Electron transport in diborides: Observation of superconductivity in ZrB2

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB(2)) with different transition metals atoms (A = Zr, Nb, Ta). The temperature dependence of resistivity, rho (T), and ac susceptibility of these samples reveal a superconducting transition of ZrB2 with T-c = 5.5 K, while NbB2 and TaB2 have been observed to be nonsuperconducting up to 0.37K. H-c2(T) is linear in temperature below T-c, leading to a rather low H-c2(0) = 0.1 T. At T close to T-c, H-c2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB2 samples, behave like simple metals in the normal state with usual Bloch-Gruneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB2, NbB2, and MgB2, respectively, rather than T-2 and T-3, as previously reported for MgB2. (C) 2001 MAIK Nauka/ Interperiodica.