5
ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 8, pp. 887–891. © Pleiades Publishing, Inc., 2006. Original Russian Text © Zh.V. Dobrokhotova, L.P. Ogorodova, E.V. Makhonina, V.S. Pervov, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 8, pp. 980–984. 887 INTRODUCTION Theoretical analysis of the nature of eutectic alloys is of key importance in materials research. One of the latest concepts in the theory of eutectic alloys is that of supramolecule [1]. Recent experimental data [2–4] lend support to the assumption that there are no signif- icant distinctions between eutectic nonautonomous phases and structures meeting the definition of supramolecular ensembles [5]. Our earlier results on binary oxide systems provide convincing evidence of self-organization in eutectic alloys, which leads to the formation of metastable non- autonomous phases similar to supramolecular ensem- bles, in which self-organization processes depend on the crystallization conditions and interaction between their components [6]. The purpose of this work was to gain experimental data supporting this view. We aimed at demonstrating that eutectic alloys, commonly thought of as heterogeneous systems (i.e., containing only autonomous phases), may contain ordered microregions (nonautonomous phases) resulting from the self-organization of incommensurate substructures. EXPERIMENTAL High-temperature solution calorimetry, the main method in this study, is the only tool capable of directly determining the enthalpy of formation of metastable and, in the general case, any nonequilibrium phases. High-temperature heat-flow calorimetry consists in measuring the thermal power as a function of time. In our experiments, we used a Tian–Calvet calorim- eter. Such instruments measure the heat flux from the cell to the jacket using evenly distributed junctions of a thermopile. The heat flux is evaluated from the emf across the thermopile. Measuring this emf, one can determine the heat released or absorbed as a result of the processes in the calorimetric cell [7]. The method consists in successively determining the heat effects of solution of the phases of interest and their constituent components in a melt and calculating the enthalpies of formation of the phases from the data thus obtained. High-temperature solution calorimetry was success- fully used earlier in unique studies aimed at determin- ing the enthalpy of formation of amorphous phases in the Zr–Cu, Zr–Ni, Al–Y, Al–Y–Ni, Zr–Cu–Al, and Zr– Ni–Al systems; ternary intermetallics in the Al–Y–Ni system; and ternary solid solutions and an icosahedral quasi-crystalline phase in the Fe–Al–Cu system [8–15]. As solvents, one can use a variety of melts. The melt composition best suited to silicate, germanate, spinel, and oxide systems is 2PbO · B 2 O 3 [7]. In our studies, the samples were dropped into the melt. This method has a number of advantages over “direct” dissolution and can be used to study phases unstable at high temperatures. The samples were ther- mostated at room temperature and then dropped into the melt. As a result, the sample temperature increased from 298 K to the dissolution temperature, and the sam- ple dissolved in the melt. In this way, we determined a sum thermodynamic function which included the enthalpy increment at the measurement temperature and the enthalpy of solution of the sample: (H 0 (í) – H 0 (298 K)) + sol H 0 (í). As in an earlier study [6], we examined binary oxide systems based on the orthorhombic phase β-PbO. The Mixing Enthalpies of Oxides in the PbO–Y 2 O 3 and PbO–MoO 3 Systems Zh. V. Dobrokhotova a , L. P. Ogorodova b , E. V. Makhonina a , and V. S. Pervov c a Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia b Moscow State University, Vorob’evy gory 1, Moscow, 119899 Russia c Moscow State University of Environmental Engineering, Staraya Basmannaya ul. 21/4, Moscow, 107866 Russia e-mail: [email protected] Received December 7, 2005 Abstract—The enthalpies of mixing of oxides in the binary systems PbO–Y 2 O 3 and PbO–MoO 3 have been determined for near-eutectic compositions by solution calorimetry. The results, combined with earlier solution calorimetry data for these systems, provide clear evidence for self-organization of the interacting substructures in the melt (formation of nonautonomous phases similar to supramolecular ensembles). DOI: 10.1134/S0020168506080164

Mixing enthalpies of oxides in the PbO-Y2O3 and PbO-MoO3 systems

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ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 8, pp. 887–891. © Pleiades Publishing, Inc., 2006.Original Russian Text © Zh.V. Dobrokhotova, L.P. Ogorodova, E.V. Makhonina, V.S. Pervov, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 8, pp. 980–984.

