**4.1.1. Solid-State Synthesis**

**1. Solid-State Synthesis**<sup>9</sup>

in **Section 4.1.1**.

**2. Wet chemical method**<sup>10</sup>

**3. Hydrothermal synthesis**<sup>11</sup>

autoclave

(**Dry Method**) usually requires rather high temperature ≥ 1200°C

(precipitation from the solution): requires a long time period (10

: involves heating of reactants with water in closed vessel, an

and the product is characterized by a gradient of composition in the grain of material. The method and special techniques, conditions and devices used to control the product properties and to improve the reaction rate and homogeneity of the products are described

hour or more) and often results in amorphous and non-stoichiometric products.

Other methods (**Section 4.1.1**) such as microwave synthesis, combustion synthesis and high

The pressure-temperature ranges of these methods are shown in **Fig. 2**. Some of the most applied techniques are described in this chapter. The methods for the preparation of single

Environmentally stressed

pressure method or deposition techniques are used much rarely.

180 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

crystals are described separately in the next **Section 4.2**.

Ultrahigh pressure

Solution

10

12

14

8

6

Log pressure (Pa)

4

2

Vacuum

**Fig. 2.** Pressure-temperature range for the material preparation [21].

11 Sometimes are included among high-pressure methods [24].

0

9

in literature [23].

0 1000

<sup>10</sup>JAFFE [18] recognized the precipitation by metathesis and the precipitation by hydrolysis.

deposition Environmentally stressed

Vapor deposition Solid state reaction

Hydrothermal

Temperature (°C)

Various alternative names, such as Shake 'n Bake Methods or Beat 'n Heat, are used for the solid state synthesis (reaction)

Sputtering Plasma

High pressure

Flux Environmentally

Melt Stressed

Ultrahigh temperature

2000

Solid-phase reactions (syntheses) are usually activated by high-temperature treatment [25]. The preparation of materials in solid-state is rather different form the synthesis of discrete molecules. The process involves the treatment of the whole lattice. Often, the post-synthesis purification of materials is not possible due to low solubility of formed phases. Hence, all effort must be made to avoid the excess of reagents. These methods are usually slow due to entire reaction which occurs in the solid state and requires the diffusion across the points of contact in a mixture [26],[27]. Special techniques can also be used based on the reduction of particle size or on the preparation of precursor in order to reduce the particle size, improve the product homogeneity and lower the temperature of thermal treatment [24].

**Fig. 3.** Reaction scheme for solid-state synthesis: (a) ceramic method [26], (b) reaction of solids under special gas atmos‐ phere [24] and (c) sealed glass tube [21],[33].

The most applied techniques12 in solid-state synthesis are [23],[26],[24],[29],[28],[30],[31],[32]:

**a. Ceramic method**<sup>13</sup> **:** is the most common way of preparation of solids (metal oxides, aluminosilicates, sulfides, nitrides, etc.) that is based on thermal treatment of compact‐ ed powder14 of two or more nonvolatile solids, which react to reach required composi‐ tion and desired microstructure of the product **Fig. 3**(**a**). Since the reaction can occur only at the interface of solids, once the surface layer has reacted, the reaction continues as the reactants diffuse from the bulk to the interface. The rate of reaction is then often limited by the diffusion hence it is important to prepare raw meal from fine and well mixed particles in order to maximize the contact area and minimize the diffusion path. It also decreases the temperature required for the thermal treatment. The repeating of regrind‐

<sup>12</sup> Solid-state synthesis is classified among physical methods (together with vapor phase synthesis, laser ablation, etc.) some of other techniques listed below (sol-gel process, precipitation method, etc.) are considered as chemical methods [28].

<sup>13</sup> Since ceramic can be fabricated by a variety of methods, some of which have their origin in early civilizations, ceramic methods must be distinguished from the ceramic fabrication processes.

<sup>14</sup> The consolidation of ceramic powder to produce a **green body** is commonly referred to as forming. The main methods include dry of semidry pressing (1), plastic forming with water or organic polymers (2) and casting from a concentrated suspension or slurry (3) [30].

ing of the sample and the repeating of its thermal treatment is usually required to improve the homogeneity of the product.

