Materials of the International Seminar of the Institute of Spectroscopy of the Russian Academy of Sciences (ISAN), dedicated to the anniversary of Professor M.N. Popova
Spectroscopy of condensed state

Chemical pressure, structural instability, and IR-active phonons in a series of rare-earth analogues of francisite Cu$_3$RE(SeO$_3$)$_2$O$_2$Cl

N. N. Novikova^a, V. A. Yakovlev^a, E. S. Kuznetsova^b, P. S. Berdonosov^b, S. A. Klimin<sup>a

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

Abstract: ATR and transmission spectra of a series of rare-earth francisite-like phases Cu$_3$RE(SeO$_3$)$_2$O$_2$Cl (RE is a rare earth element, RE = Nd, Sm, Eu, Gd, Dy, Ho, Tm, Yb) were studied. The frequencies of the IR-active phonons of the crystals under study were determined. In the dependences of phonon frequencies on the ionic radius in a series of isostructural compounds, two tendencies are observed: a hardening of frequencies due to an increase in chemical pressure and a softening of the vibration frequencies, in which rare-earth ions are involved, due to an increase in the mass of a specific rare-earth ion. In the Cu$_3$Dy(SeO$_3$)$_2$O$_2$Cl crystal, an anomalous softening of low-frequency phonons at low temperatures was observed, which was apparently associated with the structural instability of the compound.

Keywords: rare-earth francisite-like phases, optical spectroscopy, IR-active phonons, chemical pressure.

Received: 22.07.2021
Revised: 22.07.2021
Accepted: 05.08.2021

DOI: 10.21883/OS.2022.01.51889.22-21

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