Proton transport in barium stannate: classical, semi-classical and quantum regimes

Abstract

Density-functional theory calculations are performed to investigate proton transport in BaSnO3. Structural optimizations in the stable and saddle point configurations for transfer (hopping) and reorientation allow description of the high-temperature classical and semi-classical regimes, in which diffusion occurs by over-barrier motion. At lower temperature (typically below 300 K), we describe the thermally-assisted quantum regime, in which protonic motion is of quantum nature and occurs in "coincidence" configurations favored by thermal fluctuations of the surrounding atoms. Both the non-adiabatic and the adiabatic limits are examined. In the adiabatic limit, the protonic energy landscape in the coincidence configuration is very flat. Path-integral molecular dynamics simulations of the proton in the coincidence potential reveal, in the transfer case, that the density of probability of H(+) has its maximum at the saddle point, because the zero-point energy exceeds the coincidence barrier. Arguments are given that support the adiabatic picture for the transfer mechanism. In the case of reorientation, the time scales for the existence of the coincidence and for protonic motion, as estimated from the time-energy uncertainty principle by using a simple one-dimensional model, are of the same order of magnitude, suggesting that the adiabatic limit is not reached. Protonic transfer and reorientation in this oxide are therefore governed by different mechanisms below room temperature.

On the relationship between chemical expansion and hydration thermodynamics of proton conducting perovskites

“…In the denser SrTiO 3 , on the other hand, protons have been argued to take on a position along the edge of the TiO 6 polyhedra, resulting in 8 equivalent positions around each oxide ion. 31,32 Our calculations are in general agreement with those of refs [26][27][28][29][30] showing that all included compositions exhibit proton positions at, or close to, the bisector of two oxygen connecting lines (see Fig. 1).…”

“…26 In BaZrO 3 , BaSnO 3 , BaCeO 3 and SrZrO 3 , the proton takes on a position along the bisector of two O connecting lines and there are thus 4 proton positions per oxide ion. [26][27][28][29][30] While these positions are degenerate in BaZrO 3 and BaSnO 3 , they are nondegenerate in the orthorhombic BaCeO 3 and SrZrO 3 . In the denser SrTiO 3 , on the other hand, protons have been argued to take on a position along the edge of the TiO 6 polyhedra, resulting in 8 equivalent positions around each oxide ion.…”

Recent developments in the ABINIT software package

“…PIMD has been used with ABINIT to reproduce the large isotope effect on the phase transition between phase I and phase II of dense hydrogen [15], and also some aspects of diffusion at low and room temperature in proton-conducting oxides for fuel cells [16]. PIMD in the NPT ensemble is not available yet.…”

Quantum Nuclear Dynamics of Protons within Layered Hydroxides at High Pressure

“…The results of recent experiments and calculations have informed our efforts, which are herein focused on calcium hydroxides because the proton disorder should be observable at lower experimental pressures in these hydroxides 1, 6 . Although previous calculations tended to treat the nuclei in a classical way, an increasing number of studies point to the importance of nuclear quantum effects (NQEs) in protonated systems 13 , either for pure water 14–16 or for extrinsic defects for low concentrations of protons immersed in perovskites 17, 18 . NQEs ought to be our next consideration for ordered compounds rich in hydrogen, such as mineral hydroxides, in order to clarify the dynamic properties of protons in their layered and disordered structure.

Figure 1Scheme of the proton disorder in layered calcium hydroxides at low and high pressures.

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