**6. Flux method**

*Synthesis Methods and Crystallization*

**18**

**Figure 13.**

**Figure 12.**

*Bi2Cl10 octahedron surrounded by aminopyrazolium entities, showing the H-Cl contacts [36].*

*Projection of the structure of (C7H7N2)2[CuCl4].2H2O showing the alternating stacking of the organic and inorganic layers connected through hydrogen bonds. The face-to-face π▬π stacking between parallel organic* 

*molecules is noteworthy with a centroid-centroid distance of 3.968 (3) [35].*

Usually, the flux method is used to grow materials as single crystals [40]. The main objective of this method is to decrease the crystallization temperature. This technique has been used to grow high melting phosphate crystals, arsenates, oxides, minerals, and ceramic crystals which cannot be obtained by the conventional solid-state method (Section 1). In this growth technique, the basic materials (solute precursors to crystallize) are reduced to a liquid form in an appropriate flow and the growth process starts when the solution reaches critical supersaturation. The resulting supersaturation and crystal growth are achieved by flow evaporation, solution cooling, or a transport process in which the solute is caused to flow from the hottest region to the coldest region.

The understanding of the phase information about the materials is indispensable to optimize the crystal growth of multi-component system. Practically, binary or other above compositional systems can be easily described using the phase diagram. The phase diagram shows the crystallization or solidification within a material as a function of the material composition (% elements) and material temperature.

The solvent can be a single element, compound, or combination of chemical compounds. The solute is an element or a compound with a melting point generally higher than that of the solvent but in principle it is quite possible to grow crystals from eutectic systems in which the "solvent" has a higher melting point.

For more clarification, taking the example of LiPr(PO3)4 [40]. The material has been synthesized as a single crystal by the means of flux method. A mixture of Li2CO3 and Pr6O11 with stoichiometric ratio was dissolved in an excess of phosphoric acid H3PO4 (85%). The mixture was heated to 200°C for 12 h, then to 325°C for 5 days. Finally, the mixture was cooled slowly to room temperature. The single crystal was separated from the excess phosphoric acid by washing in boiling water [40].
