Mullite-type EMBO₄: synthesis, structural, optical, and vibrational properties of rare tin(II) borates with M = Al and Ga

Metal tin-(II)-borates are rarely studied mainly due to the susceptibility of either oxidation into tin(IV) or disproportionation into elemental tin(0) and tin(IV). We report mullite-type SnAlBO4 and SnGaBO4 ceramics produced by conventional solid-state synthesis in sealed quartz tubes at low pressure of 10(-7) MPa. Both compounds are isostructural to PbAlBO4 as confirmed by Rietveld refinements of powder X-ray data. The crystal structures are highly influenced by the stereochemical activity of the 5s(2) lone electron pair of the Sn2+ cation measured by the Wang-Liebau eccentricity parameter. To further consolidate the structural features Sn-119 Mossbauer, solid state NMR, Raman, IR and UV/vis spectroscopic measurements are performed. The Sn-119 Mossbauer isomer shifts and the quadrupole splitting values confirm the SnO4 coordination and Sn(II) valence states. Solid state B-11, Al-27 and Sn-119 NMR spectra provided insights into the local crystal-chemical environment. The vibrational properties are discussed from group theoretical analysis to mode assignments. SnAlBO4 and SnGaBO4, respectively, possess an electronic band gap of 3.73(9) and 3.21(4) eV calculated from the diffuse reflectance UV/Vis spectra.