Изменение вероятности оптических переходов в ионе Yb-=SUP=-3+-=/SUP=- при магнитных фазовых переходах в мультиферроике Cu-=SUB=-3-=/SUB=-Yb(SeO-=SUB=-3-=/SUB=-)-=SUB=-2-=/SUB=-O-=SUB=-2-=/SUB=-Cl

The infrared transmission spectra of Cu3Yb(SeO3)2O2Cl, an ytterbium analogue of the natural mineral francisite, have been studied in a wide temperature range. Two phase transitions (at TN = 35 K and TR = 9 K) were registered by studying the splittings of Yb3+ 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 Jmol-1. It was found that the probability of a 0-0 transition in the Yb3+ ion changes in the temperature range of both phase transitions, presumably due to spontaneous magnetostriction.