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Phase relationships at ambient temperature in the pseudobinary system Nd5Si4-Nd5Ge4 were studied by X-ray powder diffraction (XRD). Four structurally distinct phase regions exist in this system. The Nd5Si4-based solid solution in which Ge statistically substitutes for Si crystallizes in the tetragonal Zr5Si4-type structure with space group P4(1)2(1)2. The Nd5Ge4-based solid solution in which Si statistically substitutes for Ge crystallizes in the orthorhombic Gd5Ge4-type structure with space group Pnma. There are two ternary intermediate phases, which crystallize in the orthorhombic Gd5Si4-type structure with space group Pnma and the monoclinic Gd5Si2Ge2-type structure with space group P112(1)/a. The Rietveld powder diffraction profile fitting technique was used for refinement of the crystal structure. The lattice parameters, atomic positions and interatomic distances for Nd5Si4, Nd5Si2.8Ge1.2, Nd5Si2Ge2 and Nd5Ge4 were derived. (C) 2002 Elsevier Science B.V. All rights reserved.