887

INTRODUCTION

Theoretical analysis of the nature of eutectic alloysis of key importance in materials research. One of thelatest concepts in the theory of eutectic alloys is that ofsupramolecule [1]. Recent experimental data [2–4]lend support to the assumption that there are no signif-icant distinctions between eutectic nonautonomousphases and structures meeting the definition ofsupramolecular ensembles [5].

Our earlier results on binary oxide systems provideconvincing evidence of self-organization in eutecticalloys, which leads to the formation of metastable non-autonomous phases similar to supramolecular ensem-bles, in which self-organization processes depend onthe crystallization conditions and interaction betweentheir components [6]. The purpose of this work was togain experimental data supporting this view. We aimedat demonstrating that eutectic alloys, commonlythought of as heterogeneous systems (i.e., containingonly autonomous phases), may contain orderedmicroregions (nonautonomous phases) resulting fromthe self-organization of incommensurate substructures.

EXPERIMENTAL

High-temperature solution calorimetry, the mainmethod in this study, is the only tool capable of directlydetermining the enthalpy of formation of metastableand, in the general case, any nonequilibrium phases.High-temperature heat-flow calorimetry consists inmeasuring the thermal power as a function of time.

In our experiments, we used a Tian–Calvet calorim-eter. Such instruments measure the heat flux from thecell to the jacket using evenly distributed junctions of a

thermopile. The heat flux is evaluated from the emfacross the thermopile. Measuring this emf, one candetermine the heat released or absorbed as a result ofthe processes in the calorimetric cell [7].

The method consists in successively determiningthe heat effects of solution of the phases of interest andtheir constituent components in a melt and calculatingthe enthalpies of formation of the phases from the datathus obtained.

High-temperature solution calorimetry was success-fully used earlier in unique studies aimed at determin-ing the enthalpy of formation of amorphous phases inthe Zr–Cu, Zr–Ni, Al–Y, Al–Y–Ni, Zr–Cu–Al, and Zr–Ni–Al systems; ternary intermetallics in the Al–Y–Nisystem; and ternary solid solutions and an icosahedralquasi-crystalline phase in the Fe–Al–Cu system [8–15].As solvents, one can use a variety of melts. The meltcomposition best suited to silicate, germanate, spinel,and oxide systems is 2PbO · B

2

O

3

[7].In our studies, the samples were dropped into the

melt. This method has a number of advantages over“direct” dissolution and can be used to study phasesunstable at high temperatures. The samples were ther-mostated at room temperature and then dropped intothe melt. As a result, the sample temperature increasedfrom 298 K to the dissolution temperature, and the sam-ple dissolved in the melt. In this way, we determined asum thermodynamic function which included theenthalpy increment at the measurement temperatureand the enthalpy of solution of the sample:

(

H

0

(

í

) –

H

0

(298

K

)) +

sol

H

0

(

í

).

As in an earlier study [6], we examined binary oxidesystems based on the orthorhombic phase

β

-PbO. The

Mixing Enthalpies of Oxides in the PbO–Y

2

O

3

and PbO–MoO

3

Systems

Zh. V. Dobrokhotova

a

, L. P. Ogorodova

b

, E. V. Makhonina

a

, and V. S. Pervov

c

a

Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,Leninskii pr. 31, Moscow, 119991 Russia

b

Moscow State University, Vorob’evy gory 1, Moscow, 119899 Russia

c

Moscow State University of Environmental Engineering, Staraya Basmannaya ul. 21/4, Moscow, 107866 Russiae-mail: [email protected]

Received December 7, 2005

Abstract

—The enthalpies of mixing of oxides in the binary systems PbO–Y

2

O

3

and PbO–MoO

3

have beendetermined for near-eutectic compositions by solution calorimetry. The results, combined with earlier solutioncalorimetry data for these systems, provide clear evidence for self-organization of the interacting substructuresin the melt (formation of nonautonomous phases similar to supramolecular ensembles).

