Peculiarities of electronic, phonon and magnon subsystems of lanthanum and samarium tetraborides

Abstract

Experimental research was carried out to study the temperature dependences of heat capacity Cp(T = 2–300 K), lattice parameters a(Т), and ≿(Т), (5–300 K) of lanthanum and samarium tetraborides. A comparison with data obtained previously for LuB4 reveals the peculiar influence of lanthanide contraction and the rare-earths mass on the thermodynamic properties of rare earth tetraborides at low and high temperatures. Sharp anomalies were found in the heat capacity and thermal expansion for SmB4 at ТN = 25.1 K, conditioned by the phase transition into antiferromagnetic state. The more poorly defined heat capacity anomaly around 7 K is referred to the quadrupole orbital fluctuation of the atomic magnetic moments for Sm3+ ions. The electronic, lattice, and magnetic contributions to the heat capacity and thermal expansion of samarium tetraboride were defined. Our approach makes it possible to adequately approximate the lattice components of heat capacity and thermal expansion by combining the Debye and Einstein contributions, which are based on the joint analysis of calorimetric and X-ray data. The influence of the frustration of the atomic magnetic moment system for Sm3+ ions on the thermodynamic characteristics of the samarium tetraboride magnetic phase transition was revealed.