Unusual Phase Transitions and Magnetoelastic Coupling in TlFe1.6Se2 Single Crystals

摘要

Structural, magnetic, electrical transport, and heat capacity data arereported for single crystals of TlFe1.6Se2. This compound crystallizes in atetragonal structure similar to the ThCr2Si2 structure, but with vacancies inthe Fe layer. The vacancies can be ordered or disordered depending ontemperature and thermal history. If the vacancies are ordered, the basal planelattice constant increases from a to \sqrt{5}a. Antiferromagnetic order withthe Fe spins along the c-axis occurs below T_N ~ 430K as shown by singlecrystal neutron diffraction and the magnetic structure is reported. Inaddition, for the vacancy ordered crystal, two other phase transitions arefound at T_1 ~ 140K, and T_2 ~ 100K. The phase transitions at T_1 and T_2 areevident in heat capacity, magnetic susceptibility, resistivity data, a and clattice parameters, and in the unusual temperature dependence of the magneticorder parameter determined from neutron scattering. The phase transitions atT_1 and T_2 result in significant changes in the magnetic moment per iron, with1.72(6)\mu_B observed at 300K, 2.07(9)\mu_B at 140\,K, 1.90(9)\,\mu_B at115\,K, and 1.31(8)\mu_B for 5\,K if the same "block checkerboard" magneticstructure is used at all temperatures. The phase transitions appear to bedriven by small changes in the c lattice constant, large magnetoelasticcoupling, and the localization of carriers with decreasing temperature.