Ligand hyperfine coupling and electron structure features in the EPR spectra of Cu-doped multicomponent borate crystals: an interpretation based on the concept of the Jahn–Teller effect
Institute of Physics AS CR, Na Slovance 2, 18221, Prague, Czech Republic
2 O. O. Galkin Donetsk Institute for Physics and Engineering, National Academy of Sciences of Ukraine, 03028, Kyiv, Ukraine
3 Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Straße 2A, 12489, Berlin, Germany
4 Institute of Physics PAS, al. Lotnikow 32/46, 02-668, Warsaw, Poland
5 A. A. Galkin Donetsk Physico-Technical Institute, R. Luxemburg 72, 83114, Donetsk, Ukraine
Accepted: 30 November 2022
Published online: 16 December 2022
We report structural and spectroscopic properties of the yttrium and europium aluminum borates YAl3(BO3)4 and EuAl3(BO3)4 doped by divalent copper with the content of 0.1 at.%. The tested samples have been synthesized by the spontaneous solution-melt crystallization method. The X-ray diffraction analysis shows that the samples crystallize in the huntite structure with space group R32. The lattice parameters and atomic coordinates of YAl3(BO3)4: 0.1 at.% Cu and EuAl3(BO3)4: 0.1 at.% Cu single crystals are determined. It was found that the Cu ions substitute Al and locate in octahedral oxygen coordination complexes with the C2 site symmetry. During the EPR experiment, the second-order Jahn–Teller effect (or the pseudo-Jahn–Teller effect) and hyperfine structures are detected in both samples. The YAl3(BO3)4: 0.1 at.% Cu sample manifests the ligand hyperfine (or superhyperfine) splitting related to the features of the host matrix. The parameters and axes of the EPR spectrum of Cu2+ (gX, gY, gZ, AX, AY, AZ) have been determined. Analysis of the EPR spectra performed by the modified crystal field theory revealed that the three differently oriented but energetically equivalent crystallographic sites of the Cu ions result in different scenarios of the EPR spectrum formation. The distinct orientation of Cu-containing coordination complexes about the crystal axes results in occurring the signals in different parts of the EPR spectrum. The strong deviation of the EPR spectrum axes accompanying the pseudo-Jahn–Teller effect allocates directions of the best resolution for observation of both the superfine structure in EuAl3(BO3)4 and the ligand hyperfine structure in YAl3(BO3)4. The developed theoretical model of the EPR spectra relates the appearance of the ligand hyperfine splitting in the YAl3(BO3)4: 0.1 at.% Cu sample with the interaction of doped copper with the two pairs of equivalent yttrium ions from the host matrix.
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