Coexisting optical and polar domains have been found in single-crystal Ni₃TeO₆. Furthermore, these domains are intertwined—the optical orientation, directly related to structural chirality, determines the polar orientation and vice versa. A group at Rutgers, The State University of New Jersey led by Sang-Wook Cheong reports these findings in APL Materials (DOI: 10.1063/1.4927232). This is the first known report of a crystal that has been found to show intertwined optical and polar domains.

“After getting high-quality single crystals of Ni₃TeO₆ (NTO), we happened to observe domain inversion under a transmission polarized optical microscope,” Cheong tells MRS Bulletin. “Since the crystal group Ni₃TeO₆ is non-centrosymmetric, it’s natural to test the piezoresponse by PFM [piezo force microscope] scanning.” Such crystals have no center of symmetry, and show charge polarization, which is necessary for the piezoelectric effect—the propensity of certain materials to develop a surface charge when squeezed. PFM is a method of directly observing this behavior using an atomic force microscope. Optical activity is the rotation of plane-polarized light as it passes through a material.

Most crystals are found to be divided into smaller regions based on their polar/optical responses. The macroscopic behavior of a crystal is then the average behavior of all these domains. Some optical domains may rotate light counterclockwise while other domains rotate it clockwise. Similarly, polar domains have dipoles orientated in different directions. In NTO, if adjacent domains rotate light right-left, the corresponding polarization is found to be either up-down or down-up. Thus, an optical domain in NTO is also a polar domain and vice versa since their domain walls coincide. The polar domains in a hexagonal single crystal of NTO resemble the trefoil symbol for radiation danger.

The basic building block of Ni₃TeO₆. Credit: APL Materials.

The group explains this correlation through the crystal structure of the material. NTO has a corundum (Al₂O₃) crystal structure with nickel and tellurium occupying the sites traditionally reserved for aluminum. Each of the metal ions is surrounded by six oxygen atoms forming edge, vertex, or face-sharing octahedra. However, the three nickel atoms are crystallographically inequivalent with different Ni–O bond lengths. Combined with the trigonal symmetry of the crystal, this leads to an arrangement of 120° rotated triangles that are stacked on top of each other to form a helix.

In the figure, the blue triangles correspond to adjacent nickel octahedra while the red triangles correspond to nickel-tellurium. Importantly, the researchers were able to show that the chiralities of the red and blue helices do not cancel each other within a unit cell leading to a net left- or right-handedness. Also, the resulting spatial shift of the ions leads to a net polarization. Any optical domain wall that was not also a polar domain wall was found to disturb the ideal stacking for tellurium octahedra.

Most inorganic materials that have large optical activity have so far only shown chirality without polarity. NTO is the first known material that shows both. “Our findings unveil the rich coupling nature of chiral and polar order parameters and provide new insights into understanding and engineering domains in functional chiral and polar materials,” the group states in their article. This understanding of the fundamental properties of polar domain boundaries is key to the development of new economically important materials according to Chris Stock of The University of Edinburgh, who adds, “This has been evidenced by fundamental developments in disordered ferroelectrics (such as the lead-based relaxors) resulting in recent applications of memory storage such as FRAM [ferroelectric random-access memory]. The observation of these highly structured domains is really a breakthrough in materials physics and will lead to new studies on similar materials and eventually new applications.”