Magnetic phase transition and Mossbauer spectroscopy of ErNi2Mnx compounds
Investigation of ErNi2Mnx with 0.90 ≤ x ≤ 1.10 indicates that the MgCu2-type structure (with space group Fd-3m) is formed within x = 0.97−1.10. Curie temperature TC = 50 K of ErNi2Mn is significantly higher than those of the corresponding ErNi2 (TC = 7 K) and ErMn2 (TC = 15 K) compounds. Detailed analyses of DC magnetization data demonstrate that the magnetic phase transition at the Curie temperature is second order. The magnetic entropy change around TC has been found to be 4.8 J/kg K for a magnetic field change of 0 to 5 T with its relative cooling power ∼283 J/kg. 57Fe Mossbauer spectra below TC have been fitted with three sub-spectra representing three inequivalent local magnetic environments while the paramagnetic spectra are fitted using two sub-spectra for two inequivalent crystal sites (8a and 16d), which supports the conclusion based on Rietveld refinement that Mn atoms occupy at both 8a and 16d sites.
Temperature- and pressure-induced structural transitions in rare-earth-deficient (R = Y, Sm, Gd, Tb) Laves phases
Electrical resistivity, thermal expansion, and temperature-dependent x-ray diffraction measurements on compounds give mutually consistent evidence for structural phase transitions at 740 K, 550 K, 600 K, and 450 K respectively for R = Y, Sm, Gd, Tb; 0 < x < 0.05. Arguments are given as to why most of the rare-earth - nickel compounds with the 1:2 ratio do not crystallize in the simple cubic Laves phase (C15 type) but show a superstructure of the cubic Laves phase at room temperature and at ambient pressure. This superstructure with the space group and a doubled cell parameter is characterized by ordered vacancies on the R sites. It is shown that the observed structural instabilities result in transitions to the cubic Laves phase (space group ), however with disordered vacancies at high temperatures. High-pressure x-ray powder diffraction experiments show that the phase transition in shifts down to room temperature for a pressure of 27 GPa.
Elastic constants of some MAl2 single crystals
Elastic constants of single crystals of the cubic Laves phases CaAl2, YAl2, LaAl2, and GdAl2 have been measured over the temperature range 4.2–300 K. The ferromagnetic transition in GdAl2 near 170 K is evident by changes in slope in plots of the three independently measured elastic constants of GdAl2 as functions of temperature. Debye temperatures at 0 K were evaluated as 474 K for CaAl2, 484 K for YAl2, 374 K for LaAl2, 399 K for magnetically disordered GdAl2, and 406 K for magnetically ordered GdAl2. It was inferred from the data that the valence electron distributions in nonsuperconducting YAl2 and GdAl2 are much alike, but the valence electron distribution in superconducting LaAl2 is significantly different. Thus the results support a theoretical calculation indicating an admixture of 4f character into the valence bands of LaAl2.