VIBRATIONAL INVESTIGATION OF THE ANTIFERRODISTORTIVE STRUCTURAL INSTABILITIES IN THE PEROVSKITE CRYSTAL RBCAF3

The Raman spectra of the three phases of the perovskite RbCaF3 have be en recorded. A detailed group-theoretical analysis together with a rig id-ion model calculation in the cubic symmetry have been carried out i n order to identify the observed peaks. The established compatibility relations demonstrate that the orthorhombic Gamma(0,0,0) modes come fr om the Gamma(0,0,0) and X(0.5,0.5,0) tetragonal modes, themselves comi ng from the Gamma(0,0,0), R(0.5,0.5,0.5), M(0.5,0,0.5), and X(0,0.5,0) eigenmodes of the cubic Brillouin zone. These calculations, together with the analysis of experimental Raman spectra, unambiguously show th at the first-order transition that occurs at T-c = 31.5 +/- 1 K gives rise to an orthorhombic symmetry (Pnma space group associated with the a(-)a(+)a(-) tilt system in Glazer's classification) without any grou p-subgroup relation to the tetragonal one. Particular attention was al so paid to hard Raman modes which are not directly involved in the tra nsition but still provide valuable information about the local symmetr y. The temperature dependence of the integrated Raman intensity of the se hard modes is correctly described by a power law t(2 beta) in which t is the reduced temperature, (T-T-c)/T-c, and the critical exponent beta is found to be 0.27. A deviation from this behavior becomes appar ent close to transition, indicating that local structural distortions subsist over a fairly wide temperature range.