Rhombohedral Orthorhombic Phase Boundary Research Articles

Piezoelectric and dielectric properties of lead-free 0.96(K0.48Na0.535)0.96Li0.04Nb1-xSbxO3-0.04CaZrO3 ceramics with rhombohedral–orthorhombic phase boundary

ABSTRACTLead-free 0.96(K0.48Na0.535)0.96Li0.04Nb(1-x)SbxO3-0.04CaZrO3 ceramics were prepared by a conventional solid-state reaction method. The crystal structure of the ceramics was found transforming from an orthorhombic phase to a rhombohedral phase with the increase of Sb content. Furthermore, a rhombohedral-orthorhombic phase boundary was determined in 0.03 ≤ x ≤ 0.05, near which the ceramics exhibited relatively high piezoelectric properties, with maximum values of d33 ∼ 213 pC/N and kp ∼ 0.37 at x = 0.03. Our results suggest that Sb may play an important role in the construction of phase boundary as well as the improvement of piezoelectric activity for (K,Na)NbO3-based piezoelectric ceramics.

Intermediate structural state in Bi1−x Pr x FeO3 ceramics at the rhombohedral–orthorhombic phase boundary

Crystal structure of the Bi1−x Pr x FeO3 ceramics of the compositions corresponding to the threshold concentrations separating the polar rhombohedral (R3c) and anti-polar orthorhombic (Pbam) phases has been investigated with X-ray diffraction, transmission electron microscopy and differential scanning calorimetry measurements performed in a broad temperature range. The structural study specifies the peculiarities of the temperature-driven transition into the non-polar orthorhombic (Pnma) phase depending on the structural state of the compounds at room temperature. The crystal structure analysis reveals the revival of the anti-polar orthorhombic phase upon the temperature-induced transition, thus assuming that it can be considered as the bridge phase between the polar rhombohedral and the non-polar orthorhombic phases.

Microstructure and piezoelectric properties of Ho2O3 doped (K0.4Na0.6)0.95Li0.05Nb0.95Sb0.05O3 lead-free ceramics near the rhombohedral–orthorhombic phase boundary

Ho2O3 doped (K0.4Na0.6)0.95Li0.05Nb0.95Sb0.05O3 (KNLNS) lead-free piezoelectric ceramics were prepared by normal sintering to construct rhombohedral–orthorhombic polymorphic phase boundary. An orthorhombic and rhombohedral phase boundary is formed in the ceramics around x = 0.6–0.8, indicating that the doping of Ho2O3 is helpful to stabilize the low temperature rhombohedral structure. The surface and interior of the x ≤ 0.6 samples exhibit tetragonal and orthorhombic structures, respectively, which demonstrate that phase transitions can be induced by the alkali metal oxides volatilization during sintering. Anomalous broad dielectric relaxation is observed in the er-T curves near TC when x ≥ 0.8. Abnormal grain growth occurs when small amount of Ho2O3 is added but the grain size decreases at high amount of Ho2O3. The optimum electrical properties of d33 = 252 pC/N, kp = 39.67 %, er = 930.4, and Qm = 59.5 are obtained at x = 0.6. These results show that the KNLNS ceramic with 0.6 wt% Ho2O3 is a promising lead-free piezoelectric material.

Evolution of electromechanical properties of Bi1−xPrxFeO3 solid solutions across the rhombohedral–orthorhombic phase boundary: Role of covalency

Bi1−xPrxFeO3 ceramics of the compositions across the rhombohedral–orthorhombic phase boundary have been studied by X-ray diffraction and piezoresponse force microscopy. Charge density calculations and transport properties measurements have also been performed. Piezoresponse force microscopy measurements revealed a considerable increase of the piezoresponse signal for the x=0.125 compound with dominant polar rhombohedral phase and minor amount of antipolar orthorhombic phase. Electron density distribution study testified significant increase of covalency of this compound as compared with gradual delocalization of electronic wave functions observed for other Bi1−xPrxFeO3 compounds with higher praseodymium content. Alteration of charge distribution character has been confirmed by resistivity measurements.

Phase coexistence in Bi1−x Pr x FeO3 ceramics

Bi1−x Pr x FeO3 ceramics across the rhombohedral–orthorhombic phase boundary have been studied by X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. The structural phase transitions in Bi1−x Pr x FeO3 driven by doping concentration and temperature are significantly different from those in BiFeO3 compounds doped with other rare-earth elements. The features of the structural transformations have been discussed based on the specific character of the chemical bonds associated with praseodymium ions. The detailed study of the crystal structure evolution clarified the ranges of both single-phase and phase coexistence regions at different temperatures and dopant concentrations. For x = 0.125, compound extraordinary three-phase coexistence state has been observed in a narrow temperature range at about 400 °C. The results explicate driving forces of the structural transitions and elucidate the origin of the remarkable physical properties of BiFeO3-based compounds near the morphotropic phase boundary.

Electromechanical and magnetic properties of BiFeO3-LaFeO3-CaTiO3 ceramics near the rhombohedral-orthorhombic phase boundary

BiFeO3-LaFeO3-CaTiO3 ceramics have been studied by X-ray diffraction, magnetization measurements, and piezoresponse force microscopy (PFM). The compositional ranges of the polar, antipolar, and non-polar phases have been estimated. PFM measurements testify gradual decrease of piezoelectric response in Bi0.85−xLa0.15CaxFe1−xTixO3 system with Ca/Ti content increase, except a narrow concentration region near polar-antipolar phase boundary where piezoelectric signal shows maximum value. It is found that increase of dopant concentration leads to apparent decrease of the off-center Bi-O displacement and, consequently, causes a reduction of piezoelectric response. It is concluded that notable remanent magnetization in polar and non-polar structural phases is a result of the Dzyaloshinsky-Moria interaction.

Evolution of crystal structure and ferroic properties of La-doped BiFeO3 ceramics near the rhombohedral-orthorhombic phase boundary

Bi1−xLaxFeO3 ceramics (0.15⩽x⩽0.2) across the rhombohedral-orthorhombic phase boundary have been studied using X-ray and neutron diffraction, magnetization measurements and piezoresponse force microscopy (PFM). The regions of structural stability of the polar, anti-polar and non-polar phases have been identified depending on the dopant concentration and a temperature and the structural phase diagram has been further clarified. The factors influencing phase transitions (size effects, chemical bonds peculiarities, local chemical inhomogeneities, etc.) have been estimated. PFM measurements testified a maximal piezoelectric response for a compound with the dominant rhombohedral phase in a metastable state. Magnetic properties have been discussed assuming weak ferromagnetic state with a major contribution from the orthorhombic phase. An evolution of structural parameters across the phase boundary decisive for improved ferroelectric and magnetic properties has been analyzed.

Enhanced ferroelectric, magnetic and magnetoelectric properties of Bi 1− xCa xFe 1− xTi xO 3 solid solutions

We report on the enhanced electromechanical, magnetic and magnetoelectric properties of Bi 1− x Ca x Fe 1− x Ti x O 3 solid solutions. The crystal structure of the x ≈ 0.25 compounds are close to the rhombohedral–orthorhombic phase boundary, and the solid solutions are characterized by increased electromechanical properties due to the polarization extension near the polar–nonpolar border. The homogenous weakly ferromagnetic state is established at x > 0.15 doping. The chemical doping shifts the magnetic transition close to room temperature, thus enlarging the magnetic susceptibility of the compounds. The solid solutions at the morphotropic phase boundary exhibit a nearly twofold increase in piezoelectric response, whereas the magnetoelectric coupling shows five times enhancement in comparison with the parent bismuth ferrite.