(longer) bonds and (shorter) non-bonded distances (as in B 1), would be favoured over 'one-angle' contacts (as in the six-membered rings of B 3 and B4). It is our belief that this will be a more fruitful approach to the understanding of the relative stability of different structure types than current theories utilizing concepts such as radius ratio and/or ionicity.We are grateful to ZWO (the Dutch Organization for the Support of Pure Scientific Research) for the award of a Bezoekersbeurs (to MO'K) which made this work possible. We also thank Dr Z. Barnea for a prepublication copy of his paper with Whiteley and Moss. The structure of Pb3MnvO 15 has been determined and refined to an R factor of 0.047 from single-crystal data. The unit-cell parameters are: a = 17.28 (1), b = 9.98 (1), c = 13.55 (1) ,~,, Z = 8, the space group is Cmc2 r The structure can be described as a succession of layers of octahedra perpendicular to the c axis containing randomly distributed Mn nI and Mn Iv atoms. The Pb atoms constitute hexagonal close packing with O. The electrostatic repulsion due to the lone pairs gives to half of them fourfold coordination analogous to that observed in the massicot variety of PbO and to the others sixfold coordination never before observed. References

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An investigation of the BaO-SiO~-BaCt2 system has shown the existence of several new phases: BasSiO4C16, BaTSi2OTCIs, and Ba5Si206C16. Activated by Eu 2+, BasSiO4C16 is an efficient blue-emitting phosphor under u.v. and x-ray excitation. As a result of a small Stokes shift, the emission is characterized by a narrow bandwidth (30 nm at half-intensity) and a high quenching temperature.