Magnetic phase transition, crystal electric field and zero thermal expansion in ab-plane of thulium carboboride TmB2C

Temperature dependences of heat capacity C_P(T) and lattice parameters a(T), b(T), c(T) of thulium carboboride TmB₂C at 2–300 K were experimentally studied. Magnetic phase transition in the carboboride – 'paramagnetic – antiferromagnetic' at T_N = 11.0 K was revealed. By comparison with the diamagnetic LuB₂C the Schottky contribution to TmB₂C heat capacity due to the crystal electric field (CEF) was calculated. The splitting scheme of Tm³⁺ ion f – level by the crystal field was suggested. It was obtained that in the temperature range under study (5–300 K) TmB₂C carboboride thermal expansion in the basal plane is almost missing (anisotropic invar effect, linear thermal expansion coefficient α_a(T) ≃ α_b(T) ≃ 0). It is, seems, a common property of the lattice dynamics of layered carboborides. The anomalies in the thulium carboboride thermal expansion are due to the magnetic phase transition and the CEF influence.