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21Journal of Siberian Federal University. Chemistry 2 (2012 5) 125-130
УДК 536.63
High-Temperature Heat Capacity
of Bismuth Oxide and Bismuth-Zinc Double Oxide
with the Sillenite Structure
Liliy A. Irtyugo, Nataliy V. Belousova, Viktor M. Denisov*, Liubov T. Denisova, Sergei D. Kirik and Liubov G. Chumilina
Siberian Federal University 79 Svobodny, Krasnoyarsk, 660041 Russia 1
Received 05.06.2012, received in revised form 12.06.2012, accepted 19.06.2012
Experimental data on the heat capacity of bismuth oxide and bismuth-zinc double oxide with the sillenite structure (446-939 K) were presented.
Keywords: bismuth oxide, bismuth-zinc double oxide, heat capacity.
Introduction
Bismuth oxide and Bi2O3-based compounds are of practical importance in view of their valuable properties and for a long time attract considerable attention of scientists [1 - 4]. At the same time, there are certain problems relating to the obtaining materials. It is attributed to the fact that many Bi2O3 -based oxide compounds can be in metastable state [4 - 6]. Phase equilibria in such systems can be improved with the use of thermodynamic modeling. To do this would require data on the thermodynamic properties of original oxides as well as complex oxide compounds over a wide temperature interval. Although Bi2O3-based compounds are used extensively, their properties call for additional investigations. In the first place, this is true for pure Bi2O3. It forms a, p, y and 5 polymorphous modifications in different temperature ranges [7].
By now there are data on the heat capacity of Bi2O3. The values of Cp for this oxide at 298-800 K are given in [8] by Eq. (1), J/(mole-K):
Cp = a + bT - cT-2, (1)
where a = 103,60; b = 33,50 • 10-3; c = 0. From this equation it follows that Cp is a linear function of the temperature.
* Corresponding author E-mail address: antluba@mail.ru
1 © Siberian Federal University. All rights reserved
V.E. Gorbunov et al [9] have studied the heat capacity of Bi2O3 at 1 1-50 K with the use of an adiabatic microcalorimeter. Based on these results and the data of other researchers, they obtained smoothed values of this thermodynamic functions of Bi2O3 to T = 298,15 K: C=p29S,15 = 113,4 J/(mole-K); H»8i5 " Ho = 21530 J/mole; S^15 - S„ = 150,0 J/(mole-K). It should be noted that the close values ot Cp298,[5 = 113,804 ± 1,255 J/(mole-K) and SJ9nje - SO = = t51,46[ ± 4,184 J/(moleK) w^eire; J5r-ese;nted in [10l. At the saime tim^, different value o5 the enthalpy was given in [10]: H298 15 -H0 a 130540,08 ± 20<C,2 J/mole. The cause oit such difference in the magnitude of the enthalpy is only stated by the aulhoes of [9] .
To obtain consistent; jesuJts on the thermodynamic properties ofBi2O3,0. V Kuzaetoova et al [11] useddata on the low-temperature heat capacity (11 - 300 K) [7, 9, 10, 12] and high-temperature omes (■400 c 800 K) [°3] which weoo pcoeessed by the Renhetnikov's function [14].
We found mo Information on diaect cflo-mefric meas9rements °f Cp in the range o0 temperatutes foom 400 to 1003 K (io the n — 5 phare ttaf sition temperature (1003 K [4]).
The thermodynamic properties of Bi2O3 - ZnO system were inventigated 6y S. Serena et al [155], but the aim of that work was the thermodynamic analysis oi the Br rO3 - ZnO phrase diagram in the field o0 Bi2O3.
The jpinrpose of the presen] work is to measure thf heat cspacity of Bi2O3 at 403 - 960 K and BinCBej^zntk 33 at 446 - 939 K.
Experimental procedu re
Bi12(B i0357Zthnos33)019=33 ceoamics was obtained by the; solid-pfase sintering with matching optimal re gime. T he total time of the synthesis at 1008 K was 45 hours. 9 intermediate grindings were performed, accompanied by the pressing. X-ray diffraction (XRD) data were collected on a PANalytical X'Pert Pro powder diffractometer (CuKa12 radiation). The record was carried out with the use of a highspeed PrXcel fetectoa and a goaphite monochromator in anchgle range of 5 - 80° 20 with the pitch engle of 0,026°. The identifications was performey by PDFrlCDD tPewder diffrnctionfile sets 1-58. JCfDf-ICDD, Pennsylvania 19f73.3273 USA]. The lattice parametees wece refined by fhe profile-fitting method (Le Beil method): a = 10,2030(1) A, V = 1062,I4 A3, spaee group Ii3. Ttese data aee in agreement with those obtained previously [4]. Fig 1 shows a XRD pattern of Bi12[Bi0)67Zn0)33]O1)33 sample.
