Unusual phase transitions and magnetoelastic coupling in TlFe _1.6Se₂ single crystals

Structural, magnetic, electrical transport, and heat capacity data are reported for single crystals of TlFe_1.6Se₂. This compound crystallizes in a tetragonal structure similar to the ThCr₂Si 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 T₁140 K and T ₂ 100 K. The phase transitions at T₁ and T₂ 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 ₁ and T₂ result in significant changes in the magnetic moment per iron, with 1.72(6) μ_B observed at 300 K, 2.07(9) μ_B at 140 K, 1.90(9) μ_B at 115 K, and 1.31(8) μ_B 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.