Phase equilibria in the YFeO3 – YСoO3 system in air

YFe1-xСоxO3 solid solutions were prepared by glycerol-nitrate technique. The homogeneity range of solid solutions was studied within the temperature range 1173 – 1573 K. A continues series of solid solution below the decomposition temperature of YСоO3, which was shown to be equal to 1266  6 K, begins to narrow at higher temperatures 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 explained by probable changes in spin state of Co ions. Keywords


Experimental
The samples were prepared via the glycerol-nitrate technique. The starting materials -Y 2 O 3 , metallic Co, FeC 2 O 4 ·2H 2 O -were dissolved in nitric acid and then glycerol was added. Metallic cobalt was prepared from Co 3 O 4 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 Shimadzu 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.

Results and Discussion
First, the parent oxides of the studied system YFeO 3 and YСоO 3 were prepared and examined. Yttrium ferrite quenched from high temperature within the entire range (1173 -1473 K) or slowly cooled to RT possesses orthorhombically distorted perovskite structure, which is in good agreement with the results reported earlier [1][2][3][4]. Fig. 1 illustrates XRD patterns of YFeO 3 prepared at various conditions and evaluated values of unit cell parameters, as an example. The XRD pattern refined by the Rietveld method and structural model of YFeO 3 designed Single-phase yttrium cobaltite YСоO 3 was obtained only at relatively low temperatures 1173 and 1223 K. Like ferrite it possesses the orthorhombic structure (SG Pnma).
The structural parameters of YСоO 3 quenched from 1173 and 1223 K refined by the Rietveld method are listed in Table 1.
To preliminarily estimate the decomposition temperature of YСоO 3 , 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). To refine the decomposition temperature in TGA measurements, 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 YСоO 3 , 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 Co 3 O 4 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 YСоO 3 decomposes according the reaction within the range 1260 < T dec , K < 1273. This allows us to evaluate T dec (YCoO 3 ) = 1266  6 K. Prolonged annealing of the sample with nominal composition corresponding to YCoO 3 at 1373 K reveals coexisting of two binary simple oxides Y 2 O 3 and CoO.
Since YСоO 3 is only stable below 1266 K, it is likely that a continuous series of YFe 1-x Co x O 3 solid solutions cannot be obtained at higher temperatures. Indeed, continuous series of YFe 1-x Co x O 3 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 Y 2 O 3 and enriched by cobalt Fe 1-y Co y O with the rock salt structure (Fig. 5). Further increase of temperature leads to a decrease in Co content in the limiting YFe 1-x Co x O 3 solid solution ( Table 2).  (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 corresponds to a nonvariant thermodynamic equilibrium, when all participating phases coexist at fixed T and Po 2 . In contrast, scheme (2) represents the situation when Cosaturated single-phase YFe 1-x' Co x' O 3 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 solution and displacement of its composition YFe 1-x'' Co x'' O 3 as well as appearance of two secondary phases, namely, Y 2 O 3 and Co 1-y' Fe y' 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, respectively. 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 Po 2 .
Based on the results of phase composition of all studied samples the "T-composition" phase diagram of the YFeO 3 -YСoO 3 system in air was drawn (Fig. 6).  Tables 3  and 4.

Conclusions
The homogeneity range and crystal structure of YFe 1-x Со x O 3 solid solution have been studied within the entire composition range (0≤x≤1) in 1173 -1573 K temperature range. Continuous series of YFe 1-x Co x O 3 solid solutions in air can be obtained only below decomposition temperature of YCoO 3 , which was evaluated equal to 1266  6 K. Further temperature increase leads to a decrease of YFe 1-x Co x O 3 homogeneity range which is determined to be 0≤x≤0.1 at 1573 K. Phase diagram of the YFeO 3 -YСoO 3 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 unusual behavior of orthorhombic distortions in YFe 1-x Со x O 3 (x = 0.35 and 0.45) with temperature.