Phase formation in the (1- y )BiFeO 3 - y BiScO 3 system under ambient and high pressure

Magnetic Behaviour of Perovskite Compositions Derived from BiFeO3

The phase content and sequence, the crystal structure, and the magnetic properties of perovskite solid solutions of the (1−y)BiFeO3–yBiZn0.5Ti0.5O3 series (0.05 ≤ y ≤ 0.90) synthesized under high pressure have been studied. Two perovskite phases, namely the rhombohedral R3c and the tetragonal P4mm, which correspond to the structural types of the end members, BiFeO3 and BiZn0.5Ti0.5O3, respectively, were revealed in the as-synthesized samples. The rhombohedral and the tetragonal phases were found to coexist in the compositional range of 0.30 ≤ y ≤ 0.90. Magnetic properties of the BiFe1−y[Zn0.5Ti0.5]yO3 ceramics with y < 0.30 were measured as a function of temperature. The obtained compositional variations of the normalized unit-cell volume and the Néel temperature of the BiFe1−y[Zn0.5Ti0.5]yO3 perovskites in the range of their rhombohedral phase were compared with the respective dependences for the BiFe1−yB3+yO3 perovskites (where B3+ = Ga, Co, Mn, Cr, and Sc). The role of the high-pressure synthesis in the formation of the antiferromagnetic states different from the modulated cycloidal one characteristic of the parent BiFeO3 is discussed.

Comparative density functional studies of pristine and doped bismuth ferrite polymorphs by GGA+U and meta-GGA SCAN+U

We analyzed the effect of nonmagnetic dopants Al, Ga, Sc, and In at the Fe-site on the phase stability, structural, and electronic properties of different bismuth ferrite (BiFeO₃) polymorphs in the framework of density functional theory with the Hubbard U correction (DFT+U). We started our consideration from the determination of the magnitude of the U parameter. First, we calculated the structural, electronic, and magnetic properties of the rhombohedral R3c-G phase of BiFeO₃ within the generalized gradient approximation (GGA) and strongly constrained and appropriately normed (SCAN) meta-GGA for different values of the U. Next, we compared these results with those obtained within the parameter-free hybrid functional. After determining the optimal values of the Hubbard U parameter we analyzed the total energies between the selected BiFeO₃ polymorphs without and with dopants within both GGA+U and SCAN+U. For all dopants the concentration was 12.5% which was close to their solubility limit in BiFeO₃ under ambient conditions. We found that none of these dopants led to the structural phase transition. However, DFT+U calculations revealed that the doping of BiFeO₃ with Al and Ga reduced the energy barrier between R3c-G and Cm-C phases whereas for Sc and In the energy difference between both phases increased. For the orthorhombic phases the considered dopants do not affect the energy barrier between them and the rhombohedral phase. In addition, the ferroelectric polarization does not change after replacing the Fe atom by the dopant for the all considered BiFeO₃ polymorphs.

Phase Transitions in the Metastable Perovskite Multiferroics BiCrO3 and BiCr0.9Sc0.1O3: A Comparative Study

The temperature behavior of the crystal structure as well as dielectric and magnetic properties of the perovskite bismuth chromate ceramics with the 10 mol % Cr³⁺-to-Sc³⁺ substitution were studied and compared with those of the unmodified compound. Using a high-pressure synthesis, BiCrO₃ and BiCr_0.9Sc_0.1O₃ were obtained as metastable perovskite phases which are monoclinic C2/c with the √6a_p × √2a_p × √6a_p superstructure (where a_p is the primitive perovskite unit-cell parameter) under ambient conditions. At room temperature, the unit cell volume of BiCr_0.9Sc_0.1O₃ is ∼1.3% larger than that of BiCrO₃. Both perovskites undergo a reversible structural transition into a nonpolar GdFeO₃-type phase (orthorhombic Pnma, √2a_p × 2a_p × √2a_p) in the temperature ranges of 410-420 K (BiCrO₃) and 470-520 K (BiCr_0.9Sc_0.1O₃) with a relative jump of the primitive perovskite unit cell volume of about -1.6 and -2.0%, respectively. Temperature dependences of the complex dielectric permittivity demonstrate anomalies in the phase transition ranges. The Pnma-to-C2/c crossover in BiCrO₃ is accompanied by a decrease in the direct current (dc) conductivity, while in BiCr_0.9Sc_0.1O₃ the conductivity increases. The onset of an antiferromagnetic order in BiCr_0.9Sc_0.1O₃ is observed at the Néel temperature (T_N) of about 85 K as compared with T_N = 110 K in BiCrO₃. In contrast to BiCrO₃, which exhibits a spin reorientation at T_sr ∼ 72 K, no such a transition occurs in BiCr_0.9Sc_0.1O₃.

The phenomenon of conversion polymorphism in Bi-containing metastable perovskites

A post-synthesis thermal treatment of metastable phases in the high-pressure stabilised perovskite BiFe1-yScyO3 system results in the irreversible formation of polymorphs which represent novel polar and antipolar structures with interesting magnetic properties. Such annealing-stimulated polymorphism is believed to be a general phenomenon which can be found in other systems.

Theory of order–disorder phase transitions of B-cations in AB′1/2 B′′1/2O3 perovskites

Perovskite-like oxides AB′1/2 B′′1/2O3 with two different cations in the B-sublattice may experience cation order–disorder phase transitions. In many cases the degree of cation ordering can be varied by suitable synthesis conditions or subsequent sample treatment, which has a fundamental impact on the physical properties of such compounds. Therefore, understanding the mechanism of cation order–disorder phase transition and estimation of the phase transition temperature is of paramount importance for tuning of properties of such double perovskites. In this work, based on the earlier proposed cation–anion elastic bonds model, a theory of order–disorder phase transitions of B-cations in AB′1/2 B′′1/2O3 perovskites is presented, which allows reliable estimation of the phase transition temperatures and of the reduced lattice constants of such double perovskites.