Abstract
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 15at%, 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 ~ 4Å 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 870K, respectively.
Highlights
Rhombohedral perovskite bismuth ferrite, BiFeO3, combines all three ferroic orders in one structural phase [1,2]
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
Using the high-pressure synthesis method, single-phase perovskite solid solutions derived from BiFeO3, in which 50 and more at.% of iron was substituted either by the cations of transition metals (Mn3+ [10,18,19], Co3+ [20,21], Cr3+ [22]) with the ionic size similar to that of Fe3+ or by Ga3+ [23] whose size is by 8% smaller, have been produced
Summary
Rhombohedral perovskite bismuth ferrite, BiFeO3, combines all three ferroic orders in one structural phase [1,2]. Using the high-pressure synthesis method, single-phase perovskite solid solutions derived from BiFeO3, in which 50 and more at.% of iron was substituted either by the cations of transition metals (Mn3+ [10,18,19], Co3+ [20,21], Cr3+ [22]) with the ionic size similar to that of Fe3+ or by Ga3+ [23] whose size is by 8% smaller, have been produced. Combination of the antipolar displacements of bismuth and the ++-- oxygen octahedral tilting results in a 2ap 4ap 2 2ap superstructure (ap ~ 4 Å is the pseudocubic unit-cell parameter) of the Pnma phase in BiFe0.50Sc0.50O3 [24,25] This antipolar phase exhibits a long-range G type of antiferromagnetic order with a weak ferromagnetic component below about 220 K. The revealed compositional sequence in the high-pressure prepared perovskite BiFe1-yScyO3 phases is discussed in comparison with the respective sequence in the BiFeO3-BiMnO3 system
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