After the discovery of the superconductivity in the sulphur-hydrogen system at temperatures ~200 K [1], there was a renewal of an interest towards the fundamental properties of hydrides.

Until present, the highest hydrogen content among all hydrides was observed in the La-H system for LaH₁₀ superhydride [2]. Fcc-LaH₁₀ was proven to have exceptional superconductive properties, with claimed T_c value to be above 260 K [3], or about 250 K in later, more accurate measurements [4]. These observations confirmed earlier theoretical predictions of high-temperature superconductivity in fcc-LaH₁₀ [5,6].

In present work we report on the XRD study of the La-H and La-D systems in pressure range 127-163 GPa, where we observed a number of LaH(D)_x phases, with most of them being new. These include, in the order of decreasing volumes:

a) Distorted primitive simple cubic sc-LaH_~12 with a=3.30Å at 159 GPa.

b) fcc-LaD_~11 with a=5.26Å at 127GPa.

c) P4/nmm-LaD₁₁, which has a tetragonally distorted fcc lattice with a=3.73Å c=5.10Å at 142GPa, predicted in [5].

d) fcc-LaH(D)₁₀ with a=5.10Å at 150GPa, discovered in [2].

e) orthorhombically distorted fcc-LaD₁₀ with a=3.74Å b=5.18Å c=3.60Å (tentative space group Pmmn, atomic coordinates La 2a (0 0 0.18)) at 131GPa.

f) hcp-LaH(D)₁₀ with a=3.62Å c=5.91Å at 150GPa.

g) hcp-II-LaH(D)_~10 with a=3.75Å c=5.58Å and unusually small c/a≈1.49 ratio at 131GPa.

h) I4/mmm-LaD₄ with a=2.77Å c=5.96Å at ~153GPa, predicted for LaH₄ in [6].

i) Cmcm-LaD₃ with a=2.8Å b=6.6Å c=4.5Å at ~153GPa, predicted for LaH₃ in [5].

j) fcc-LaH₃ with a=4.36Å at ~152GPa.

k) numerous unidentified phases, most of them irreproducible.

Among presently observed superhydrides in the La-H(D) system, only Cmcm-LaD₃, I4/mmm-LaD₄ and P4/nmm-LaD₁₁ were predicted by theory accurately enough to consider their compositions fixed. The rest of the phases are either unpredicted or their crystal structures (fcc or hcp) occur simultaneously in different hydrides, preventing reliable composition determination via comparison of their V(P) dependencies with ab-initio calculations. An alternative approach, employed here, is to construct the V(H(D)/La) plot from the volumes of individual phases. Such volumes should follow a linear dependence on H(D)/La, if the Vegard law applies to the La-H(D) system. To compensate for different pressures, at which the new superhydrides were observed, we adjusted all volumes to 150 GPa (corresponding to the vibron positions in the gas phase  ν(H₂)=4045cm^-1 and ν(D₂)=2970 cm^-1), using the same bulk modulus as for La, which equation of state was measured in [2]. The resulting volumes are plotted in Figure 1 as a function of H(D)/La composition. All volumes follow nearly the same linear dependence with the hydrogen-induced volume expansion of 1.8 ų/H atom, justifying the estimations of their compositions, listed above.

Figure 1. Lattice volumes of various lanthanum superhydrides as a function of their composition at 150 GPa. The symbols are moved apart horizontally from their stoichiometric values to avoid cluttering.

1. A. P. Drozdov et al, Nature 2015, 525, 73.
2. Z.M. Geballe et al, Angew. Chem. Int. Ed. 2018, 57, 688–692
3. M. Somayazulu et al, Phys. Rev. Lett. 2019, 122, 027001
4. A. P. Drozdov et al, arXiv:1812.01561.
5. F. Peng et al, Phys. Rev. Lett. 2017, 119, 107001
6. H. Liu et al, PNAS 2017, 114(27), 6990-6995