New alkaline-earth amidosulfates and their unexpected decomposition to S4N4.

Abstract

The amidosulphates Mg(NH2SO3)2·4H2O (P21/c), Mg(NH2SO3)2·3H2O (P1̄), Ca(NH2SO3)2·4H2O (C2/c), Ca(NH2SO3)2·H2O (P212121), Sr(NH2SO3)2·4H2O (C2/c), Sr(NH2SO3)2·H2O (P21/c) and Ba(NH2SO3)2 (Pna21) could be obtained as cm-sized crystals from aqueous solutions of the corresponding metal carbonates, hydroxides and amidosulphonic acid, respectively, by careful control of the crystallisation conditions. β-Sr(NH2SO3)2 (Pc) and α-Sr(NH2SO3)2 (P21) could be obtained by careful thermal dehydration of Sr(NH2SO3)2·H2O. Their crystal structures were determined by single-crystal XRD and revealed a rich structural diversity with a significant tendency to form non-centrosymmetric crystals. The compounds were characterised by powder XRD, FT-IR, Raman and UV/vis spectroscopy and thermogravimetry. Temperature programmed single-crystal XRD, powder XRD and Raman spectroscopy, as well as DFT calculations were employed to aid the interpretation of vibrational and thermal properties. For the first time, SHG measurements were performed on metal amidosulphates, revealing the SHG intensities of β-Sr(NH2SO3)2 and Ba(NH2SO3)2 that were comparable to quartz and KDP. Thermal decomposition was additionally studied by the preparation of reaction intermediates, serendipitously revealing the formation of S4N4 as the decomposition product. This unprecedented reaction represents the first sulphur nitride synthesis process that neither employs a sulphur halide nor elemental sulphur.