Comparative study of terbium tellurides Tb₂Te₅ and TbTe₃

Two terbium tellurides, TbTe₃ and Tb₂Te₅, were studied by means of thermodynamics, ultrafast pump-probe spectroscopy, and torque magnetometry. While the crystal structure and some physical properties of TbTe₃ were established previously, the crystal structure of Tb₂Te₅ is reported based on the low-temperature (100 K) single crystal x-ray diffraction experiment. Tb₂Te₅ crystallizes in orthorhombic Cmcm space group with a = 4.3009(17), b = 43.107(18), c = 4.3016(17) Å, V = 797.5(6) ų, and Z = 4. In contrast to TbTe₃, which experiences three successive magnetic phase transitions, Tb₂Te₅ orders antiferromagnetically in two steps at T_N1 = 9.0 K and T_N2 = 6.8 K, both readily suppressed by an external magnetic field. The third transition in TbTe₃ is due to the interaction of the magnetic subsystem with the charge density waves. The interaction of magnetic and electronic subsystems in Tb₂Te₅ has been revealed by the pump probe. Torque measurements of TbTe₃ show that the magnetic moments of Tb are oriented predominantly in the ac plane at high temperatures and switch to the b axis at low temperatures. In Tb₂Te₅, the magnetic moments of Tb are oriented predominantly in the ac plane at low temperatures.