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The published crystallographic data on cesium, rubidium, and potassium phosphates crystallizing in the NaZr 2 (PO 4 ) 3 (NZP) and langbeinite structural types are summarized and correlated. The existence of new phosphates, analogs of langbeinite mineral, is predicted. The phosphates of the suggested compositions are prepared and studied by X-ray and neutron diffraction and by IR spectroscopy. Phosphates of the formulas A 2 RM(PO 4 ) 3 , A 2 B 0.5 Zr 1.5 (PO 4 ) 3 , and ABR 2 (PO 4 ) 3 have a cubic cell, space group P2 1 3. The unit cell parameters of the phosphates in these series vary only slightly with variation of the cationic composition. Variations in the bond lengths and bond angles in the langbeinite structure depending on the cation are estimated from the results of structural studies. Cesium can be incorporated in cubic framework phosphates in an amount of up to 38 wt %. The langbeinite structure is characterized by wide possibilities of isomorphous substitutions involving large alkali and alkaline-earth metal cations arranged in the framework voids and small cations of p, d, and f elements in oxidation states 2+, 3+, and 4+, arranged in the framework positions. A specific role of lanthanides in formation of the langbeinite-type framework is noted.

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