Phase formation in the (1-y)BiFeO3-yBiScO3 system under ambient and high pressure

Formation and thermal stability of perovskite phases in the BiFe1-yScyO3 system (0≤y≤0.70) were studied. When the iron-to-scandium substitution rate does not exceed about 15 at.%, the single-phase perovskite ceramics with the rhombohedral R3c symmetry (as that of the parent compound, BiFeO3) can be prepared from the stoichiometric mixture of the respective oxides at ambient pressure. Thermal treatment of the oxide mixtures with a higher content of scandium results in formation of two main phases, namely a BiFeO3-like R3c phase and a cubic (I23) sillenite-type phase based on γ-Bi2O3. Single-phase perovskite ceramics of the BiFe1-yScyO3 composition were synthesized under high pressure from the thermally treated oxide mixtures. When y is between 0 and 0.25 the high-pressure prepared phase is the rhombohedral R3c with the 2ap2ap23ap superstructure (ap ~ 4 Å is the pseudocubic perovskite unit-cell parameter). The orthorhombic Pnma phase (2ap4ap22ap) was obtained in the range of 0.30≤y≤0.60, while the monoclinic C2/c phase (6ap2ap6ap) is formed when y=0.70. The normalized unit-cell volume drops at the crossover from the rhombohedral to the orthorhombic composition range. The perovskite BiFe1-yScyO3 phases prepared under high pressure are metastable regardless of their symmetry. At ambient pressure, the phases with the compositions in the ranges of 0.20≤y≤0.25, 0.30≤y<0.50 and 0.50≤y≤0.70 start to decompose above 970, 920 and 870 K, respectively. Graphical Abstract