DOI:

10.1134/S0020168506080164

Page 2: Mixing enthalpies of oxides in the PbO-Y2O3 and PbO-MoO3 systems

888

INORGANIC MATERIALS

Vol. 42

No. 8

2006

DOBROKHOTOVA et al.

principal requirement of the systems to be studied waseutectic melting in the composition range of this study.

We investigated the systems PbO–Y

2

O

3

[16] in thecomposition range 5–11 mol % Y

2

O

3

and PbO–MoO

3

[17] in the range 50–90 mol % MoO

3

. In these systems,the heat capacity of crystalline eutectic materials pre-

pared by nonequilibrium crystallization exceeds theadditivity-rule value, and heat capacity is a nonmono-tonic function of composition over the entire tempera-ture range studied [6].

Samples for determining the enthalpy of solutionwere prepared by heating a glassy material at a rate of5

°

C/min from –20

°

C to the maximum peak tempera-ture of crystallization of the second phase in the system[6], followed by quenching.

RESULTS AND DISCUSSION

The reliability of solution calorimetry data is sup-ported by the following results.

One of the samples in the PbO–MoO

3

system hadthe composition PbMoO

4

. The enthalpies of formation

f

H

0

of this compound and its constituent oxides areavailable from the IVTANTERMO Database [18]:

They can be used to calculate the enthalpy of formationof lead molybdate from its constituent oxides:

(1)

The enthalpies of solution

sol

H

0

of PbMoO

4

andPbO were measured at 973 K (Tables 1, 2). The data forMoO

3

were reported earlier by Kiseleva et al. [19]:

The enthalpy of formation of PbMoO

4

from its con-stituent oxides [reaction (1)] was calculated as

H

1

=

H

2

+

H

3

H

4

, where

H

1

is the enthalpy of forma-tion of PbMoO

4

from its constituent oxides, and

H

2

,

H

3

, and

H

4

are the enthalpies of solution of PbO,MoO

3

, and PbMoO

4

, respectively. We obtained

H

0

(298

K

) = –80.2

±

7.9

kJ/mol, in agreement withpublished data within the present experimental uncer-tainty.

PbO–Y

2

O

3

system.

This system has a simple eutec-tic at 5 mol % Y

2

O

3

, with no compounds [16]. Themelting point of the eutectic is 1103 K. For two compo-sitions in this system (5 and 10 mol % Y

2

O

3

), we mea-

Compound ∆fH0(298 K), kJ/mol

PbMoO4 –1051.9 ± 1.7

PbO –219.29 ± 0.71

MoO3 –745.17 ± 0.46

PbO + MoO3 = PbMoO4,

∆fH0(298 K) = –87.5 ± 2.3 ‘kJ/mol.

Oxide∆solH

0(973 K) + (H0(973 K) – H0(298 K)),kJ/mol

MoO3 –5.7 ± 1.9 [19]

PbO 57.2 ± 2.4

PbMoO4 131.7 ± 7.3

Table 1.

Experimental data on PbMoO

4

dissolution

Sampleweight, mg

sol

H

0

(973 K) + (H0(973 K) – H0(298 K))

cal/g kJ/mol

15.928 88.023 135.198

15.961 87.921 135.042

8.824 82.756 127.109

14.648 83.453 128.179

13.465 88.312 135.642

9.318 84.000 129.019

Table 2. Experimental data on PbO dissolution

Sampleweight, mg

∆solH0(973 K) + (H0(973 K) – H0(298 K))

cal/g kJ/mol

11.891 60.523 56.520

18.618 61.518 57.450

7.660 63.249 59.066

8.343 60.633 56.623

9.862 63.862 59.639

10.289 59.791 55.837

12.551 61.490 57.423

Table 3. Experimental data on PbO–5 mol % Y2O3 dissolution

Sampleweight, mg

∆solH0(973 K) + (H0(973 K) – H0(298 K))

