LOCAL DISORDER AND STRUCTURAL PHASE-TRANSITION IN RB1-XCSXCAF3-NI+ CRYSTALS STUDIED BY EPR

The EPR spectra of different Ni+ centres in Rb1-xCsxCaF3 (0 less than or equal to x less than or equal to 1) crystals have been analyzed in order to study the local disorder around Ni+ probes. Different. types of Ni+ centres have been tentatively identified. Their spin Hamiltonia n parameters have been obtained, adjusting computer simulated spectra to the experimental ones. Changes in the Ni+-F- distance are observed. They are due both to Cs+ substitutions in the next nearest neighbour shell around Ni+ as well as to a global effect of the whole crystal. F rom the temperature dependence of these EPR spectra the critical tempe rature of the cubic to tetragonal structural phase transition that app ears in these crystals has been determined as a function of x. A criti cal exponent beta approximate to 0.3 has been found in the x = 0 and x = 0. 1 samples. (C) 1998 Elsevier Science Ltd. All nights reserved.