49 L. B. Vedmid’, V. M. Dimitrov, O. M. Fedorova Institute of metallurgy, Ural Branch of Russian Academy of Sciences, 101 Amundsena st., Ekaterinburg, 620016, Russian Federation elarisa100@mail.ru Low-temperature synthesis under low oxygen pressure and thermodynamic properties of YbFe 2 O 4–δ The mixed-valence compound YbFe 2 O 4–δ was synthesized using a modified method that allows employing gaseous atmosphere with a controlled ratio of inert gas and oxygen. The stability range for YbFe 2 O 4–δ was determined at 1090 °C under reducing conditions. Thermodynamic characteristics for the formation of YbFe 2 O 4–δ from the simple oxides and from elements in the tem- perature range 700–910 °C have been calculated. The obtained results allow specifying the low-temperature part of Р-Т-Х diagram for the Yb-Fe-O system. Keywords: oxygen partial pressure, mixed-valence compound, thermodynamic properties, complex oxides, oxygen nonstoichiometry Received: 27.11.2017. Accepted: 26.03.2018. Published: 10.05.2018. © Vedmid’ L. B., Dimitrov V. M., Fedorova O. M., 2018 D O I: 1 0. 15 82 6/ ch im te ch .2 01 8. 5. 1. 03 Vedmid’ L. B., Dimitrov V. M., Fedorova O. M. Chimica Techno Acta. 2018. Vol. 5, No. 1. P. 49–54. ISSN 2409–5613 Introduction Complex oxides RFe2O4 (R = Er, Tm, Yb, Lu), which demonstrate both fer- roelectric and ferromagnetic properties, are presenting a special class of materials called multiferroics. These materials are promising for application in energy-saving and storage devices [1]. The information concerning physicochemical properties for these complex oxides is important for their synthesis and practical applications. One of the representatives of such materials is YbFe2O4. Taking into account that YbFe2O4 con- tains iron in two oxidation states, Fe3+ and Fe2+, it is obvious that one of the important parameters for its synthesis is oxygen par- tial pressure in gaseous phase. Since the compound is stable only within narrow interval of temperature and oxygen partial pressure [2, 3], it is very important to es- tablish a synthesis protocol and to find the temperature and oxygen partial pressure ranges of its stability. Based on the ionic radii of rare earth ions R3+ [2], the whole set of R-Fe-O systems can be divided into four groups. The Yb-Fe-O system belongs to the forth group D, in which R2Fe3O7 exists together with RFeO3, R3Fe5O12 and RFe2O4. The decrease of temperature may cause a decomposition of R2Fe3O7 and, as a result, the Yb-Fe-O system would be converted to the group C [4]. The synthe- sis of mixed-valence compounds in the R-Fe-O systems belonging to the C and D groups is traditionally performed under low oxygen partial pressure employing H2 and CO2 mixtures [2–4]. In works [2–4] raw mixtures of R2O3 and Fe2O3 were an- nealed at temperature 1200 °C in flow of gas with low oxygen activity. A precise 50 control and maintenance of oxygen par- tial pressure in such gaseouse mixtures in contact with oxide sample is not an easy task. The information about the possib i- lity of YbFe2O4 synthesis at temperatures lower than 1200 °C is absent. The purpose of the present study is the determination of conditions for preparation of YbFe2O4 at temperatures lower than 1200 °C, and investigation of its physicochemical pro- perties in the low-tempearture range. Experimental YbFe2O4–δ was synthesized by the ad- vanced method in which gas mixture of argon and oxygen was used. Reducing con- ditions were provided by maintaining the required value of oxygen partial pressure using the electrochemical method [5]. An interaction of a sample with the gaseous Ar-O2 mixture can only result in oxygen exchange, while using of CO2-H2 mixture may lead to the formation of carbides. Preliminarily dried powders of Fe2O3(purity ≥ 98%) and Yb2O3(purity 99,9%) were mixed in equimolar amounts and grinded in an agate mortar for an hour, and then pressed into pellets with the di- ameter of 10 mm. The heat treatment was performed inside the airtight apparatus with a confined internal space (Fig.  1). A  sample in a crucible 1 was placed in the quartz reactor 2 mounted in the fur- nace 3. Air was evacuated by the vacuum pump 4 out of the apparatus which was followed by refilling with argon. Oxygen partial pressure in the gas phase inside