RB-87 NMR-STUDY OF THE CUBIC TO TETRAGONAL PHASE-TRANSITION IN RBCAF3

The static and dynamical phenomena occurring around the structural phase transition at 195.90±0.15 K in RbCaF3 have been investigated by means of Rb87 Fourier-transform NMR measurements. The second-order shift of the central line due to the static quadrupole perturbation has been related to the electric field gradient arising at the Rb site below Tc and the temperature dependence of the angle of rotation φ of the CaF6 octahedra has been obtained. The Rb87 spin-lattice relaxation rate above Tc has been related to the rotational fluctuations of the octahedra induced by the critical-soft-mode branches, from which information on the symmetry and the anisotropy of the fluctuations are derived. The analysis of the results shows: (i) below 170 K the temperature dependence of φ is well described by the classical mean-field approximation, with critical exponent β=12 and extrapolated transition temperature T0=220 K; (ii) above about 170 K a nonclassical critical behavior is evidenced, the apparent critical exponent being not equal to 13, suggesting a possible changeover in the dimensionality of the correlations of the rotations; (iii) the rotational fluctuations appear to be predominantly of R25 symmetry but strongly anisotropic, i.e., only slightly correlated in adjacent (100) planes as is consistent with the hypothesis of softening of a large portion of the R-M branch. Order-of-magnitude estimates for the static angle of rotation as well as for the degree of anisotropy of the fluctuations are also obtained and discussed. © 1979 The American Physical Society.