The chemistry of the phosphates of barium and tetravalent cations in the 1:1 stoichiometry

The chemistry of phosphates of barium and tetravalent cations [BaMIV(PO4)2] is reviewed. Such phosphates crystallise in the C2/m space group for MIV=Ti, Zr, Hf, Ge, Sn, and Mo, and in the P21/n space group for BaTh(PO4)2. The existence of BaMIV(PO4)2 in which MIV=Pb, Ce, and U is further evaluated. Several aspects, such as phase transitions in the compounds with yavapaiite structure, solid solutions of BaMIV(PO4)2 compounds and practical applications are briefly discussed.