KMO4O6 REVISITED - EVIDENCE FOR MO-6 CLUSTER-CENTERING INTERSTITIAL ATOMS IN THE HIGH-PRESSURE PHASE KMO4O6-II

The c-axis lengths of various ''KMo4O6'' samples, prepared by high-tem perature, high-pressure decomposition of K2MoO4, vary in the range fro m 2.908 to 2.943 Angstrom, all of them being larger than the correspon ding c-axis for KMo4O6 prepared by electrolysis of K2MoO4/MoO3 melts ( c = 2.879 Angstrom). From a single-crystal structure refinement of a c rystal with c = 2.917 Angstrom we gained evidence that the different l engths of the c-axes in the high-pressure phases KMo4O6-II are caused by additional atoms which center otherwise empty Mo-6 octahedra. Assum ing these interstitial atoms to be oxygens, the results of bond valenc e and bond order sum calculations are clearly improved. The influence of different synthetic routes to KMo4O6, as well as some peculiarities observed during the structure refinement for KMo4O6 samples of differ ent origin, are discussed.