The nucleation of a new phase, the epitactic15 and topotactic16 phenomenon's (oriented nucleation), crystal growth, phase transformation17 and the sintration18 are common during the thermal treatment.19

	- **Directly applied pressure** (external pressure, pressure sintering, or pressure-assisted sintering): includes the techniques known as hot pressing (pressure is applied uniax‐

<sup>15</sup> There is a structural similarity between the substrate and the nucleus that is limited to 2D interface and referred to as epitaxy [23].

<sup>16</sup> There is a structural similarity between the substrate and the nucleus (like for epitaxy15)) that extends to 3D for topotaxy [23].

<sup>17</sup> Phase transformation has usually significant effects on the reaction rate. The reaction rate is strongly increased at the temperatures near the phase transformation because the mobility of atoms is also increased. This phenomenon is termed as **Hedvall effect** [29].

<sup>18</sup> Sintering is defined as the bonding of adjacent surfaces in a mass of powder or a compact by heating [29]. In general there are three types of sintering process including Solid-Phase (Dry) Sintering (1), Liquid-Phase Sintering (2) and Reactive (Reaction) Sintering (3) [29]. The process can also be divided according to applied conditions and densification practice to Conventional Sintering (1), Microwave Sintering (2) and Pressure Sintering (3). The stages of the sintering process include: (1) initial stage (formation and growth of necks), (2) intermediate stage (pores reached their equilibrium shapes, continuous porosity), (3) final stage (pores reached their equilibrium shapes, isolated (enclosed) porosity) [30]. The process can also be divided according to the mass transport mechanism to viscous sintering and diffusion sintering (further divided according to dominant type of diffusion to surface diffusion, volume diffusion, intergranular diffusion, grain-boundary diffusion, but gas transport (diffusion) of matter can also occur).

<sup>19</sup> Generally, the term **firing** is used when the processes occurring during thermal treatment of green body are fairly complex, as in many traditional ceramics produced from clay-based materials. In less complex cases the term sinter‐ ing18 is used [30].

ing of the sample and the repeating of its thermal treatment is usually required to improve

**b. Sealed tube** (**pipe**) **method** (**reaction**): is applied in the cases when direct reaction under ambient conditions (in air at one atmosphere pressure) cannot be performed due to high volatility of reactants, air sensitivity of starting materials and/or products, or the desire to form a compound with a metal in an unusually low oxidation state. Typically for this type of reactions, the components are loaded into a glass (method was first applied by DE SÉNARMONT [33], **Fig. 3**(**c**)) or quartz ampoule (tube) in a glass box, evacuated and sealed off by melting the glass/quartz using a blow torch. The whole tube is heated to required temperature and time. Cooled tube is then broken up to get the product. The reaction of material inside with the tube may cause that the side-wall of tube is weaken. In the combination with pressure in the tube, these reactions can be hazardous. The synthesis can be also performed with metal capsules (solvothermal reactions, **Section 4.1.3**) sealed

**c. Synthesis under controlled** (**special**) **atmosphere**: the preparation of some compounds must be carried out under a special atmosphere, but not necessarily at high pressures. The oxidation (O2), inert (Ar, N2) or reduction atmosphere (CO/CO2 or H2/H2O) is used to

**d. Solid-state synthesis under high-pressure**: high pressure can be applied [24],[30],[34],

<sup>15</sup> There is a structural similarity between the substrate and the nucleus that is limited to 2D interface and referred to as

16 There is a structural similarity between the substrate and the nucleus (like for epitaxy15)) that extends to 3D for topotaxy

<sup>17</sup> Phase transformation has usually significant effects on the reaction rate. The reaction rate is strongly increased at the temperatures near the phase transformation because the mobility of atoms is also increased. This phenomenon is termed

