Vanadates metallic pyro

Pyrovanadates, R 4V207 or 2R a0.Va05.—The alkali pyrovana-dates are prepared by dissolving the equivalent quantity of vanadium pentoxide in solutions of alkalis, or by the spontaneous decomposition in solution of the alkali orthovanadates. Pyrovanadates of other metals are obtained by fusing vanadium pentoxide with the salts or hydroxides of the metals in molecular proportions, or, when they are sufficiently insoluble, by double decomposition between an alkali pyro-vanadate and a salt of the metal required. 

Metavanadates, R VOs or R 20.V205.—These salts are more stable than either the ortho- or pyro-vanadates. Solutions of the latter yield metavanadates on being evaporated or by treatment with carbon dioxide. The alkali metavanadates are prepared directly by dissolving vanadium pentoxide in the calculated quantity of alkali hydroxide. The metavanadates of other metals are prepared by fusing vanadium pentoxide with the oxide or carbonate of the metal in calculated quantity,11 or by the action of a soluble salt of the metal... 

The MRVjOy (M = alkali metal) compounds have two structure types pyro-chlore and weberite (Chincholar, 1972). The pyrochlore structure is formed with lithium and for smaller rare earths also with sodium. The potassium compounds and sodium compounds with larger rare earth ions seem to have weberite structure, which is an orthorhombic distortion of cubic pyrochlore (Gabe and Chincolkar, 1979). The corresponding pentavalent niobates and tantalates have similar structural behaviour to the vanadates. 

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