The Manganite−Water Interface

Abstract

The properties of bulk manganite, its freshly cleaved (010) surface, and this surface exposed to water monolayers at both low and high coverage and to liquid water at 300 K, are examined using density-functional theory (DFT) by the PW91 density functional as well as using the new PM6 semiempirical electronic-structure method. The bonds between the (010) layers are calculated to be very weak, of average energy -2.7 kcal mol-1, explaining the ease at which manganite surfaces cleave. Upon cleavage, a surface reconstruction is predicted that produces ferroelectrically ordered surface layers, and the surface manganese atoms are predicted to display different chemical properties depending on the nature of the oxygen atom to which they bind in the subsurface layer. Water is predicted to be only physisorbed to the surface, with this process acting to lift the surface reconstruction. At high water coverage, the differences between the two types of surface manganese atoms are also lost. Simulations at 300 K indicate that less than half of the surface manganese atoms coordinate to the fluid at 300 K while only two-thirds of the manganite oxygen atoms on the outside of the surface coordinate. No dominate liquid structure is found, suggesting that dielectric continuum models may be useful in understanding surface chemistry, but it is clear that water-water rather than water-surface interactions dominate the nature of the interface.