The thermodynamic stability of three near-degenerate phases of platinum dioxide
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FF7.8.1
The thermodynamic stability of three near-degenerate phases of platinum dioxide Shuping Zhuo1, 2 Karl Sohlberg 2, * 1 School of Chemical Engineering, Shandong University of Technology, Zibo 255049, P.R. China 2 Department of Chemistry, Drexel University, Philadelphia, PA 19104, U.S.A.
ABSTRACT The thermodynamic stability of the three nearly energy degenerate crystal structures of PtO2 is studied here with first-principles-based calculations of their free energies. For P = 0 the α-(CdI2) structure is the thermodynamically stable phase at low temperature, while the β-(CaCl2) structure is stable at high pressure. The β'- (rutile) structure represents an unstable fixed point on the potential energy surface, or is possibly just barely bound. These results reconcile seemingly contradictory findings and answer longstanding questions about PtO2. INTRODUCTION Platinum dioxides are widely used as versatile catalysts[1, 2] and in a number of optical and electrochemical applications[3]. Due to this broad importance, there have been a number of studies on platinum dioxides. Many platinum dioxides described in the literature, however, have not been well characterized so that for some of them only limited and contradictory information exists[4]. This decidedly incomplete understanding of the structures of the platinum dioxides may be summarized as follows: Experimental studies have revealed three crystal polymorphs of PtO2, i.e., α-PtO2 (CdI2-type structure)[4-8], β-PtO2 (CaCl2-type structure)[4, 8-12] and β’-PtO2 (rutile-type structure)[13]. Theoretical calculations have predicted the existence of the α-[14] and β-PtO2 structures[14-16]. A Raman spectroscopy study of β-PtO2 was interpreted as providing evidence of a phase transition from β- to the β’- structure at high temperature[17]. Although the previously reported experimental and theoretical studies support the existence of α-, β- and β’-type PtO2 structures, the relative stability among these is still a matter of debate. To date, theoretical studies of phase stability in the platinum dioxides have relied on comparing the electronic energies for individual structures, which is a zero-temperature and zero-pressure technique[18] and furthermore neglects the contribution of zero-point vibrational energy. For some phases the electronic energy differences are sufficiently large that the results predicted based only on electronic energies may be reasonably accepted. For near-energy-degenerate phases, however, these calculations can give misleading predictions for the relative stability. In this work, we present the first investigation of the relative stability of three near-energydegenerate crystal structures of PtO2 that is based on computed Gibbs free energies, incorporating the contribution of the vibrational energy as well as vibrational and configurational entropy. The results obtained here reconcile seemingly contradictory interpretations of earlier experimental and theoretical investigations of this material.
FF7.8.2
COMPUTATIONAL METHOD The theoretical calcula
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