High temperature X-ray diffraction studies on sodium yttrium fluoride

Abstract

The high temperature behaviour of two forms of sodium yttrium fluoride, i.e. Na 1.5Y 1.5F 6 (hexagonal) and NaYF 4 (cubic), have been studied using high temperature X-ray diffraction (HT-XRD) in the temperature range of 293–1073 K in vacuum. The studies revealed that both modifications are stable up to 773 K. The temperature variation of the lattice parameters as well as the coefficients of the average lattice thermal expansion of the compounds have been evaluated in the range 293–773 K. The coefficient of the average volume thermal expansion of NaYF 4 ( α v=81.68×10 -6 K −1) was found to be much higher than that of Na 1.5Y 1.5F 6 ( α v=66.25×10 −6 K −1). At higher temperatures (>1000 K), Na 1.5Y 1.5F 6 (hexagonal) and NaYF 4 (cubic) decompose into a new phase, probably due to loss of NaF. The phase evolution has been identified in situ.