Abstract
The 195 K cubic-to-tetragonal structural phase transition (SPT) in RbCaF3 crystals has been investigated by electron nuclear double resonance (ENDOR) using Mn2+ as a probe. The rotational angle of the fluorine octahedra, which is the local order parameter of the transition, has been determined at 40 K for the first and second shells of F- ions surrounding the Mn2+ impurities. The values obtained are 5.1 degrees +or-0.2 degrees for the first shell and 7.4 degrees +or-1 degrees for the second shell. The thermal evolution of the first shell angle at temperatures close to that of the SPT has also been studied. The results have been compared with those in RbCdF3:Mn2+. The first shell angle measured at 40 K is similar to the one in RbCdF3:Mn2+. The second shell angle is almost double that in RbCdF3:Mn2+. Covalency effects through Cd2+ ions are suggested to account for this difference.
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