<sup>18</sup> Sintering is defined as the bonding of adjacent surfaces in a mass of powder or a compact by heating [29]. In general there are three types of sintering process including Solid-Phase (Dry) Sintering (1), Liquid-Phase Sintering (2) and Reactive (Reaction) Sintering (3) [29]. The process can also be divided according to applied conditions and densification practice to Conventional Sintering (1), Microwave Sintering (2) and Pressure Sintering (3). The stages of the sintering process include: (1) initial stage (formation and growth of necks), (2) intermediate stage (pores reached their equilibrium shapes, continuous porosity), (3) final stage (pores reached their equilibrium shapes, isolated (enclosed) porosity) [30]. The process can also be divided according to the mass transport mechanism to viscous sintering and diffusion sintering (further divided according to dominant type of diffusion to surface diffusion, volume diffusion, intergranular diffusion,

19 Generally, the term **firing** is used when the processes occurring during thermal treatment of green body are fairly complex, as in many traditional ceramics produced from clay-based materials. In less complex cases the term sinter‐

**• Directly applied pressure** (external pressure, pressure sintering, or pressure-assisted sintering): includes the techniques known as hot pressing (pressure is applied uniax‐

and topotactic16

and the sintration18

phenomenon's (oriented

are common during

the homogeneity of the product.

the thermal treatment.19

by welding.

[35]:

epitaxy [23].

as **Hedvall effect** [29].

ing18 is used [30].

[23].

The nucleation of a new phase, the epitactic15

nucleation), crystal growth, phase transformation17

182 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

prepare the compounds in required oxidation state.

grain-boundary diffusion, but gas transport (diffusion) of matter can also occur).

**Fig. 4.** Schematic diagram of hot pressing process [30], hot isostatic pressing [36] and the example of metallic capsule method [37].

ially or biaxially to the powder in a die, **Fig. 4**(**a**)) and sinter forging (similar to hotpressing but without confining the sample in a die).

**• Indirectly applied pressure** can be applied through inert (hot isostatic pressing, HIP, **Fig. 4**(**b**)) or reactive gas, through ultrasound, by milling equipment (mechanochemi‐ cal (powder) synthesis and activation) and by detonation.

The method is particularly important for the preparation of dense sample of ceramics with high degree of covalent bonding such as SiC and Si3N4.

Normal process of compaction of powder material involves uniaxial pressing in a die followed by sintering for densification. However better densification can be achieved by exerting a uniform pressure from all directions through a fluid medium onto the powder material retained in container (die), i.e. the pressure is generated by heating of medium. The process is termed as isostatic compaction, and as **hot isostatic pressing** (HIP) if performed at high temperature. The diffusion of medium from the container to the sample is avoided by its encapsulation by metal or glass (metallic or glass capsule method, **Fig. 4**(**c**) [36],[37].

In the case of **ultrasound (cavitation) methods** the phenomenon known as cavitation20 takes place. Traveling the ultrasound wave leads to high pressure volume (compression) of the liquid which is followed by low pressure. Sudden expansion (rarefaction) leads to the formation of tiny bubbles. The bubbles expand to an unsustainable size and then collapse. The expansion and the collapse of bubbles create very localized hot spots, which reach instanta‐ neous pressures of more than 100 MPa and temperatures of up to 5000°C (**Section 9.2.2**) [24], [28].

By **detonation methods** usually the nano-sized particles are prepared. The detonation is a superfast (with the velocity exceeding that of sound) exothermic reaction through the substance. The detonation wave consisting of the shock front, chemical reaction zone and the region where the products are scattered, spreads at constant rate due to continuous supply of energy from the chemical transformation of new portions of the explosive to the shock wave. The temperature (in the range from 2000 to 5000 K) and the pressure (shock wave) are reached by detonation in a suitably strong vessel. For example, the change of pressure at the end of the reaction zone ranges from 9.5 to 30 GPa for hexogen. This method was used for the prepara‐ tion of nano-sized synthetic diamond,21 graphite, boron nitride, etc. [24],[38].

