Phase equilibria in the YFeO3 – YСoO3 system in air Chimica Techno Acta ARTICLE published by Ural Federal University 2021, vol. 8(1), № 20218108 journal homepage: chimicatechnoacta.ru DOI: 10.15826/chimtech.2021.8.1.08 1 of 7 Phase equilibria in the YFeO3 – YСoO3 system in air A.V. Bryuzgina, A.S. Urusova * , I.L. Ivanov, V.A. Cherepanov Institute of Natural Science and Mathematics, Ural Federal University, Lenin av. 51, Yekaterinburg, 620000 Russia * Corresponding author: Anastasia.Podzorova@urfu.ru This article belongs to the regular issue. © 2021, The Authors. This article is published in open access form under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract YFe1-xСоxO3 solid solutions were prepared by glycerol-nitrate tech- nique. The homogeneity range of solid solutions was studied within the temperature range 1173 – 1573 K. A continues series of solid solu- tion below the decomposition temperature of YСоO3, which was shown to be equal to 1266  6 K, begins to narrow at higher tempera- tures and becomes equal to 0 ≤ x ≤ 0.1 at 1573 K. The phase diagram of the YFeO3 – YСoO3 system in the “T – composition” coordinates was divided into three fields. Similar to the parent ternary oxides, all single-phase YFe1-xСоxO3 solid solutions possess orthorhombically distorted perovskite structure (Pnma space group). Unusual behavior of orthorhombic distortions in YFe1-xСоxO3 with temperature was ex- plained by probable changes in spin state of Co 3+ ions. Keywords solid solutions perovskite crystal structure phase diagram Received: 14.03.2021 Revised: 10.04.2021 Accepted: 11.04.2021 Available online: 13.04.2021 1. Introduction Yttrium ferrite and yttrium cobaltite with the perovskite structure and their mixed derivates attract much attention due to a set of electrical and magnetic properties [1-12]. Although their structural features and some functional properties were widely studied [1-16], there is lack infor- mation concerning phase equilibria and homogeneity ranges of YFe1-xСоxO3 solid solutions. It is worth noting that phase equilibria in the Y – Fe – O system as well as thermodynamic characteristics of ternary oxides inside including YFeO3 were studied in detail [17-21]. Also, de- tailed information available for the Fe – Co – O system [22-24]. Much less information can be found for the Y – Co – O system and the thermal stability of YCoO3 [16, 25]. Thus, the aim of present work was determination of ho- mogeneity ranges of YFe1-xСоxO3 solid solutions as a func- tion of temperature in air and establishing of phase equi- libria in the YFeO3 – YСoO3 system. 2. Experimental The samples were prepared via the glycerol-nitrate tech- nique. The starting materials — Y2O3, metallic Co, FeC2O4·2H2O — were dissolved in nitric acid and then glyc- erol was added. Metallic cobalt was prepared from Co3O4 by reduction in hydrogen flow at 923 K. The solution was carefully heated to dryness. The obtained residue was slowly heated up and annealed at 1173 K for 20 h. Final annealing of the samples were performed at required temperatures in