We study the structural phase transition, originally associated with the highest superconducting critical temperature T_c measured in high-pressure sulfur hydride. A quantitative description of its pressure dependence has been elusive for any ab initio theory attempted so far, raising questions on the actual mechanism leading to the maximum of T_c. Here, we estimate the critical pressure of the hydrogen bond symmetrization in the Im\(\bar{3}\)m structure, by combining density functional theory and quantum Monte Carlo simulations for electrons with path integral molecular dynamics for quantum nuclei. We find that the T_c maximum corresponds to pressures where local dipole moments dynamically form on the hydrogen sites, as precursors of the ferroelectric Im\(\bar{3}\)m-R3m transition, happening at lower pressures. For comparison, we also apply the self-consistent harmonic approximation, whose ferroelectric critical pressure lies in between the ferroelectric transition estimated by path integral molecular dynamics and the local dipole formation. Nuclear quantum effects play a major role in a significant reduction (≈50 GPa) of the classical ferroelectric transition pressure at 200 K and in a large isotope shift (≈25 GPa) upon hydrogen-to-deuterium substitution of the local dipole formation pressure, in agreement with the corresponding change in the T_c maximum location.

我们研究结构相变，最初与高压硫氢化物中测量的最高超导临界温度T _c有关。对于迄今为止尝试的任何从头理论来说，其压力依赖性的定量描述都是难以捉摸的，这对导致T _c最大值的实际机制提出了疑问。在这里，我们通过结合密度泛函理论和电子的量子蒙特卡罗模拟以及量子核的路径积分分子动力学来估计 Im \(\bar{3}\) m 结构中氢键对称的临界压力。我们发现T _c最大值对应于氢位点上动态形成局部偶极矩的压力，作为铁电 Im \(\bar{3}\) m-R3m 转变的前体，发生在较低压力下。为了进行比较，我们还应用了自洽谐波近似，其铁电临界压力位于路径积分分子动力学估计的铁电转变和局部偶极子形成之间。核量子效应在 200 K 时经典铁电转变压力的显着降低（约 50 GPa）以及氢到氘取代局部偶极子形成压力时的大同位素转变（约 25 GPa）中发挥着重要作用，与T _c最大位置的相应变化一致。