Structural analysis and superconductivity of CeFeAsO${}_{1-x}$H${}_x$

We performed the neutron powder diffraction (NPD) and synchrotron x-ray diffraction measurements on CeFeAsO${}_{1-x}$(D,H)${}_x$ ($x$ $=$ 0.0 − 0.48) as a representative of 1111-type family of iron-based superconductors $Ln$FeAsO${}_{1-x}$H${}_x$ ($Ln$ $=$ lanthanoid). Deuterated and hydrogenated samples (CeFeAsO${}_{1-x}$D${}_x$ and CeFeAsO${}_{1-x}$H${}_x$) were synthesized by the solid-state reaction of a metal oxide, arsenides, and a hydride and a deuteride source under an applied pressure of 2 GPa. No distinct differences were found between the structural and superconducting properties of the hydride and deuteride samples. Rietveld analyses of the NPD patterns demonstrated that deuterium exclusively substitutes on the oxygen sites in the 1111-type structures according to the nominal composition. Bulk superconductivity was observed over a wide $x$ region (0.1 < $x$ < 0.4) and the superconducting dome had a rather flat shape with a maximum $T_{\mathrm{c}}$ $=$ 47 K at around $x$ $=$ 0.25. It was concluded from density functional theory calculations and comparison with the superconducting dome of the fluorine-substituted system that the charge state of the hydrogen substituting the oxygen sites was −1. The relationship between the lattice parameter $a$ and $T_{\mathrm{c}}$ in our samples prepared from metal hydrides is almost the same as that reported previously for samples prepared from cerium hydroxide. These results strongly suggest that H${}^{-}$ ions exclusively occupy the oxygen sites in both samples, regardless of the hydrogen species in the starting material.

Figure 1

(a) Fitted NPD pattern of CeFeAsO${}_{0.7}$D${}_{0.3}$ at RT and (b) deuterium contents obtained by Rietveld analyses of NPD patterns and oxygen content obtained by EPMA as a function of nominal composition $x$.

Figure 2

Structural and superconductivity data. (a) Variations in lattice parameters ($a$ and $c$) as a function of $x$; (b) atomic positions $z$ ($z_{\mathrm{Ce}}$ and $z_{\mathrm{As}}$) of CeFeAsO${}_{1-x}$H${}_x$ and CeFeAsO${}_{1-x}$D${}_x$ measured by SXRD and NPD; (c) ρ–$T$ profile of CeFeAsO${}_{1-x}$H${}_x$ (solid line) and CeFeAsO${}_{1-x}$D${}_x$ (open circles) in underdoped (left, $x$ $=$ 0.00 − 0.10) and overdoped states (right, $x$ $=$ 0.22 − 0.48) systems; and (d) shielding volume fractions of CeFeASO${}_{1-x}$H${}_x$ and CeFeAsO${}_{1-x}$D${}_x$ estimated from $M$–$H$ curve at 2 K and 10 Oe.

Figure 3

$x$–$T$ diagrams of CeFeAsO${}_{1-x}$H${}_x$ and CeFeAsO${}_{1-x}$D${}_x$ superimposed by data reported for CeFeAsO${}_{1-x}$F${}_x$ (Ref. 18).

Figure 4

Electronic structures: (a) structural model for DFT calculations; “+” and “−” on the Ce and Fe atoms indicate the signs of the spin magnetic moment. (b) Total DOS and atomic PDOS of CeFeAsO, CeFeAsO${}_{0.75}$F${}_{0.25}$, and CeFeAsO${}_{0.75}$H${}_{0.25}$, obtained by DFT calculations. The origin was set at the Fermi level.

Figure 5

Critical temperatures of LnFeAsO${}_{1-x}$H${}_x$ (Ln $=$ Ce and Sm) as a function of lattice parameters $a$, superimposed by those of LnFeAsO${}_{1-y}$(OH)${}_x$ LnFeAsO${}_{1-y}$, and LnFeAsO${}_{1-x}$F${}_x$.(Refs. 12, 18, 28,43, 46, 47, 48, 49, 50, 51.)