Crystalline structure and the superconducting properties of NbB2+x

The effect of boron excess in the structure and superconducting properties of NbB2 is reported. Rietveld refinements of the x-ray diffraction patterns indicate that boron excess induces significant changes in the Nb–B bond length, increasing the c-axis. In contrast, the B–B bond length remains essentially constant. Magnetization behaviour was studied in the temperature range from 2 to 15 K. We found that for (B/Nb)exp 2.20(2) of boron excess samples display superconductivity with a maximum TC of about 9.8 K at (B/Nb)exp = 2.34(1) .H igh pressure measurements in samples with two different boron contents reveal that TC decreases at different ratios, dTC/dP. Superconducting parameters were determined, indicating that NbB2+x is a type II superconductor. We correlated the change of TC with the evolution of th es tructural parameters and found that it coincides with theoretical predictions of band structure. (Some figures in this article are in colour only in the electronic version)