Kinetics and mechanism of free-surface vaporization of zinc, cadmium and mercury oxides analyzed by the third-law method

Article abstract of DOI:10.1016/j.tca.2003.08.024

On the basis of critical comparison of experimental and theoretical values of the E parameter and investigation of the retardation effect of oxygen on the evaporation rate of ZnO, CdO and HgO, it was concluded that the dissociative evaporation of ZnO and HgO proceeds with releasing of atomic oxygen (O) as a primary product of decomposition. By contrast, the mechanism of dissociative evaporation of CdO corresponds to the equilibrium reaction with releasing of molecular oxygen (O₂) as a primary product of decomposition. As was shown, this difference in mechanisms is not related with interatomic O-O distances in these oxides. From the analysis of crystal structure for 12 different oxides, which evaporate with releasing of atomic oxygen, and for 13 compounds, which evaporate with releasing of molecular oxygen, it was revealed that the first mechanism is observed for all oxides with the cubic crystal structure. It was proposed that a decisive role in this difference belongs to a local symmetry in the position of O atoms.

Full text of DOI:10.1016/j.tca.2003.08.024

Thermochimica Acta 411 (2004) 187–193
Kinetics and mechanism of free-surface vaporization of zinc, cadmium
and mercury oxides analyzed by the third-law method
Boris V. L’vova,∗, Valery L. Ugolkovb, Fedor F. Grekovc
a
Department of Analytical Chemistry, St. Petersburg State Polytechnic University, St. Petersburg 195251, Russia
Laboratory of Thermodynamics and Kinetics, Institute of Silicate Chemistry, St. Petersburg 199155, Russia
b
c
Department of Electronics Materials Technology, St. Petersburg State Polytechnic University, St. Petersburg 195251, Russia
Received 15 August 2003; accepted 18 August 2003
Abstract
On the basis of critical comparison of experimental and theoretical values of the E parameter and investigation of the retardation effect
of oxygen on the evaporation rate of ZnO, CdO and HgO, it was concluded that the dissociative evaporation of ZnO and HgO proceeds
with releasing of atomic oxygen (O) as a primary product of decomposition. By contrast, the mechanism of dissociative evaporation of CdO
corresponds to the equilibrium reaction with releasing of molecular oxygen (O2) as a primary product of decomposition. As was shown, this
difference in mechanisms is not related with interatomic O–O distances in these oxides. From the analysis of crystal structure for 12 different
oxides, which evaporate with releasing of atomic oxygen, and for 13 compounds, which evaporate with releasing of molecular oxygen, it
was revealed that the first mechanism is observed for all oxides with the cubic crystal structure. It was proposed that a decisive role in this
difference belongs to a local symmetry in the position of O atoms.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Decomposition mechanism; ZnO evaporation kinetics; CdO evaporation kinetics; HgO evaporation kinetics; Retardation effect of O2; Effect of
crystal structure
1. Introduction
oration in MoO3 (from pale yellow to blue) in the process of
ZnO, HgO, CuO and PtO2 decomposition in vacuum. The
reason of this difference remains an enigma.
Kinetics and thermodynamics of vaporization of ZnO,
CdO and HgO has been discussed recently in two funda-
mental books [1,2]. Nevertheless, many questions remain to
be answered. These three oxides for IIB Group of metals
evaporate with formation of only gaseous products and, at
first sight, the interpretation of the mechanism of their va-
porization creates no problems. However, in fact, this is not
the case. In spite of rather high number of works devoted to
the investigation of evaporation kinetics of these oxides, the
E parameters of the Arrhenius equation reported in [3–20]
vary in the range of factor 2.4 for ZnO and 1.5 for HgO
(Table 1). In contrast to the reversible dissociation of solid
CdO up to Cd atoms and molecular oxygen (O2), the decom-
position of ZnO and HgO yields atomic oxygen (O). This
was proved more than 40 years ago by Harano [4] by col-
The purposes of this work included: thermogravimetric
determination of the E parameters for ZnO and HgO vapor-
ization by the third-law method; theoretical calculation of
the enthalpies for ZnO, CdO and HgO dissociative evapora-
tion; investigation of the retardation effect of oxygen on the
evaporation rate of ZnO, CdO and HgO oxides and, what is
the most important, analysis of the possible effect of crystal
structure of these and some other oxides on the mechanism
of their evaporation (up to molecular or atomic oxygen).
