**3. Hydrothermal method**

*Synthesis Methods and Crystallization*

decompose the desired compound.

copy (EDX) (**Figure 3**).

intensity and the width of the diffracted X-rays.

H2O, only the oxides remain. The mixture is again ground at the outlet of the oven

**Crystal growth:** After the germination phase, and under the effect of a concentration gradient, the cations have just migrated to the germs, forming well-ordered layers. This migration is favored by heating at very high temperature. After cooling, the crystals are separated from the stream by hot and sometimes boiling water. The disadvantages of this method are that it is very slow and needs a lot of energy. In fact, the reaction occurs at high temperatures (500–2000°C) for several hours and for same time for several days. The heating at these temperatures may

Experimentally, oxides and nitrates are bad reagents in the synthesis of single crystals, and they often give crystals with small size which is insufficient to do the x-ray single crystal diffraction. The mechanical grinding can be used to decrease the

The cooling rate is a very important factor to obtain a single crystal with good crystallinity. The cooling rate should be as slow as possible and at least up to 50°C below the crystallization temperature. The choice of the size and the confirmation of the crystallinity of single crystals are initially done using a binocular magnifier (**Figure 2**) then by using the polarizing microscope. This choice is confirmed by the

**Table 1** summarized that same materials have been obtained as single crystals by the means of solid-state reaction. The different parameters of the synthesis (reagents, pre-treatment temperature, temperature of synthesis, pre-treatment

The resolution of the structure same crystals needs the knowledge of its compositions by using elementary analysis such as the energy-dispersive X-ray spectros-

grain sizes and increase the specific surface then increase the reactivity.

time, synthesis time, and cooling rate) are regrouped in the table.

to make it more homogeneous and to minimize the grain size.

**12**

**Figure 3.**

*and composition of the single crystal.*

*SEM micrograph and EDX analysis of a single crystal of Na2CoP1.5As0.5O7 [8] showing the morphology, size,* 

The synthesis of the single crystal by the means of hydrothermal method occurs usually in water at temperatures between 180 and 300°C. The reactor can be an autoclave (**Figure 4**) or a sealed glass tube (**Figure 5**). The pressure is controlled by the gas law [P = *f*(T)]. The pressure of same reactors can be controlled, and it can reach a value of 850 GP. Several materials have been synthesized using the hydrothermal method.

The hydrothermal conditions of an aqueous medium correspond to temperatures and pressures above 100°C and 1 bar, respectively. These conditions allow to considerably modify the chemistry of the cations in solution. They favor the formation of complex metastable structures of lower symmetry and involving smaller variations in enthalpy and entropy than under "normal" conditions [18, 19]. Hydrothermal conditions are also those of the geological processes during which many minerals were formed. In the laboratory, such conditions are achieved by heating a solution in a closed enclosure (autoclave and sealed glass tube) at temperatures of the order of 200–400°C.

The thermodynamic properties of water up to temperatures of 1000°C and pressures of several tens of kilobars are well known [18]. Quantitative data are collected in numerous review articles [18–21]. There are three essential points to remember.


$$
\log \mathbf{K}\_{\mathbf{e}} = -(3018 \,\mathrm{/T}) - 3.55 \,\tag{1}
$$

**Figure 5.** *Sealed glass tube.*

**Figure 6.**

*Variation of the dielectric constant of water as a function of the temperature and the pressure [22].*

**Figure 7.** *Variation of the ionic product Ke of water as a function of the temperature and the density of the liquid [24].*

The phosphate AgNi3PO4(HPO4)2 [25] has been obtained after 3 weeks of heating at 300°C in sealed glass tube filled with the mixture to about 25% in volume (**Figure 5**). The phase has been prepared from an aqueous solution of AgNO3, Ni(NO3)2.6H2O, and H3PO4 in the atomic ratio Ag:Ni:P = 2:1:2.

In the other hand, most of the single crystals of the borophosphate family have been obtained by the hydrothermal rout. Kniep et al. [26] have prepared a lot of new borophosphates as a single crystal and they have developed an approach of the

**15**

autoclave at 150°C for 2 days.