**Mechanochemical synthesis**22 is a solid-state synthesis method that takes advantage of the perturbation of surface-bonded species by pressure to enhance the thermodynamic and kinetic reactions between solids. The pressure can be applied at ambient temperature by friction and impact via milling equipment (**Fig. 5**) ranging from low energy ball mills to high energy stirred mills.23 The main advantage of this method is the simplicity and low cost. The method was successfully used for the synthesis of oxides, phosphates, carbides, complexes, intermeta‐ lides [44],[45],[46],[47],[48],24 alloys, etc.

<sup>20</sup> Generally, the cavitation can be divided into four types on the basis of the mode of generation of cavitation conditions: (1) acoustic cavitation (sound waves of high frequency 16 kHz–1 MHz), (2) Hydrodynamic cavitation (pressure variation is obtained by changing the geometry of the system), (3) optic cavitation (passing of photons of high intensity) and (4) particle cavitation (produced by the bombardments of other types of elementary particles, e.g. protons) [28].

<sup>21</sup> The method for the detonation transformation of graphite into diamond was earlier developed at the Institute of Chemical Physics of the Academy of Sciences of the USSR [38].

<sup>22</sup> Mechanochemical synthesis (reaction) of solids in the presence of water can be considered as hydrothermal one [25].

<sup>23</sup> Mechanochemistry is a branch of chemistry which is concerned with chemical and physico-chemical changes of substances of all stages of aggregation due to the influence of mechanical energy (OSTWALD [39]). Colloid mills can be classified into three main groups with regard to the mechanism utilized for production of dispersion: beater-type mills (1), the smooth-surface type (2) and the rough-surface type. Beater-type mills include the original Plauson machine and some modified mills [48].

<sup>24</sup> Also intermetallic compounds, i.e. substances composed of two or more metallic elements with given stoichiometry and structure. Different atomic species occupy different lattice sites [44].

In the case of **ultrasound (cavitation) methods** the phenomenon known as cavitation20

184 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

[28].

mills.23

tion of nano-sized synthetic diamond,21

**Mechanochemical synthesis**<sup>22</sup>

lides [44],[45],[46],[47],[48],24

some modified mills [48].

place. Traveling the ultrasound wave leads to high pressure volume (compression) of the liquid which is followed by low pressure. Sudden expansion (rarefaction) leads to the formation of tiny bubbles. The bubbles expand to an unsustainable size and then collapse. The expansion and the collapse of bubbles create very localized hot spots, which reach instanta‐ neous pressures of more than 100 MPa and temperatures of up to 5000°C (**Section 9.2.2**) [24],

By **detonation methods** usually the nano-sized particles are prepared. The detonation is a superfast (with the velocity exceeding that of sound) exothermic reaction through the substance. The detonation wave consisting of the shock front, chemical reaction zone and the region where the products are scattered, spreads at constant rate due to continuous supply of energy from the chemical transformation of new portions of the explosive to the shock wave. The temperature (in the range from 2000 to 5000 K) and the pressure (shock wave) are reached by detonation in a suitably strong vessel. For example, the change of pressure at the end of the reaction zone ranges from 9.5 to 30 GPa for hexogen. This method was used for the prepara‐

perturbation of surface-bonded species by pressure to enhance the thermodynamic and kinetic reactions between solids. The pressure can be applied at ambient temperature by friction and impact via milling equipment (**Fig. 5**) ranging from low energy ball mills to high energy stirred

successfully used for the synthesis of oxides, phosphates, carbides, complexes, intermeta‐

<sup>20</sup> Generally, the cavitation can be divided into four types on the basis of the mode of generation of cavitation conditions: (1) acoustic cavitation (sound waves of high frequency 16 kHz–1 MHz), (2) Hydrodynamic cavitation (pressure variation is obtained by changing the geometry of the system), (3) optic cavitation (passing of photons of high intensity) and (4)

21 The method for the detonation transformation of graphite into diamond was earlier developed at the Institute of

22 Mechanochemical synthesis (reaction) of solids in the presence of water can be considered as hydrothermal one [25]. 23 Mechanochemistry is a branch of chemistry which is concerned with chemical and physico-chemical changes of substances of all stages of aggregation due to the influence of mechanical energy (OSTWALD [39]). Colloid mills can be classified into three main groups with regard to the mechanism utilized for production of dispersion: beater-type mills (1), the smooth-surface type (2) and the rough-surface type. Beater-type mills include the original Plauson machine and

24 Also intermetallic compounds, i.e. substances composed of two or more metallic elements with given stoichiometry

particle cavitation (produced by the bombardments of other types of elementary particles, e.g. protons) [28].

alloys, etc.

