THE TETRAGONAL TO ORTHORHOMBIC STRUCTURAL PHASE-TRANSITION IN RBCAF3 SINGLE-CRYSTALS - INFLUENCE ON THE LOCAL ENVIRONMENT OF DIFFERENT NICKEL PROBES

The tetragonal to orthorhombic structural phase transition in RbCaF3 c rystals has been investigated by electron paramagnetic resonance using Ni3+, Ni2+ and two types of Ni+ center as probes. This transition is sample dependent and was only observed in some of the crystals. It has a first order character and shows a thermal hysteresis with transitio n temperatures of about 20 K and 40 K during cooling down and warming up processes, respectively. In the tetragonal phase the fluorine octah edra surrounding the Ni+ ions undergo an alternate rotation around one of the [100] cubic directions. The rotation axis changes to a [111] d irection in the orthorhombic phase. In both cases the rotation angle i s about 2.5 degrees. Tetragonal Ni2+ centers whose axes are along a [1 00] direction are transformed, because of the transition, to axial cen ters with the z axis approximately in the {110} planes and at about 23 degrees from the nearest [110] direction. The phase transition also p roduces a decrease in the symmetry of tetragonal Ni3+ centers. No defi nite model has been obtained for these centers in the ortherhombic pha se, although it seems that their symmetry is lower than axial. Besides , all the Ni+ and Ni2+ centers observed in the tetragonal phase are tr ansformed in the transition while some of the Ni3+ centers remain tetr agonal. (C) 1997 Elsevier Science Ltd.