Pressure-induced superconductivity in the metal thiophosphate ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$

We report on the pressure-tuning of the physical properties of a metal thiophosphate ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$. Resistance measurements evidence a semiconducting-to-metallic transition at around 39 GPa and immediate emergence of superconductivity. The superconducting critical temperature $T_C$ monotonously increases from 3.2 to 5.1 K until the highest studied pressure 54 GPa. Over the whole pressure range, the pristine monoclinic lattice symmetry is not changed. Moreover, the resonance Raman scattering is observed in the range of 8–12 GPa depending on the excitation energy used. These findings establish ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ as a new platform to explore intrinsic metallization and superconductivity, which may promote developments of relevant applications.

Figure 1

(a) XRD pattern for a ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ single crystal. Inset: an optic image of the single crystal. (b) Energy-dispersive x-ray spectroscopy and (c) Raman spectrum for the ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ single crystal.

Figure 2

(a) Temperature-dependent resistance curves R(T) for a ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ single crystal in the pressure range of (a) 23.3–33.3 GPa, and (b) 39.4–54.0 GPa. The inset of (b) displays the zoomed-in view of the R(T) curves in the low-temperature range.

Figure 3

Magnetic field evolution of the resistive superconducting transition measured at selected pressures of (a) 45.8 GPa, and (b) 54.0 GPa. (c) Temperature-dependent upper critical field ${\mu }_0{H}_{c2}\left(T\right)$ extracted from the data in (a) and (b). For determination of the superconducting critical temperature, $T_c^{90\%}$ where resistance drops to 90% of the normal-state resistance are used. The solid lines correspond to the fitting to the Werthamer-Helfand-Hohenberg (WHH) model [48].

Figure 4

(a) Synchrotron x-ray diffraction (XRD) patterns under pressures up to 55.9 GPa, which were measured on powdered ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ crystals at room temperature. For 0.8 GPa, the red line presents the fitting to the data and the blue vertical bars indicate the diffraction peak positions. (b) Pressure dependences of lattice parameters $a$, $b$, and $c$. (c) Pressure dependence of the lattice volume V. The solid red line represents the fitting curve based on the third-order Birch-Murnaghan equation of state.

Figure 5

Raman spectra (Run 1) measured on the ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ single crystal (a) from 0.2 to 16.7 GPa and (b) from 16.7 to 49.5 GPa. The measurements were performed with a laser wavelength of $\lambda =532$ nm (or laser energy 2.33 eV) at room temperature. (c) Pressure-dependent Raman shifts for all the detected modes.

Figure 6

The pressure evolution phase diagram for ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$. The left axis represents the values of resistance at 300 K. The right axis corresponds to the onset superconducting critical temperature ${T_c}^{\mathrm{onset}}$ and zero-resistance temperature ${T_{\mathrm{c}}}^{\mathrm{zero}}$. The insets display optical micrographs of the ${\mathrm{Pb}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ sample at different pressures.