The effect of rare-earth substitution on the Debye temperature of inorganic phosphors

Abstract

In the quest to predict photoluminescent efficiency in rare-earth substituted inorganic phosphors, research has shown that materials consisting of ordered, rigid crystal structures tend to possess the highest photoluminescent quantum yields. A compound's Debye temperature (ΘD), which can be calculated using ab initio calculations, is an ideal proxy for quantitatively comparing structural rigidity among different inorganic compounds, allowing potentially efficient phosphors to be selected from large crystal structure databases. However, the high computational cost of these calculations limits estimating ΘD for unsubstituted host crystal structures only. It is assumed that the low substitution concentration of the rare-earth luminescent center does not significantly influence a material's Debye temperature. This work evaluates the validity of this approximation by examining the effect of luminescent center substitution on a host structure's ΘD. Two well-known phosphors, (Y1-xCex)3Al5O12 (x = 0 - 0.05) and Ba1-δEuδMgAl10O17 (δ= 0 - 0.15), were synthesized with varying rare-earth concentrations, while ΘD was computationally estimated and then determined by ultrasonic pulse-echo speed-of-sound and low-temperature heat capacity measurements. The ensuing results provide key implications for using ΘD as a proxy for structural rigidity in substituted inorganic compounds.