A Si-29 MAS-NMR study of Cd-7[Ge6Si]O-21 pyroxmangite

Synthetic (SiGe)-pyroxenoids are often observed to have superperiods. Wheth er or not these superperiods grow in connection with Si-Ge ordering is a fu ndamental question. The size difference between Ce and Si tetrahedra leads to CdGeO3 having the pyrox-mangite structure, whereas CdSiO3 has the wollas tonite structure. Consequently, considerable strain is expected for a pyrox enoid with a disordered Ge-Si distribution. A Si-29 MAS-NMR study of a Cd-p yroxmangite of nominal composition Cd-7[Ge6Si]O-7 points to considerable Si clustering, probably as GeSiSiSiGe triples corresponding to the wollastoni te-like units of the siebener chain. It is proposed that such ordering reli eves strain within the tetrahedral chain. Residual strain is also relieved by insertion of an extra pair of GeO4 tetrahedra into the pyroxene-like com ponent of the siebener-chain units, leading to single neuner-chain units wh ich terminate the superperiods. This growth pattern is cyclic. The presence of several types of superperiod may reflect similar energetics for differe nt Ge-Si-ordering patterns within the siebener chains. Ordering of Si-rich unit cells and of unit cells having no Si is proposed as the reason for the occurrence of the superperiods in Cd-pyroxmangite.