**2. Solid-state method**

The solid-state method is the simplest method. The number of acting parameters is relatively few but difficult to control. This method is commonly used for the synthesis of the single crystals and polycrystalline powders of phosphates and arsenates of transition metals and monovalent cations [1–17].

The crystallization is a phenomenon which generally occurs during a phase change and accompanied by a thermal effect. It is carried out in two stages: germination and growth. The germination begins at a point where the phases are not in equilibrium, a condition can be favored by several factors such as crucible wall, impurity, amorphous. This step consists of the appearance within the reaction mixture of "germs." The growth takes place in several stages: reorganization of the atoms, adsorption on the surface of the solid, diffusion, and fixation of the atoms on their final sites. Successive layers therefore aggregate on the faces of the crystal which sees its volume increase.

The solid-state synthesis can be done into two steps:

**Preliminary treatment:** This step consists in weighing the desired quantities of the precursors and then grinding them in an agate mortar. The powder obtained is placed in a crucible or a porcelain basket (**Figure 1**) and preheated between 350 and 400°C for a few hours (**Figure 1**) This operation allows the decomposition of the starting reagents and removes the volatile products such as NH3, NO2, CO2, and

**11**

**Single crystal**

LiCo As 2 O3 10 NaCo As 2 O3 10 Na4Co7(AsO4)6

K0.13Na3.87MgMo

Na2.77

K0. 86Na1.14CoP

Na CoP 2

1.5As0.5

Na1.25Co2.187Al1.125(AsO4)3

Na Co 3

2(As0. 52P0. 48)O As 4 O2 7

K0.405Bi0.865AsO4

(Na0. 71Ag0. 29) CoP 2

Ag4Co7(AsO4)6

Na4Li0. 62Co5. 67Al0. 71(AsO4)6

Ag3.68Co2(P

Na Co 2 Na7Li0.8

**Table 1.**

*Same materials have been obtained as single crystals by the means of solid-state reaction.*

K0.2Co5(As

O3 10)2(As

O2 7)2

2(MoO4)3

O2 7)2

Na CO 2

3 + LiOH.H

2 AgNO3 + Co(NO3)2·6H

NaNO3+ Co(NO3)2·6H

NaNO3 + Co(CH

*T1, pre-treatment temperature; T2, temperature of synthesis; t1, pre-treatment time; t2, synthesis time; R, cooling rate.*

 COO) 3

2.4H O + As 2

O2 5 *The different parameters of the synthesis (reagents, pre-treatment temperature, temperature of synthesis, pre-treatment time, synthesis time, and cooling rate) are regrouped in the table.*

2

O + (NH

4) Mo 6 O7 24

O + NH 2

H4 2PO4

O + Co(NO

3)2.6H O + Al 2

O2 3 + NH

H4 AsO 2 4

400 350 350 400

670

12 h

7 days

5 K/h

[17]

650

24 h

4 days

5 K/h

[16]

610

12 h

3 days

5 K/h

[15]

870

24 h

5 days

5 K/h

[14]

O2 7

O7

O2 7

K1.52Fe2.57(AsO4)4

O3 12

Na CO 2 Na CO 2

3 + K CO 2 NaNO3 + KNO3 + Co(NO3)2.6H

NaNO3 + Co(NO3)2.6H

Na CO 2

Na CO 2

3 + Co(NO3)2.6H

K CO 2

3 + Bi

NaNO3 + AgNO3 + Co(NO3)2·6H

AgNO3 + Co(NO3)2·6H

 O + As 2

O2 5

O + NH 2

H4 2PO4

O2 3 + NH

H4 AsO 2 4

O + NH 2

H4 AsO 2

4 + NH

H4 2PO4

350 400 400 400

1005

24 h

5 days

5 K/h

[13]

620

24 h

5 days

5 K/h

[12]

850

12 h

30 days

5 K/h

[11]

700

12 h

3 days

5 K/h

[10]