air for 96 h with grinding after each 12 h. The samples were quenched to room temperature (RT) by removing them from a furnace and placing them to a cold massive copper plate. Phase identification was carried out by means of X-ray powder diffraction (XRD) using a Shi- madzu XRD 7000 diffractometer (Cu Kα radiation, 2θ = 20°–90°, 0.02 deg/min, 5 s/point). High temperature XRD measurements were performed using a HTK 1200N (Anton Paar, Austria) high temperature chamber installed at the diffractometer. Unit cell parameters were calculated using Celref 3 software. The structure was refined by full-profile Rietveld analysis using Fullprof 2017 software. TGA measurements were performed using a STA 409 PC instrument (Netzsch) within the temperature range of 300 – 1373 K in air. 3. Results and Discussion First, the parent oxides of the studied system YFeO3 and YСоO3 were prepared and examined. Yttrium ferrite quenched from high temperature within the entire range (1173 – 1473 K) or slowly cooled to RT possesses ortho- rhombically distorted perovskite structure, which is in good agreement with the results reported earlier [1-4]. Fig. 1 illustrates XRD patterns of YFeO3 prepared at vari- ous conditions and evaluated values of unit cell parame- ters, as an example. The XRD pattern refined by the Rietveld method and structural model of YFeO3 designed http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2021.8.1.08 http://creativecommons.org/licenses/by/4.0/ Chimica Techno Acta 2021, vol. 8(1), № 20218108 ARTICLE 2 of 7 Fig. 1 XRD patterns of YFeO3 quenched from various temperatures in air: (a) 1373 K; (b) 1173 K and (c) slowly cooled to room tem- perature and unit cell parameters evaluated by the Rietveld re- finement using the “Diamond 3.2” software is shown in Fig. 2. Single-phase yttrium cobaltite YСоO3 was obtained on- ly at relatively low temperatures 1173 and 1223 K. Like ferrite it possesses the orthorhombic structure (SG Pnma). The structural parameters of YСоO3 quenched from 1173 and 1223 K refined by the Rietveld method are listed in Table 1. To preliminarily estimate the decomposition tempera- ture of YСоO3, TGA measurements in a dynamic mode with a heating rate of 3.2 K/min were performed. A sharp drop in the mass of the sample was detected at 1300 K (Fig. 3). Table 1 The structural parameters of YСоO3 quenched from 1173 and 1223 K refined by the Rietveld method SG Pnma: Y(4c)(x; 0.25; z); Co(4a)(0; 0; 0); O1(4c)(x; 0.25; z); O2(8d)(x; y; z) Т, К 1173 1223 a, Å 5.41656(5) 5.41784(5) b 7.36230(7) 7.36195(7) c, Å 5.13591(5) 5.13604(5) V, (Å) 3 204.811(1) 204.855(3) x(Y) 0.4316(2) 0.4315(2) z(Y) 0.0183(3) 0.0184(3) x(O1) 0.5287(1) 0.5285(2) z(O1) 0.5956(2) 0.5972(1) x(O2) 0.2002(1) 0.1988(1) y(O2) 0.0487(8) 0.0487(8) z(O2) 0.3051(2) 0.3047(1) d(Y)–(O1), Å 2.234(8) 2.227(7) d(Y)–(O2), Å 2.437(6) 2.439(6) d(Y)–(O2), Å 2.266(6) 2.263(6) d(Y)–(O2), Å 2.558(6) 2.557(6) d(Co)–(O1), Å 1.911(2) 1.913(2) d(Co)–(O2), Å 1.939(6) 1.933(6) d(Co)–(O2), Å 1.941(7) 1.949(6) Rp, % 10.7 13.2 Rwp, % 12.3 15.5 Rexp, % 9.18 11.3 RBr, % 3.58 