Aluminium substitution induced superstructures in Mg1-xAlxB2 (x = 0.0 to 0.50): An X-ray diffraction study

摘要

The physical property characterization of Al doped Mg1-xAlxB2 system with x =0.0 to 0.50 is reported. The results related to phase formation, structuraltransition, resistivity R(T) and magnetization M(T) measurements are discussedin detail. It is shown that the addition of electrons to MgB2 through Alresults in loss of superconductivity. Also seen is a structural transitionassociated with the collapse of boron layers reflected by the continuousdecrease in the c parameter. The main emphasis in this paper is on slow scanX-ray diffraction (XRD) results, which confirm the existence of asuperstructure along the c-direction for the x = 0.50 sample. The appearance ofsome additional peaks, viz. [103], [004], [104] and [112], results in doublingof the lattice parameter along the c-axis. This possibly indicates thealternative ordering of Al and Mg in MgAlB4 separated by hexagonal boron layersbut still maintaining the same hexagonal AlB2 type structure.