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Rare-earth hydrides can exhibit high-temperature superconductivity under high pressure. Here, we apply a crystal structure prediction method to the current record-holding Tc material, LaH10, and a candidate for even higher Tc, YH10. We find a pressure-induced phase transition from the experimentally observed cubic LaH10 phase to a new hexagonal phase at around 420 GPa. This hexagonal phase could explain experimental observations of hcp impurities in fcc samples. We find that YH10 forms similar structures to LaH10 and discuss the sensitivity of superconductivity calculations to the computational parameters used. I. INTRODUCTIONHydrogen was predicted to be a room-temperature superconductor at very high pressure in 1968 [1], but the pressures required to metallise hydrogen are difficult to obtain. Hydrides have been suggested to have lower metallisation pressures than pure hydrogen due to chemical pre-compression [2] and therefore might become superconducting at more readily accessible pressures. This idea has motivated a surge of research examining potential superconductivity in high-pressure hydrides. There are several reviews summarising recent developments in this field [3][4][5][6][7][8][9].Theoretical studies of ScH 3 , LaH 3 [10], YH 3 , YH 4 and YH 6 [11-13] identified hydrides of rare-earth elements as potential high-temperature superconductors. Firstprinciples structure searching studies of rare-earth hydrides have reported structures with high hydrogen content adopting clathrate (cage-like) structures [14,15]. Of particular note, a T c of 264-286 K was calculated for F m 3m LaH 10 at 210 GPa [14], while the analogous YH 10 structure was calculated to have T c = 305-326 K at 250 GPa. Slight distortions of the cubic LaH 10 phase were found to lead to C2/m and R 3m structures at lower pressures [16,17], though Ref. [18] showed that quantum effects render F m 3m as the true ground state.These predictions were followed by experimental measurement of critical temperatures reaching 260 K in LaH 10 at 20]. These record-breaking measurements built on previous successful studies of superconductivity in H 3 S [21,22]. The high-T c phase was determined to be a structure with an fcc arrangement of La atoms, giving support to theoretical predictions.

Searching for superconducting hydrides has so far largely focused on finding materials exhibiting the highest possible critical temperatures (Tc). This has led to a bias towards materials stabilised at very high pressures, which introduces a number of technical difficulties in experiment. Here we apply machine learning methods in an effort to identify superconducting hydrides which can operate closer to ambient conditions. The output of these models informs structure searches, from which we identify and screen stable candidates before performing electron-phonon calculations to obtain Tc. Hydrides of alkali and alkaline earth metals are identified as particularly promising; a Tc of up to 115 K is calculated for RbH12 at 50 GPa and a Tc of up to 90 K is calculated for CsH7 at 100 GPa.

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