On the stability of hexagonal Ge7Nb10

Our studies on the Ge-Nb system reveal the existence of a hexagonal compound with the composition Ge7Nb10 that is stable with respect to disproportionation and which was found to crystallize in the space group P63/mmc. A hexagonal phase of the approximate composition Ge3Nb5 has been mentioned by different authors but has been commonly regarded as C- or N-stabilized and therefore not included in the binary phase diagram. A single crystal X-ray study of the compound revealed a Mn5Si3-type based structure where half of the 2b interstitial positions are filled with Ge. X-ray powder diffraction studies and electron probe microanalysis were used to determine the composition ranges of the phases. Various experiments were carried out in order to confirm the thermodynamic stability of this compound and to rule out stabilization by hetero-elements. These studies included the use of different starting materials, different sample preparation techniques and different annealing procedures. The results show strong indications, that Ge7Nb10 is formed as a binary phase which is stable with respect to disproportionation and should therefore be included in the binary phase diagram. Π2001 Elsevier Science B.V.