Chemical Physics of the Academy of Sciences of the USSR [38].

and structure. Different atomic species occupy different lattice sites [44].

The main advantage of this method is the simplicity and low cost. The method was

graphite, boron nitride, etc. [24],[38].

is a solid-state synthesis method that takes advantage of the

takes

**Fig. 5.** The first mechanochemical reactor25 (a): mortar (A), iron collar (B), pestle (C), handle (D) and rubber tube (E) [32],[39], Plauson-Oderberg (beater-type [48]23) colloid mill for wet milling (b) and longitudinal section through Plau‐ son colloidal mill [47] (c).

**Mechanochemical** activation involves the dispersion of solids and their deformation. These processes cause the generation of defects in solids, and also accelerate the migration of defects in the bulk, increase the number of contacts between particles and renew the contacts [25],[32], [34],[39],[40],[41],[42],[43],[49].

The advanced preparation techniques used for the reduction of particle size, activation of starting material or preparation of nano-scale26 precursor are a broad group of methods with complicated nomenclature. The most known methods are:

**i. Sol-gel process** [26],[30],[50],[51],[52]: in this method, a solution of metal com‐ pounds (usually alkoxides of M(OR)4 type, such as TMOS (Si(OCH3)4), TEOS (Si(OC2H5)4), TEOT (Ti(OC2H5)4), etc.,27 or (RO)xMR'4−x type, such as (H3CO)3SiCH3, (H3CO)3Si(CH=CH2), etc.28) or suspension of very fine particles in a liquid (sol) is converted into rigid gel by removing the solvent or by adding a component which causes the gel to solidify. Two different sol-gel processes can be distinguished,29 depending on whether the sol or the solution of alkoxides (alkoxide methods) is used **Fig. 6**(**a**,**b**).

<sup>25</sup> Constructed by PARKER [39] for the study of solid–solid reaction of the type: Na2CO3 + BaSO4 → Na2SO4 + BaCO3 [41]. Earlier works were concerned with the decomposition of solids by high pressure and by grinding in a mortar with the pestle [42],[43].

<sup>26</sup> Nano it the Greek word for dwarf. In the International System of Units (SI) it is the decimal multiple 10−9 used as prefix. Nanoscience refers to the range from one to several hundred manometers and the nanotechnologies are the technologies in which atoms are manipulated in quantities of one to several thousand atoms. Nanoscience probably first gained the attention in 1959 in the lecture of the American Nobel laureate of 1965 in physics R. FEYNMAN, who stated: *…that the day was no far off, when substances could be assembled at an atomic level* [27].

<sup>27</sup> Tetrametoxysilane, tetraetoxysilane, tetraethyl orthotitanate, etc.

<sup>28</sup> Methyltrimethoxysilane, vinyltrimethodxysilane, etc. **R**´ can also be reactive (polymerizable) groups such as (3 aminopropyl)trietoxysilane (APTES) or (3-glycidopropyl)trimetoxysilane (GLYMO) which can be used to prepare interconnected inorganic-organic networks. These materials are known as ORMOCERS (organically modified ceramics).

**Fig. 6.** Basic flow charts for sol-gel processes using a sol (a) and a solution of alkoxides (b). Schematic diagram of the structure of particulate gel formed from a sol (c) and polymeric gel from a solution (d) [30].

