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  Stein, W.-D.; Liebertz, J.; Becker, P.; Bohaty, L.; Braden, M.: Structural investigations of the tetraborates MB₄O₇ (M = Pb, Sr, Ba). The European Physical Journal B 85 (2012), p. 236/1-5

10.1140/epjb/e2012-21062-y

Abstract:
The crystal structures of the tetraborates MB₄O₇ were investigated at room temperature for M=Sr, Pb, Ba and at 100~K for M=Pb, Ba. The influence of the lone electron pair of Pb²⁺ in PbB₄O₇ can be studied by comparing the isostructural compounds PbB₄O₇ and SrB₄O₇. The lone pair shows a clear displacement of the lead atom which increases upon cooling. In contrast with the Sr and Pb compounds, BaB₄O₇ crystallizes in a centrosymmetric structure. However, this structure seems to be on the border of stability, as we find strong internal stress.