the apparatus was maintained and controlled by the oxygen cell 5, which consists of the oxygen sensor 6 and the oxygen pump 7. The oxygen cell was operated by the pro- grammable oxygen pressure controller 8. Since the thermal condition for each sample synthesis was tuned individually, but the oxygen cell worked at the limited tempearture range 600–900  °C (the op- timal temperature is about 800  °C) two temperature zonesin the apparatus were created: one for the oxygen cell (furnace 9) and another for samples (furnace 3), re- taining a common gas space. Syntheses of YbFe2O4–δ were carried out at 1090 °C for 48 hours. It should be noted that it is the highest possible temperature that can be used in this apparatus because quartz was used as the reactor material. The value of oxygen partial pressure in the furnace 9 was set in the interval pO2 = 10 –11.04–10–12.84 atm. It’s worth to note that in the region of low oxygen pressures pO2 is defined by the equilibrium constant of reaction (1) and also depends on temperature [6]: H2O  ½ O2 +H2 (1) Due to this fact the oxygen pressure in the furnace 3 differed from pO2 which was achieved and measured in the furnace 6. The syntheses of samples in the furnace 3 at 1090 °C were performed in the interval pO2 = 10 –11.04–10–12.84 atm. The circulation pump 10 was utilized to distribute the gas mixture with a controlled oxygen pressure evenly inside the apparatus. Manometer- vacuummeter 11 was used to measure the total gas pressure. The samples were quenched by pulling them out of the high temperature zone of the reactor into the space cooled by the flow of cold water. The phase composition of quenced samples was determined by X-Ray diffrac- tion (XRD) using a Shimadzu XRD7000C diffractometer (Cu  Kα radiation) in the range of angles 20°< 2θ < 80° with a step of 0.2°. The primary processing of diffrac- tion data was performed with the software 51 package for the diffractometer XRD-7000, and calculation of the unit cell parameters was carried out using the X-ray Diffraction Tabulated Process (RTP) program. Ther- modynamic characteristics of YbFe2O4–δ in relatively low temperature range and the values of oxygen nonstoichiometry for the samples obtained at various oxygen partial pressure were determined with the vacuum circulation apparatus [7]. Results and discussion According to the XRD data, the ob- tained YbFe2O4–δ samples were single-phase (Fig. 2). All of them possess rhombohedral structure (space group R3m). The unit cell parameters were in a good agreement with those collected from the database ICDD, PDF4, card No 01–070–1734 [8] (Fig. 2). The stability range for YbFe2O4 with re- spect to the oxygen partial pressure was de- termined at the fixed temperature 1090 °C. The decrease in oxygen pressure down to pO2 = 10 –12.84 atm leads to the partial de- composition of initial YbFe2O4 oxide to Yb2O3 and FeO phases. On the other hand, when the oxygen partial pressure had been increased up to pO2 = 10 –11.04 atm, XRD indicated that YbFe2O4 coexisted with the oxidized phases YbFeO3 and Fe3O4 (Fig. 3). Fig. 2. Initial X-ray diffraction patterns of YbFe2O4 Fig. 3. A fragment of the phase diagram for the Yb–Fe–O system at 1090 °C. The phases are: А–YbFe2O4±δ, Р–YbFeO3±δ, G – Yb3Fe5O12, W – FeO, M – Fe3O4 Fig. 1. A scheme of the apparatus for synthesis of materials at the required value of oxygen partial pressure. 1 – a crucible with a sample, 2 – quartz reactor, 3 – big furnace, 4 – vacuum forepump, 5 – oxygen cell, 6 – oxygen sensor, 7 – oxygen pump, 8 – oxygen partial pressure controller, 9 – small furnace, 10 – circulation pump, 11 – manometer-vacuum meter 52 Thus, it was experimentally determined that in the Yb–Fe–O system at 1090  °C YbFe2O4–δ exists within the range of oxygen pressures pO2 = 10 –11.04–10–12.84 atm. This result significantly widens the temperature range for YbFe2O4–δ stability, as compared to previous studies [2]. The absolute values of oxygen nonstoi- chiometry for samples prepared at various oxygen partial pressures were determined by means of gravimetric analysis via the hydrogen reduction of the samples to the stable simple oxides in the vacuum circula- tion setup as follows: YbFe2O4–δ + 0.5 H2 = 0.5 Yb2O3 + + 2 FeO + (0.5 – δ)H2O (2) The absolute value of