Evidence of local structural distortions and subtle thermal disorder in transparent photochromic yttrium oxyhydride

The structural properties of photochromic yttrium oxyhydride powder in its transparent state were examined using x-ray diffraction and temperature-dependent extended x-ray absorption fine structure spectroscopy (EXAFS) combined with reverse Monte Carlo (RMC) simulations. The refinement of the x-ray powder diffraction pattern, employing the Rietveld method, indicates that yttrium oxyhydride crystallizes in the nanocrystalline phase with the cubic space group Fm-3m (225), at room temperature. The lattice parameter was determined as a = 5.404(3) Å, and the nanocrystallite size was estimated at d = 16(2) nm. The partial radial distribution functions (RDFs) g(r) for Y-O, O-O, and Y-Y atom pairs were obtained from the results of the RMC simulations of the Y K-edge EXAFS spectra measured at three temperatures (10, 150, and 300 K). The analysis of the RDFs reveals a subtle impact of the thermal disorder and splitting of the second coordination shell of yttrium atoms (the Y-Y RDF), remaining at all temperatures. This observation, also supported by our density functional theory calculations, suggests the presence of local structural distortions associated with yttrium sites, which do not affect the long-range crystal order.