Superprotonic phase transition ofCsHSO4: A molecular dynamics simulation study

Abstract

The superprotonic phase transition ͑phase II → phase I; 414 K͒ of cesium hydrogen sulfate, CsHSO 4 , was simulated using molecular dynamics with the "first principles" MSXX force field ͑FF͒. The structure, binding energy, and vibrational frequencies of the CsHSO 4 monomer, the binding energy of the ͑H 2 SO 4 ͒ 2 dimer, and the torsion barrier of the HSO 4 − ion were determined from quantum mechanical calculations, and the parameters of the Dreiding FF for Cs, S, O, and H adjusted to reproduce these quantities. Each hydrogen atom was treated as bonded exclusively to a single oxygen atom ͑proton donor͒, but allowed to form hydrogen bonds to various second nearest oxygen atoms ͑proton acceptors͒. Fixed temperature-pressure ͑NPT͒ dynamics were employed to study the structure as a function of temperature from 298 to 723 K. In addition, the influence of several force field parameters, including the hydrogen torsional barrier height, hydrogen bond strength, and oxygen charge distribution, on the structural behavior of CsHSO 4 was probed. Although the FF does not allow proton migration ͑i.e., proton jumps͒ between oxygen atoms, a clear phase transition occurred as demonstrated by a discrete change of unit cell symmetry ͑monoclinic to tetragonal͒, cell volume, and molar enthalpy. The dynamics of the HSO 4 − group reorientational motion also changed dramatically at the transition. The observation of a transition to the expected tetragonal phase using a FF in which protons cannot migrate indicates that proton diffusion does not drive the transition to the superprotonic phase. Rather, high conductivity is a consequence of the rapid reorientations that occur in the high temperature phase. Furthermore, because no input from the superprotonic phase was employed in these simulations, it may be possible to employ MD to hypothesize superprotonic materials.

Particle morphology, electrical conductivity, crystal and electronic structures of hydrothermally synthesized (Ce,Sr)PO4

“…4), it was also found that all the oxygen had large anisotropic displacements even at room temperature in contrast with the cations, and that their directions were perpendicular to the P-O bonds, indicating rotations of the PO 3À 4 tetrahedra. Such a significant rotation of an XO nÀ 4 moiety (X = P and S) was often observed in the case of oxoacid-based proton conductors [17][18][19][20][21][22]. Table 3 lists volumes, quadratic elongations, k, [23] and bond angle variances, r 2 , [23] of the PO 3À 4 tetrahedra and bond valence sums (B. V. S.) [24] at the P sites, calculated from the structure parameters.…”

Lithium hydroxide dihydrate: A new type of icy material at elevated pressure

“…The time scale of ∼1 ps has been observed previously in superprotonic conductors as a typical time scale associated with the hydrogen hopping. 22,[27][28][29] The time averaged structure of LiOH · 2H 2 O at 500 K is consistent with the (initial) static structure (Table II): all atoms including the hydrogens are within ∼0.04 Å. The structure of LiOH · 2H 2 O appears to be stable at 500 K: ∼50% of the static internal coordinates are within one standard deviation of each other and the free enthalpies of the ab initio and the MD-calculated phase agree to within ∼0.6 meV/atom.…”

A review on synthesis and characterization of solid acid materials for fuel cell applications

“…This mechanism differ from CHS, where the rate of proton hopping is lower than the rate of sulphate reorientation [42,43]. In theory, proton transport in oxo-acids such as phosphoric acid exhibit weak proton transport due to strong hydrogen bonds within the material.…”