Crystal Chemistry and Thermodynamics of HREE (Er, Yb) Mixing in aXenotime Solid Solution

摘要

Rare earth elements (REEs), the 15 naturally occurring lanthanides plusyttrium and scandium, are ubiquitously used in modern life as they are criticalcomponents of many advanced devices and technologies. However, the demand forREEs is not equal, with the heavy rare earth elements (HREEs) having a higher demand.Xenotime (HREEPO4) is an important HREE ore mineral and globally is an economicalsource of HREE. Most of the crystallographic and thermodynamic properties ofxenotime endmembers have been elucidated by calorimetric, solubility, and high-pressure studies. Yet, in natural systems, endmembers are rarely encountered, andinstead, REE solid solutions are more commonly observed. In this work, we characterizethe crystal chemistry, thermodynamics of HREE mixing, and high-temperature materialbehaviors and thermochemistry of a synthetic erbium (Er)-ytterbium (Yb) binaryxenotime solid solution (Er(x)Yb(1-x)PO4) using a suite of experimental techniques,including X-rayfluorescence spectroscopy, synchrotron X-ray powder diffractionimplemented with Rietveld analysis, Fourier transform infrared spectroscopy coupledwith attenuated total reflectance, Raman spectroscopy, thermogravimetric analysis coupled with differential scanning calorimetry,and high-temperature oxide melt drop solution calorimetry. Our results shed light on the formation of natural xenotimes and lay thefoundation for their industrial applications as thermal coating materials

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