2. Theoretical
2.1. Theoretical values of the E parameter
∗
In the case of a solid compound S decomposed in vacuum
into gaseous products A and B, that is
Corresponding author. Present address: Politechnicheskaya ul.17/3
apt 123, St. Petersburg 194021, Russia. Tel.: +7-812-552-7741; fax:
+7-812-247-4384.
E-mail address: borislvov@rambler.ru (B.V. L’vov).
S (s) → aA (g) + bB (g)
(1)
0040-6031/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2003.08.024

188
B.V. L’vov et al. / Thermochimica Acta 411 (2004) 187–193
Table 1
The literature data on evaporation kinetics of ZnO, CdO and HgO
Oxide
ZnO
Sample
Powder
Crystal
Crystal face
Atmosphere
Vacuum
T (K)
Point number
24
E (kJ mol−1
)
Ref.
[3]
1133–1273
180
21
Zn-reach
O-reach
Vacuum
Vacuum
1389–1531
1423–1555
20
15
375.4
392.7
13
21
[7]
Film
Crystal
Crystal
Vacuum
1215–1385
1236–1361
1238–1341
6
10
12
318.0
326.7
328.4
[9]
Zn-reach
O-reach
Spinela
Pellet
Monolayer
Crystal
Vacuum
Vacuum
1608–1773
1300–1800
1100–1200
1400
6
355.6
312.0
428
364b
[10]
[11]
[12]
[13]
10
1
20
CdO
HgO
Powder
Monolayer
N2 (1 atm)
Ar (1 atm)
1150–1374
650–800
17
10
240
233
7
20
[14]
[12]
Powder
Powder
Powder
Powder
Powder
Air
733–763
652–730
701–783
643–834
673–773
298
4
240.6
162.3–201.3
216.5
195
170
200.8c
[15]
[16]
[17]
[18]
[19]
[20]
Vacuum
Vacuum
Vacuum
Vacuum
9
60
7
5
8
a
(Zn0.2Co0.8)O·Al2O3.
b
c
Calculated by the third-law method from the data reported by Galuzzo and Searcy [7].
Theoretical value calculated from thermodynamic data.
the E parameter for reaction (1) is equal to:
received recently [13] strong experimental confirmation.)
Taking into account Eqs. (2) and (5), Eq. (4) can be reduced
to the equation:
ꢀrH◦T
E =
(2)
ν
ꢀ
ꢁ
ꢀrS◦T
ν
Here, ν is the total number of moles of gaseous products,
(a + b), and ꢀrH◦T is the change of the enthalpy in reaction
(1). The latter value is calculated by the obvious equation:
E = T
− R ln Peq
(6)
In this case, a measurement at only one temperature is
sufficient for the determination of E but knowledge of abso-
lute values of the entropy of all components in the reaction
is necessary for the calculation. Tables 2–4 contain these
values for all the oxides under investigation.
ꢀrH◦T = ꢀfHT◦ (A) + bꢀfHT◦ (B) − ꢀfH◦T (S)
(3)
The magnitudes of thermodynamic functions (the entropy
and enthalpy) for the components of decomposition reac-
tions and for the reactions as a whole (marked by bold) at
different temperatures are listed in Tables 2–4. A possible
error (S.D.) in these calculations of ꢀrH◦T values is within
Table 2
1–2 kJ mol−1
.
Thermodynamic functions for ZnO decomposition [21,22]
Functiona
T (K)
2.2. The third-law method for the experimental
determination of the E parameter
1200
1300
ST◦
Zng
ZnOs
O2
189.8
191.5
The third-law method is based on the direct application
of the basic equation of chemical thermodynamics:
111.7
249.9
190.5
116.1
252.8
192.2
O
ꢀrH◦T = T(ꢀrS◦T − R ln KP )
(4)
ꢀrS◦T
268.6
1
1
267.6
1
1
where ꢀrS◦T is the entropy change and KP is the equilibrium
constant for the reaction (1) in terms of the partial pressures,
P, of the gaseous products:
ꢀf H◦T
Zng
ZnOs
O2
149.3
−305.6
38.4
151.4
−300.1
42.0
KP = PAa × PBb
(5)
O
272.5
274.6
ꢀrH◦T
727.4
726.1
(The equilibrium character of decomposition reactions
that is at the basis of the physical approach to the interpre-
tation of the kinetics of thermal decomposition of solids has
All S◦T values are in J mol−1 K−1 and allꢀH◦T values are in kJ mol−1
.
a
The uncertainties indicated here and in the text correspond to the S.D.

