Negative capacitance in boracites: a structure-properties study

Boracites are a family of crystals of chemical formula M3B7O13X, with a structure composed of a framework of boron-oxygen tetrahedra, within which M and X ions are intercalated. Despite the first observation of the coupling between magnetism and ferroelectric properties (magnetoelectricity) being made in a boracite in the 60s, the oxyhalide system has gathered limited attention, largely due to the strong and versatile properties observed in perovskites. Recently, boracites have regained interest due to the existence of charged domain walls. Domain walls are few unit-cells-thick interfaces separating domains of homogenous ferroic orders such as polarisation, magnetisation, or strain. They can be injected, annihilated, and repositioned by external bias such as electric field of stress making them interesting for novel computing applications such as memristors.

Some of the charged walls in Cu-Cl boracite are particularly perplexing as they move the “wrong way” under the application of an electric field; this generates a negative capacitance contribution to the dielectric response [3]. No such anomalous domain wall motion has ever been reported in any other ferroelectric system. Indeed, no physical mechanism has yet been proposed to explain such an energetically counterintuitive phenomenon and so further investigations are needed.

To improve the understanding of the origin of this negative capacitance, this project aims to grow new single crystals of boracite with different compositions, followed by the thorough characterisation of the structure and the dielectric and elastic properties of the material, especially at the phase transition. A better understanding of the role of the metal in the structure and on the phase transition will allow to propose a new model for negative capacitance in this system and will informed the future design of materials and devices.