Structural and high-frequency resistive characteristics of MgB2 in the range 0-110 MHz at 5-300 K

The structure and electrical resistivity of polycrystalline MgB2 are investigated in the interval 5-300 K. Its impedance is investigated in the same temperature range in the frequency region 9-110 MHz. It is shown that the lattice type and the superconducting-phase symmetry of MgB2 are invariant in the whole temperature interval. In the region of superconducting transition temperature Tc = 39.5 K, a structural instability is observed, which is accompanied by a scatter in the lattice parameters measured. It is suggested that the crystal deformation with varying temperature is significantly anisotropic. The measurements of the temperature and frequency dependences of the surface resistance Rs(T, f) in the superconducting state have revealed a transition from the Pippard nonlocal limit at T << Tc to a London local one near Tc. At T/Tc < 0.76, Rs(T) is well described by the exponential dependence exp (- Δ (T) / k T) in accordance with the BCS theory