Unusual phase transitions and magnetoelastic coupling in TlFe_(1.6)Se_2 single crystals

摘要

Structural, magnetic, electrical transport, and heat capacity data are reported for single crystals of TlFe_(1.6)Se_2. This compound crystallizes in a tetragonal structure similar to the ThCr_2Si_2 structure, but with vacancies in the Fe layer. The vacancies can be ordered or disordered depending on temperature and thermal history. If the vacancies are ordered, the basal plane lattice constant increases from a to 5a. Antiferromagnetic order with the Fe spins along the c axis occurs below T_N ≈ 430 K, as shown by single-crystal neutron diffraction, and the magnetic structure is reported. In addition, for the vacancy-ordered crystal, two other phase transitions are found at T_1 ≈ 140 K and T_2 ≈ 100 K. The phase transitions at T_1 and T-2 are evident in heat capacity, magnetic susceptibility, resistivity data, a and c lattice parameters, and the unusual temperature dependence of the magnetic order parameter determined from neutron scattering. The phase transitions at T_1and T_2result in significant changes in the magnetic moment per iron, with 1.72(6) μB observed at 300 K, 2.07(9) μB at 140 K, 1.90(9)μ B at 115 K, and 1.31(8) μB for 5 K if the same "block checkerboard" magnetic structure is used at all temperatures. The phase transitions appear to be driven by small changes in the c lattice constant, large magnetoelastic coupling, and the localization of carriers with decreasing temperature.