Process for the removal of which is caused by defects stained centers of a - and more crystals having the perovskite structure or perovskite a similar structure

摘要

In the preparation of colourless monocrystals or polycrystals which have the formula LnAO3 where Ln is a trivalent ion of a rare earth element (defined as an element having an atomic number between 57 (Lanthanum) and 71 (Lutecium) inclusive) and A is one of the trivalent ions Ti3+, V3+, Cr3+, Al3+ and Ga3+, which have a crystalline structure analogous to or resembling that of perovskite, and which are obtained from a homogenized powder allowed to crystallize by the application of Verneuil's method, oxidizable ions capable of displaying a valency higher than 3 and having no absorption band (or few absorption bands) in the visible spectrum are incorporated together with the ions Ln and/or A in the powder; these incorporated ions subsequently are oxidized into their highest valency state to give colourless crystals, such oxidation taking place either during the application of Verneuil's method to form crystals from the powder or after the application of Verneuil's method when the crystal may be reheated in air or in oxygen at a temperature between 500 DEG C. and 1500 DEG C. But for the incorporation and oxidation of these ions, crystallization of the powder would give coloured crystals. Incorporation of the oxidizable ions may be effected either by using a solution of appropriate salts capable of yielding one or more of the ions V5+, Mn4+, Mo6+, Ru4+, Ce4+, Tb4+, W6+, Re6+ and Ir4+, or by mixing with the homogenized powder appropriate oxides capable of yielding one or more of the ions Ti4+, Zr4+, Nb5+, Hf4+, Ta5+ and Th4+. Reference is made to Specification 1,042,586, in which Specification is described Verneuil's method of crystal formation.