Introduction
Rhombohedral perovskite bismuth ferrite, BiFeO 3 , combines all three ferroic orders in one structural phase [1,2]. A polar order results from parallel off-centre displacements of Bi 3+ cations, a magnetic order is induced by exchange interactions between Fe 3+ cations in oxygen octahedra, and an elastic order originates from correlated rotations (tilts) of these octahedra.
Besides, bismuth ferrite is one of very few type-I perovskite multiferroics that can be produced using the conventional preparation methods, particularly the ceramic technique [2,3]. However, in spite of the availability and importance of bismuth ferrite for studies of the multiferroic phenomena, this compound can hardly be used in practice. Because of modulation of the spin structure, the macroscopic magnetization of BiFeO 3 is averaged to zero [4]. Furthermore, the Curie temperature and the Neel temperature of bismuth ferrite are too high (1083 K and 643 K, respectively [2]), which makes ineffective an exploitation of the lattice-magnetic coupling effect in the operating range of the majority of electronic devices. Pure BiFeO 3 is difficult to prepare since numerous parasitic phases form in the Bi 2 O 3 -Fe 2 O 3 system at near the same conditions as the perovskite phase does [3]. A problem of stabilization of perovskite phase as well as an adjustment of physical properties are usually solved by means of modification of chemical composition. Indeed, using the conventional preparation methods, the perovskite solid solutions with gradual substitution of Bi 3+ by a trivalent cation (as a rule, a rare-earth one) were successfully obtained. At the relatively small substitution rates (typically, below [10][11][12][13][14][15] at.%), the crystal structure remained rhombohedral described by the R3c space group as that of the parent compound. An increase of the substitution rate results in formation of other perovskite phases (Ref [5] and references therein). In most cases, these phases coexist over wide compositional ranges. Through the Bi-site substitutions, the polar order and the tilting configuration of oxygen octahedra were modified, while the magnetic ordering temperature remained basically the same as that in BiFeO 3 . Wide-range substitutions in the Fe site using the conventional methods were not very successful. In case of cations with either higher oxidation state (Ta 5+ , Ti 4+ ) [6,7] or lower oxidation state (Mg 2+ ) [8] than that of Fe 3+ , the substitution rates of order of few at.% only were achieved. Although some physical properties (e.g., electrical resistivity) were improved, those substitutions have induced no change in the crystal structure symmetry. Even when the substituting cation was Mn 3+ , which is very similar to Fe 3+ in terms of size and electronic configuration, the maximal rate did not exceed about 30 at.% [9,10] and crystal structure symmetry of the obtained solid solutions remained the rhombohedral, R3c.
It is known that although most of Bi-containing compositions of both the BiB 3+ O 3 -type and the BiB 2+ 0.5 B 4+ 0.5 O 3 -type do not crystallize in perovskite structure at ambient pressure, they can be obtained as metastable perovskite phase by means of quenching from 1000-1500 K under a pressure of 4-6 GPa (the high-pressure synthesis) [11][12][13][14][15][16]. It should be pointed out that the magnitude of the applied pressure does matter: in particular, a pressure of several tens of MPa was not enough to synthesize single-phase solid solutions of BiFe 0.5 B 3+ 0.5 O 3 , where B 3+ =Cr, Mn, Sc, Y [17]. Using the high-pressure synthesis method, single-phase perovskite solid solutions derived from BiFeO 3 , in which 50 and more at.% of iron was substituted either by the cations of transition metals (Mn 3+ [10,18,19], Co 3+ [20,21], Cr 3+ [22]) with the ionic size similar to that of Fe 3+ or by Ga 3+ [23] whose size is by 8% smaller, have been produced.
Structural phases different from the parent rhombohedral R3c have been obtained and thoroughly studied. In all the above-mentioned solid solutions, the normalized unit-cell volume was found to decrease with increase of rate of substitution of Fe 3+ by those trivalent cations.
We have recently reported on a high-pressure synthesis of the perovskite solid solution with the BiFe 0.5 Sc 0.5 O 3 composition [24]. One should noticed that the ionic size of Sc 3+ is by 24% bigger than that of Fe 3+ . Structure of the as-prepared samples was found to be orthorhombic Pnma similar to that reported for the BiFe 1-x Mn x O 3 solid solutions in the range of 0.15≤x≤0.40 synthesized under high pressure [10]. Combination of the antipolar displacements of bismuth and the ++--oxygen octahedral tilting results in a 2a p 4a p 22a p superstructure (a p ~ 4 Å is the pseudocubic unit-cell parameter) of the Pnma phase in BiFe 0.50 Sc 0.50 O 3 [24,25].
This antipolar phase exhibits a long-range G type of antiferromagnetic order with a weak ferromagnetic component below about 220 K. It was revealed that the heating to about 770 K followed by cooling down to room temperature results an irreversible crossover from the orthorhombic Pnma phase to the orthorhombic Ima2 one via the intermediate high-temperature rhombohedral R3c phase. The Ima2 polymorph of BiFe 0.5 Sc 0.5 O 3 demonstrates the same type of magnetic order below 220 K as that of its antipolar polymorph and complex polar structure which makes this phase one of rare examples of canted ferroelectrics [24]. One can expect new structural phases and other types of polymorphism in solid solutions of the BiFe 1-y Sc y O 3 system with the Fe/Sc ratio different from 1:1.
In this paper, we report on study of mutual solubility in the quasibinary (1-y)BiFeO 3 -yBiScO 3 system. Samples with the nominal composition of 0.10≤y≤0.70 were thermally treated at ambient pressure and under high pressure. Crystal structure of the obtained phases was characterized using methods of x-ray and neutron powder diffraction. The revealed compositional sequence in the high-pressure prepared perovskite BiFe 1-y Sc y O 3 phases is discussed in comparison with the respective sequence in the BiFeO 3 -BiMnO 3 system. Thermal treatments under high pressure were performed using an anvil press DO-138A with a press capacity of 6300 kN. Both the raw oxide mixtures and the product of their presynthesis at ambient pressure (as described above) were explored. The powders were pressed into pellets of 4.5 mm in diameter and about 4 mm height. In order to avoid penetration of graphite from the tubular heater to the sample a protective screen of molybdenum foil was used. The samples were synthesized/sintered at 6 GPa and 1170-1470 K. The high-pressure treatment time was between 9 and 1 min (depending on temperature).

