Neutron-Diffraction and Mössbauer Effect Study of the Tb₂Fe₁₇₋ₓAlₓ Solid Solutions

The magnetic properties of a series of Tb ₂ Fe _17-x Al _x solid solutions, with nominal x compositions of 0, 2, 3, 4, 5, 6, 7, and 8, have been studied by neutron diffraction and Mössbauer spectroscopy. Neutron-diffraction data indicate that the compounds all crystallize with the Th ₂ Zn ₁₇ structure and that the aluminum atoms are excluded from the 9d site and show a distinct preference for the 6c site only for an aluminum content greater than 6. The unit-cell volume increases by approximately 1% per aluminum atom substituted in the formula unit. The magnetic moment per formula unit, measured at 295 K, shows very little change for x less than or equal to 4, but decreases rapidly with increasing aluminum content for higher values of x. Mössbauer spectral results indicate that all the samples are ferromagnetically ordered at 85 K. However, at 295 K Tb ₂ Fe ₉ Al ₈ is paramagnetic and Tb ₂ Fe ₁₀ Al ₇ is either paramagnetic or has at most very small ferromagnetic moments. An analysis of the magnetic spectra with a basal magnetic model is successful for x values of 5 or less; however, at higher x values an axial model for the magnetization is required, indicating the presence of a spin reorientation with increasing aluminum content and decreasing temperature. The weighted average hyperfine field decreases approximately linearly by 21 kOe per substituted aluminum atom at 85 K and more rapidly at 295 K. As expected, the isomer shifts increase with increasing aluminum content as a result of interatomic charge transfer and intraatomic iron 4s-3d electronic redistribution.