⁸⁷Rb nuclear-magnetic-resonance study of the cubic to tetragonal phase transition in RbCaF₃

Abstract

The static and dynamical phenomena occurring around the structural phase transition at Tc 195.90±0.15 K in RbCaF₃ have been investigated by means of ⁸⁷Rb 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 T_c and the temperature dependence of the angle of rotation φ of the CaF₆ octahedra has been obtained. The ⁸⁷Rb spin-lattice relaxation rate above T_c 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 T 170 K the temperature dependence of φ is well described by the classical mean-field approximation, with critical exponent β=1/2 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 1/3, 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.