Superconductivity and phase separation in electrochemically hydrogenized ${\mathrm{K}}_{1-\delta }{\mathrm{Cr}}_3{\mathrm{As}}_3{\mathrm{H}}_x$

We report preparation, crystal structure, and physical properties of a quasi-one-dimensional Cr-based arsenide hydride ${\mathrm{K}}_{1-\delta }{\mathrm{Cr}}_3{\mathrm{As}}_3{\mathrm{H}}_x$. Through an electrolysis using essentially nonsuperconducting samples as the cathode, additional hydrogen atoms can be successfully intercalated up to $x=0.45$ and, consequently, the in-plane and interplane $\mathrm{Cr}\text{–}\mathrm{Cr}$ bond distances in the chains of face-sharing Cr octahedra increase by 3.7% and 1.5%, respectively. The electrochemically hydrogenized samples show a broad superconducting transition at $T_{\mathrm{c}}=5.8$ K, a record in the K-Cr-As-H system, with nearly full magnetic shielding at 1.8 K. The electronic specific-heat coefficient extracted from the specific-heat measurement is as high as ${\gamma }_{\mathrm{n}}=47 \mathrm{mJ}$ ${\mathrm{K}}^{-2} \mathrm{mol}$ ${\mathrm{Cr}}^{-1}$, suggesting a stronger electron correlation that is likely to be associated with the expansions of $\mathrm{Cr}\text{–}\mathrm{Cr}$ bonds. Meanwhile, the dimensionless specific-heat jump $\mathrm{\Delta }C/\left({\gamma }_{\mathrm{n}}{T}_{\mathrm{c}}\right)$ is only 0.30, about 20% of the expected value in the BCS weak-coupling scenario. Furthermore, the normal-state magnetism is characterized by Curie-Weiss paramagnetism with an enhanced effective localized moment of 1.33 ${\mu }_{\mathrm{B}}/\mathrm{Cr}$, suggesting that a nonsuperconducting phase with localized spins dominates. The ${}^1\mathrm{H}$ nuclear magnetic resonance measurement reveals two different spin-lattice relaxations, corresponding to superconducting and localized-spin phases, respectively. All the results point to phase separation with minority superconducting phase and majority nonsuperconducting phase in the quasi-one-dimensional ${\mathrm{K}}_{1-\delta }{\mathrm{Cr}}_3{\mathrm{As}}_3{\mathrm{H}}_x$ system.

Figure 1

Chemical composition analysis for the electrochemically hydrogenized ${\mathrm{K}}_{1-\delta }{\mathrm{Cr}}_3{\mathrm{As}}_3{\mathrm{H}}_x$ sample using energy-dispersive x-ray spectra (EDS). (a) A typical EDS with electron beam focused on a crystalline rod. (b), (c) SEM images in which EDS line scans (in bold green) along and across the aligned crystalline rods were performed. The dashed lines are guides to the eye showing variations of the chemical composition.

Figure 2

Rietveld refinement profile of powder x-ray diffraction of the electrochemically hydrogenized ${\mathrm{KCr}}_3{\mathrm{As}}_3{\mathrm{H}}_{0.45}$. The middle inset shows the crystal structure projected along the $c$ axis. The right inset highlights the ${\mathrm{Cr}}_3{\mathrm{H}}_{0.45}$ face-sharing-octahedron chains with H atoms at the centers.

Figure 3

Temperature dependence of electrical resistivity for the electrochemically hydrogenized ${\mathrm{KCr}}_3{\mathrm{As}}_3{\mathrm{H}}_{0.45}$ polycrystalline sample. The red solid line gives the data fit with the formula shown. The inset displays the closeup which clearly shows the superconducting transition.

Figure 4

Temperature dependence of dc magnetic susceptibility at low temperatures for ${\mathrm{KCr}}_3{\mathrm{As}}_3{\mathrm{H}}_{0.45}$. FC and ZFC denote field cooling and zero-field cooling, respectively. The inset shows the closeup from which the superconducting onset transition and spin-glass transition can be simultaneously seen (marked by the arrows).

Figure 5

Field dependence of magnetization at different temperatures for ${\mathrm{KCr}}_3{\mathrm{As}}_3{\mathrm{H}}_{0.45}$. The inset shows the closeup at low fields.

Figure 6

Temperature dependence of magnetic susceptibility (left axis) and its reciprocal (right axis) for ${\mathrm{KCr}}_3{\mathrm{As}}_3{\mathrm{H}}_{0.45}$. Red lines show the Curie-Weiss fitting. The small anomaly at around 50 K, commonly observed for porous samples exposed in air, is attributed to the antiferromagnetic transition of the residual condensed oxygen.

Figure 7

Low-temperature specific-heat data for ${\mathrm{KCr}}_3{\mathrm{As}}_3{\mathrm{H}}_{0.45}$, plotted in $C/T$ vs $T^2$. The data of superconducting ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ and nonsuperconducting ${\mathrm{KCr}}_3{\mathrm{As}}_3$ are also presented for comparison. The lower inset shows temperature dependence of the electronic part ($C-C_{\mathrm{ph}}$) scaled by ${\gamma }_{\mathrm{n}}T$.

Figure 8

${}^1\mathrm{H}$ NMR results for ${\mathrm{KCr}}_3{\mathrm{As}}_3{\mathrm{H}}_{0.45}$. (a) Spin-lattice relaxation curve of ${}^1\mathrm{H}$ nuclei and the corresponding data fitting with two components of $T_1^{\mathrm{long}}$ and $T_1^{\mathrm{short}}$. Inset: ${}^1\mathrm{H}$ NMR spectrum at 265 K with two-component fitting. Red and blue lines stand for two distinct fitting components and green dashed line stands for the total fitting curve. (b) Temperature dependence of ($T_1{T)}^{-1}$ for both the long and short components.