B.V. L’vov et al. / Thermochimica Acta 411 (2004) 187–193
189
Table 3
ported by the results of theoretical analysis [13] and by the
results of application of the second- and third-law methods
to the determination of the E parameter for 22 different de-
composition reactions [13,23–25].
As can be seen from a consideration of Eq. (6), the self-
cooling effect results in the overestimation of the E val-
ues calculated by the third-law method. Therefore, it is
preferable to use for the calculation the result(s) of low-
temperature measurement when this effect is minimal.
Thermodynamic functionsa for CdO decomposition at 1300 K [21,22]
ST◦
Cdg
CdOs
O2
198.2
131.1
252.8
192.2
O
ꢀrS◦T
193.5
1
2
ꢀf H◦T
Cdg
CdOs
O2
O
132.6
−204.8
42.0
2.3. The retardation effect of excess of gaseous
product on the evaporation
274.6
ꢀrH◦T
358.4
As it follows from Eqs. (5) and (7), in case of dissociative
evaporation of oxide in accord with the reaction:
All S◦T values are in J mol−1 K−1 and all ꢀH◦T values are in kJ mol−1
.
a
The uncertainties indicated here and in the text correspond to the S.D.
MO (s) → M (g) + 0.5O2
(8)
at two different external partial pressures of oxygen, P1(O2)
and P2(O2), the evaporation rates should be related as
The equivalent pressure of the gaseous product is re-
lated to the absolute rate of decomposition in vacuum, J (in
kg m−2 s−1), by the Hertz–Langmuir equation [23]:
ꢀ
ꢁ0.5
J1
J2
P2(O2)
P1(O2)
=
(9)
√
√
ꢀ
ꢁ
(2πRT)1/2
J
a MA + b MB
Peq
=
(7)
γ(a+b)
Mr
In case of dissociative evaporation of oxide in accord with
the reaction:
Here, MA, MB and Mr are the molar masses of the gaseous
products A and B and reactant S and γ = 101,325 Pa atm−1
is the conversion factor from pascals to atmospheres used
to calculate partial pressures in chemical thermodynamics.
The factor (a + b) in denominator and additional factor in
brackets are installed in this equation to take into account
the fact that the J value actually includes the flows of two
gaseous products (A and B) with the different molar masses.
Our analysis of results reported in [2,21] for several tens
of substances has shown that the data calculated by the
third-law method are in general the order of magnitude more
precise than those calculated by the second-law or equiva-
lent Arrhenius-plots method. This conclusion has been sup-
MO(s) → M(g) + O
(10)
the difference in the partial pressures of O2 at temperatures
lower than 1500 K (when the dissociation degree of O2 into
atomic oxygen is negligible) should have no effect on the
evaporation rate, i.e.
J1
= 1
(11)
J2
Therefore, a comparison of the evaporation rates of oxide
at two different external partial pressures of O2 can be used
as a criterion for choosing the true mechanism of dissociative
evaporation.
Table 4
3. Experimental
Thermodynamic functions for HgO decomposition [21,22]
Functiona
T (K)
The experiments were carried out with a Netzsch STA 429
instrument on the TG and DSC measuring head. The actual
measured quantities were the mass change of the sample per
time unit, ꢀm/ꢀt, and the absolute crucible temperature. An
open alumina crucible 5.7 mm inner diameter and 4.0 mm
high was used as a sample container. The reacting materials
were high purity powders of ZnO, CdO and HgO. A pow-
der sample (20 mg) introduced into a crucible was leveled
and pressed manually (about 1 kg mm−2) into a flat pellet.
The total (outer) surface area of pellet was calculated taking
into account the crucible diameter and the width of pellet
(estimated from the sample mass and the apparent density
of powder measured separately).
600
700
800
ST◦
Hgg
HgOs
O2
189.5
192.5
195.5
105.7
226.3
176.0
114.2
231.4
179.2
121.0
235.8
182.0
O
ꢀrS◦T
259.8
1
1
257.5
1
1
256.5
1
1
ꢀf H◦T
Hgg
HgOs
O2
67.8
−75.5
17.9
69.8
−69.9
21.2
71.9
−64.0
24.5
O
260.0
262.1
264.2
ꢀrH◦T
403.3
401.8
400.1
In experiments on determination of the E parameter, the
sample chamber was evacuated to a residual pressures about
5 × 10−4 or 5 × 10−8 atm with the use, respectively, of only
All S◦T values are in J mol−1 K−1 and all ꢀH◦T values are in kJ mol−1
.
a
The uncertainties indicated here and in the text correspond to the S.D.

