Order–Disorder Transitions in Cerium Hydrides, Manifested in Temperature-Dependent Raman Scattering Spectroscopy

Phononic and electronic Raman scattering of CeH_2+x (x = 0.25, 0.50, 0.79, and 0.94) were explored within the temperature range of 77–345 K. The experimental analysis was theoretically supported by DFT-calculated phonon dispersion curves and embedded cluster model ab initio calculations of the energies of Ce³⁺ crystal field states and is correlated with the broadly investigated “twin system” of lanthanide hydrides, which exhibits a highly similar temperature–composition phase diagram. The Raman spectrum of CeH_2.94 is dominated by a band with a frequency placed within an energy gap of the calculated phonon dispersion curves of BiF₃-type cubic CeH₃. This result goes along with structural modification, driven by H displacement from the expected Wyckoff positions, and in particular with the additional orthorhombic phase, which theoretically demonstrated LaH_∼3 to be the dynamically stable phase among the two phases. Octahedral H-sublattice rearrangements, strongly affected by repulsive H–H interactions, are known to drive the temperature-dependent order–disorder transformations in lanthanide hydrides. Hydrides with 0.7 < x < 0.8 were previously shown to undergo an electronic (metal–semiconductor) transition at ∼245 K, which is accompanied by a structural (cubic–tetragonal) phase transition. Here, we systematically demonstrate the ability of Raman scattering to closely follow this transition for CeH_2.79 by monitoring the tetrahedral H-sublattice phonons, as well as by following changes in the crystal field scheme around the Ce³⁺ ions. In the lanthanum “twin system” of CeH_2.25 and CeH_2.50, the structured long-range tetragonal order formed below ∼200 K is manifested in the temperature evolution of its diffraction patterns. However, although first-order Raman scattering spectra of solids with well-defined structures are composed of Brillouin Γ-point symmetry-allowed phonons, the CeH_2.25 Raman spectra practically follow the phonon density of states (PDOS), previously found by INS of La hydrides. This scheme raises an enigma in the Raman scattering analysis of “disordered” hydride systems, which will be presented and discussed. In CeH_2.50, a switch of the spectral profile is detected from PDOS-expressed spectrum to Γ-point-expressed spectrum commences at ∼200 K. Spectral changes in the crystal field scheme, also found in this temperature range, are argued, according to embedded cluster calculations, to stem from the modified landscape of octahedral H vacancies.