3.10 Rf, % 6.85 3.11 χ 2 1.783 1.877 Fig. 2 XRD pattern refined by the Rietveld method and structural model of YFeO3 along the b axis designed using the “Diamond 3.2” software. To refine the decomposition temperature in TGA meas- urements, a static mode was used. The following protocol was used: a single-phase sample was heated at a rate of 1 K/min to 1110 K and equilibrated at this temperature for 8 h. Then the temperature was increased in a step of 20 K and the sample was kept at a fixed temperature until a constant mass was established. No significant mass changes were detected at T  1260 K. The next step to 1280 K results in a dramatic weight loss. XRD analysis of the sample quenched after annealing at 1273 K, which was originally a single-phase YСоO3, showed the presence of significant amounts of yttrium oxide and cobalt oxide (II) as secondary phases (Fig. 4). It should be mentioned that small amount of Co3O4 forms due to partial oxidation of CoO while cooling since latter is thermodynamic stable form of cobalt oxide at 1273 K in air. Thus, one can conclude that YСоO3 decomposes accord- ing the reaction YСоO3 = ½ Y2O3 + CoO + ¼ O2 (1) within the range 1260 < Tdec, K < 1273. This allows us to evaluate Tdec (YCoO3) = 1266  6 K. Fig. 3 TGA curve for single-phase YСоO3 in air measured in a dy- namic mode with a heating rate of 3.2 K/min Chimica Techno Acta 2021, vol. 8(1), № 20218108 ARTICLE 3 of 7 Fig. 4 XRD patterns of the samples with nominal composition of YCoO3, fired and quenched from various temperatures in air: (a) 1373 K; (b) 1323 K; (c) 1273 K; (d) 1223 K; (e) 1173 K. Prolonged annealing of the sample with nominal com- position corresponding to YCoO3 at 1373 K reveals coexist- ing of two binary simple oxides Y2O3 and CoO. Since YСоO3 is only stable below 1266 K, it is likely that a continuous series of YFe1-xCoxO3 solid solutions cannot be obtained at higher temperatures. Indeed, continuous series of YFe1-xCoxO3 solid solutions in the range of 0  x  1 was obtained at 1173 K. The homogeneity range at 1273 K was evaluated as 0  x  0.9. The sample with x=0.95 contained together with perovskite phase also Y2O3 and enriched by cobalt Fe1-yCoyO with the rock salt struc- ture (Fig. 5). Further increase of temperature leads to a decrease in Co content in the limiting YFe1-xCoxO3 solid solution (Table 2). Table 2 The homogeneity range value for the YFe1-xCoxO3 solid solutions at various temperatures Т, K homogeneity range Т, K homogeneity range 1173 0x1 1373 0x0.45 1273 0x0.9 1423 0x0.3 1323 0x0.68 1473 0x0.2 Fig. 5 XRD patterns for YFe1-xCoxO3 (x=0.9, 0.95) equilibrated at 1273 К and quenched to RT Thermal decomposition of Co-saturated solid solution with the temperature increase can be shown by following reaction: YFe1-x’Cox’O3  YFe1-x’’Cox’’O3 + Co1-y’Fey’O +Y2O3 + zO2 (2) were x’ > x’’ and y’ corresponds to the Fe-saturated solid solution at a fixed temperature. It is worth noting that the process described by scheme (2) differs significantly from the one occurs according to equation (1). The latter corre- sponds to