Quantum and temperature effects on the crystal structure of superhydride ${\mathrm{LaH}}_{10}$: A path integral molecular dynamics study

By classical and path integral molecular dynamics simulations, we study the pressure-temperature ($P\text{−}T$) phase diagram of ${\mathrm{LaH}}_{10}$ to clarify the impact of temperature and atomic zero-point motions. We calculate the XRD pattern and analyze the space group of the crystal structures. For 125 GPa $\le P\le$ 150 GPa and $T=300\mathrm{K}$, we show that a highly symmetric $Fm\overline{3}m$ structure, for which superconductivity is particularly favored, is stabilized only by the temperature effect. On the other hand, for $T=200\mathrm{K}$, the interplay between the temperature and quantum effects is crucial to realize the $Fm\overline{3}m$ structure. For $P=100\mathrm{GPa}$ and $T=300\mathrm{K}$, we find that the system is close to the critical point of the structural phase transition between the $Fm\overline{3}m$ structure and those with lower symmetries.

Figure 1

$P_b$ [the number of the Suzuki-Trotter decomposition in Eq. (3)] dependence of the Gibbs free energy at $(P,T)=(150\text{GPa},300\text{K})$ and (150 GPa, 200 K). The last 250 fs of the simulation are divided into ten bins for 200 K. Then, the average and standard deviation of the sum of the energy and the $PV$ term for each bin are calculated. For 300 K, we took 750 fs for the MD calculation to reduce the error. Regarding the entropy term, we evaluated it from the velocity of each atom [60]. We used a $4\times 4\times 4\mathit{k}$ mesh in the PIMD simulation here.

Figure 2

Time evolution of the volume of the ${\mathrm{LaH}}_{10}$ crystal for $(P,T)=(150\text{GPa},300\text{K})$ in the classical MD simulation.

Figure 3

Crystal structure of ${\mathrm{LaH}}_{10}$ obtained from the classical MD simulation for $(P,T)=(150\text{GPa},200\text{K})$ (a) and that for (150 GPa, 300 K) belonging to the space group of $Fm\overline{3}m$ (b).

Figure 4

XRD patterns of ${\mathrm{LaH}}_{10}$ crystals obtained by classical MD around $2\theta =6.5^{\circ }$ with the wavelength of $\lambda =0.3344\AA$.

Figure 5

XRD patterns of ${\mathrm{LaH}}_{10}$ crystals obtained by PIMD around $2\theta =6.5^{\circ }$ with the wavelength of $\lambda =0.3344\AA$.

Figure 6

Phase diagram of the crystal structure of ${\mathrm{LaH}}_{10}$ crystal structure determined by the classical MD (a) and PIMD (b).