Starting with a sol, gelled material consists of identifiable colloidal particles which were joined together by the surface forces to form a network **Fig. 6**(**c**). When the solution of metal-alkoxides is used (**d**), the gel consist of a network of polymer chains formed via the reaction of hydrolysis (**Eq. 3**30) and condensation (**Eq. 4**31):

$$\text{M(OR}\,\text{)}\_{4} + \text{H}\_{2}\text{O}^{\prime} \leftrightarrow \text{M(OR}\,\text{)}\_{3}\text{O}^{\prime}\text{H} + \text{ROH}\tag{3}$$

<sup>29</sup> This classification [30] does not set aside often used group of semi-alkoxide methods (using the mixture of soluble salt and metal alkoxide), Pechini type polymeric gel methods (liquid mix techniques) as well as modified Pechini methods. The Pechini process usually uses soluble nitrates, acetates, chlorides, carbonates, isopropoxides or other metal compounds which are dissolved in the solution of citric acid (in general in polycarboxylic acids) and ethylene glycol (in general glycol). The polycondensation reaction leads to the polymeric gel accommodating the stable chelates of metal cations [50]:

30 The first step is the hydrolysis, the equilibrium constant for higher degree of hydrolysis decreases depending on the nature of –OR (increasing length of hydrocarbon chain and its branching). The process is also affected by the temperature, the solvent composition and applied water to alkoxide ratio, the type of catalysis, the application of ultrasound energy, etc. [52].

<sup>31</sup> Due to the formation of ROH molecule is termed as "alcohol producing condensation", i.e. the reaction between alkoxy (nonhydrolyzed) and hydrolyzed groups. The reaction with two hydrolyzed groups leads to the formation of –M–O–M– bridge and water, i.e. is termed as "water producing condensation".

$$\text{M(OR)}\_{3}\text{OH} + \text{M(OR)}\_{4} \leftrightarrow \text{(RO)}\_{3} - \text{M} - \text{O} - \text{M} - \text{(RO)}\_{3} + \text{ROH} \tag{4}$$

Drying of gel leads to xerogel. The process is usually followed by shrinkage and formation of cracks. The thermal treatment of xerogel often involves the pyrolysis and calcination. If monolith is needed (aerogel [51]) the supercritical drying is usually applied.


Starting with a sol, gelled material consists of identifiable colloidal particles which were joined together by the surface forces to form a network **Fig. 6**(**c**). When the solution of metal-alkoxides is used (**d**), the gel consist of a network of polymer chains

29 This classification [30] does not set aside often used group of semi-alkoxide methods (using the mixture of soluble salt and metal alkoxide), Pechini type polymeric gel methods (liquid mix techniques) as well as modified Pechini methods. The Pechini process usually uses soluble nitrates, acetates, chlorides, carbonates, isopropoxides or other metal compounds which are dissolved in the solution of citric acid (in general in polycarboxylic acids) and ethylene glycol (in general glycol). The polycondensation reaction leads to the polymeric gel accommodating the stable chelates of metal cations [50]:

30 The first step is the hydrolysis, the equilibrium constant for higher degree of hydrolysis decreases depending on the nature of –OR (increasing length of hydrocarbon chain and its branching). The process is also affected by the temperature, the solvent composition and applied water to alkoxide ratio, the type of catalysis, the application of ultrasound energy,

31 Due to the formation of ROH molecule is termed as "alcohol producing condensation", i.e. the reaction between alkoxy (nonhydrolyzed) and hydrolyzed groups. The reaction with two hydrolyzed groups leads to the formation of –M–O–M–

**Fig. 6.** Basic flow charts for sol-gel processes using a sol (a) and a solution of alkoxides (b). Schematic diagram of the structure of particulate gel formed from a sol (c) and polymeric gel from

Hydrolysis and condensation

Solution of metal alkoxides

**(a) (b)**

Sol (solution of polymers) Gelation

"Polymeric" gel

) and condensation (**Eq. 4**<sup>31</sup>

Particle Polymer Liquid

"Particulate" gel

( ) <sup>2</sup> ( ) M OR H O M OR O H ROH 4 3 + ¢ « ¢ + (3)

):

"Polymer" gel

formed via the reaction of hydrolysis (**Eq. 3**<sup>30</sup>

bridge and water, i.e. is termed as "water producing condensation".

