Chisholm, Calum R. I. and Jang, Yun Hee and Haile, Sossina M. and Goddard, William A., III (2005) Superprotonic phase transition of CsHSO4: A molecular dynamics simulation study. Physical Review B, 72 (13). Art. no. 134103. ISSN 1098-0121 http://resolver.caltech.edu/CaltechAUTHORS:CHIprb05
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The superprotonic phase transition (phase II --> phase I; 414 K) of cesium hydrogen sulfate, CsHSO4, was simulated using molecular dynamics with the "first principles" MSXX force field (FF). The structure, binding energy, and vibrational frequencies of the CsHSO4 monomer, the binding energy of the (H2SO4)2 dimer, and the torsion barrier of the HSO<sub>4</sub><sup>-</sup> 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 donor), but allowed to form hydrogen bonds to various second nearest oxygen atoms (proton acceptors). Fixed temperature-pressure (NPT) 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 CsHSO4 was probed. Although the FF does not allow proton migration (i.e., proton jumps) between oxygen atoms, a clear phase transition occurred as demonstrated by a discrete change of unit cell symmetry (monoclinic to tetragonal), cell volume, and molar enthalpy. The dynamics of the HSO<sub>4</sub><sup>-</sup> 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.
|Additional Information:||©2005 The American Physical Society (Received 14 July 2004; revised 21 January 2005; published 7 October 2005) We thank Lang Yang for her help with the probability distribution functions in Figs. 10 and 11. Financial support was provided by the National Science Foundation and acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund, for partial support of this research.|
|Subject Keywords:||caesium compounds; hydrogen compounds; molecular dynamics method; ab initio calculations; binding energy; quantum theory; hydrogen bonds; enthalpy; solid-state phase transformations|
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|Deposited On:||11 Jan 2006|
|Last Modified:||26 Dec 2012 08:44|
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