Abstract
The impact of structural symmetry on electrical charge transport, magnetism and electrocatalytic activity has been investigated in isoelectronic materials Ca2LaMn2O7 and Sr2LaMn2O7. These oxides form the so-called Ruddlesden-Popper structure, comprising bilayer stacks of MnO6 octahedra, where Ca/Sr/La occupy the spaces within and between the stacks. The change in ionic radius from Ca2+ (1.18 Å) to Sr2+ (1.31 Å) results in a change in structural symmetry in these systems. Ca2LaMn2O7 has an orthorhombic structure with Cmcm space group, whereas Sr2LaMn2O7 features a tetragonal, I4/mmm, structure. The higher symmetry results in a significant variation of electrical, magnetic and electrocatalytic properties. Sr2LaMn2O7 shows significantly greater charge transport in the entire temperature range of 25–800 °C, owing to a larger angle of Mn–O–Mn conduction pathway. In addition, the electrocatalytic properties of Sr2LaMn2O7 for hydrogen-evolution reaction (HER) and oxygen-evolution reaction (OER) are enhanced as compared to Ca2LaMn2O7. This is manifested in the improved overpotential for both HER and OER, as well as enhanced reaction kinetics. The improved OER/HER activity is also a function of structural features that produce greater electrical conductivity, which in turn affects the electrocatalytic properties.
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