cal/g kJ/mol

14.200 79.344 74.136

13.586 81.730 76.359

11.963 74.308 69.446

10.479 80.995 75.672

12.587 74.399 69.510

11.083 75.849 70.865

11.324 67.957 63.491

15.736 80.875 75.560

6.492 69.712 65.131

Page 3: Mixing enthalpies of oxides in the PbO-Y2O3 and PbO-MoO3 systems

INORGANIC MATERIALS Vol. 42 No. 8 2006

MIXING ENTHALPIES OF OXIDES IN THE PbO–Y2O3 AND PbO–MoO3 SYSTEMS 889

sured the enthalpy of solution at 973 K using samplesquenched after crystallization (Tables 3, 4):

The results, together with the data on dissolution ofthe constituent oxides (∆solH0(973 K) of PbO was deter-mined above, and that of Y2O3 was taken from [20]),were used to determine ∆H0(298 K) from oxides forPbO–5 mol % Y2O3 and PbO–10 mol % Y2O3: –15.6 ±9.3 and –5.2 ± 7.3 kJ/mol, respectively.

Thus, the enthalpy of formation of the eutectic com-position from the constituent oxides in this system dif-fers from zero by more than the experimental uncer-tainty, whereas in heterogeneous systems this quantitymust be zero, as is the case in PbO–10 mol % Y2O3.

Using the temperature-dependent heat capacity ofthe eutectic composition determined earlier in a limitedtemperature range [6], we calculated the entropy of for-mation of PbO–5 mol % Y2O3 from the constituentoxides. The heat capacity data were extrapolated toabsolute zero using the fitting procedure described pre-viously [21], which makes it possible to calculate theabsolute 298-K entropy from low-temperature Cp(T)data in a limited temperature range. The S0(298 K) ofthe eutectic composition is 64.8 ± 3.6 J/(mol K), and theentropy of formation of PbO–5 mol % Y2O3 from itsconstituent oxides is –5.4 ± 3.9 J/(mol K). The low neg-ative value of the entropy of formation from oxides sug-gests that, under the above (nonequilibrium) crystalli-zation conditions, a new, ordered phase may be formedin the eutectic composition from the constituent oxides.This correlates with the finding, reported earlier [6],that, under appropriate crystallization conditions, anonautonomous phase may be formed in this system asa combination of autonomous phases. Using the Cp(T)data for the eutectic composition and constituent oxidesand the ∆S0(298 K) and ∆H0(298 K) of formation of theeutectic composition from the constituent oxides, weobtained temperature-dependent ∆G0 of this reaction(Fig. 1). It can be seen that, with increasing tempera-ture, the thermodynamic stability of the nonautono-mous phase (ordered microregions) decreases.

PbO–MoO3 system. In this system, we studied com-positions in the range 50–90 mol % MoO3 (50 mol %MoO3 corresponds to PbMoO4). The eutectic betweenPbMoO4 and MoO3 melts at 933 K and lies at79.5 mol % MoO3 [17]. using solution calorimetry, weinvestigated two compositions: (I) 56.1 mol %PbMoO4 + 43.9 mol % MoO3 (69.5 mol % MoO3 in the

mol % Y2O3∆solH

0(973 K) + (H0(973 K) – H0(298 K)),kJ/mol

0 57.2 ± 2.4

5 71.1 ± 7.8

10 59.2 ± 6.0

100 23.9 ± 1.1 [20]

PbO–MoO3 system), (II) 25.8 mol % PbMoO4 +74.2 mol % MoO3 (79.5 mol % MoO3 in the PbO–MoO3 system) (Tables 5, 6). Note that composition IIcorresponds to the eutectic in this system and its melt-ing point (933 K) is below the measurement tempera-ture (973 K). Consequently, the raw experimental datafor this composition can be represented as

∆solH0(í) + ∆mH0(í) + (H0(í) – H0(298 K)).

