**5.3. The atom and nanovoid as two entities of a crystal**

The destruction of the crystal does not take place in the spectral experiment. Assume ε is the absortion photon energy, εpr is the predissociation energy and Dsubl is the energy of crystal dissociation. If that is true, then the function

$$\varepsilon = \text{f}(\mathbf{D}\_{\text{sub} \mathbb{V}} \ \mathbf{Z}\_{\text{\prime}} \varepsilon\_{\text{pr}})\_{\text{\prime}}$$

describes the interaction of radiation with the crystal where the predissociation states may be realized (see §4 in this chapter). Value Z in this function makes it possible supposing that the nanovoids between atoms of Z formula units are providing the space necessary for predissociation of particles belonging to the crystal cell. Let's show a role of nanovoids in predissociation of particles using an example of crystals with three diferent structures.

**1.** As an example, we consider the cell of periclase, Mg4O4, while *Z*=4. The cell can increase its potential energy through the resonance absorption of a photon with energy ε. Then, according to Fig. 7b we can write

$$(\mathbf{M} \mathbf{g}\_4 \mathbf{O}\_4)^0 = (\mathbf{M} \mathbf{g}\_4 \mathbf{O}\_4)^\* + \varepsilon\_{\nu}$$

where (Mg4O4) 0 is the basic state of the cell and (Mg4O4)\* is an excited state.

The anions of oxygen form cubic close packing [65]. There are octahedral and tetrahedral voids. All the octahedral voids are filled in with cations; all the tetrahedral voids are empty. If four tetrahedral voids are temporarily filled in due to photon excitement, the part of the solid (4MgO) will lose the particles' placement symmetry, and we will write

$$2\text{ Mg}\_4\text{O}\_4\text{(crystal)} = 4\text{ MgO}\text{ (non-crystal)} + \text{Z}\Delta H / N\_{\text{A}^+} $$

Comparing those states one can acknowledge that if the states

When the laser radiation power is less than 109

place, i.e. the value εkin=0. Assume also that

132 Solar Cells - New Approaches and Reviews

may be realized.

where (Mg4O4)

a transition

to Fig. 7 we can write

0

W/m2

capacities, for example, in the spectral experiment, the destruction of the crystal does not take

ε= f(Dsubl, Z,εpr),

describes the interaction of radiation with the solid in regards to which predissociation states

As an example, we consider the cell of periclase (Mg4O4), while *Z*=4. The cell can increase its potential energy through the resonance absorption of a photon with energy ε. Then, according

e

Mg O crystal = 4 MgO quasi-gas + Z H /N , 4 4 () ( ) subl A D (20)

(crystal)

is the basic state of the cell and (Mg4O4)\* is an excited state. We can write

( )( ) <sup>0</sup> Mg O = Mg O \* + , 4 4 4 4

(Mg4O4)0and Mg4O4

are energetically identical, the predissociation of the crystal cell in Fig. 7 may be presented as

(Mg4O4)\*→4 MgO (quasi-gas)

Shown below are the particles filling the crystallographic cells; this process is accompanied by the formation of nanovoids in the space of an ideal crystal. Filling in these nanovoids can damage the cells' symmetry. If the particles can temporarily move into these nanovoids after photon absorption, we have a prerequisite for the particles' predissociation as a stage of the

The destruction of the crystal does not take place in the spectral experiment. Assume ε is the absortion photon energy, εpr is the predissociation energy and Dsubl is the energy of crystal

antenna process without any damage to the symmetry of the crystal as a whole.

Comparing Eq. (19) and (20) one can acknowledge that if the states

between energetically identical states if ε=ZΔ*H*subl/*N*A.

**5.3. The atom and nanovoid as two entities of a crystal**

dissociation. If that is true, then the function

, a crystal melts. At still lower radiation

(19)

$$(\mathrm{Mg\_4O\_4})^0 \text{and } \mathrm{Mg\_4O\_4} \text{(crystal)}$$

are energetically identical, the predissociation of the crystal cell in Fig. 7b may be presented as a transition of the particle into the nanovoid with the following dispersion in the crystal

$$(\mathrm{Mg\_4O\_4})^\* \rightharpoonup 4 \,\mathrm{MgO (non-crystal)} \rightharpoonup 4 \,\mathrm{MgO (quasi-gas)}$$

between energetically identical states if ε=4Δ*H*subl/*N*A. Thus, in Mg4O<sup>4</sup> cell of magnium oxide crystal there are 2Z=8 tetrahedral voids providing their space to 4 Mg atoms at the moment of pre-dissociation of Mg-O chemical bonds in spectral experiment.

