![]() Larger-scale structural inhomogeneities are known to strongly influence physical phenomena and applications. These defects produce states at various levels within the band gap, which are likely responsible for transport within the insulator. The resistive switching observed in a variety of metal oxides including NiO 20, 21, 22, 23 appears to be due to intrinsic point defects. Nickel oxide’s antiferromagentic order is predicted to persist even in the presence of the cation vacancy and resulting half-metallicity 17. However, the interaction of vacancies resulting in half-metallicity in the same spin channel is predicted to be energetically favourable 19. In the latter, single defects can produce half-metallicity in either spin channel locally. Focusing on monoxides, cation vacancies can induce fascinating properties such as ferromagnetism in the case of CoO 16 and half-metallicity in the case of MnO 17, CoO 18 and NiO 17. The macroscopic properties of most metal oxides are extremely sensitive to the presence of point defects. Consequently, even high-quality single crystals, as judged by diffraction techniques, can contain a relatively high concentration of point defects. Point defects are considered to be scattered homogeneously throughout the crystal and appear in diffraction spectra as a background signal with the reflex structure unaffected. However, comprehensive investigation of crystal structure requires the combination of methods providing statistically averaged information and high spatial resolution electron and scanning probe microscopy techniques. The standard statistical approach of X-ray diffractometry allows one to obtain information averaged over ensembles in the kinematical 12, 13, semi-dynamical 14 or dynamical approaches 15. Lab based X-ray sources can probe the coherent domain size and overall crystalline quality 10, 11 but cannot easily distinguish between or study particular two-dimensional defects. Crystal lattice defects in FZM single crystals such as domain boundaries, dislocation walls, twins and stacking faults affect the structure of X-ray diffraction (XRD) reflexes 9. Despite the high quality of single crystals grown by FZM, different types of structural and chemical inhomogeneities can exist. NiO sees application in spin valves 2, supercapacitors 3, hole transport layer in perovskite solar cells 4, water splitting (Fe doped NiO) 5 and gas sensing 6.įloating zone melting (FZM) is one of the most suitable techniques for the growth of high quality metal oxide single crystals (for detailed review see 7, 8). ![]() However, nickel vacancies are common and give rise to an effective p-type doping 1. NiO is antiferromagnetic and a charge transfer insulator. Metal oxides display a wide variety of physical properties, stimulating interest from the point of view of fundamental physics and device engineering. The different inhomogeneities are understood in terms of the structural relaxation induced by ordering of nickel vacancies, which is predicted to be favourable. Density functional theory calculations-considering the spatial and electronic disturbance induced by the favourable nickel vacancy-reveal a nanoscale distortion comparable to STM and TEM observations. X-ray photoelectron spectroscopy and electron energy-loss spectroscopy indicate the crystal is Ni deficient. A comprehensive transmission electron microscopy (TEM) study reveals inhomogeneities ranging from domains of varying size, misorientation of domains, variation of the lattice constant and bending of lattice planes. In turn, these domains are visualised by atomic force and scanning tunneling microscopy (STM), respectively. X-ray and low-energy electron diffraction reveal domains on the submicron- and nanometer-scales, respectively. ![]() A range of inhomogeneities are observed by diffraction and microscopy techniques. Investigating the correlation between point defects and domain structure can provide a deeper understanding of their formation and structure, which potentially allows one to tailor domain structure and the dynamics of the aforementioned applications. The properties and structure of domains dictate the dynamics of resistive switching, water splitting and gas sensing, to name but a few. In this work we present a comprehensive study of the domain structure of a nickel oxide single crystal grown by floating zone melting and suggest a correlation between point defects and the observed domain structure.
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