<sup>A</sup> Drying <sup>B</sup> Drying

186 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

Dried gel

Dense product Firing

Gelation

"Particulate" gel

a solution (d) [30].

etc. [52].

Sol (suspension of fine particles)

Hydrolysis

<sup>32</sup> The precipitation method is used for the preparation of precursor that is further treated by solid-state synthesis. The techniques of direct precipitation of apatite are described in **Section 4.1.2**.

<sup>33</sup> Therefore often associated with combustion techniques [56].

<sup>34</sup> The process is analogous to the process of frontal polymerization in which localized polymerization reaction zone propagates thorough the mixture of solution of a monomer and initiator due to the heat diffusion and the occurrence of exothermic reaction [59].

**vii. Spray pyrolysis** [62],[63]: small droplets of a solution containing desired precursor are introduced into the hot zone to the furnace to obtain required product. According to applied conditions in different stages of the thermal cycle (**Fig. 7**) the aerosol will form non-coherent powder and solid particles.

**Fig. 7.** Thermal cycle of spray pyrolysis [63].

**viii. Freeze drying** [35],[64]: the method discovered in 1965 that consists of fast freezing of the precursor solution (ensures preservation of maximum chemical homogeneity achieved in the starting solution), sublimation of solvent and final calcination. The main characteristics of the freeze drying method are that the drying is not accompa‐ nied by the coagulation of particles and the shrinkage of particles usually does not occur. The method produces fine and reactive powder of high purity

The reaction schemes for the solid-state synthesis of apatite structured compounds were published by KNYAZEV et al [65]:

$$\begin{aligned} \text{4.5 } \text{M}^{\text{il}} \text{(NO}\_{3}\text{)}\_{2} \cdot \text{nH}\_{2}\text{O} + \text{0.5 } \text{M}^{\text{il}} \text{L}\_{2} \cdot \text{mH}\_{2}\text{O} + \text{3(NH}\_{4}\text{)}\_{2} \text{HPO}\_{4} \rightarrow \\ \text{M}^{\text{il}}\_{3} \text{(PO}\_{4}\text{)}\_{3} \text{L} + 9 \text{ NO}\_{2} + 2.25 \text{ O}\_{2} + 6 \text{ NH}\_{3} + \text{(4.5 + 4.5n + 0.5m)} \text{H}\_{2}\text{O} \end{aligned} \tag{5}$$

where *M*II = Ca, Sr, Ba, Cd, Pb and *L* = OH, F, Cl, Br and I.

$$\begin{aligned} \text{13 }\text{M}^{\text{II}}\text{(NO}\_{3}\text{)}\_{2}\cdot\text{nH}\_{2}\text{O}+2\text{ }\text{NH}\_{4}\text{VO}\_{3}\rightarrow\text{M}\_{3}^{\text{II}}\text{(VO}\_{4}\text{)}\_{2}+6\text{ }\text{NO}\_{2}+\text{1.5 }\text{O}\_{2} \\ +2\text{ }\text{NH}\_{3}+\text{(3n+l) }\text{H}\_{2}\text{O} \end{aligned} \tag{6}$$

$$\text{M.5 M}\_{\text{s}}\text{\textdegree(VO}\_{4}\text{)}\_{\text{2}} + \text{0.5 M}^{\text{u}}\text{L}\_{\text{2}} \cdot \text{mH}\_{\text{2}}\text{O} \rightarrow \text{M}\_{\text{s}}^{\text{u}}\text{\textdegree(VO}\_{4}\text{)}\_{\text{3}}\text{L} + \text{0.5 m H}\_{\text{2}}\text{O} \tag{7}$$

where *M*II = Ca, Sr, Ba, Cd, Pb and *L* = F, Cl and Br.

