Local Lattice Dynamics in the Mg0.5Al0.5B2 Superconductor

We have studied temperature evolution of the local as well as the average crystal structure of MgB2 and Mg0.5Al0.5B2 using real-space atomic pair distribution function (PDF) measured by high resolution neutron powder diffraction in a wide temperature range of T=10–600 K. The mean square relative displacements (MSRD) of atomic B–B, B–Mg (B–Al) pairs are compared with mean-square displacements (MSD) to calculate atomic correlations. In spite of the enhanced atomic disorder in Mg0.5Al0.5B2, where the boron–boron, and boron–magnesium pair motions are found to be small, we find that the same atomic correlations in MgB2 assume even slightly lower values and remain nearly constant in a wide temperature range of 0–600 K. This anomalous behavior and its physical interpretation provoke new questions on our understanding to the local lattice dynamics in this material.