Microscopically structural studies of lithium niobate powders

Abstract

On the basis of the crystallographic characteristics of lithium niobate (LN) crystals, Law of Bravais and Pauling's third rule (i.e. Polyhedral Sharing Rule) are employed with the aim to find the relationship between the crystal structure and morphological faces of LN powders. In order to validate our analytical results, we have successfully synthesized LN powders and measured the corresponding X-ray powder diffraction. Our results show that the structural analysis is consistent with the experimental data and is helpful and effective for us to control the single-crystal growth and to design superstructures at the specific plane, starting from the viewpoint of the microscopic behaviors of constituent chemical bonds and polyhedra in the crystallographic frame.

Introduction

Lithium niobate LiNbO₃ (LN) is an important functional crystal material that has been widely used in the modern science and technology due to its large pyroelectric, piezoelectric, electro-optical and photo-elastic coefficients and good optical, electric, acousto-optical and electro-mechanical properties, etc. Recently, with the development of the application of functional devices, the synthesis of LN powders attracts much attention and becomes an interesting topic in the field of LN crystals, since materials scientists have focused on the first step of the growth of LN single crystals (the synthesis of LN powders) in order to obtain LN single crystals with a high quality and large size, which can match different kinds of practical needs.

Over the past several decades, LN powders were traditionally synthesized by the conventional high temperature solid-state method at 1200 °C from the raw powders of Li₂CO₃ and Nb₂O₅, which often results in the growth of inhomogeneous and uncontrollable large grains, the formation of additional phases and possible loss of the crystal stoichiometry due to the easy volatilization of lithium at the high temperature. In recent years, a number of the wet chemical syntheses have been reported for the synthesis of LN powders, which include the hydrothermal route [1], peroxide route [2], water-soluble complex method [3], solvethermal route [4] and metal alkoxides method [5]. However, there still exist some problems, such as the uneasy control of reaction conditions and long reaction time. The combustion synthesis is an attractive technique for producing LN powders [6], whose advantages are that an organic compound (e.g. urea) gives out heat that can be effectively supplied to the raw reagent of Li₂CO₃ and Nb₂O₅ powders, the supplied energy can speed up the chemical reaction, decrease the reaction time, and also reduce the reaction temperature (∼500–600 °C). Moreover, this method does not need to use any organic and inorganic solvents in the whole reaction process, on the other hand, the generation of impurities during this processing can be greatly avoided.

The crystallographic structure of LN crystals consists of distorted oxygen octahedra by sharing their common faces (along the c-axis) or edges (at the ab plane), which forms a trigonal (c-axial) lattice. The succession of the cations in the columns of octahedra along a three-fold axis (c-axis) is as follows: Li, Nb and an empty octahedron. In the ferroelectric phase, both Nb⁵⁺ and Li⁺ cations are displaced from the centers of their respective octahedra. Additionally, in the three dimensional frame, NbO₆ octahedra link each other by sharing their common corners and similarly, LiO₆ octahedra each other by sharing their common corners [7], [8]. The crystallographic structure of LN crystals is shown in Fig. 1. Although the structure of LN crystals has been extensively studied for several decades, however, the characteristic structure of LN powders has not yet been investigated. In this work, the crystallographic structure of the LN powders is studied in detail on the basis of crystal structure properties. And the structural analysis is also validated by the characterization result of the obtained LN powders that are synthesized by an organic combustion method.