Superconductivity at 253 K in lanthanum-yttrium ternary hydrides

Polyhydrides offer intriguing perspectives as high-temperature superconductors. Here we report the high-pressure synthesis of a series of lanthanum-yttrium ternary hydrides: cubic hexahydride \((La,Y)H_{6}\) with a critical temperature \(T_{C}\) = 237 +/- 5 K and decahydrides \((La,Y)H_{10}\) with a maximum \(T_{C}\) ~\({253 K}\) and an extrapolated upper critical magnetic field \(B_{C2(0)}\) up to \({135 T}\) at 183 GPa. This is one of the first examples of ternary high-\(T_{C}\) superconducting hydrides. Our experiments show that a part of the atoms in the structures of recently discovered \({Im3m}\)-\(YH_{6}\) and \({Fm3m}\)-\(LaH_{10}\) can be replaced with lanthanum (~70 %) and yttrium (~25 %), respectively, with a formation of unique ternary superhydrides containing incorporated \(La@H_{24}\) and \(Y@H_{32}\) which are specific for \({Im3m}\)-\(LaH_{6}\) and \({Fm3m}\)-\(YH_{10}\). Ternary La-Y hydrides were obtained at pressures of 170-196 GPa via the laser heating of $P6_{3}$${/mmc}\( lanthanum-yttrium alloys in the ammonia borane medium at temperatures above 2000 K. A novel tetragonal \)(La,Y)H_{4}\( was discovered as an impurity phase in synthesized cubic \)(La,Y)H_{6}\(. The current-voltage measurements show that the critical current density \)J_{C}\( in \)(La,Y)H_{10}\( may exceed \)2500 A/mm^{2}\( at 4.2 K, which is comparable with that for commercial superconducting wires such as \){NbTi}\(, \)Nb_{3}$${Sn}$. Hydrides that are unstable in a pure form may nevertheless be stabilized at relatively low pressures in solid solutions with superhydrides having the same structure.