Revised structural phase transitions in the archetype KMnF3 perovskite crystal

摘要

Reinvestigation of the structural and vibrational properties of the typical perovskite KMnF3, exhibiting two antiferrodistortive structural phase transitions, was performed using x-ray powder diffraction and Raman spectroscopy in the range between 30 and 300 K. The x-ray-diffraction work has unambiguously shown that a monoclinic distortion (a(-)b(+)c(-) tilt system P2(1)/m space group) is observed at low temperature below T-C2 = 91 K. This result corresponds with the Raman temperature study which shows that this transition, in spite of its first-order character, can be associated with a group-subgroup relation between tetragonal and monoclinic symmetries. Additionally, existence of a large structural disorder far above the cubic to tetragonal transition (T-C1 = 186 K) is suggested by the two following experimental indications: (i) persistence of hard modes of the tetragonal phase in the cubic symmetry, and (ii) existence of Raman broad bands in normally inactive ideal cubic phase, which an interpreted by the folding of the whole phonons branches of the cubic Brillouin zone. This last observation allows us to follow the evolution of the cubic R-15' soft mode versus temperature, based only on the Raman-scattering data, in full agreement with previous inelastic neutron data. The results of Raman investigations into KMnF3 are discussed in more general framework of structural disorder in perovskite systems which exhibit anisotropic correlation between octahedra. [S0163-1829(99)00522-6]. [References: 44]