(La,Th)H₁₀: Potential High-T_c (242 K) Superconductors Stabilized Thermodynamically below 200 GPa

The recent experimental discovery of high-pressure superconductivity in (La,Y)H₁₀, (La,Ce)H₉, (La,Ce)H₁₀, (Y,Ce)H₉, and (La,Nd)H₁₀ confirms that ternary rare-earth clathrate hydrides are promising candidates for high-temperature superconductors. This study theoretically demonstrates the ability of actinide-metal thorium (Th), rare-earth-metal lanthanum (La), and hydrogen to form stable ternary hydrides under 200 GPa. Using the evolutionary algorithm combined with the first-principles calculations and strong-coupling Eliashberg approach, we predicted the pressure-dependent ternary phase diagram of La_xTh_yH_z, particularly, the case of (La_1-xTh_x)H_n [or designated as (La,Th)H_n for simplicity]. Our calculations revealed that hydrogen-rich phases such as (La,Th)H₁₀ are thermodynamically stable below 200 GPa, with phase decomposition occurring in (La,Th)H₉ above 150 GPa. Moreover, the electron-phonon coupling (EPC) calculations show that the (La,Th)H₁₀ series could function as potential superconductors, of which I4/mmm-La₃ThH₄₀ (at 200 GPa) exhibited a large EPC constant of λ = 2.46 with the highest transition temperature (T_c) of 242 K.