*Representation of the inorganic part [Mo8O26]*

*[27]. The two parts are liked by a hydrogen bond.*

**4. Reaction at high pressure**

*Synthesis Methods in Solid-State Chemistry DOI: http://dx.doi.org/10.5772/intechopen.93337*

*Projection of the crystal structure of (C13H28N2)2[Mo8O26] along c-axis [27].*

**Figure 8.**

**Figure 9.**

borophosphate crystal chemistry. They have classified the different existing materials in this family as the B/P ratio and as the coordination number of the bore [26]. It is possible to obtained hybrid (organic/inorganic) materials by using this synthesis method. For example, the hybrid material with general formula Bis[4,4′- (propane-1,3-diyl)dipiperidinium]β-octamolybdate (VI) [27] (**Figures 8** and **9**) has been synthesized as single crystals by using the hydrothermal method in an

*4− and the organic part (C13H28N2)2+ of (C13H28N2)2[Mo8O26]* 

The effect of pressures on the crystal structure of same materials as the transformation of the ZnO from wurtzite to rock salt from 9 to 13 GPa is well known [28]. Another example is the transformation of the olivine structure at high pressure

#### **Figure 8.**

*Synthesis Methods and Crystallization*

**Figure 5.** *Sealed glass tube.*

**Figure 6.**

**14**

**Figure 7.**

*Variation of the ionic product Ke of water as a function of the temperature and the density of the liquid [24].*

*Variation of the dielectric constant of water as a function of the temperature and the pressure [22].*

The phosphate AgNi3PO4(HPO4)2 [25] has been obtained after 3 weeks of heating at 300°C in sealed glass tube filled with the mixture to about 25% in volume (**Figure 5**). The phase has been prepared from an aqueous solution of AgNO3,

In the other hand, most of the single crystals of the borophosphate family have been obtained by the hydrothermal rout. Kniep et al. [26] have prepared a lot of new borophosphates as a single crystal and they have developed an approach of the

Ni(NO3)2.6H2O, and H3PO4 in the atomic ratio Ag:Ni:P = 2:1:2.

*Projection of the crystal structure of (C13H28N2)2[Mo8O26] along c-axis [27].*

#### **Figure 9.**

*Representation of the inorganic part [Mo8O26] 4− and the organic part (C13H28N2)2+ of (C13H28N2)2[Mo8O26] [27]. The two parts are liked by a hydrogen bond.*

borophosphate crystal chemistry. They have classified the different existing materials in this family as the B/P ratio and as the coordination number of the bore [26].

It is possible to obtained hybrid (organic/inorganic) materials by using this synthesis method. For example, the hybrid material with general formula Bis[4,4′- (propane-1,3-diyl)dipiperidinium]β-octamolybdate (VI) [27] (**Figures 8** and **9**) has been synthesized as single crystals by using the hydrothermal method in an autoclave at 150°C for 2 days.

#### **4. Reaction at high pressure**

The effect of pressures on the crystal structure of same materials as the transformation of the ZnO from wurtzite to rock salt from 9 to 13 GPa is well known [28]. Another example is the transformation of the olivine structure at high pressure

#### *Synthesis Methods and Crystallization*

from the hexagonal close packing into the cubic close packing of the spinel structure [29]. Upon high pressure conditions (6 GPa, 1173 K) olivine-like LiMAsO4 (M = Fe, Co, Ni) transforms to spinel-like compounds where Li+ and M+2 ions randomly occupy 16d octahedral positions and the As+5 cations occupy the tetrahedral 8a sites [29] (**Figure 10**).

Since 2006, the prediction of the structure at high pressures became an area of intense activity thanks to the development of the new computer program USPEX [30] by Oganov et al. The code was used with success to predict many new crystal structures, and the results were confirmed by the synthesis of same predicted materials such as Na-Cl system: Na3Cl, Na2Cl, Na3Cl2, Na4Cl3, NaCl3, and NaCl7 [31] and H-Cl system: H2Cl, H3Cl, H5Cl, and H4Cl7 [32, 33]. This result allows the discovery of new generation of materials where the core electrons can participate in the formation of chemical bonds. Thus, obviously, we will have very interesting physical and chemical properties.
