Positional, isotopic mass and force constant disorder in molybdate glasses and their parent metal oxides as observed by neutron diffraction and Compton scattering

Abstract

Abstract Neutron Compton scattering and neutron diffraction, augmented by ab initio modelling, have been applied for the characterisation of disorder in molybdate glasses, 20MoO3 + 30Nd2O3 + 50B2O3, 40MoO3 + 30Nd2O3 + 30B2O3, and 50MoO3 + 25Nd2O3 + 25B2O3, along with their parent metal oxides, B2O3, MoO3 and Nd2O3. Softening of the atom-projected vibrational densities of states (apVDOSes) was observed for all constituent nuclei in the metal oxide systems, with respect to the ab initio harmonic lattice dynamics predictions. For the oxygen, the mode softening was attributed to force-constant disorder, and for the boron to the isotopic mass disorder. A universal scale of disorder in oxide glasses has been proposed. The scale relies on the assumption that the amount of disorder-induced phonon softening can be bracketed by two extreme situations: (i) a completely disordered system with no confining potential whose momentum distribution is described by the Maxwell-Boltzmann momentum, and (ii) the compositional average of harmonic lattice dynamics predictions for individual metal oxide systems. The highest degree of disorder on this scale was observed for the boron in the molybdate glasses with the highest amount of B2O3. The distributions of total (summed over all nuclei) effective force constants were found to be at least an order of magnitude wider than their counterparts calculated for the parent metal oxide systems, indicating a much greater degree of positional disorder-induced force constant disorder in the molybdate glasses. The sum of all mean effective forces acting on all constituent nuclei in the molybdates was found to be decreasing with the increasing amount of the glass-former B2O3, clearly showing a systematic softening of the structure of the glasses. The biggest contributions to the total average effective mean force in all three molybdates were found for the molybdenum and neodymium.