Thermal and Nuclear Quantum Effects at the Antiferroelectric to Paraelectric Phase Transition in KOH and KOD Crystals

Abstract

Crystalline KOH undergoes an antiferroelectric (AFE) proton ordering phase transition at low temperatures, which results in a monoclinic bilayer structure held together by a network of weak hydrogen bonds (HBs). The Curie temperature shifts up when the compound is deuterated, an effect that classical MD is not able to catch. For deeper insights into the transition mechanism, we carry out ab initio MD simulations of KOH and KOD crystals by including quantum effects on the nuclei through Feynman path integrals. The geometric isotope effect and the evolution of the lattice parameters with temperature agree with the experimental data, while the purely classical description is not appropriate. Our results show that deuteration strengthens the HBs in the low- T AFE ordered phase. The transition is characterized by the flipping of OH/OD groups along a bending mode. Above the transition, the system is driven into a dynamical disordered paraelectric phase.

Routine Molecular Dynamics Simulations Including Nuclear Quantum Effects: From Force Fields to Machine Learning Potentials

“…The efficiency and massive parallelization capabilities of the newly introduced platform now allows the inclusion of explicit nuclear quantum effects in very large systems. This should be particularly relevant for simulations in extreme conditions of pressure or temperature where NQEs can be massive ,,, or to investigate disorder effects in solids for which NQEs can be determinant and large supercells required. − Importantly, being able to simulate quantum nuclei enables the study of isotope effects that are simply not reachable using classical MD. , Finally, it opens up the possibility of investigating quantitatively the importance of NQEs in biological processes , and the subtleties of hydrogen-bonded systems . While the methods for quantum MD are now readily available in Tinker-HP, it will be necessary to reparametrize some of the force fields to avoid double counting of implicit and explicit NQEs, as was shown in the case of Q-AMOEBA for water .…”

Simulation of Nuclear Quantum Effects in Condensed Matter Systems via Quantum Baths

“…Nuclear quantum effects, however, also often come into play at ambient conditions, typically when the thermal energy of chemical bonds in molecules is comparable or smaller than the associated vibrational ZPE or when characteristic interatomic distances in the system are comparable to the de Broglie wavelength of the nuclei. These conditions are met, in particular, in systems containing light atoms and most notably hydrogen [4,5], which is a ubiquitous constituent of inorganic compounds and a basic element in biological systems. Thus, nuclear quantum effects influence, for example, the stability of crystal polymorphs of pharmaceutical interest, the spectroscopy of ice and water, or enzymatic reactions in living organisms [6][7][8][9].…”

The quantum taste of hydrogen

“…Many other such examples exist: for instance KOH and KOD [9] exhibit a 30K temperature di↵erence for a structural phase transition at ambient pressure; NaOH is paraelectric without change as temperature is decreased, while NaOD undergoes a phase transition to an antiferroelectric structure at 150K [10][11][12][13]. SrTiO 3 also shows isotopic e↵ects [14] upon O 16 -O 18 substitution.…”