EPR Investigations of a Structural Phase Change in RbCaF₃, RbCdF₃, and TlCdF₃

Abstract

EPR investigations of the structural phase change which occurs in the compounds RbCaFa: Mn2+, TlCdF3:Fez+, and RbCdF3:Fe3+ were made. For RbCdF3 we have determined the critical index and measured the order parameter using two different probes, namely the Fe3+ ion and the Vk center.

Local release factors of Cr3+ and Ni2+ centers in the tetragonal phase of RbCdF3 crystal

“…(ii) ENDOR measurement [8] suggested a local rotational angle fE3:6ð1Þ for RbCdF 3 :Cr 3þ at TE17 K: If the estimated DR=R > in Ref. [8] is used, we have k l E1:5: The local release factor is consistent with that obtained from the assumption that the volume of the initial cubic cell and of the derived pseudo-cubic due to the MX 6 octahedral rotation are the same [5,20]. However, the assumption is not suitable (and so ka1:5) in many pure and doped ABX 3 crystals (see Table 1 and kE0:7 for SrTiO 3 [20]).…”

“…In general, the tetragonal distortion [which is related to DR=R > ¼ ðR jj À R > Þ=R > ¼ ðk À 1 2 Þf 2 in the tetragonal phase is small. So, the phase transition is often studied by EPR spectra using transition-metal or rare-earth ions as probes, because the EPR zero-field splitting D is sensitive to the tetragonal distortion of the octahedron surrounding a paramagnetic ion [4,5]. However, since the charge and/or size of the probe ion differ from those of the replaced host ion, the local phase transition behavior (characterized by k l and f l ) of impurity centers may be different from that in the pure crystal.…”

The temperature dependence of the structures of RbCaF3, and the determination of the low temperature phases

“…-While, in the perovskite compounds family, the ferroelectric phase transitions are now well described by the orderdisorder model proposed by Comes for BaTi03 [27], the antiferrodistorsive ones, related to octahedra tilts, remain an unsolved problem. The order-disorder description proposed by Rousseau [3] figure 4 together with the experimental and the classical displacive ones [26]. One can see the improvement brought by the orderdisorder description.…”

“…For this computation the very large thermal motion of fluorine must be taken into account [19]. When [26] (dashed line) and the order-disorder model [3] (solid line). This correction for thermal librations is similar to the one proposed by Glazer [20].…”

Influence of the crystal field on bent configuration of VK centres in AMF3 compounds

“…1 [5,61 and on RbCdF, [7]. In order to get information on the VK centre structure in crystals with noticeably different lattice parameters, we 1) 72017-Le Mans Cedex, France.…”