Mossbauer study of crystallographic and magnetic phase transitions, phonon softening, and hyperfine interactions in Zr(Al_xFe_1-x)₂

Mossbauer-spectroscopy studies of ⁵⁷Fe in Zr(Al_xFe_1-x)₂ (x=0-0.825) at various temperatures, from 90 to 612 K have been performed. For x<0.25 the systems are of cubic Laves phase structure and order magnetically above room temperature. Above x=0.25 the systems have a hexagonal structure and do not order magnetically above 90 K. The Mossbauer spectra in the paramagnetic state are all very well described by a model that assumes that the isomer shift and quadrupole interaction are linearly dependent on the number of Al first-nearest neighbors to the iron nucleus (0.056 mm/s and -7% per Al neighbor, respectively). The samples x=0.083, 0.167, and 0.20 according to the measured Mossbauer recoil-free fractions, display increasing phonon softening. The effective Debye temperature decreased from FTHETA_D≈440(30) K for x=0 to FTHETA_D≈310(30) K for x=0.2. The compounds with x≥0.25 harden again and FTHETA_D increases to 395(20) K.