190
B.V. L’vov et al. / Thermochimica Acta 411 (2004) 187–193
Table 6
rotation pump or rotation and oil and diffusion pumps. All
measurements have been conducted at continuous pumping
under isothermal conditions. The heating rate of the sample
from the room temperature to intermediate one (20 K lower
than the desired temperature) was 20 K min−1 and from in-
termediate to the desired temperature was 2 K min−1. At the
beginning of each measuring cycle, the system was heated
at the temperature chosen, usually during 10 min, to reach a
constant rate of the decomposition. The changes of the mass
and surface area of crystals and powders during this period
were taken into account. A decrease of the surface area, as
was checked experimentally, was proportional to (1 − α)2/3
where α is the decomposition degree. (This dependence can
be interpreted as a combined result of the reduction of num-
ber and size of particles in the process of decomposition.)
Temperature was measured with Pt–Pt10%Rh thermocou-
ple placed with its junction immediately below the crucible.
Temperature variations in the process of mass and change
measurements (usually, during 20–30 min) did not exceed
0.2 K. A single measurement of the decomposition rate
took entirely about 2–3 h.
The absolute value of the decomposition rate for powder
samples was estimated using the method proposed in our pre-
vious work [24]. It consists in the evaluation of the absolute
decomposition rate of a powder sample (reduced to the unit
of the outer surface area of a pellet formed by the powder
sample in a cylindrical crucible). The value received is low-
ered by the correction (empirical) factor and then used for
the calculation of the E parameter by the third-law method.
The value of this factor (2.8 0.4), as was shown in [23,25],
does not depend on the temperature, residual pressure of gas
in the reactor, grain size and mass of a powder sample.
The experiments on the retardation effect of oxygen on the
decomposition of Cd and Hg oxides were carried out under
constant volume conditions in two different gases: nitrogen
and air at atmospheric pressure. In this case, only relative
losses of mass were measured under isothermal conditions.
In case of ZnO evaporation, the similar experiments were
performed in vacuum, at two different pressures of residual
air in the furnace: 5×10−4 and 5×10−8 atm (or about 10−4
and 10−8 atm of oxygen).
Kinetics of isothermal decomposition of oxides in atmosphere of nitrogena
and air
Oxide
T (K)
ꢀm/ꢀtb (␮g s−1
)
Ratio (air/N2)
Experiment
Nitrogen
Air
Theory
CdO
1302
1302
1254
1
1
1
1.3
1.8
0.53
0.16
0.20
0.053
0.12
0.11
0.10
0.022
0.022
0.022
HgO
709
709
1
1
1.20
1.18
1.14
1.17
0.95
0.99
1.00
1.00
a
Pure nitrogen was introduced after evacuation of the furnace chamber
up to Pair = 5 × 10−4 atm or PO = 1 × 10−4 atm.
2
b
Averaged value for 40 min in all cases.
4. Results and discussion
4.1. Mechanisms of ZnO, CdO and HgO evaporation
The experimental conditions and results of determination
of the E parameter for ZnO and HgO in vacuum by the
third-law method are presented in Table 5. Table 6 contains
the results of comparative experiments on the retardation
effect of O2 on the evaporation rate of CdO and HgO.
The following conclusions can be deduced from the anal-
ysis of these data:
(i) The complete independence of the evaporation rate for
ZnO on the residual partial pressure of air in the furnace
(ca. 10−4 and 10−8 atm of O2) and for HgO in air and
nitrogen (0.21 and 10−4 atm of O2) supports the mech-
anism of evaporation described by reaction (10). In case
of reaction (8), the expected difference in the evapo-
ration rates should reach, as it follows from Eq. (9), a
factor of 100 and 46 for ZnO and HgO, respectively.