Experimental
In order to estimate the stability limits of the perovskite phases prepared under high pressure, the obtained ceramic samples were annealed at temperatures between 770 and 1120 K 5 with a 50 K step. The samples were put a furnace heated to the certain temperature and quenched in air after a 2-h dwell. An x-ray diffraction (XRD) study of the powdered samples was performed using either a DRON-3 diffractometer (phase analysis) or a PANalytical X'Pert MPD PRO diffractometer (phase analysis and the crystal structure characterization) in Cu Kα radiation at room temperature.
Neutron powder diffraction data were collected at the ISIS pulsed neutron and muon facility of the Rutherford Appleton Laboratory (UK), on the WISH diffractometer located at the second target station [26]. The sample was loaded into a 3-mm cylindrical vanadium can and measured at room temperature.
Rietveld refinement of the crystal structure was performed using the FullProf program [27].
Microstructure of the fractured surface and local chemical composition of the highpressure prepared ceramics before annealing and after each annealing step were studied by scanning electron microscopy (SEM, Hitachi S-4100) equipped with an energy dispersive spectroscopy (EDS) detector.

Phase formation in the BiFe 1-y Sc y O 3 system under ambient pressure
At a first step of the study, the thermal treatment of all the BiFe 1-y Sc y O 3 samples (0.10≤y≤0.70) was performed at the certain temperature, 1040 K, which is the optimal temperature for synthesis of polycrystalline BiFeO 3 from simple oxides [28]. The treatment duration was 10 min.
It was found from analysis of the XRD patterns that the products of the thermal treatment represented mixtures of two main phases with the ratio between them dependent on the relative content of scandium ( Figure 1). Perovskite phase, which was present in the most amount in the sample of the BiFe 0.90 Sc 0.10 O 3 nominal composition, demonstrates the same rhombohedral distortion of the crystal lattice as that in the parent compound, BiFeO 3 . However, the reflections are shifted to a lower angle range indicating a partial substitution of iron by scandium. Second phase was identified to be cubic, a sillenite-type, described by the I23 space group [29]. This structure type is derived from a γ-modification of Bi 2 O 3 , in which from 5 to 10 at.% of bismuth is in crystallographic position that is not equivalent to that of other bismuth and can be substituted by other elements, in particular, by iron and/or scandium [30][31][32][33].
Attempts were undertaken to change the phase ratio in favour of the perovskite phase. It turned out that double homogenization, increase of the reaction time, and variation of the heating/cooling rates resulted in changes in neither quantitative ratio nor qualitative content of the observed phases. We managed to obtain a single-phase perovskite solid solution with y=0.10 when the treatment temperature was increased to 1140 K. However, the thermal treatment of the BiFe 1-y Sc y O 3 samples with a higher scandium content at 1140, 1170 and 1220 K resulted in no desired effect: the relative amount of the perovskite phase in the product was almost the same as that in the material treated at 1040 K. Increase of the treatment temperature has resulted only in a transformation of the phase based on γ-Bi 2 O 3 into the phase based on β-Bi 2 O 3 with the tetragonal symmetry [30,34,35] The XRD patterns of a sample with y=0.50 treated at 1170 K and then at 1220 K is shown in Figure 2. It is seen that the sample treated at 1220 K followed by quenching consists of two phases, namely the rhombohedral perovskite phase and the tetragonal phase. An annealing of the sample at temperature below 1170 K led to a transition from the phase back to the I23 phase ( Figure 2). The observed reversibility of the transformation indicates the same chemical composition of these phases.

