Chemical Pressure, Structural Instability, and IR-Active Phonons in a Series of Rare-Earth Analogues of Francisite Cu3RE(SeO3)2O2Cl

ATR and transmission spectra of a series of rare-earth francisite-like phases Cu3RE(SeO3)2O2Cl (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 Cu3Dy(SeO3)2O2Cl crystal, an anomalous softening of low-frequency phonons at low temperatures was observed, which was apparently associated with the structural instability of the compound.