Abstract
A detailed study on the single-molecule ferroelectric property of Preyssler-type polyoxometalates (POMs), [M3+P5W30O110]12− (M = La, Gd, and Lu), is performed by density functional theory calculations. Linked to one H2O molecule, the cation (M3+) encapsulated in the cavity of the Preyssler framework is off-centered, and it generates a permanent dipole, which is essential for a ferroelectric ground state. Accompanied with a 180° rotation of H2O, the switching of M3+ between two isoenergetic sites on both sides of the cavity results in a calculated barrier of 1.15 eV for Gd3+, leading to the inversion of electric polarization. The height of the barrier is in good agreement with the experimentally measured barrier for the Tb3+ ion, whose ionic radius is similar to Gd3+. The total polarization value of the crystal is estimated to be 4.7 µC/cm2 as calculated by the modern theory of polarization, which is quite close to the experimental value. Considering that the order of contributions to the polarization is M3+–H2O > counter-cations (K+) > [P5W30O110]15−, the interconversion of M3+–H2O between the two isoenergetic sites is predicted to be the main origin of ferroelectricity with a polarization contribution of 3.4 µC/cm2; the K+ counter-cations contribute by 1.2 µC/cm2 and it cannot be disregarded, while the framework appears to contribute negligibly to the total polarization. Our study suggests that a suitable choice of M3+–H2O could be used to tune the single-molecule ferroelectricity in Preyssler-type polyoxometalates.
Highlights
INTRODUCTIONFerroelectricity (FE) is a characteristic of some electro-active materials that possess a robust spontaneous polarization, which can be switched under an external electric field. Due to the nonlinear nature, hysteretic behavior, and catalytic properties, ferroelectric materials (FEs) have been widely employed as capacitors, sensors, tunnel junctions, catalysts, and nonvolatile memory. Up to now, many important FEs belong to the class of perovskites, such as barium titanate (BaTiO3) and lead zirconate titanate. FE has been described in many other materials, and the development of computing techniques and algorithms drives tremendous efforts to look for higher-performance ferroelectric materials
Considering that the order of contributions to the polarization is M3+–H2O > counter-cations (K+) > [P5W30O110]15−, the interconversion of M3+–H2O between the two isoenergetic sites is predicted to be the main origin of ferroelectricity with a polarization contribution of 3.4 μC/cm2; the K+ counter-cations contribute by 1.2 μC/cm2 and it cannot be disregarded, while the framework appears to contribute negligibly to the total polarization
We have only studied the displacement of the M3+–H2O unit from the center of the cavity to stable intermediate connected to the initial position (Site-A) or A′, but as can be seen in the broken solid curve of Fig. 4, the two potential energy surfaces cannot be connected as the orientation of the water molecule is opposite for the two cases
Summary
Ferroelectricity (FE) is a characteristic of some electro-active materials that possess a robust spontaneous polarization, which can be switched under an external electric field. Due to the nonlinear nature, hysteretic behavior, and catalytic properties, ferroelectric materials (FEs) have been widely employed as capacitors, sensors, tunnel junctions, catalysts, and nonvolatile memory. Up to now, many important FEs belong to the class of perovskites, such as barium titanate (BaTiO3) and lead zirconate titanate. FE has been described in many other materials, and the development of computing techniques and algorithms drives tremendous efforts to look for higher-performance ferroelectric materials.. While experimental studies have been addressed for Preyssler systems, a detailed analysis based on density functional theory (DFT) for the microscopic mechanism at the origin of this “single-molecule” ferroelectricity is still lacking due to the complexity of the system. This motivated us to close this gap. We report DFT calculations carried out to explore the effect of a water molecule inside the cavity and the origin and main contributions to the ferroelectric property experimentally observed in the [M3+@P5W30]12− system with M = La, Gd, and Lu. Gd3+ has a similar ionic radius as Tb3+ but is computationally much simpler because of the half-filled 4f shell. It is certainly non-negligible while the contribution of the [P5W30]15− framework is much smaller and can safely be ignored
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