CRYSTAL-FIELD POTENTIALS OF PRFE2SI2 AND PRFE2GE2 AS DEDUCED FROM INELASTIC NEUTRON-SCATTERING AND SPECIFIC-HEAT MEASUREMENTS

Inelastic neutron scattering experiments and specific heat measurements have been performed on polycrystalline samples of PrFe(2)X(2) (X = Si, Ge), a tetragonal intermetallic system showing a low-temperature antiferromagnetic ordering of the Pr sublattice, with no local magnetic moment on the iron site. The temperature variation of the specific heat, and the values of the magnetic entropy, indicate for both compounds a crystal-field energy level scheme characterized by two closely spaced singlets, split by about 1 meV for X = Ge and by about 2.5 meV for X = Si, with the other levels positioned above 7 meV. These conclusions are confirmed by the observation of only one excitation peak in the magnetic neutron scattering response in this energy range. In the paramagnetic phase, the inelastic peak is centred at 2.4(1) meV for X = Si and at 0.8(1) meV for X = Ge. Below TN, the peak position shifts to 2.8(1) meV for X = Si, and to 2.7(1) meV for X = Ge. The experimental findings are explained by a crystal- and molecular-field model with two low-lying singlets, coupled by exchange in the ordered phase. The different size of the molecular field in the two compounds, with respect to the crystal-field splitting of the low-lying singlets, causes the large difference in the ordered moments of the Pr3+ ions (1.41 mu(B) for Si and 2.75 mu(B) for Ge). The asymmetric shape of the peaks in the neutron scattering function has been attributed to the presence of two branches of magnetic excitons in these crystal-field systems showing manifestly Van Vleck-induced antiferromagnetism of singlet-singlet type.