Subsolidus phase relations in the Tl2MoO4-Ho2(MoO4)3-Zr(MoO4)2 system have been studied by X-ray diffraction and differential thermal analysis methods. The system has been shown to contain molybdates with the compositions Tl5HoZr(MoO4)6 (5 : 1 : 2) and Tl2HoZr2(MoO4)6.5 (2 : 1 : 4). A new ternary molybdate TlHoZr0.5(MoO4)3 was discovered by the method of spontaneous solution-melt crystallization through spontaneous nucleation, and its crystal structure has been determined to be: sp. gr. R 3, a = b = 13.3665(2) Å, c = 11.9844(4) Å, V = 1854.31(7) Å3, Z = 6. The mixed three dimensional framework of the crystal structure is composed of Mo tetrahedra sharing O vertices with (Ho,Zr)O6 octahedra. The thallium atoms occupy wide channels in the framework. Crystals of binary molybdate of Ho2Zr2(MoO4)7 composition were grown by solution-melt crystallization and their crystal structure were determined to be: sp. gr. С2/c, a = 20.6668(4), b = 9.8193(2), c = 13.8187(3) Å, b = 113.678(1)о, .V = 2568.20(9)Å3, Z = 4. The crystal structure is composed of three types of polyhedra: МоО4 tetrahedra, ZrO6 octahedra, and eight-vertex HoO8.

Keywords: Phase equilibria, synthesis, Thallium, Holmium, Zirconium, Molybdates, Crystal structure, Space group.