Hydrogen substitution effect on the superconductivity of LnNiAsO (Ln = La–Nd)

Abstract

Hydrogen substitution at the oxygen sites of 1111-type layered nickel oxyarsenides LnNiAsO (Ln=lanthanoid) was successfully performed using a high-pressure synthesis technique using LnH2 as a key component of the starting mixture. In addition to the superconducting LaNiAsO (Tc = 2.4 K) and non-superconducting CeNiAsO, which have already been reported, PrNiAsO (Tc = 1.6 K) and NdNiAsO (non-superconducting above 0.5 K) were successfully synthesized using this technique. Electron-probe micro-analysis (EPMA) and thermogravimetry analysis-mass spectroscopy (TG-MS) indicated that up to 18% of the oxygen sites could be substituted by hydrogen. Although the Tc of LaNiAsO1−xHx increased to 3.7 K for x = 0.07–0.17 and rapidly decreased to 1.7 K at x = 0.18, no enhancement in the Tc value above 2 K was observed in any of the other LnNiAsO1−xHx systems. Isovalent substitution at the La sites of La3+ for either Pr3+ or Nd3+ and hydrogen substitution of the oxygen sites produced a chemical pressure effect, causing the lattice parameters to decrease. An enhancement and sudden drop in the Tc values, however, was observed following hydrogen substitution, suggesting that the hydrogen substitution process supplied additional electron density to the NiAs layer, resulting in enhancement of the Tc value.