Neodymium ultraphosphates and process for their preparation

Process for the preparation of compounds having the formula

• Me.sub.x Nd.sub.1-x P.sub.5 O.sub.14
• in the form of untwinned crystals wherein Me represents scandium, gallium, yttrium, indium, lanthanum, cerium, gadolinium lutetium, thallium and/or uranium, and x is a number between 0 and 0.999, which comprises heating Nd.sub.2 O.sub.3 and Me.sub.2 O .sub.3 in a vessel consisting of fine gold or of an inert, dense, carbon-base material in a closed system having a zone which is held at a lower temperature with an excess of high-purity, anhydrous normal or heavy phosphoric acid or diphosphoric or polyphosphoric acid, the deuterium being able to be wholly or partially replaced by tritium, with the proviso that a deuteriated or tritiated acid must be used if x .times. 0, at a temperature between about 500 and 600.degree. C., regulating the crystal growth by regulating the vapor pressure during crystal growth until the desired crystal size is achieved and then separating excess phosphoric acid or polyphosphoric acid, as the case may be.