**2.** As an example we consider the cell of sphalerite, Zn4S4, while *Z=*4. The cell can increase its potential energy through the resonance absorption of a photon with energy ε. Then, according to Fig. 7b we can write

$$\left(\mathrm{Zn}\_4\mathrm{S}\_4\right)^0 = \left(\mathrm{Zn}\_4\mathrm{S}\_4\right)^\* + \varepsilon,\tag{21}$$

where (Zn4S4) 0 is the basic state of the cell and (Zn4S4)\* is an excited state. Sulfur anions form cubic close packing with tetrahedral voids [65]. Half of them are filled in with zinc cations, while the second half is empty. When four tetrahedral voids are temporarily filled in due to photon excitement, the part of the solid (4ZnS) will lose the particles' placement symmetry, and we will write

$$\text{Zn}\_4\text{S}\_4\text{(sphalerite)} = 4\text{ ZnS}\text{ (non-crystal)} + 4\Delta H/N\_{\Lambda'} \tag{22}$$

Comparing states (21) and (22), one can acknowledge that if the states

$$(\text{Zn}\_4\text{S}\_4)^0 \text{and } \text{Zn}\_4\text{S}\_4 \text{(crystal)}$$

are energetically identical, the predissociation of the crystal cell in Fig. 7b may be presented as a transition

$$\text{(Zn}\_4\text{S}\_4\text{)\*}-4\text{ ZnS (non-crystal)}-4\text{ ZnS (quasi-gas)}$$

between energetically identical states if ε=4Δ*H*subl/*N*A. So, in the cell Zn4S4 of sphalerite there are 2Z=8 tetrahedral voids, half of which is populated by 4 zinc atoms. Remaining 4 voids can provide their space to Zn atoms at the moment of pre-dissociation of no more than 4 chemical bonds Zn-S in spectral experiment.

**3.** Following this example, we consider the cell of wurtzite, Zn2S2, while *Z=*2. The cell can increase its potential energy through the resonance absorption of a photon with energy ε. Then, according to Fig. 7b we can write

$$(\mathbf{Z}\mathbf{n}\_2\mathbf{S}\_2)^{0\_{\mathrm{im}}} (\mathbf{Z}\mathbf{n}\_2\mathbf{S}\_2)^\* + \varepsilon\_{\omega\_{\mathrm{eq}}}$$

where (Zn2S2) 0 is the basic state of the cell and (Zn2S2)\* is an excited state.

Sulfur anions form cubic close packing with tetrahedral voids [65]. Half of them are filled in with zinc cations, while the second half is empty. When four tetrahedral voids are temporarily filled in due to photon excitement, the part of the solid (4ZnS) will lose the particles' placement symmetry, and we will write

$$\text{(Zn}\_2\text{S}\_2\text{)\*-} \text{-2 ZnS (non-crystal)} \rightarrow \text{2 ZnS (quasi-gas)}$$

between energetically identical states if ε=2Δ*H*subl/*N*A. Thus, in Zn2S2 of wurtzite there 2Z=4 tetrahedral voids half of which is populated by 2 atoms of zinc. Remaining 2 nanovoids can provide their space to Zn atoms at the moment of pre-dissociation of no more than 2 chemical bonds Zn-S in spectral experiment.

These examples of crystal-chemical description of spectral experiment allow explaining that the atom transition from one terahedral void to another is the principal essence of crystal predissociation under photons' action. Suppose that among absorbed photons there are photons which are satisfying the condition ε=εpr. Then the original function ε=f (Dsubl, Z, εpr) may be presented as follows:

$$1/\lambda \text{= constZ} \,\Delta H\_{\text{subl}}$$

that relates the wavelength λ of the ultraviolet radiation absorbed by the crystal to its subli‐ mation and dissociation enthalpies Δ*H* [10, 43].
