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An x‐ray diffraction study of the substitution of gallium in Tb₂Fe₁₇ to form the Tb₂Fe_17−xGa_x solid solutions indicates that the compounds adopt the rhombohedral Th₂Zn₁₇ structure. The unit cell volume and the a‐axis lattice parameter increase linearly with increasing gallium content. The c‐axis lattice parameter increases linearly from x=0 to 6 and then decreases between x=7 and 8. Magnetic studies show the Curie temperature increases by ∼150° above that of Tb₂Fe₁₇ to reach a maximum between x=3 and 4, and then decreases with further increases in x. Neutron diffraction studies of Nd₂Fe₁₅Ga₂ and Tb₂Fe_17−xGa_x, with x equal to 5, 6, and 8, indicate that the gallium completely avoids the 9d site, occupies the 6c ‘‘dumbell’’ site only at high values of x and strongly prefers the 18f site at high values of x. The magnetic neutron scattering indicates both that the terbium sublattice magnetization couples …