Effects on transition temperature and Raman spectra of substitution of Cr and Al for Mg in MgB2

Abstract

AbstractMg1−x Mx B2 (M = Cr, Al) polycrystalline bulk samples with 0 ≤ x ≤ 5% have been synthesized by a solid state reaction and studied by X‐ray diffraction, SEM and Raman spectroscopy. By comparing with the results of Sc‐substituted MgB2, it is found that all these atomic substitutions for Mg depress Tc, and Cr and Sc substitutions depress Tc even more quickly than the one of Al doping. It is suggested that the Tc changes of the M‐substituted MgB2 can be understood by the competing/cooperating effects of the hole‐filling and the disorder. The Raman peak linewidth decreases for Al‐substituted MgB2, which represents an opposite trend to the linewidth change of Mg1−x Crx B2 and shows an evidence of the disorder caused by the atomic substitution and the different arrangement of the substituted atoms in the lattice. Meanwhile, different frequency changes of the Raman peak for the Cr‐ and Al‐substitute MgB2 are discovered and understood by the competing/cooperating effects of the electron‐phonon coupling, the grain size, and the lattice contraction. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)