Pressure-Stabilized Tin Selenide Phase with an Unexpected Stoichiometry and a Predicted Superconducting State at Low Temperatures

Tin-selenium binary compounds are important semiconductors that have attracted much interest for thermoelectric and photovoltaic applications. As tin has a $+2$ or $+4$ oxidation state and selenium has an oxidation number of $-2$, only SnSe and ${\mathrm{SnSe}}_2$ have been observed. In this work, we show that the chemical bonding between tin and selenium becomes counterintuitive under pressures. Combining evolutionary algorithms and density functional theory, a novel cubic tin-selenium compound with an unexpected stoichiometry $3:4$ has been predicted and further synthesized in laser-heated diamond anvil cell experiments. Different from the conventional SnSe and ${\mathrm{SnSe}}_2$ semiconductors, ${\mathrm{Sn}}_3{\mathrm{Se}}_4$ is predicted to be metallic and exhibit a superconducting transition at low temperatures. Based on electron density and Bader charge analysis, we show that ${\mathrm{Sn}}_3{\mathrm{Se}}_4$ has a mixed nature of chemical bonds. The successful synthesis of ${\mathrm{Sn}}_3{\mathrm{Se}}_4$ paves the way for the discovery of other IV-VI compounds with nonconventional stoichiometries and novel properties.

Figure 1

Enthalpies of formation of the predicted stable compounds in the Sn-Se binary system. Solid lines represent the convex hulls at different hydrostatic pressures.

Figure 2

Phonon dispersions of the predicted ${\mathrm{Sn}}_3{\mathrm{Se}}_4$ compound at hydrostatic pressures of 0 GPa (a) and 10 GPa (b). The negative values represent imaginary phonon frequencies.

Figure 3

Le Bail fitting plot of in situ XRD patterns of ${\mathrm{Sn}}_3{\mathrm{Se}}_4$ under 16.4 GPa and 1225 K. The experimental and simulated data are shown in black circles and red solid curves, respectively; their differences are shown in black solid curves and the Bragg positions are indicated by vertical bars. The peaks of MgO, the thermal isolator, are masked and shown in gray circles.

Figure 4

Projected electronic band structures along high symmetry paths (left) and projected DOS (right) of SnSe (a), ${\mathrm{Sn}}_3{\mathrm{Se}}_4$ (b), and ${\mathrm{SnSe}}_2$ (c), at 10 GPa. Fermi level is located at 0 eV. Different circle sizes correspond to the projected weights of different orbitals.

Figure 5

(a) The conventional unit cell of ${\mathrm{Sn}}_3{\mathrm{Se}}_4$; red and grey spheres represent the Sn and Se atoms, respectively. Charge density distributions in SnSe (b), ${\mathrm{Sn}}_3{\mathrm{Se}}_4$ (c), and ${\mathrm{SnSe}}_2$ (d) at 10 GPa; the corresponding coordination polyhedra are also given.