Defect Mode in LaB6.

The specific heat of high quality La^NB₆ (N = 10, 11, natural) single crystals is investigated in a wide range of temperatures 2 - 300 K. The obtained data allow to estimate correctly (i) the electronic γ ·T term of specific heat (γ ≈ 2.4 mJ/(mol·K²)), (ii) the contribution from quasilocal vibrating mode of La³⁺ ions (θE ≈ 150 - 152 K), (iii) the Debye-type term from rigid boron cages (θ_D ≈ 1160 ± 40 K). Our data also suggest an additional defect-mode component (iv) which may be attributed to a contribution of 1.5% boron vacancies in LaB₆. The obtained results may be interpreted in terms of formation of two level systems, which appear when La³⁺ ions are displaced from their centrosymmetric positions in the cavities of rigid boron cages, apart from randomly distributed boron vacancies in the LaB₆ matrix. [ABSTRACT FROM AUTHOR]