Temperature dependence of the Fe2+ Mossbauer parameters in triphylite (LiFePO4)

Abstract

The ferrous Mossbauer parameters are determined for synthetic triphylite (LiFePO 4 ) over a wide temperature range between 8 and 900 K. A sharp magnetic order-disorder transition is observed at (52.5 ± 0.5) K. In the paramagnetic state, the Mossbauer spectra are adequately analyzed by one Fe 2+ quadrupole doublet with a narrow line width of 0.285 mm/s at room temperature. In the magnetic state, the spectra show up to eight absorption lines, which is typical for Fe 2+ . These spectra are analyzed through diagonalization of the full nuclear-interaction hamiltonian. The hyperfine field B hf is found to be collinear to the local principal axis of the electric field gradient (EFG) tensor. The saturation value for B hf is 125.5 kOe. For temperatures below 52 K, the adjusted value for the asymmetry parameter of the EFG is η ≈ 0.8 and for the quadrupole splitting Δ E Q = 3.06 mm/s. This latter value is perfectly in line with the value at 80 K (paramagnetic regime). The temperature dependence of the quadrupole splitting is interpreted within the 5 D orbital energy level scheme of Fe 2+ by a crystal field calculation based on the point symmetry of the Fe 2+ site in triphylite. The variation of the hyperfine field with temperature is interpreted within the molecular field approximation, however, taking exchange magnetostriction into account. From the temperature dependence of the isomer shift the characteristic Mossbauer temperature Θ M is determined to be 418 K and the intrinsic isomer shift δ I = 1.46 mm/s. From these, the Mossbauer fraction at room temperature is calculated to be 0.785.