  The high critical temperature phase of lanthanum hydrides deserves special
attention because this phase displays the highest observed critical
temperature. We focus on the evaluation of the critical temperature, isotope
coefficient, and their pressure dependence of this phase (Fm3m phase of the
LaH10 compound). By combining the method of two coupling constants, describing
the interaction with the optical and acoustic modes, and two characteristic
frequencies with first-principle calculations we are able to analyze the role
of the light hydrogen in the amplitude of the critical temperature in this
unique compound. Specifically, the critical temperature of 254 K and the
isotope coefficient of 0.45 are evaluated at high pressure, and the results are
in good agreement with the experimental data. The peculiar decrease in the
value of the critical temperature upon further increase in pressure is
explained by the presence of a flat region on the Fermi surface and the
appearance of a two-gap structure, which should be experimentally observable by
tunneling spectroscopy.
