A ${}^{119}$sn mössbauer-spectroscopic characterization of the diamagnetic birefringence material sn${}_2$b${}_5$o${}_9$cl

The crystal structure of diamagnetic borate chloride Sn${}_2$B${}_5$O${}_9$Cl exhibits two crystallographically independent tin sites which both show pronounced lone-pair activity of the tin atoms. This is reflected in substantial quadrupole splitting in the ${}^{119}$Sn Mössbauer spectrum. The isomer shifts of 3.887(8) and 4.137(7) mm s${}^{-1}$ clearly indicate divalent tin. In agreement with bond valence calculations, the Sn1 atoms have a lower charge and the higher isomer shift, compatible with a higher electron density at the tin nuclei.