Impact of ionic quantum fluctuations on the thermodynamic stability and superconductivity of LaBH$_8$

• URL: http://arxiv.org/abs/2206.07439v1
• Date: Wed, 15 Jun 2022 10:25:20 GMT
• Title: Impact of ionic quantum fluctuations on the thermodynamic stability and superconductivity of LaBH$_8$
• Authors: Francesco Belli (1,2) and Ion Errea (1,2,3) ((1) Fisika Aplikatua Saila, Gipuzkoako Ingeniaritza Eskola, University of the Basque Country (UPV/EHU), (2) Centro de F\'isica de Materiales (CSIC-UPV/EHU), (3) Donostia International Physics Center (DIPC))
• Abstract summary: A metastable high-symmetry Fm$bar3$m phase of LaBH$_8$ gives hopes to reach high superconducting critical temperatures. Our results suggest that low pressure metastable phases with covalently bonded symmetric XH$_8$ units will be destabilized by ionic quantum fluctuations.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: The recent prediction of a metastable high-symmetry Fm$\bar{3}$m phase of LaBH$_8$ gives hopes to reach high superconducting critical temperatures at affordable pressures among ternary hydrogen-rich compounds. Making use of first-principles calculations within density functional theory and the stochastic self-consistent harmonic approximation, we determine that ionic quantum fluctuations drive the system dynamically unstable below 77 GPa, a much higher pressure than the 45 GPa expected classically. Quantum anharmonic effects stretch the covalent B-H bond in the BH$_8$ units of the structure and, consequently, soften all hydrogen-character modes. Above 77 GPa Fm$\bar{3}$m LaBH$_8$ remains metastable and, interestingly, its superconducting critical temperature is largely enhanced by quantum anharmonic effects, reaching critical temperatures around 170 K at the verge of the dynamical instability. Our results suggest that low pressure metastable phases with covalently bonded symmetric XH$_8$ units will be destabilized by ionic quantum fluctuations.

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