Defect Mode in LaB_{6}

Abstract

The specic heat of high quality La N B6 (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 specic heat (γ ≈ 2.4 mJ/(mol·K 2 )), (ii) the contribution from quasilocal vibrating mode of La 3+ ions (ΘE ≈ 150 152 K), (iii) the Debye-type term from rigid boron cages (ΘD ≈ 1160 ± 40 K). Our data also suggest an additional defectmode 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 La 3+ ions are displaced from their centrosymmetric positions in the cavities of rigid boron cages, apart from randomly distributed boron vacancies in the LaB6 matrix.

Lattice instability and enhancement of superconductivity in YB6

“…We present also the chemical composition found for the investigated YB6 crystals, and the mass densities g C m and gm obtained from C(T ) and hydrostatic measurements, respectively. [44]. It is also in agreement with the data of Θ D ≈ 1160-1190 K obtained in [2,45,48] for the analog non-magnetic higher boride − lutetium dodecaboride (LuB 12 ), and comparable to Θ D ≈ 1250-1370 K deduced for β-boron in X-ray diffraction studies [48].…”

“…It is also in agreement with the data of Θ D ≈ 1160-1190 K obtained in [2,45,48] for the analog non-magnetic higher boride − lutetium dodecaboride (LuB 12 ), and comparable to Θ D ≈ 1250-1370 K deduced for β-boron in X-ray diffraction studies [48]. Along with Einstein component, which leads to maximum on (C − γT − C D )/T 3 vs. T curves near 20 K [see Fig.14(a)], we observed two additional features on these dependence − one near 10 K and another below 4 K. The separation of these low-temperature contributions was made in the same manner as it was done for LaB 6 in [43,44], where the heat capacity below 20 K was associated with two additive two-level components attributed to vibrations of rare earth ions in the vicinity of boron vacancies (see two-level systems TLS 1 and TLS 2 in Fig.14(a), and also [49]). These two TLS terms were described by the Schottky formula…”

“…The barrier height of TLS 2 [see Fig.14(b) and Table IV] was found to be ∆E 2 ∼ 50 K, and this value does not practically depend on the concentration of intrinsic defects in crystals No.1-No.3. The obtained relative concentration of cells with a double-well Table IV] is quite small when compared with the amount of boron di-vacancies previously detected in LaB 6 [43,44]. Indeed, the estimation of the concentration of boron di-vacancies in case of their random distribution in the RB 6 matrix leads to n d (B) = n v (B)(1 − [1 − n v (B)] z ) ≈ 3.5 ×10 −3 (where z = 4 is the coordination number in the boron sub-lattice), which with a good accuracy corresponds to the TLS 1 concentration N 1 found for sample No.3 [Table IV].…”

Towards a Single Chemical Model for Understanding Lanthanide Hexaborides

“…13 The presence of an anomalous peak points towards the existence of an additional energy scale of motion. 30 On the basis of the two-minima model and statistical thermodynamics, we previously proposed an analytic expression for Cv in the low-T regime, which can be applied to EuB6: 25 (1)…”

Inhomogeneous superconductivity in LuxZr1−xB12 dodecaborides with dynamic charge stripes

“…The presence of two-level systems with a barrier of ΔE 2 ~ 90 K was reliably demonstrated also in cage-glass compound LaB 6 and in Ce x La 1−x B 6 solid solutions basing on low-temperature heat capacity measurements [80], [81]. Furthermore, a pseudogap [82] and a low-frequency peak in inelastic light scattering spectra [83], [84] were found in LaB 6 and the barrier in the DWP was attributed to the formation of a pseudogap in disordered metallic systems [80], [81].…”