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 crystal structure and some physical properties of TbTe3 were established previously, the crystal structure of Tb2Te5 was solved only in this work in the orthorhombic space group Cmcm with the parameters of unit cell a = 4.3120(5), b = 41.0305(76) and c = 4.2979(8) {\AA}. In contrast to TbTe3, which experiences three successive magnetic phase transitions, Tb2Te5 orders antiferromagnetically in two steps at TN1 = 9.0 K and TN2 = 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.