High-pressure synthesis of superconducting Nb{1-x}B2 (x = 0-0.48) with the maximum Tc = 9.2 K

TL;DR

This study reports the synthesis of superconducting Nb{1-x}B2 with Tc up to 9.2 K under high pressure, highlighting the effect of non-stoichiometry and synthesis conditions on superconductivity.

Contribution

It demonstrates that high-pressure synthesis induces superconductivity in NbB2 with varying non-stoichiometry, revealing the relationship between composition, structure, and Tc.

Findings

Superconductivity observed above x=0.04 in Nb1-xB2.

Maximum Tc of 9.2 K at x=0.24 for Nb1-xB2.

Tc varies with sintering pressure from 7 K to 9 K.

Abstract

Superconductivity with Tc above 9 K was found in metal-deficient NbB2 prepared under 5 GPa, while no clear superconductivity was observed down to 3 K in stoichiometric NbB2. The superconductivity was observed above x = 0.04 in Nb1-xB2. and the lattice parameters also changed abruptly at x = 0.04. As x increased, the transition temperature Tc slightly rose and fell with the maximum value of 9.2 K at x = 0.24 for the samples sintered at 5 GPa and 1200 C. The Tc-value changed in the range from 7 K to 9 K, depending on the sintering pressure. A series of Ta1-xB2 (0 =< x =< 0.24) was also synthesized under high pressure to examine a special effect of high-pressure synthesis.