Investigation of synthetic Mg1.3V1.7O4 spinel with MgO inclusions:: Case study of a spinel with an apparently occupied interstitial site

A magnesium vanadate spinel crystal, ideally MgV2O4, synthesized at I bar, 1200 degrees C and equilibrated under FMQ + 1.3 logf(O2), condition, was investigated using single-crystal X-ray diffraction, electron microprobe, and electron backscatter diffraction (EBSD). The initial X-ray structure refinements gave tetrahedral and octahedral site occupancies of Wg(0.966)square(0.034)) and (M)(V0.73+V0.1094+Mg0.180), respectively, along with the presence of 0.053 apfu Mg at an interstitial octahedral site (16c). Back-scattered electron (BSE) images and electron microprobe analyses revealed the existence of an Mg-rich phase in the spinel matrix, which was too small (<= 3 pm) for an accurate chemical determination. The EBSD analysis combined with X-ray energy dispersive spectroscopy (XEDS) suggested that the Mg-rich inclusions are periclase oriented coherently with the spine] matrix. The final structure refinements were optimized by subtracting the X-ray intensity contributions (similar to 9%) of periclase reflections, which eliminated the interstitial Mg, yielding a structural formula for spinel Mg-T(M)(V1.3163+V0.3164+Mg0.316)O-4. This Study provides insight into possible origins of refined interstitial cations reported in the literature for spinel, and points to the difficulty of using only X-ray diffraction data to distinguish a spinel with interstitial cations from one with coherently oriented MgO inclusions.