  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.
