CRYSTALLIZATION OF CaTi₄(PO₄)₆ FROM CALCIUM-TITANIUM BOROPHOSPHATE GLASSES

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Preparation of the CaTi₄(PO₄)₆-type glass-ceramic materials by controlled crystallization of calcium-titanium borophosphate glasses was studied in the compositional series of 50CaO-10B₂O₃-40P₂O₅+xTiO₂ (x=0-56 mol% TiO₂) and 30CaO-30B₂O₃-40P₂O₅+xTiO₂ (x=0-24mol% TiO₂). The glasses crystallize under heating within the temperature region of 700-800°C and the surface nucleation mechanism prevails in all the TiO₂-doped glasses over the internal one. The major compounds formed by crystallization were CaTi₄(PO₄)₆. Ca₂P₂O₇ and BPO₄. Changes in the XRD patterns after leaching with 1M HCl or 1M HNO₃ snowed that the acids dissolve most of all the other crystalline phases than CaTi₄(PO₄)₆. In this way a porous skeleton with CaTi₄(PO₄)₆ phase is formed the specific surface area of which decreases with increasing TiO₂ content in the starting glasses.