Crystal Structure, Optical Properties, and Electronic Structure of Calcium Strontium Tungsten Oxynitrides CaxSr1–xWO2N

Abstract

Novel calcium strontium tungsten oxynitrides CaxSr1–xWO2N (x = 0.25 and 0.5) have been synthesized. The crystal and electronic structures, electron-density distribution, and optical properties of CaxSr1–xWO2N and CaxSr1–xWO4 (x = 0, 0.25, and 0.5) have been investigated by neutron, synchrotron, and X-ray powder diffraction; transmission electron microscopy energy-dispersive spectroscopy (TEM-EDS); scanning electron microscopy; UV–visible reflectance measurements; and ab initio density functional theory (DFT)-based calculations. Precursor materials CaxSr1–xWO4 (x = 0, 0.25, 0.5, and 1) with a scheelite-type structure were prepared by solid-state reactions, and heated at 900 °C for 5 h under an ammonia flow. The main phase in the product for the composition x = 1 was metallic tungsten W, whereas cubic Pm3̅m perovskite-type oxynitrides CaxSr1–xWO2N were obtained for the compositions x = 0, 0.25, and 0.5. The unit-cell parameter a of the cubic perovskite-type CaxSr1–xWO2N obtained from the Rietveld analysis of synchrotron X-ray and neutron powder diffraction data decreases with an increase of Ca concentration x (0 ≤ x ≤ 0.5), which indicates the substitution of Ca for Sr. The existence of nitrogen in CaxSr1–xWO2N was confirmed by (1) the refined occupancy factor in the Rietveld analysis of neutron data and (2) EDS. The maximum-entropy-method electron-density analysis combined with the DFT calculations indicates W–N and W–O covalent bonds in CaxSr1–xWO2N, which are formed by the overlapping of W 5d and anion 2p orbitals. The minimum electron density at the W–N bond is higher than that at the W–O one, which indicates that the W–N bond is more covalent due to the smaller difference in the electronegativity between W and N atoms compared to the W and O ones. The oxidation number of W in CaxSr1–xWO2N was estimated to be 5.2 by bond valence sum, which indicates the W5+ ion with the 5d1 electron configuration. Precursor oxides CaxSr1–xWO4 with W6+ having the 5d0 electron configuration are white and insulating, whereas the CaxSr1–xWO2N oxynitrides with the W5+ ion having the 5d1 configuration are black and exhibit metallic character. These results indicate the insulator–metal transition from the d0 oxide CaxSr1–xWO4 to the d1 oxynitride CaxSr1–xWO2N.