The enthalpy of fusion ∆mH0 was assumed to beequal to the heat effect of crystallization (–24.6 ±1.5 kJ/mol) with the opposite sign (Fig. 2):

The experimental data, in combination with the dataon dissolution of MoO3 and PbMoO4 (∆solH0(973 K) of

mol % MoO3∆solH0(973 K) + (H0(973 K)

– H0(298 K), kJ/mol

100 –5.7 ± 1.9

74.2 (composition II) 14.4 ± 8.5

56.1 (composition I) 51.0 ± 7.9

0 (PbMoO4) 131.7 ± 7.3

Table 4. Experimental data on PbO–10 mol % Y2O3 disso-lution

Sampleweight, mg

∆solH0(973 K) + (H0(973 K) – H0(298 K))

cal/g kJ/mol

7.989 68.994 64.518

17.310 65.192 60.963

8.257 64.005 59.852

22.466 61.407 57.423

10.308 62.218 58.181

11.585 59.960 56.070

19.162 61.391 57.408

1000800600400200

0

–4

–8

–12

–16

T, K

∆G, kJ/mol

Fig. 1. Gibbs energy of formation from oxides as a functionof temperature for the eutectic composition in the PbO–Y2O3 system.

Page 4: Mixing enthalpies of oxides in the PbO-Y2O3 and PbO-MoO3 systems

890

INORGANIC MATERIALS Vol. 42 No. 8 2006

DOBROKHOTOVA et al.

MoO3 was taken from [19], and that of PbMoO4 wasdetermined above), were used to determine ∆H0(298 K)from MoO3 and PbMoO4 for compositions I and II:3.6 ± 8.5 and 15.4 ± 9.3 kJ/mol, respectively.

Thus, the enthalpy of formation of the eutectic com-position from the end-members in this system differs

from zero by more than the experimental uncertainty,whereas in heterogeneous systems this quantity mustbe zero, as is the case with composition I.

As in the case of the eutectic composition in thePbO–Y2O3 system, we calculated the 298-K absoluteentropy of composition II using low-temperature Cp

data obtained in a limited temperature range. We foundfor the eutectic composition S0(298 K) = 77.9 ±4.6 J/(mol K) and the entropy of formation from MoO3

and PbMoO4 ∆S0(298 K) = –35.7 ± 5.9 J/(mol K). Thenegative entropy of formation suggests that, under theabove (nonequilibrium) crystallization conditions, anew, ordered phase may be formed in the eutectic com-position from the end-members.

Between 298 K and Tm, the ∆G0(í) of the eutecticcomposition in the PbMoO4–MoO3 system is positive.

Earlier data [6] suggest that, in the PbMoO4–MoO3system, nonautonomous phases may be formed,whereas calculations for the PbO–Y2O3 system indicatethe formation of microregions (a nonautonomousphase) as combinations of autonomous phases underappropriate crystallization conditions. These resultsaccount for the difference between the two systems inthe thermodynamic characteristics of formation of theeutectic composition from the end-members.

CONCLUSIONS

Using solution calorimetry, we determined theenthalpies of mixing of oxides for near-eutectic compo-sitions in binary systems. The results provide clear evi-dence of self-organization in eutectic melts, whichleads to the formation of metastable nonautonomousphases similar to supramolecular ensembles. Self-orga-nization processes depend on the crystallization condi-tions and interaction between the components of thesystem.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research, project nos. 05-03-32434 and06-03-32879.

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Table 6. Experimental data on PbO–79.5 mol % MoO3 dis-solution

Sampleweight, mg

∆solH0(973 K) + (H0(973 K) – H0(298 K))

cal/g kJ/mol

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–1

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Q, mW

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Fig. 2. Heating curve of a glassy PbO–79.5 mol % MoO3sample.

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Page 5: Mixing enthalpies of oxides in the PbO-Y2O3 and PbO-MoO3 systems

INORGANIC MATERIALS Vol. 42 No. 8 2006

MIXING ENTHALPIES OF OXIDES IN THE PbO–Y2O3 AND PbO–MoO3 SYSTEMS 891

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