumption of the space groups of D2h18, D4h5 and D4h17, respectively.