CC BY
7Journal of Siberian Federal University. Chemistry 3 (2014 7) 312-315
УДК 536.63
High-Temperature Heat Capacity of Erbium Cuprate
Liubov Т. Denisova*, Natalya V. Belousova, Viktor М. Denisov, Liubov G. Chumilina and Sergey D. Kirik
Siberian Federal University 79 Svobodny, Krasnoyarsk, 660041, Russia
Received 24.06.2014, received in revised form 15.07.2014, accepted 08.08.2014
Data on the molar heat capacity of Er2Cu2O5 (359 - 974 K) were obtained by differential scanning calorimetry. The Cp = f(T) experimental data were used to determine thermodynamic properties of this compound.
Keywords: thermodynamic properties, erbium cuprate.
Высокотемпературная теплоемкость купрата эрбия
Л.Т. Денисова, Н.В. Белоусова, В.М. Денисов, Л.Г. Чумилина, С.Д. Кирик
Сибирский федеральный университет Россия, 660041, Красноярск, пр. Свободный, 79
Методом дифференциальной сканирующей калориметрии получены экспериментальные данные по молярной теплоемкости Ег2Си205 (359-974 К). По экспериментальным данным Ср = (Т) рассчитаны термодинамические свойства оксидного соединения.
Ключевые слова: термодинамические свойства, купрат эрбия.
© Siberian Federal University. All rights reserved Corresponding author E-mail address: [email protected]
Introduction
For a long time interest in metal oxide compounds including Er2Cu2O5 crystals does not weaken [15]. In spite of the attention to this compound, many of its features have not been studied. In particular it applies to the thermal properties. The heat capacity of Er2Cu2O5 was investigated at temperatures 2-30 K [3]. The information on the standard Gibbs free energy of formation of this compound is given in [2]. The optimization of obtaining conditions for such compounds presupposes thermodynamic studies which are feasible with the thermodynamic databases.
The purpose of this work is to measure the high-temperature heat capacity and to calculate the thermodynamic properties of Er2Cu2O5 using the data obtained.
The Er2Cu2O5 samples were prepared by a solid-phase synthesis technique. Taking into account peculiarities of CuO behavior at high temperatures, the weighed Er2O3 and CuO samples were previously ignited at 1173 K [6]. On homogenization and following pressing, the tablets were annealed in air at 1273 K during 25 h with 5 intermediate grindings and pressings. XRD-spectra of the samples were collected on X'Peyt Pro (PANalytical, CuKa radiation). From the X-ray diffraction patterns, we found the lattice parameters by the full-profile analysis wtthout yeferving to the structyre. The oPtained data is shown in Fig. 1. The synthesized samples had an orthorhombic structure (space group Pna2j) with the lattice parameters a = 10.777(1) A, b = 3.4711(4) A, c = 12.443(1) A. These values are consistent with the data oP [7]: a = 10.7839(2) A, b = 3.4745(1) A, c = 12.4434(3) At, and coiircife completely with tire data of [l].
The heat capacity Cp was measured in platinum crucibles with an STA 449 C Jupiter instrument (NETZSCH). The used technique was similar to that employed in [9, 10]. The temperature range examined (359 - 974 13) was choren l ased on difleeential thermal analysis (DTA) data obtained with the 7TA 414)9 C Jupiter.
Experimental
I
JU
10 20 30 40 50 60 70 80
2 ©; grad
Fig. 1. X-ray diffraction pattern of Er2Cu205 at 298 K
Results and discussion
The results of experime nts onthe measurements of the he at capac ity of ErpCu2O5 are presented in Fig. 2. As follows from the graphs, Cp increases systematically with increasing temperature. The Cp =//C) dependence was approximated "by a Maier-Kelly-type equation:
Cp = a + b• 10 - - c-105r2 = 200. 50 + 34.5010-3T- 7.38405T -2. (1)
Triis makes ir possibte eo detebmine the enthalpy inceement (Hb -Htey and entropy change (St - Sr5-) us:in.g- knnw n the rmody namic delations. The sesults aire; pbese nte d in /he Table 1.
At temperatures above 500 K, the heat capacity exceeds the classical Dulong-Petit limit 3Rs, where R is the universal gas constant, and s is the number of atoms in the Er2Cu2O5 formula unit (s = 9). Using the experimental data on Cp fo r Er2C u2O 5 at low temperatures, we determined the Debye temperature. It was found to be 400 K.
K
o r
T, K
Fig. 2. Temperature dependences of the heat capacity of Er2Cu2O5
Table; 1. Thermodynamic properties o5Er0Cu2O5
T, K Cp, J/(mole K) (//4 -/y3500 ),kJ/mole ( 95 - 95500 ) , J/(mole K)
359 212.9 - -
4100 214.33 81.747 23. 10
450 216.0 19.52 418.43
500 217.7 30.36T 71.2 9
550 219.5 41.29 92 . 12
600 221.22 56.51 111.3
6650 222.9 63.41 129.1
700 224.6 74.(60 145.6
"750 22(6.4 85.17 161.2
800 228.1 97.24 175.9
050 229.8 118.7 189.7
900 231 .5 720.2 202.9
950 233.3 111.8 215.5
235 2JU 225 220 215 aid
The obtained values of Cp cannot be compared with the results of other authors, since there is no such information in the literature. Nevertheless, taking into consideration a correlation between the oxide composition and their heat capacity [11], it can be no ted that in the system Er2O3 - CuO the values of C°p (in units of J/(g-K)) increase in the series Er^A (0.28) - E^Of (0.39) - CuO (0.53). The values of C/ for Er2O3 and CuO weee taken from [02]. Such change of Cp in /he Er2O3 - CuO syrtem agrees with the atomic mass effect: the phonon frequencies must be lower in the case of the oxides with the high content of Er2O3.
Conclusion
The hea2 capacity of Er2Cu2O5 has been studied first experimentally in the temperature range 359974 K. IP has been established that there is a correlation It)ntween the composition of the Er2O3 - CuO system andthe specific heat capacity of the oxide compounds of this system.
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