$$4.5\ \mathrm{M}^{\mathrm{II}}\mathrm{CO}\_{3} + 3\ \mathrm{CrO}\_{3} + 0.5\ \mathrm{M}^{\mathrm{II}}\mathrm{L}\_{2}\cdot\mathrm{m}\mathrm{H}\_{2}\mathrm{O} \rightarrow \mathrm{M}\_{\mathrm{x}}^{\mathrm{II}}\mathrm{(CrO}\_{4}\mathrm{)}\_{3}\mathrm{L} + 4.5\ \mathrm{CO}\_{2} + 0.75\ \mathrm{O}\_{2} + 0.5\ \mathrm{m}\mathrm{H}\_{2}\mathrm{O}$$

where *M*II = Ca, Sr and *L* = F and Cl.

$$\begin{aligned} &4.5 \text{ Ba}^{0} \text{CO}\_{3} + 3 \text{ MnO}\_{2} + 0.5 \text{ Ba}^{0} \text{L}\_{2} \cdot \text{mH}\_{2}\text{O} + 0.75 \text{ O}\_{2} \rightarrow \text{Ba}\_{5}^{\text{H}} \text{(MnO}\_{4}\text{)}\_{3} \text{L} \\ &+ 4.5 \text{ CO}\_{2} + 0.5 \text{ mH}\_{2}\text{O} \end{aligned} \tag{9}$$

The temperature effect (**Table 1**) observed during the synthesis includes [65]:

**1.** synthesis;

**vii. Spray pyrolysis** [62],[63]: small droplets of a solution containing desired precursor

form non-coherent powder and solid particles.

188 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

**Fig. 7.** Thermal cycle of spray pyrolysis [63].

published by KNYAZEV et al [65]:

II

**viii. Freeze drying** [35],[64]: the method discovered in 1965 that consists of fast freezing

occur. The method produces fine and reactive powder of high purity

( ) ( ) ( ) ( )

4.5 M NO nH O 0.5 M L mH O 3 NH HPO

( ) ( )

( ) ( ) II II II

II II

II II

where *M*II = Ca, Sr, Ba, Cd, Pb and *L* = OH, F, Cl, Br and I.

( )

2 NH 3n 1 H O

3 2

where *M*II = Ca, Sr, Ba, Cd, Pb and *L* = F, Cl and Br.

The reaction schemes for the solid-state synthesis of apatite structured compounds were

3 2 2 2 22 4 4

×+ × + ®

3 2 43 3 4 2 2 2 2

+ ++ (6)

3 4 2 3 22 54 <sup>2</sup> 1.5 M VO 0.5 M L mH O M VO L 0.5m H O + ×® + (7)

×+ ® + +

+ + + ++ + (5)

54 2 2 3 3 2

M PO L 9 NO 2.25 O 6 NH 4.5 4.5n 0.5m H O

3 M NO nH O 2 NH VO M VO 6 NO 1.5 O

of the precursor solution (ensures preservation of maximum chemical homogeneity achieved in the starting solution), sublimation of solvent and final calcination. The main characteristics of the freeze drying method are that the drying is not accompa‐ nied by the coagulation of particles and the shrinkage of particles usually does not

are introduced into the hot zone to the furnace to obtain required product. According to applied conditions in different stages of the thermal cycle (**Fig. 7**) the aerosol will



**Table 1.** Temperature of synthesis (Ts), polymorphic transition (Ttr), decomposition (Td) and melting (Tm) of some apatitestructured compounds [65].

The temperatures of these effects for some apatites are listed in **Table 1**.

The phase transformation and the thermal expansion coefficient of apatite-structured compound with the composition given by the formula **M**5(**XO**4)3**Z**q (M = Ca, Sr, Cd, Ba, Pb) were investigated by CHERNORUKOV et al [66]. Pb-containing apatites are shown to undergo the phase transition involving the reduction in unit-cell symmetry from hexagonal to monoclin‐ ic. The thermal expansion anisotropy in the hexagonal phases increases in the order:

$$\mathbf{Ca} \lhd \mathbf{Sr} \lhd \mathbf{Ba} \lhd \mathbf{Pb} \lhd \mathbf{Cd},$$

and the monoclinic phases are less anisotropic but have larger thermal expansion coeffi‐ cients in comparison with the hexagonal phases.
