The broad-band reflection spectra of YB 6 and YbB 6 hexaborides with Jahn–Teller instability of the boron cage have been measured at room temperature. An optical conductivity analysis has revealed, along with the Drude electronic components, heavily overdamped collective modes, which are notable in YB 6 for high dielectric contributions, Δε = 2000–5700. The fraction of nonequilibrium charge carriers in YB 6 , which is at the boundary of structural instability in the hexaboride family, reaches 85–90%, whereas this fraction in doped YbB 6 semiconductor is not higher than 25%. It has been shown that unlike the predictions of the topological Kondo insulator model, the surface “metallization” in Yb 2+ B 6 crystals can be explained by additional doping of a surface layer with Yb 3+ ions.

A detailed investigation of contributions to the magnetization of nonmagnetic YB 6 , LaB 6 , and YbB 6 hexaboride single crystals has been performed, and a procedure for their separation has been proposed. It has been shown that a low value of electronic susceptibility χ e ( T ) in YB 6 and LaB 6 hexaborides seems to be associated with a small effective mass of band carriers, m * ~ 0.5 m 0 . As a result of this, the Pauli component and Landau diamagnetism cancel each other. It has been found that χ e ( T ) varies in the intervals T < T * ( T * ~ 50 K) and T > 150 K, which are attributed to an order–disorder transition below T * and a carriers-related contribution due to the Jahn–Teller structural instability of the boron frame work.

Detailed measurements of the Hall effect in the Ho0.8Lu0.2B12 antiferromagnetic compound (Néel temperature TN = 5.75 K) in magnetic fields up to 80 kOe oriented in the (110) plane at temperatures of 1.9–6.6 K are performed. It is established with the contribution separation procedure that the anisotropic positive contribution, which is responsible for the double inversion of the sign of the Hall resistance, dominates in the antiferromagnetic state of Ho0.8Lu0.2B12 in fields of 30–50 kOe. A sharp decrease in the amplitude of the isotropic negative contribution at the transition to the antiferromagnetic phase is found. The nature of the detected anomalies in the antiferromagnetic metal with dynamic charge stripes and spin-wave component of the magnetic structure is discussed.

Using Fourier-transform infrared spectroscopy and optical ellipsometry, room temperature spectra of complex conductivity of single crystals of hexaborides GdxLa1-xB6, x(Gd) = 0, 0.01, 0.1, 0.78, 1, are determined in the frequency range 30-35000 cm(-1). In all compounds, in addition to the Drude free-carrier spectral component, broad excitations are discovered with unusually large dielectric contributions Delta epsilon = 7 000-15 000 and non-Lorentzian line shapes. It is suggested that the origin of the excitations is connected with the dynamic cooperative Jahn-Teller effect of B-6 clusters. Analysis of the spectra together with the results of dc and Hall resistivity measurements show that only 25-50% of the conduction band electrons are contributing to the free carrier ac conductivity, with the rest being involved in the formation of an overdamped excitation, thus providing a possible explanation, in terms of nonequilibrium (hot) electrons in these hexaborides, of both the remarkably low work function of thermoemission of Gd(x)La(1-x)B(6 )and the non-Fermi-liquid behavior in GdB6 crystals.