PbGaBO₄, an orthoborate with new structure type

Hyunsoo Park and Jacques Barbier, Acta Cryst. (2001). E57, i82–i84

Binary compounds have simple structures with relatively uninteresting properties. Ternary compounds, such as the perovskites, have more complex structures with more interesting properties such as ferroelectricity. Recently, inorganic crystallographers have been studying compounds with four or more elements whose structures are even more complex and continue to stay one step ahead of the ability of theory to predict them or their properties. PbGaBO₄ is a good example. It consists of chains of edge-sharing GaO₆ octahedra linked by PbO₄ and BO₃ groups. Because the BO₃ groups share corners with two different GaO₆ octahedra in the same chain, they cause the octahedra to twist (see the figure). One consequence is that the bonds to O₃ are stretched leaving O₃ underbonded (bond valence sum = 1.79). To compensate, the Ga–O1 bond is shortened, leaving it overbonded (bond valence = 2.23). Although it would be difficult to predict this structure a priori, one can at least understand the twists in the chains and the distortions of the GaO₆ octahedra.

Reviewed by I. David Brown, Brockhouse Inst., McMaster U. Canada