(ii) The strong retardation effect of oxygen on the evapo-
ration rate of CdO (Table 6) proves the mechanism of
its evaporation described by reaction (8). Five-fold dis-
crepancy between experimental and theoretical values
of the retardation effect can be related with the higher
partial pressure of O2 in nitrogen (as compared with
10−4 atm used in calculations) because of desorption of
Table 5
Experimental conditions and results of determination of the E parameter for oxide decomposition in vacuum by the third-law method
ꢀrS◦T /ν
(J mol−1 K−1
)
E (kJ mol−1
)
a
b
c
Oxide
Pair (atm)
T (K)
m0
(mg)
S0
(mm2)
αm
Sm
ꢀm/ꢀtc
Jcorr
(kg m−2 s−1
Peq (atm)
(mm2)
(␮g s−1
)
)
ZnO
5 × 10−8
5 × 10−8
5 × 10−8
5 × 10−4
5 × 10−4
1255.3
1255.5
1255.4
1257.6
1257.2
20.0
20.0
20.0
20.0
20.0
57.30
57.30
57.30
57.30
57.30
0.0182
0.0047
0.0045
0.0009
0.0018
56.60
57.12
57.13
57.27
57.23
0.220
0.073
0.100
0.052
0.061
1.39 × 10−6
4.56 × 10−7
6.25 × 10−7
3.30 × 10−7
3.80 × 10−7
8.25 × 10−9
2.71 × 10−9
3.71 × 10−9
1.96 × 10−9
2.25 × 10−9
134.1
134.1
134.1
134.1
134.1
362.6
374.3
371.0
378.3
376.7
HgO
5 × 10−4
681.0
711.0
40.0
40.0
56.41
56.41
0.0551
0.2119
53.37
48.13
1.82
6.28
1.22 × 10−5
3.01 × 10−8
129.0
128.7
185.9
185.8
5 × 10−4
4.66 × 10−5
1.18 × 10−7
a
The decomposition degree by the time of measurement.
The surface area by the time of measurement calculated by the equation Sm = S0(1 − αm)2/3
b
c
.
Jcor = (ꢀm/ꢀt)/(2.8 Sm).

B.V. L’vov et al. / Thermochimica Acta 411 (2004) 187–193
191
Table 7
The ultimate theoretical and experimental values of the E parameter
Decomposition reaction
T (K)
E (kJ mol−1
)
Refs.
Theory
Experiment
ZnO (s) → Zn (g) + O
CdO (s) → Cd (g) + 0.5O2
HgO (s) → Hg (g) + O
1256
1300
700
363
239
201
1
1
1
373
240
186
6
7
1
This work
[14]
This work
oxygen from the furnace wall in the process of 40 min
heating in N2 atmosphere under constant volume con-
ditions.
its successive decomposition was discussed in detail
by L’vov in [20] with illustration of this peculiarity
by the corresponding Arrhenius plot (taken from the
literature).
(iii) As can be seen from Table 7, the averaged values of
the E parameter for ZnO and HgO found in this work
and the E parameter for CdO measured by Gilbert and
Kitchener [14] are in good agreement with the theo-
retical values of ꢀrH◦T /ν listed in Tables 2 and 4 for
reaction (10) in cases of ZnO and HgO and for reac-
tion (8) in case of CdO. A small underestimation (8%)
of the E parameter in case of HgO evaporation can be
explained by interference from the secondary decom-
position reaction of lower oxide:
(iv) Our experimental results for ZnO and HgO (Table 7) are
in agreement with the literature data listed in Table 1.
If to omit the extreme results reported in [3,12] and in
[15,19], the averaged values (344 30 kJ mol−1 (n =
7) for ZnO and 189 22 kJ mol−1 (n = 5) for HgO)
become rather close to our data. The value of E =
364 kJ mol−1 for ZnO, which was calculated in [13] by
the third-law method from the data reported by Galuzzo
and Searcy [7], practically coincides with the theoreti-
cally predicted value (363 1 kJ mol−1).
Hg2O (s) → 2Hg (g) + O
(12)
The above results make it possible to conclude that the
mechanism of dissociative evaporation of ZnO and HgO
corresponds to reaction (10) and atomic oxygen (O) is the
primary product of their decomposition. By contrast, the
mechanism of dissociative evaporation of CdO corresponds
which, in its turn, forms at low temperatures (<500 K)
as a result of oxidation of free atoms of Hg by atomic
oxygen in the interface between the two solid phases
(HgO/Hg2O). The interference of Hg2O formation and
Table 8
Crystal structure and evaporation mechanism for some oxides and sulfates
Oxide
Singonya
Space group
Minimum O–O distance (Å)
Primary product
Refs.