a nonvariant thermodynamic equilibrium, when all participating phases coexist at fixed T and Po2. In con- trast, scheme (2) represents the situation when Co- saturated single-phase YFe1-x’Cox’O3 solid solutions in the left-hand side exist at T’, and an increase of temperature to T’’ = T’ + T causes a depletion of cobalt in solid solu- tion and displacement of its composition YFe1-x’’Cox’’O3 as well as appearance of two secondary phases, namely, Y2O3 and Co1-y’Fey’O. The left-hand side and the right-hand side in the scheme (2) represent the phase composition in the system at different temperatures, T’ and T’’=T’+T, re- spectively. Thus, scheme (2) describes nonequilibrium process that occurs due to the change in thermodynamic parameter, in this case it is temperature. A similar process can take place at fixed temperature due to the decrease in Po2. Based on the results of phase composition of all studied samples the “T-composition” phase diagram of the YFeO3 – YСoO3 system in air was drawn (Fig. 6). All single-phase YFe1-xCoxO3 solid solutions quenched from all studied temperatures possess the orthorhombic structure, like parent ternary oxides. The influence of temperature on the crystal structure of YFe1-xCoxO3 (x = 0.35 and 0.45) was studied by in situ high tempera- ture (HT) XRD measurements. The structural parameters of YFe1-xCoxO3 (x = 0.35 and 0.45) at various temperatures refined by the Rietveld method are listed in Tables 3 and 4. Temperature dependencies of the unit cell parameters and unit cell volume demonstrate visible non-linearity (Fig. 7). As a rule, the distortions of crystal structure tend to decrease with the temperature rise; however, the ortho- Fig. 6 Phase diagram of the YFeO3 – YСoO3 system in air Chimica Techno Acta 2021, vol. 8(1), № 20218108 ARTICLE 4 of 7 Table 3 The structural parameters of YFe0.65Со0.35O3 at various temperatures refined by the Rietveld method using HT-XRD measurements SG Pnma : Y(4c)(x; 0.25; z); Fe/Co(4a)(0; 0; 0); O1(4c)(x; 0.25; z); O2(8d)(x; y; z) Т, K 298 473 623 673 973 1273 1373 a, Å 5.53089(6) 5.53716(4) 5.54493(4) 5.54861(4) 5.58349(4) 5.61551(6) 5.62316(5) b, Å 7.51120(8) 7.52574(5) 7.54017(6) 7.54575(6) 7.59046(6) 7.63524(8) 7.64818(6) c, Å 5.23159(6) 5.24225(4) 5.25323(4) 5.25755(4) 5.29060(4) 5.32228(6) 5.33169(4) V, (Å) 3 217.339(4) 218.451(3) 219.636(3) 220.125(3) 224.222(3) 228.197(4) 229.300(3) x(Y1) 0.4316(2) 0.4324(2) 0.4332(2) 0.4330(2) 0.4339(2) 0.4347(2) 0.4350(2) z(Y1) 0.0181(3) 0.0182(3) 0.0174(3) 0.0174(3) 0.0172(3) 0.0165(3) 0.0163(3) x(O1) 0.534(1) 0.533(1) 0.532 (1) 0.531(1) 0.533(1) 0.531(2) 0.5329(2) z(O1) 0.603 (1) 0.605(2) 0.604(2) 0.605(2) 0.605(2) 0.606(2) 0.606(2) x(O2) 0.196(1) 0.195(1) 0.195(1) 0.194(1) 0.193(1) 0.192(1) 0.192(1) y(O2) 0.054(7) 0.053(7) 0.053(7) 0.053(7) 0.055(7) 0.054(7) 0.054(7) z(O2) 0.307(1) 0.306(1) 0.307(1) 0.309(9) 0.309(1) 0.308(1) 0.307(1) d(Y)–(O1), Å 2.287(7) 2.307(7) 2.314(7) 2.323(7) 2.331(7) 2.360(8) 2.371(8) d(Y)–(O2), Å 2.479(5) 2.490(5) 2.503(5) 2.512(6) 2.529(6) 2.553(6) 2.557(6) d(Y)–(O2), Å 2.270(5) 2.271(5) 