Temperature dependence of local structural changes around transition metal centers Cr3+ and Mn2+ in RAl3(BO3)4 crystals studied by EMR.

Theoretical investigations are carried out to determine the temperature dependence of the local structural parameters of Cr 3+ and Mn 2+ ions doped into RAl 3 (BO 3 ) 4 (RAB, R = Y, Eu, Tm) crystals. The zero-field splitting (ZFS) parameters (ZFSPs) obtained from the spin Hamiltonian (SH) analysis of EMR (EPR) spectra serve for fine-tuning the theoretically predicted ZFSPs obtained using the semi-empirical superposition model (SPM). The SPM analysis enables to determine the local structure changes around Cr 3+ and Mn 2+ centers in RAB crystals and explain the observed temperature dependence of the ZFSPs. The local monoclinic C 2 site symmetry of all Al sites in YAB necessitates consideration of one non-zero monoclinic ZFSP (in the Stevens notation, b 2 1 ) for Cr 3+ ions. However, the experimental second-rank ZFSPs ( D = b 2 0 , E = 1 / 3 b 2 2 ) were expressed in a nominal principal axis system. To provide additional insight into low symmetry aspects, the distortions (ligand's distances Δ R i and angular distortions Δ θ i ) have been varied while preserving monoclinic site symmetry, in such way as to obtain the calculated values ( D , E ) close to the experimental ones, while keeping b 2 1 close to zero. This procedure yields good matching of the calculated ZFSPs and the experimental ones, and enables determination of the corresponding local distortions. The present results may be useful in future studies aimed at technological applications of the Huntite-type borates with the formula RM 3 (BO 3 ) 4 . The model parameters determined here may be utilized for ZFSP calculations for Cr 3+ and Mn 2+ ions at octahedral sites in single-molecule magnets and single-chain magnets. [ABSTRACT FROM AUTHOR]