New Metal-rich Phosphides of Niobium, Tantalum and Tungsten

Abstract

PREVIOUS investigations^1–5 of the niobium–phosphorus and tantalum–phosphorus systems indicate that only two intermediate phases occur in each system, a monophosphide (which shows a varying amount of stacking disorder in the structure) and a diphosphide. These results are based on investigations of the products formed when the metals react with phosphorus at temperatures below 1,100° C. In the tungsten–phosphorus system, the occurrence of tungsten monophosphide and a high and a low temperature form of tungsten diphosphide has been observed^6–9. No further intermediate phases appear to be formed by direct reaction between tungsten and phosphorus below 1,100° C. It has been reported, however, that tungsten phosphides containing less than 50 atom per cent phosphorus can be prepared by fused salt electrolysis. Thus, Hsu et al.¹⁰ obtained a crystalline compound of composition W₃P by electrolysis of a fused sodium metaphosphate bath containing tungsten trioxide and sodium chloride at temperatures below 900° C. This compound was described as having the body-centred tetragonal iron triphosphide-type structure with cell dimensions a = 9.890 Å; c = 4.808 Å.