2.272(6) 2.267(6) 2.268(6) 2.282(6) 2.287(6) d(Y)–(O2), Å 2.633(5) 2.634(5) 2.635(5) 2.633(5) 2.657(6) 2.666(6) 2.672(6) d(Fe/Co)–(O1), Å 1.963(2) 1.969(2) 1.971(2) 1.973(2) 1.985(2) 1.997(2) 2.002(3) d(Fe/Co)–(O2), Å 1.980(5) 1.976(5) 1.982(5) 1.987(5) 2.004(6) 2.003(6) 2.004(6) d(Fe/Co)–(O2), Å 2.002(5) 2.010(5) 2.012(6) 2.016(5) 2.030(6) 2.052(6) 2.056(6) (Fe/Co)–(O1)–(Fe/Co),° 146.09(3) 146.25(2) 146.07(7) 146.00(1) 145.92(3) 145.8(2) 145.61(7) (Fe/Co)–(O2)–(Fe/Co),° 145.91(6) 145.62(2) 145.92(2) 145.47(9) 144.92(6) 145.15(4) 145.25(7) Rp, % 11.8 12.2 12.5 12.9 13.4 13.8 14.0 Rwp, % 15.0 15.2 15.2 15.7 15.8 16.0 16.2 Rexp, % 10.6 10.6 10.7 10.7 11.0 11.3 11.4 RBr, % 4.25 4.83 4.98 4.91 5.01 5.38 5.09 Rf, % 3.43 3.87 4.05 3.89 4.26 4.80 4.91 χ 2 1.985 2.057 2.036 2.148 2.043 2.002 2.019 Table 4 The structural parameters of YFe0.55Со0.45O3 at various temperatures refined by the Rietveld method using HT-XRD measurements SG Pnma : Y(4c)(x; 0.25; z); Fe/Co(4a)(0; 0; 0); O1(4c)(x; 0.25; z); O2(8d)(x; y; z) Т, K 298 623 723 873 1223 1373 a, Å 5.51229(9) 5.52600(9) 5.53528(9) 5.55559(9) 5.60219(9) 5.61552(9) b, Å 7.48536(13) 7.51456(13) 7.52654(13) 7.54977(13) 7.60614(13) 7.62717(13) c, Å 5.21595(9) 5.23834(9) 5.24821(9) 5.26589(9) 5.30639(9) 5.32121(8) V, (Å) 3 215.218(2) 217.524(4) 218.648(7) 220.869(6) 226.111(7) 227.910(7) x(Y1) 0.4324(3) 0.4343(3) 0.4343(3) 0.4345(3) 0.4349(3) 0.4356(3) z(Y1) 0.0178(4) 0.0170(4) 0.0168(5) 0.0166(4) 0.0164(5) 0.0158(5) x(O1) 0.5328(18) 0.5319(18) 0.5304(18) 0.5319(18) 0.5332(18) 0.5340(19) z(O1) 0.6004(19) 0.5999(20) 0.5984(20) 0.6031(20) 0.6030(20) 0.602(2) x(O2) 0.1935(15) 0.1929(15) 0.1926(15) 0.1952(15) 0.1925(15) 0.1948(16) y(O2) 0.0547(10) 0.0530(9) 0.0524(10) 0.0527(10) 0.0538(10) 0.0541(10) z(O2) 0.3050(15) 0.3045(15) 0.3062(15) 0.3039(15) 0.3070(15) 0.3053(16) d(Y)–(O1), Å 2.25(1) 2.25 (1) 2.26(1) 2.24 (1) 2.26(1) 2.27(1) d(Y)–(O1), Å 2.287(1) 2.306(1) 2.316 (1) 2.324(1) 2.338(1) 2.341(2) d(Y)–(O2), Å 2.4729(8) 2.4979(8) 2.512(8) 2.505(8) 2.540(8) 2.537(9) d(Y)–(O2), Å 2.250(8) 2.260(8) 2.263(8) 2.284(8) 2.278(8) 2.293(9) d(Y)–(O2), Å 2.629(8) 2.630(7) 2.626(8) 2.645(8) 2.662(8) 2.678(8) d(Fe/Co)–(O1), Å 1.952(3) 1.958(3) 1.959(3) 1.972(3) 1.987(3) 1.992(3) d(Fe/Co)–(O2), Å 1.959(8) 1.959(8) 1.968(8) 1.974(8) 1.996(8) 2.002(9) d(Fe/Co)–(O2), Å 2.014 (8) 2.022(8) 2.021(8) 2.023(8) 2.046(8) 2.0447(9) (Fe/Co)–(O1)– (Fe/Co),° 147.017(6) 147.24(2) 147.798(6) 146.33(2) 146.222(6) 146.418(6) (Fe/Co)–(O2)– (Fe/Co),° 145.537(6) 145.995(6) 145.861(6) 146.534(6) 145.349(6) 145.868(6) Rp, % 15.3 15.4 15.4 15.6 15.8 16.7 Rwp, % 18.2 18.2 18.3 18.3 18.5 19.4 Rexp, % 11.7 11.5 11.5 11.6 11.9 12.3 RBr, % 4.38 4.56 4.53 4.76 5.19 5.36 Rf, % 3.53 3.92 3.81 4.53 4.99 5.30 χ 2 2.429 2.503 2.528 2.471 2.398 2.481 Chimica Techno Acta 2021, vol. 8(1), № 20218108 ARTICLE 5 of 7 Fig. 7 The unit cell parameters and unit cell volume of YFe1-xCoxO3 (x=0.35 and 0.45) versus temperature rhombic distortion parameters calculated by the formula [26, 27]: 𝐷𝑜𝑟𝑡ℎ = 1 3 ∑ | 𝑎𝑖 − �̅� �̅� | × 100% 3 𝑖=1 (3) where 