The localized vibrations of hydride ions paired with divalent rare-earth impurities in calcium fluoride

Abstract

Hydrogen or deuterium has been diffused into calcium fluoride crystals containing various rare earths. The fundamental and second harmonic vibrational transitions of the hydride ions have been examined by infrared absorption measurements at 77 K and 4.2 K. It is found that a fraction of the hydride ions form close pairs with the rare-earth impurities, and the localised vibrations of these hydride ions give rise to satellite absorption lines around those from similar ions in unperturbed fluoride ion sites. For crystals containing ytterbium and europium impurities thermal treatments alone produce centres which are interpreted as Yb²⁺-H^-(s) and Eu²⁺-H^-(s' pairs of two types, depending on whether the hydride ion occupies a first or second neighbour F^-(s) site.