The hcai capacity (Cp[ mersuremenfs were carried out in platinum crucibles by ihe differential scanning ciorimetry bith r STA nr9 C J^pciitei: device (]SrEr3'^ZZS<[[;Er;)|i. Thie (3t!;;j5^:r^:i;iiental procedure was i^iL:m.31isr described one [16, 17].
Results and discussion
The (Cp temperature dependence of bismuth oxide is presented in Fig. 2. It can be seen that, in the temperature rrnge uoder investigation, the values of Cp increase regularly and the depentlence Cp = f(T) does not exhibit ana extrema. Neas 100r K the magnitudes of Cp, iri[i<s sltarply and a jump of Cp is observed at the a — 5 phase transieion temperature (not shown ie Fig. 2). The smoothed values of Cp = f(T) can be described by Eq. 2, J/(moleK):
Cp = eer,79 + 24,46 -e0-rT. (2)
29, rpan
Fig. 1. XRD pattern of Bi i2[Bi0,67Zn0,33]Oi9,33 sample
Fig. 2. Influence of the temperature on the heat capacity ofBi2O3. 1 - our data, 2! - [11]
The comparison of this expression and Eq. 1 shows that they differ slightly. It is not inconceivable that this fact is accounted for the different temperature interval: 298-800 K (Eq. 1) and 403 - 960 K (Eq. 2). At the same time our experimental values of Cp agree satisfactory with those calculated in [11] (Fig. 2).
Data on the heat capacity of Bii2(Bi0)67Zno)33)O19)33 is presented inFig. 3. The values of Cp are seen to increase almost linearly. with increased temperature. "The obtained temperature dependence of Cp takes the form, J/(mole-K):
Cp=7t6,54+ 155,78-If-3 T. (3)
A comparison of our data with the results obtained in [18] demonstrates (Fig. 3) that they are reasonably close but our values are sl ightly below.
According to the equilibrium diagram, Bi^Bi^Zn^O^ compound melts incongruently [15, 19] (data of [19] are shown in Fig. 4). Because of this, G.S. Suleimenova and V.M. Skorikov obtained single crystals of this compound from Bi2O3-based solution-melt [18]. As noted in the process, the crystals cancontain impurities of s metastable phase f*-phase of Bi2O3) as a result of some excess of Bi2O3 in the initial mix shipulated by the techniques of the crystal growing from the solutioa-melt and by the melt superheating. This is likely to be responsible for the difference between our data and the ahlues presenled io [18] (Fig. 3).
Discussing Bi12MxO2i±8 cryrtals haviag a silleatte steucture tie authors of [18] supposes that a lattice contribution in the heat capacity of Bi12GeO20 and Bi12SiO20 is less than in the case of other crystals and the values of the heat capacity is above for crystals with lattice distortions. By [4, 20], the sillenite structural type has approximately 60 individual phases, the generalized formula of which can be presented as Bi12MxO20±8 (M are elements of the II-VIII groups of Mendeleev's periodic table). These compounds crystallize in the cubic system (space group I23, SG № 197). Bi12GeO20 has an "ideal" lattice but the rest of the crystals contain structural defects. The greatest disorder was noticed for the lattice of compounds with M3+ and M2+ cations, in which 1/2 or 2/3 of the regular [MO4]-tetrahedra are substituted by the umbrella-type [BiO3E] groups with the simultaneous formation of oxygen vacancies and the occurrence of [BiO4E] polyhedra [20].
It may be suggested that the aforesaid conditions a specific character of the heat capacity of sillenites including Bi12(Bi0)67Zn0)33)O19)33.
Conclusion
Data on the high-temperature heat capacity of Bi2O3 (403 - 960 K) and Bi12(Bi0 67Zn0 33)O1933 (446 - 939 K) were refined and extended.
TX
Fig.4. The phase diagram of Bi2O3 - ZnO system
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Высокотемпературная теплоемкость оксида висмута и висмут цинкового оксида со структурой силленита
Л.А. Иртюго, Н.В. Белоусова, В.М. Денисов, Л.Т. Денисова, С.Д. Кирик, Л.Г. Чумилина
Сибирский федеральный университет Россия 660041, Красноярск, пр. Свободный, 79
Получены экспериментальные данные теплоемкости для оксида висмута и висмут цинкового оксида со структурой силленита (446-939 К).
Ключевые слова: оксид висмута, висмут цинкового оксида, теплоемкость.
