A review on lithium niobate single crystals and polycrystals has been prepared. Both the classical and recent literature on this topic is revisited. It is composed of two parts with several sections. The current part discusses the available defect models (intrinsic), the trends found in ion-doped crystals and polycrystals (extrinsic defects), the fundamentals on dilute magnetic oxides, and their connection to ferromagnetic behavior in lithium niobate.

A review of lithium niobate single crystals and polycrystals in the form of powders has been prepared. Both the classical and recent literature on this topic are revisited. It is composed of two parts with sections. The current part discusses the earliest developments in this field. It treats in detail the basic concepts, the crystal structure, some of the established indirect methods to determine the chemical composition, and the main mechanisms that lead to the manifestation of ferroelectricity. Emphasis has been put on the powdered version of this material: methods of synthesis, the accurate determination of its chemical composition, and its role in new and potential applications are discussed. Historical remarks can be found scattered throughout this contribution. Particularly, an old conception of the crystal structure thought as a derivative structure from one of higher symmetry by generalized distortion is here revived.

Existent methods for determining the composition of lithium niobate single crystals are mainly based on their variations due to changes in their electronic structure, which accounts for the fact that most of these methods rely on experimental techniques using light as the probe. Nevertheless, these methods used for single crystals fail in accurately predicting the chemical composition of lithium niobate powders due to strong scattering effects and randomness. In this work, an innovative method for determining the chemical composition of lithium niobate powders, based mainly on the probing of secondary thermodynamic phases by X-ray diffraction analysis and structure refinement, is employed. Its validation is supported by the characterization of several samples synthesized by the standard and inexpensive method of mechanosynthesis. Furthermore, new linear equations are proposed to accurately describe and determine the chemical composition of this type of powdered material. The composition can now be determined by using any of four standard characterization techniques: X-Ray Diffraction (XRD), Raman Spectroscopy (RS), UV-vis Diffuse Reflectance (DR), and Differential Thermal Analysis (DTA). In the case of the existence of a previous equivalent description for single crystals, a brief analysis of the literature is made.

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Changes in structural and magnetic properties have been systematically induced in lithium niobate (LiNbO 3) powders, Fe-doped with different concentrations and thermally treated in oxidized and reduced states. A rather strong ferromagnetic response at room temperature with a saturation magnetization of 0.96 Am 2 kg −1 was obtained for the higher utilized doping concentration, which is of the order of 1% mol. This may be considered a first report of the manifestation of ferromagnetism in nanocrystalline lithium niobate powders within the regime of very low Fe-doping concentrations. Post-thermal treatment in a controlled atmosphere is key for inducing and detecting this behavior, which can also be explained as the effective recombination of Fe impurities with oxygen vacancies in the surface of the material. Mechanochemical-calcination was employed for the synthesis of LiNbO 3 powders and after that, a diffusion process of 0.44%, 0.89%, 1.47% and 2.20% mass of Fe 2 O 3 was used in the Fe-doping. Oxidation and reduction processes were performed using a controlled atmosphere of ultra-high purity oxygen and hydrogen, respectively. X-ray diffraction and Raman spectroscopy were employed to characterize the materials. The magnetic properties were studied using Vibration Sample magnetometry and Electron Spin Resonance spectroscopy.

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