In BCS-type superconductors Zr N B 12 (T C ≈ 6 K) and Lu N B 12 (T C ≈ 0.42 K) heat capacity C(T ) and magnetization measurements have been carried out on high quality single crystals with various boron isotopes (with N = 10, 11, and with natural composition of 10 B and 11 B). Parameters of the superconducting and normal states have been deduced from this study, allowing comparison between these two dodecaborides. It was shown that ZrB 12 is a type-II superconductor in which the Ginzburg-Landau parameter varies in the range κ = 0.8-1.12. A detailed analysis of specific heat in the normal state of R 10 B 12 , R 11 B 12 and R nat B 12 (where R = Zr, Lu) has revealed three Einstein type vibration modes of Zr 4+ -ions in ZrB 12 with characteristic energies θ E1 (Zr N B 12 ) ≈ 200 K and θ E2,3 (Zr N B 12 ) ≈ 450 K, N. Sluchanko ( ) •

The superconducting and normal state characteristics of yttrium hexaboride (YB6) have been investigated for the single crystals with a transition temperatures Tc ranging between 6 K and 7.6 K. The extracted set of microscopic parameters [the coherence length ξ(0) ∼ 320÷340Å, the penetration depth λ(0) ∼ 1100÷1600Å and the mean free path of charge carriers l = 31÷58Å, the Ginzburg-Landau-Maki parameters κ1,2(0) ∼ 3.3÷4.8 and the superconducting gap ∆(0) ∼ 10.3÷14.8 K] confirms the type II superconductivity in "dirty limit" (ξ≫l ) with a medium to strong electron-phonon interaction (the electron-phonon interaction constant λ e-ph = 0.93÷0.96) and s-type pairing of charge carriers in this compound [2∆(0)/kB Tc ≈ 4]. The comparative analysis of charge transport (resistivity, Hall and Seebeck coefficients) and thermodynamic (heat capacity, magnetization) properties in the normal state in YB6 allowed to detect a transition into the cageglass state at T * ∼ 50 K with a static disorder in the arrangement of the Y 3+ ions. We argue that the significant Tc variations in the YB6 single crystals are determined by two main factors: (i ) the superconductivity enhancement is related with the increase of the number of isolated vacancies, both at yttrium and boron sites, which leads to the development of an instability in the hexaboride lattice; (ii ) the Tc depression is additionally stimulated by the spin polarization of conduction electrons emerged and enhanced by the magnetic field in the vicinity of defect complexes in the YB6 matrix.

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