On the stability of hexagonal Ge7Nb10

Our studies on the Ge-Nb system reveal the existence of a hexagonal compoun d with the composition Ge7Nb10 that is stable with respect to disproportion ation and which was found to crystallize in the space group P6(3)/mmc. A he xagonal phase of the approximate composition Ge3Nb5 has been mentioned by d ifferent authors but has been commonly regarded as C- or N-stabilized and t herefore not included in the binary phase diagram. A single crystal X-ray s tudy of the compound revealed a Mn5Si3-type based structure where half of t he 2b interstitial positions are tilled with Ge. X-ray powder diffraction s tudies and electron probe microanalysis were used to determine the composit ion ranges of the phases. Various experiments were carried out in order to confirm the thermodynamic stability of this compound and to rule out stabil ization by hetero-elements. These studies included the use of different sta rting materials, different sample preparation techniques and different anne aling procedures. The results show strong indications, that Ge7Nb10 is form ed as a binary phase which is stable with respect to disproportionation and should therefore be included in the binary phase diagram. (C) 2001 Elsevie r Science B.V. All rights reserved.