oxygen non- stoichiometry was calculated using equa- tion (3): δ = − ⋅ ⋅ M m M m15 9994 15 9994. . ,red red (3) where М is the molecular weight of the studied oxide, Мred is the total molecular weight of solid products in the reduction reaction with allowance for the stoichio- metric coefficients, 15.9994 is the atomic weight of oxygen, mred is the weight of solid products after reduction reaction at the temperature of the experiment and m is the sample weight. Detailed description of the procedure is given in [7]. A decrease of the oxygen partial pressure during synthesis results in formation of oxygen vacancies and consequent change of the Fe2+/Fe3+ ratio. At the same time the unit cell para- meter a increases while the oxygen partial pressure is reduced (Fig. 4, Table 1). The obtained relationship between oxy- gen nonstoichiometry δ, which is influ- enced by oxygen partial pressure and the unit cell parameter a (Fig. 5), might be used as a criterion for oxygen nonstoichiometry δ estimation using only XRD results. The measurements of YbFe2O4–δ ther- modynamic stability have been carried out using the vacuum circulation apparatus by static method [7] in the temperature range 700–910 °C. The high temperature Table 1 Conditions of YbFe2O4–δ synthesis Sample lg(pO2) (atm) at 1090 °C Unit cell parameter a, c (Å) Oxygen non- stoichiometry δ 1 –11.4 а = 3.4506(4) с = 25.0773(19) V = 258.576 0.033 2 –11.6 а = 3.4519(4) с = 25.0655(20) V = 258.656 0.038 3 –11.8 а = 3.4528(4) с = 25.0865(20) V = 259.011 0.041 4 –12 а = 3.4538(4) с = 25.0855(21) V = 259.157 0.048 5 –12.2 а = 3.4547(4) с = 25.0715(20) V = 259.133 0.061 53 dissociation process of YbFe2O4 at low oxy- gen partial pressure within the range pO2 = 10–16.4–10–20.8 atm as a first approximation was written neglecting oxygen nonstoichi- ometry of ytterbium ferrite and oxygen nonstoichiometry of simple iron oxide as follows: YbFe2O4= 0.5 Yb2O3 + + 2 FeO + 0.25 O2 (4) A linear function was fitted to the mea- sured values of equilibrium oxygen partial pressure of the reaction (4) vs temperuture (filled point in Fig. 6), and the following equation was obtained: lg(pO2, Pa) = 11.78–27350/T±0.04 (5) Unfilled point 2 (Fig. 6) corresponds to the equilibrium oxygen partial pressure measured at 1200 °C by Kimizuka et al. [3]. One can observe good agreement of our results with those obtained earlier. Finally, Eq. (5) was recalculated to the standard Gibbs energy for reaction (4): ΔGOT = 130.895 – 0.0564T ± ± 0.99 kJ/mol (6) Since the coefficients in Eq. (6) can be treated as ΔH0T and ΔS 0 T , respectively, for reaction (4), the changes of standard en- tropy and enthalpy for YbFe2O4 formation from elements can be calculated taken into account the values of the thermodynamic functions for the simple oxides’ (Yb2O3 and FeO) formation [9,10]: ΔH0f (YbFe2O4) = –1575.767 kJ/mol, ΔS0f = 274.23 kJ/mol/K. Conclusions It was found that YbFe2O4 can be pre- pared at 1090 °C within the oxygen partial pressure range pO2 = 10 –11.04–10–12.84 atm. The obtained results allow specifying the low-temperature part of Р-Т-Х diagram for the Yb-Fe-O system, which is very im- portant from the practical point of view since it allows choosing the conditions for YbFe2O4 synthesis and usage. Using the static method in the vacuum circula- Fig. 5. Dependence of oxygen nonstoichiometry on unit cell parameter a for YbFe2O4–δ 3.451 3.452 3.453 3.454 3.455 0.030 0.035 0.040 0.045 0.050 0.055 0.060 0.065 a δ Fig. 6. Equilibrium oxygen partial pressure of YbFe2O4 dissociation (reaction 4) versus reciprocal temperature: 1 – our data, 2 –reported in [3] 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 -20 -18 -16 -14 -12 104/T,K 1 2 Fe 3 O 4 /FeO lg P O2 , atm Fig. 4. Dependence of the unit cell parameter a and c on oxygen partial pressure for YbFe2O4±δ synthesis 54 tion apparatus, the decomposition oxygen partial pressure was measured. Finally, the changes of standard entropy and enthalpy of YbFe2O4 formation from elements in the temperature range 700–910  °C were calculated. Acknowledgements The study was done in accordance with the state quota for IMET UB RAS, theme No. 0396-2015-0075, using equipment of CCU “Ural-M”. References 1. Pyatakov AP, Zvezdin AK. 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