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The subsolidus phase relations of the PrOx-CaO-CuO pseudo-ternary system sintered at 950-1000degreesC have been investigated by X-ray powder diffraction. In this system, there exist one compound Ca10Pr4Cu24O41, one Ca2Pr2Cu5O10-bascd solid solution, seven three-phase regions and two two-phase regions. The crystal structures of Ca10Pr4Cu24O41, and Ca2Pr2Cu5O10-based solid solution have been determined. Compound Ca10Pr4Cu24O41 crystallizes in an orthorhombic cell with space group D-2h(20) - Cccm, Z = 4. Its lattice parameters are a = 11.278(2) Angstrom, b = 12.448(3) Angstrom and c = 27.486(8) Angstrom. The crystal structure of Ca2Pr2Cu5O10-based solid solution is an incommensurate phase based on the orthorhombic NaCuO2 type subcell. The lattice parameters of the subcell of the Ca2.4Pr1.6Cu5O10 are a(0) = 2.8246(7) Angstrom, b(0) = 6.3693(5) Angstrom, c(0) = 10.679(1) Angstrom, and those of the orthorhombic superstructure are with a = 5a(0), b = b(0), c = 5c(0). The Ca2.4Pr1.6Cu5O10 structure can also be determined by using a monoclinic supercell with space group C-2h(5) - P2(1)/c, Z = 4, a = 5a(0), b = b(0), c = c(0)/sin beta and beta = 104.79(1)degrees or 136.60(1)degrees, V = 5a(0)b(0)c(0). (C) 2004 Elsevier Inc. All rights reserved.