3 + Co(NO3)2 .6H

O + NH 2

H4 AsO 2 4

O + NH 2

H4 2PO4

O + NH 2

H4 2PO4

3 + Fe(NO3)3.9H

O + NH 2

H4 AsO 2 4

3 + K CO 2

3 + (NH4) Mo 2 O4

13 + Mg(NO3)2.6H

O2

400 400 400 350 350

800

12 h

3 days

5 K/h

[9]

620

12 h

3 days

5 K/h

[8]

660

12 h

4 days

5 K/h

[7]

850

24 h

3 days

5 K/h

[6]

600

12 h

5 days

5 K/h

[5]

**Reagents**

> Li CO 2

3 + CoCl2.6H NaNO3 + Co(NO3)2 .6H

NaNO3 + Co(NO3)2 .6H

O + NH 2

H4 AsO 2 4

 O + As 2

O2 5

O + NH 2

H4 AsO 2 4

**T1 (°C)**

350 400 350

750

24 h

3 days

5 K/h

[4]

670

24 h

3 days

5 K/h

[3]

730

12 h

3days

5 K/h

[2]

**T2 (°C)**

**t1**

**t2**

**R**

**Ref.**

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

**Figure 1.** *Equipment used in solid-state synthesis.*

**Figure 2.** *Image of a single crystal of the phase Na1.25Co2.187Al1.125(AsO4)3 [9] seen under the binocular magnifying glass.*


**Table 1.**

*Same materials have been obtained as single crystals by the means of solid-state reaction.*

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

*Synthesis Methods and Crystallization*

The solid-state method is the simplest method. The number of acting param

eters is relatively few but difficult to control. This method is commonly used for the synthesis of the single crystals and polycrystalline powders of phosphates and

The crystallization is a phenomenon which generally occurs during a phase change and accompanied by a thermal effect. It is carried out in two stages: ger

mination and growth. The germination begins at a point where the phases are not in equilibrium, a condition can be favored by several factors such as crucible wall, impurity, amorphous. This step consists of the appearance within the reaction mixture of "germs." The growth takes place in several stages: reorganization of the atoms, adsorption on the surface of the solid, diffusion, and fixation of the atoms on their final sites. Successive layers therefore aggregate on the faces of the crystal which

**Preliminary treatment:** This step consists in weighing the desired quantities of the precursors and then grinding them in an agate mortar. The powder obtained is placed in a crucible or a porcelain basket (**Figure 1**) and preheated between 350 and 400°C for a few hours (**Figure 1**) This operation allows the decomposition of

arsenates of transition metals and monovalent cations [1–17].

The solid-state synthesis can be done into two steps:

the starting reagents and removes the volatile products such as NH



2, CO2, and

3, NO

**2. Solid-state method**

sees its volume increase.

**10**

**Figure 2.**

**Figure 1.**

*Equipment used in solid-state synthesis.*

*Image of a single crystal of the phase Na1.25Co2.187Al1.125(AsO*

*4 )*

*3 [9] seen under the binocular magnifying glass.*

H2O, only the oxides remain. The mixture is again ground at the outlet of the oven to make it more homogeneous and to minimize the grain size.

**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 decompose the desired compound.

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

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 intensity and the width of the diffracted X-rays.

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

The resolution of the structure same crystals needs the knowledge of its compositions by using elementary analysis such as the energy-dispersive X-ray spectroscopy (EDX) (**Figure 3**).

#### **Figure 3.**

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

**13**

**Figure 4.** *Autoclave.*

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

peratures of the order of 200–400°C.

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 hydro-

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 tem-

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

• The dielectric constant of water drops when the temperature increases. It increases by pressure increase [22] (**Figure 6**). The hydrothermal solutions are therefore characterized by low dielectric constants and the electrolytes which are completely dissociated under normal conditions preferentially form pairs

• The viscosity of water decreases with the increase of temperature [23], which leads to greater mobility of the dissolved species than under normal conditions.

• The ionic product of water increases strongly with temperature [24] (**Figure 7**). The conductivity measurements allow establishing the law of variation of the

log K / T <sup>e</sup> =- - (3018 3.55 ) (1)

of ions or complexes of low electrostatic charge.

ionic product as a function of temperature

**3. Hydrothermal method**

thermal method.

remember.
