6 research outputs found
Spontaneous and induced ferroelectricity in the BiFe1−xScxO3 perovskite ceramics
High-pressure synthesis method allows obtaining single-phase perovskite
BiFe1-xScxO3 ceramics in the entire concentration range. As-prepared compositions with x from 0.30 to 0.55 have the antipolar orthorhombic Pnma structure
but can be irreversible converted into the polar rhombohedral R3c or the polar
orthorhombic Ima2 phase via annealing at ambient pressure. Microstructure
defects and large conductivity of the high-pressure-synthesized ceramics make it
difficult to study and even verify their ferroelectric properties. These obstacles can
be overcome using piezoresponse force microscopy (PFM) addressing ferroelectric behavior inside single grains. Herein, the PFM study of the BiFe1-xScxO3
ceramics (0.30 ≤ x ≤ 0.50) is reported. The annealed samples show a strong PFM
contrast. Switching of domain polarity by an electric field confirms the ferroelectric nature of these samples. The as-prepared BiFe0.5Sc0.5O3 ceramics
demonstrate no piezoresponse in accordance with the antipolar character of the
Pnma phase. However, application of a strong enough electric field induces
irreversible transition to the ferroelectric state. The as-prepared BiFe0.7Sc0.3O3
ceramics show coexistence of ferroelectric and antiferroelectric grains without
poling. It is assumed that mechanical stress caused by the sample polishing can
be also a driving force of phase transformation in these materials alongside
temperature and external electric field.publishe
Annealing-Dependent Morphotropic Phase Boundary in the BiMg0.5Ti0.5O3–BiZn0.5Ti0.5O3 Perovskite System
The annealing behavior of (1-x)BiMg0.5Ti0.5O3–xBiZn0.5Ti0.5O3 [(1-x)BMT–xBZT] perovskite solid solutions synthesized under high pressure was studied in situ via X-ray diffraction and piezoresponse force microscopy. The as prepared ceramics show a morphotropic phase boundary (MPB) between the non-polar orthorhombic and ferroelectric tetragonal states at 75 mol. % BZT. It is shown that annealing above 573 K results in irreversible changes in the phase diagram. Namely, for compositions with 0.2 < x < 0.6, the initial orthorhombic phase transforms into a ferroelectric rhombohedral phase. The new MPB between the rhombohedral and tetragonal phases lies at a lower BZT content of 60 mol. %. The phase diagram of the BMT–BZT annealed ceramics is formally analogous to that of the commercial piezoelectric material lead zirconate titanate. This makes the BMT–BZT system promising for the development of environmentally friendly piezoelectric ceramicspublishe
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−yB 3+yO3 perovskites (where B 3+ = 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.publishe
Antisymmetric exchange in La-substituted BiFe0.5Sc0.5O3 system: symmetry adapted distortion modes approach
Neutron powder diffraction measurements on the 35 % La-substituted Bi1-xLaxFe0.5Sc0.5O3-composition revealed that the samples obtained under high-pressure (6 GPa) and high-temperature (1500 K) conditions crystalize into a distorted perovskite structure with the orthorhombic Pnma symmetry and the unit cell para-meters: a(0) = 5.6745(2) angstrom, b(0) = 7.9834(3) angstrom and c(0) = 5.6310(2) angstrom. A long-range magnetic ordering takes place below 220 K and implies a G-type magnetic structure with the moments 4.10(4)mu(B) per Fe aligned predominately along the orthorhombic c-axis. The space group representation theory using the orthorhombic symmetry yields four bi-linear coupling schemes for the magnetic order parameters imposed by antisymmetric exchange interactions. The couplings are analysed based on symmetry adapted distortion modes defined in respect of the undistorted cubic perovskite structure. The approach allows a quantitative estimation of the coupling strength. It is shown that the experimentally found spin configuration combines the magnetic order parameters coupled by the atomic displacement modes with the largest amplitudes. The results indicate that the antisymmetric exchange is the dominant anisotropic term which fully controls the direction of the Fe3+ spins in the distorted perovskite lattice
Complex antipolar root 2 x 4 x 2 root 2 structure with Pnma symmetry in BiFeO3 and BiFe1/2Sc1/2O3: First-principles calculations
First principles calculations are done for a root 2 x 4 x 2 root 2 Pnma structure, which has been recently discussed in several attempts to describe experiments in complex magnetoelectric perovskites and which experimentally was shown to compete with several ferroelectric phases. This makes these materials extremely attracting as switchers, starters, field-stimulated capacitors, high-voltage converters, transmitters, etc. The relative energies of the root 2 x 4 x 2 root 2 Pnma structure have been calculated from first principles and analyzed as a function of pressure in BiFeO3. The stability of two polymorphs of the root 2 x 4 x 2 root 2 Pnma structure has been studied for solid solution BiFe1/2Sc1/2O3. The main distortions and relative energies of these two polymorphs in BiFe1/2Sc1/2O3, in terms of Pm (3) over barm parent symmetry, have been calculated from first principles as well
The orthorhombic-tetragonal morphotropic phase boundary in high-pressure synthesized BiMg0.5Ti0.5O3–BiZn0.5Ti0.5O3 perovskite solid solutions
(1–x)BiMg0.5Ti0.5O3− xBiZn0.5Ti0.5O3 [(1− x)BMT–xBZT] ceramics of perovskite solid solutions, in which BMT
and BZT are lead-free structural analogs of PbZrO3 and PbTiO3, respectively, have been synthesized under high
pressure. It was found that the as-prepared compositions with a relative BZT content of <75 mol% are orthorhombic (space group Pnnm), while those with a BZT content above this value are tetragonal (P4mm). In the
solution with x = 0.75, both phases coexist forming a morphotropic phase boundary (MPB). The compositional
dependence of the normalized unit cell volume exhibits a ~5% jump at x = 0.75. At the same time, the
microstructure of the obtained (1–x)BMT− xBZT ceramics shows no particular variation with the chemical
composition over MPB. Piezoresponse force microscopy measurements indicate the ferroelectric state of the
studied materials and allowed one to estimate their intrinsic piezoelectric coefficients.publishe