Synthesis of the BiFe 1-y Sc y O 3 perovskite phases under high pressure
High-pressure synthesis of the BiFe 1-y Sc y O 3 (0.20≤y≤0.70) ceramics directly from the oxide mixture (without thermal treatment at ambient pressure) turned out to be unsuccessful:   In order to compare and analyse the structure distortions over the whole compositional range studied, the lattice parameters obtained from the refinements were recalculated into the primitive perovskite unit-cell parameters [39]. We used the following relations for the basis  The same sequence of the structural phases has been observed in ceramics of the BiFe 1- x Mn x O 3 system prepared under high-pressure: the rhombohedral R3c phase for x from 0 to about 0.10 is followed by the orthorhombic Pnma one which transforms into the monoclinic C2/c phase between x=0.60 and 0.80 [10,18]. It should be stressed here that, in contrast with the V p (y) dependence of the BiFe 1-y Sc y O 3 perovskites ( Figure 5), the normalized unit-cell volume in the BiFe 1-x Mn x O 3 system decreases with increase of the Fe-substitution rate. In spite of this distinction in kind, in both these systems, V p drops when crossing from the R3c range to the Pnma range. This is another evidence that crystal symmetry of Bi-based perovskites is determined mainly by the local atomic coordination of bismuth rather than geometrical criteria such as a tolerance-factor value [40,41]. Orthorhombic Pnma phase with the 2a p 4a p 22a p superstructure exist in wide ranges of the Fe-substitution in bismuth ferrite. Moreover, the same phase is observed in some BiFeO 3 -based perovskites prepared at ambient pressure where bismuth is partly substituted by a rare earth cation [5,42,43]. However, no such phase has been detected in undoped BiFeO 3 although numbers of structural studies over wide ranges of either temperature or pressure have been undertaken. It seems that the Pnma (2a p 4a p 22a p ) structural type requires more than one type of cation in at least one of the cation sites of a perovskite derived from BiFeO 3 .
In order to estimate the limit of solubility of BiScO 3 in BiFeO 3 at ambient pressure, values of the average parameter (a av =V p 1/3 ) of the rhombohedral perovskite phase in all the obtained BiFe 1-y Sc y O 3 samples were calculated and compared. Figure 7 presents the a av values as a function of scandium content for the R3c phase both in the high-pressure prepared ceramics and in the two-phase product obtained by means of the thermal treatment at ambient pressure as described in Part 3.1. Compositional dependence of lattice parameter of the second phase in the product, namely the cubic I23 one based on γ-Bi 2 O 3 , is shown in Figure 7 as well.
Parameters of both phases prepared at ambient pressure grow with increase of the nominal scandium content that is agreement with a gradual substitution of iron by a larger cation. It should be noticed that although the growth become slower with increasing y, it does not saturate suggesting that the substitution continues over the whole compositional range studied.
By extrapolation of the a av (y) dependence for the rhombohedral perovskite phase in the twophase product until intersection with the respective dependence for the high-pressure prepared phase (Figure 7), the maximal scandium content in the BiFe 1-y Sc y O 3 solid solution that can be obtained as a single phase at ambient pressure was estimated to be close to 0.15.

Stability of the high-pressure prepared BiFe 1-y Sc y O 3 perovskites at ambient pressure
The perovskite ceramics prepared under high pressure were step-by-step annealed as indicated in Experimental followed by XRD analysis after each step. The annealing temperature was increased until reflections attributed to the cubic sillenite-type phase based on γ-Bi 2 O 3 were detected in the XRD patterns. The thermal stability limit was defined as the temperature which is 50 K lower than that when the sillenite-type phase appears. The stability limit temperature of perovskite phase in the BiFe 1-y Sc y O 3 system was found to depend on the scandium content and decreases as y increases (Table 2). Thus, the perovskite phases of the BiFe 1-y Sc y O 3 composition (0.20≤y≤0.70) prepared under high pressure are metastable regardless of their crystal symmetry. The phases remained in the high-pressure prepared samples after annealing at temperatures by 100 K above their stability limits were identified to be the rhombohedral perovskite phase and the cubic sillenite-type one (Figure 8). The perovskite/sillenite phase ratio in the annealed samples was found to be the same as that in the products of the respective nominal compositions thermally treated at ambient pressure as described in Part 3.1. Moreover, the crystal lattice parameters of the corresponding phases were equal. Thus, the phases appeared after annealing of the high-pressure prepared ceramics and the phases formed as a result of thermal treatment of the oxide mixtures the same nominal composition at ambient pressure are identical.
Typical SEM images of fractured surface of the high-pressure synthesized samples before and after annealing at temperature above the stability limit are shown in Figure 9.
Irregular-shape grains with signs of melting were observed in the as-prepared ceramics. No correlation between the chemical composition and the average grain size of the BiFe 1-y Sc y O 3 ceramics has been detected. Grains with a rectangular prism shape were observed in the SEM images of the annealed samples whose XRD patterns demonstrated considerable amounts of the phase based on γ-Bi 2 O 3 . Such a grain shape is in a good agreement with the cubic symmetry of this phase.

Conclusions
The maximal iron-to-scandium substitution rate in BiFeO 3 that can be achieved using the