Thermal expansion and phase purity of commercial MgB_2

摘要

It is becoming increasingly clear that impurity phases and non-stoichiometry are commonplace in MgB_2, which becomes superconducting below 39 K [1]. It is now also well established that these effects influence T_c, residual resistance levels and other technologically important parameters [2-5]. In the present communication, we study two commercial samples, both of which present significant levels of impurities, and we measure the thermal expansion for the purest sample. MgB_2 adopts the anisotropic AlB_2 structure type and its structural properties have been investigated as a function of pressure [6-8] and, independently of our study, of temperature [9,10]. Both of the previous temperature-dependent measurements were carried out by neutron time-of-flight powder diffraction and covered the temperature ranges 11 to 297 K (Jorgensen et al. [9]), and 4 to 53 K (Margadonna et al.[10]).