Spectroscopy of condensed matter

Yb$^{3+}$ optical-transitions probability change at magnetic phase transitions in the multiferroic Cu$_3$Yb(SeO$_3$)$_2$O$_2$Cl

S. A. Klimin^a, E. S. Kuznetsova^b, P. S. Berdonosov<sup>b

a</sup> Institute of Spectroscopy, Russian Academy of Sciences, 108840 Moscow, Troitsk, Russia
^b Lomonosov Moscow State University, 119991 Moscow, Russia

Abstract: The infrared transmission spectra of Cu$_3$Yb(SeO$_3$)$_2$O$_2$Cl, an ytterbium analogue of the natural mineral francisite, have been studied in a wide temperature range. Two phase transitions (at $T_{\mathrm{N}}$ = 35 K and $T_{\mathrm{R}}$ = 9 K) were registered by studying the splittings of Yb$^{3+}$ Kramers doublets. It is shown that the low-temperature transition is spin-reorientational one. An analysis within the framework of the mean field theory allows us to conclude that the single-ion magnetic anisotropy of ytterbium does not coincide with the anisotropy of the copper magnetic subsystem. An estimate of the energy required for the reorientation of the copper magnetic subsystem has been obtained, it is 864 J mol$^{-1}$. It was found that the probability of a 0–0 transition in the Yb$^{3+}$ ion changes in the temperature range of both phase transitions, presumably due to spontaneous magnetostriction.

Keywords: francisite, multiferroic, optical spectroscopy, $f$–$f$ transitions, Kramers doublets.

DOI: 10.21883/OS.2023.07.56125.4397-23

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