Al substitution in MgB2 crystals:: Influence on superconducting and structural properties -: art. no. 174506

Single crystals of Mg1-xAlxB2 have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with x < 0.1 were obtained as a result of the optimization of an already developed MgB2 crystal growth procedure. A systematic decrease of the c-axis lattice constant with increasing Al content, when the a-axis lattice constant is practically unchanged, was observed. Variation of the critical temperature on Al content in Mg1-xAlxB2 crystals was found to be slightly different than that one observed for polycrystalline samples, since even a very small substitution of 1%-2% of Al leads to the decrease of T-c by about 2-3 K. X-ray and high resolution transmission electron microscopy investigations indicate the appearance of the second precipitation phase in the crystals with x > 0.1. This is in a form of nonsuperconducting MgAlB4 domains in the structure of a superconducting Mg1-xAlxB2 matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, H-c2, for H parallel to c for the samples with small x, a significant reduction of the H-c2 anisotropy at lower temperatures, and a decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with T-c in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing T-c down to 20 K. It may be related to an appearance of the precipitation phase in the Mg1-xAlxB2 structure.