Li2O
Cu2O
Ag2O
MgO
CaO
SrO
CdO
MnO
FeO
I
I
I
I
I
I
I
I
I
I
I
II
225
224
224
225
225
225
225
225
225
225
225
129
2.31
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
[12]
[28]
[4,29]
[12]
[12]
[12]
[12,14]
[12]
[28]
[28]
3.68, 4.25
4.09, 4.72
2.98
3.39
3.63
3.34
3.14
3.06
3.01
CoO
NiO
PbO
2.95
1.98
[28]
[12]
CaO2
SrO2
II
II
II
II
II
II
III
IIIa
III
IV
IV
IV
V
139
139
139
136
136
135
182
164
186
62
4.62
3.55
5.12
2.86
3.19
3.28
2.91
2.74
2.60
3.50
2.47
2.44
2.62
O
O
O
O
O
O
O
O
O
O
O
O
O
[30]
[30]
[30]
[31]
[31]
[31]
[31]
[4]
[4,31]
[4,20]
[23]
[23]
[4]
BaO2
GeO2
SnO2
Pb3O4
SiO2
PtO2
ZnO
HgO
MgSO4
BaSO4
CuO
63
62
15
a
I: cubic, II: tetragonal, III: hexagonal, IIIa: trigonal, IV: rhombic and V: monoclinic.

192
B.V. L’vov et al. / Thermochimica Acta 411 (2004) 187–193
to equilibrium reaction (8) and the molecular oxygen (O2)
is the primary product of its decomposition.
For all other compounds of different (from cubic) singony
(II, III, IIIa, IV or V), the release of oxygen occurs in the
form of free O atoms. As for PbO, the release of molecu-
lar oxygen can be related with the anomalously small O–O
distance (1.98 Å) in comparison with that for other oxides.
We are not ready now to propose any quantitative expla-
nation of this phenomenon. The only obvious conclusion
consists in correlation of these differences with the struc-
ture symmetry. It can be proposed that a decisive role here
belongs to a local symmetry in the position of O atoms.
Fragments of structure, presented in Fig. 1, illustrate the
differences in symmetry of O atoms in crystal structure of
Cd, Zn and Hg oxides. For those oxides, where this sym-
metry is highest and environment is close to isotropic, there
is the molecular mechanism of dissociation. Oxygen atoms,
which are in low – should be low – symmetrical positions,
release their sites without recombination. It is probable
that there are some differences in electronic structure of
these atoms responsible for the mechanism of recombina-
tion. To conclude, this interesting problem calls for further
experimental and theoretical investigation.
4.2. Effect of crystal structure on the evaporation
mechanism
It seems, the most important problem now is to explain
this difference in mechanisms of evaporation. Our first at-
tempt was to relate this difference with O–O distances in
corresponding crystals. To estimate these values, we used
the PowderCell Program developed by Kraus and Nolze [26]
and Tables [27]. The minimum O–O distances found with
the help of this program for ZnO, CdO and HgO were as
follows: 2.60, 3.34 and 3.50 Å. In all cases, these values are
much higher than the internuclear distance in O2 molecule
(1.21 Å). However, this is not retarding the release of O2
molecules in the process of CdO evaporation. It means that
some other factors are responsible for the difference in mech-
anisms.
To investigate the situation in more detail, we collected all
available data on the evaporation mechanisms of 23 differ-
ent oxides (and two sulfates) and correlated them with their
crystal structure (Table 8). When all these compounds were
arranged into two groups differed in the releasing mecha-
nism of oxygen, some remarkable differences in their crys-
tal structure have been appeared. As can be seen from these
data, all oxides, which evaporate with the release of molec-
ular oxygen, except for PbO, are of the cubic singony (I).
5. Conclusions
The mechanism of evaporation of ZnO and HgO with
releasing of atomic oxygen (O) as a primary product of
decomposition and the mechanism of evaporation of CdO
with releasing of molecular oxygen (O2) was proved by
good agreement of experimental values of the E parameters
with theoretical values of the enthalpies for corresponding
reactions and also, by results of investigation of the retar-
dation effect of oxygen on the evaporation rate of ZnO,
CdO and HgO. From the analysis of crystal structure for 12
different oxides, which evaporate with releasing of atomic
oxygen, and for 13 compounds, which evaporate with re-
leasing of molecular oxygen, it was revealed that (i) the
first mechanism is observed for all oxides with the cubic
crystal structure and (ii) the difference in mechanisms is
not related with interatomic O–O distances in these oxides.
It was proposed that a decisive role here belongs to a local
symmetry in the position of O atoms. These conclusions de-
serve further investigation and application to decomposition
studies for other compounds (sulfides, nitrides, etc).
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