First-principle investigation of hybrid improper ferroelectricity of <i>n</i> = 2 Ruddlesden-Popper Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf)

Abstract

Recently, perovskite ferroelectric photovoltaic materials have been studied extensively. Traditional photovoltaic device usually uses the internal electric field formed by PN junction to realize the separation of photogenerated carriers to form the photovoltaic effect, while ferroelectric material, due to the existence of spontaneous polarization, can spontaneously realize the separation of photogenerated electrons and holes without the formation of PN junction, presenting the ferroelectric photovoltaic effect. Chalcogenide perovskite with suitable band gap and visible light absorption is expected to be a new generation of ferroelectric photovoltaic materials. However, its application is limited due to the lack of ferroelectric properties. Hybrid improper ferroelectricity (HIF) in layered perovskites has opened a new way for developing the new ferroelectrics. In contrast to the proper ferroelectricity in which the polarization is the main order parameter as the driving force, the improper ferroelectricity possesses the ferroelectric polarization that becomes a secondary order parameter induced by other orders. In this work, we study the ground state, electronic structure and hybrid improper ferroelectricity of <i>n</i> = 2 Ruddlesden-Popper (RP) Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf ) based on the first principles. The total energy calculations and phonon spectrum analysis show that the ground state of Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (B = Zr, Hf ) is of <i>A</i>2<sub>1</sub><i>am</i> polar phase. The hybrid improper ferroelectricity originates from the coupling between two rotation modes of <i>B</i>Se<sub>6</sub> octahedron. Electronic structure calculations show that Sr<sub>3</sub>Zr<sub>2</sub>Se<sub>7</sub> and Sr<sub>3</sub>Hf<sub>2</sub>Se<sub>7</sub> are semiconductors with direct band-gaps, which are around 1.56 eV and 1.84 eV, respectively. The ferroelectric polarization values calculated by the Berry phase method are around 12.75 μC/cm<sup>2</sup> and 9.69 μC/cm<sup>2</sup>, respectively. The contribution of each atomic layer to the ferroelectric polarization is investigated when the Born effective charge method is used. The results show that the polarization of Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf ) mainly comes from the Sr-Se atomic layers. To sum up, Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf ) show strong ferroelectric polarization and good visible light absorption characteristics and they are expected to be candidates of a new generation of ferroelectric photovoltaic materials.