Preface

Solid materials can be crystalline, amorphous, or glassy. A crystalline material is a solid whose constituents (atoms, molecules, or ions) are assembled in a regular pattern. Crystallization is a chemical reaction that makes it possible to obtain crystalline materials. Crystallized materials have three forms: single crystal, polycrystalline powder, and thin film. The phenomenon that allows crystals to grow to dimensions greater than a few millimeters is called crystal growth.

The preparation of a single crystal is the best way to obtain new materials [1]. However, the search for new phases following this path is not easy, especially for non-oxide materials. In addition, the parameters to be controlled during the synthesis differ from one method to another. For example, it was found that the solid-solid method is the simplest method to implement, but most of the parameters in this type of reaction are uncontrollable. However, in the hydrothermal method, it is possible to control the concentration, pH, solution/solvent ratio, temperature, and pressure. The crystals are initially characterized by their sizes, colors, and morphologies. Small dimensions and twins are the problems most encountered during the experiment and of course have known phases.

To obtain new phases, researchers can change the stoichiometric proportions of the reactants and the controllable parameters of the reaction process. In recent years, it has become possible to predict the existence of a crystal structure using the Universal Structure Predictor: Evolutionary Xtallography (USPEX) code [2]. Currently, this code is used to predict the existence of new phases in two-element chemical systems and some three-element chemical systems. It is currently undergoing further development to accommodate the study of larger systems; this new code is known as COPEX [3] (co-evolutionary crystal structure prediction algorithm for complex systems). USPEX has made it possible to predict the possibility of obtaining new phases at high pressure such as NaCl3 and NaCl7 [4]. In these types of materials, the inner core electrons participate in the formation of chemical bonds. Other predicted materials showed superconductivity properties at temperatures close to room temperature [5, 6]. With this code, the experimental researcher can reduce the number of experiments. In addition, it is possible to proceed directly to the synthesis of polycrystalline powder or thin-film, once crystallographic data of the new phase is obtained.

The crystallization process may be used in chemistry, physics, or materials science to prepare materials for special applications such as batteries, fuel cells, and optics. Crystallized materials may be prepared in the laboratory as powder or thin film and in some cases as a single crystal. In industry, crystallization is an elementary process necessary to obtain some pure products. The crystallization process is used also in biology to crystallize protein and in pharmacology to prepare some drugs. The crystal growth of materials is a technique used to generate material in an orderly arrangement of atoms, especially when the material shows good properties worthy of further studies such as optical properties. In this case, the growth of the materials in crystalline form allows for the elimination of defects and improvement of material properties.

**Youssef Ben Smida**

National Centre of Research in Materials Science, Technology Park of Borj Cedria, Borj Cedria, Tunisia

> **Riadh Marzouki** King Khalid University, Abha, Saudi Arabia

> > **V**

[1] Y.B. Smida, R. Marzouki, S. Kaya, S. Erkan, M.F. Zid, A.H. Hamzaoui, Synthesis methods in solid-state chemistry, Synthesis Methods and Crystallization, IntechOpen2020.

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