On Electron Transport in ZrB12, ZrB2 and MgB2

We report on measurements of the temperature dependence of resistivity, $\rho(T)$, for single crystal samples of ZrB$_{12}$, ZrB$_{2}$ and polycrystalline samples of MgB$_{2}$. It is shown that cluster compound ZrB$_{12}$ behaves like a simple metal in the normal state, with a typical Bloch -- Gr\"uneisen $\rho(T)$ dependence. However, the resistive Debye temperature, $T_{R}=300 K$, is three times smaller than $T_{D}$ obtained from specific heat data. We observe the $T^{2}$ term in $\rho(T)$ of these borides, which could be interpreted as an indication of strong electron-electron interaction. Although the $\rho (T)$ dependence of ZrB$_{12}$ reveals a sharp superconductive transition at $T_{c}=6.0 K$, no superconductivity was observed for single crystal samples of ZrB$_{2}$ down to $1.3 K$.