MN2- LOCAL ORDER-PARAMETER OF THE STRUCTURAL PHASE-TRANSITION MEASURED BY ENDOR( AS A PROBE IN RBCAF3 )

The 195K cubic-to-tetragonal structural phase transition (SPT) in RbCa F3 crystals has been investigated by electron nuclear double resonance (ENDOR) using Mn2+ as a probe. The rotational angle of the fluorine o ctahedra which is the local order parameter of the transition, has bee n determined at 40 K for the first and second shells of F- ions surrou nding the Mn2+ impurities. The values obtained are 5.1 degrees +/- 0.2 degrees for the first shell and 7.4 degrees +/- 1 degrees for the sec ond shell. The thermal evolution of the first shell angle at temperatu res close to that of the SPT has also been studied. The results have b een 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+ i ons are suggested to account for this difference.