IUCr journals

PbGaBO4, an orthoborate with new structure type

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

[Figure 1] View of the PbGaBO4 structure approximately along the [100] direction. Strong angular distortions are associated with the bridging borate groups.
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