Due to their unique structure properties, most of the electrides that possess extra electrons locating in interstitial regions as anions are insulators. Metallic and superconducting electrides are very rare under ambient conditions. We systematically search possible compounds in Ca–S systems stabilized under various pressures up to 200 GPa, and investigate their crystal structures and properties using first-principles calculations. We predict a series of novel stoichiometries in Ca–S systems as potential superconductors, including P21/m Ca3S, P4mbm Ca3S, Pnma Ca2S, Cmcm Ca2S, Fddd CaS2, Immm CaS3 and C2/c CaS4. The P4mbm Ca3S phase exhibits a maximum T c value of ∼20 K. It is interesting to notice that the P21/m Ca3S and Pnma Ca2S stabilized at 60 and 50 GPa behave as superconducting electrides with critical temperatures T c of 7.04 K and 0.26 K, respectively. More importantly, our results demonstrate that P21/m Ca3S and Pnma Ca2S are dynamically stable at 5 GPa and 0 GPa, respectively, indicating a high possibility to be quenched to ambient condition or synthesized using the large volume press.

New stable stoichiometries in K–Ga systems are firstly investigated up to 100 GPa by the unbiased structure searching techniques. Six novel compositions as K4Ga, K3Ga, K2Ga, KGa, KGa2 and KGa4 are found to be thermodynamically stable under pressure. Most of the predicted stable phases exhibit metallic character, while the F d 3 ¯ m KGa phase behaves as a semiconductor with a bandgap ∼1.62 eV. Notably, the gallium atoms exhibit different interesting morphologies; e.g., Ga2 units, zigzag chains, six rings and cage. We further investigate the bonding nature of K–Ga systems with help of electron localization function and Bader charge analyses. Strong covalent bonding characteristics are found between the Ga and Ga atoms, and ionic bonding patterns are observed between the K and Ga atoms. Meanwhile, we notice charge transferring from the K atom to the Ga atom in the K–Ga systems. The present results can be helpful for understanding the diverse structures and properties of K–Ga binary compounds at high pressures.

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