𝐷𝑜𝑟𝑡ℎ is the orthorhombic distortion parameter, %; 1 = a; 2 = b; 3 = c/√2 and �̅� = (a×b×c/√2) 1/3 , exhibit visible anomalies within 600 – 1200 K (Fig. 8). Such be- havior could not be explained by the change in oxidation states of 3d metals. TGA analysis of YFe1-xCoxO3 (x = 0.35 and 0.45) reveals tiny mass changes while heating, which means that both oxides possess almost stoichiometric oxy- gen content within entire temperature range. Thus, the oxidation state of 3d atoms is equal to 3+ and remains unchanged with the increase of temperature. Another rea- son that can induce structural transformations can be changes in spin states. Such temperature-induced changes in the spin state of Co 3+ ion in LnCoO3 perovskites were reported by Raccach and Goodenough [28] and in later publications [7, 29-31]. Low-spin (t 6 2g) state can trans- forms into intermediate-spin (t 5 2ge 1 g) state and finally into high-spin (t 4 2ge 2 g) state. Although the possibility of spin state changes for Fе 3+ ion is still questionable and its high- spin state is more favorable [2, 5], possible spin-state transition of Co 3+ ion can cause the observed anomalies. However, this needs to be checked by further independent experiments. It is worth noting that Co substitution for Fe reveals much stronger effect on changes in the unit cell volume due to the size effect (𝑟Fe3+(HS) VI = 0.645 Å, 𝑟Co3+(LS) VI = 0.545 Å, 𝑟Co3+(HS) VI = 0.61 Å [32]) rather than tem- perature. The decrease in the unit cell volume of YFe1-xCoxO3 with the cobalt content (Fig. 9) is much more significant in comparison with the temperature depend- ence. Fig. 8 The pseudo-cubic unit cell parameter (ap) and the ortho- rhombic distortion parameter (D) for YFe1-xCoxO3 (x = 0.35 and 0.45) versus temperature Fig. 9 The unit cell volume of YFe1-xCoxO3 versus Co content (x) Chimica Techno Acta 2021, vol. 8(1), № 20218108 ARTICLE 6 of 7 4. Conclusions The homogeneity range and crystal structure of YFe1-xСоxO3 solid solution have been studied within the entire composition range (0≤x≤1) in 1173 – 1573 K tem- perature range. Continuous series of YFe1-xCoxO3 solid so- lutions in air can be obtained only below decomposition temperature of YCoO3, which was evaluated equal to 1266  6 K. Further temperature increase leads to a decrease of YFe1-xCoxO3 homogeneity range which is determined to be 0≤x≤0.1 at 1573 K. Phase diagram of the YFeO3 – YСoO3 system in air comprise of 3 phase fields. Partial substitution of Co for Fe has not changed the orthorhombic perovskite structure. Possible change in the spin state of Co 3+ ions is a presumable reason for the unu- sual behavior of orthorhombic distortions in YFe1-xСоxO3 (x = 0.35 and 0.45) with temperature. Acknowledgments This work was supported in parts by the Ministry of Sci- ence and Higher Education of Russian Federation (№ АААА-А20-120061990010-7) and A.V.B. was supported with a stipend for young scientists and PhD students from the President of Russian Federation (№ SP-3689.2019.1). References 1. 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