BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Physics and Applications of Bismuth Ferrite

A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Perspective on the Development of Lead-free Piezoceramics

Potassium-sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries.

Ferroelectrics are essential components in a wide range of applications, including ultrasonic transducers, sensors, and actuators. In the single crystal form, relaxor-PbTiO3 (PT) piezoelectric materials have been extensively studied due to their ultrahigh piezoelectric and electromechanical properties. In this article, a perspective and future development of relaxor-PT crystals are given. Initially, various techniques for the growth of relaxor-PT crystals are reviewed, with crystals up to 100 mm in diameter and 200 mm in length being readily achievable using the Bridgman technique. Second, the characterizations of dielectric and electromechanical properties are surveyed. Boundary conditions, including temperature, electric field, and stress, are discussed in relation to device limitations. Third, the physical origins of the high piezoelectric properties and unique loss characteristics in relaxor-PT crystals are discussed with respect to their crystal structure, phase, engineered domain configuration, macr...

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High performance ferroelectric relaxor-PbTiO3 single crystals: Status and perspective

A piezoelectric material is one that generates a voltage in response to a mechanical strain (and vice versa). The most useful piezoelectric materials display a transition region in their composition phase diagrams, known as a morphotropic phase boundary, where the crystal structure changes abruptly and the electromechanical properties are maximal. As a result, modern piezoelectric materials for technological applications are usually complex, engineered, solid solutions, which complicates their manufacture as well as introducing complexity in the study of the microscopic origins of their properties. Here we show that even a pure compound, in this case lead titanate, can display a morphotropic phase boundary under pressure. The results are consistent with first-principles theoretical predictions, but show a richer phase diagram than anticipated; moreover, the predicted electromechanical coupling at the transition is larger than any known. Our results show that the high electromechanical coupling in solid solutions with lead titanate is due to tuning of the high-pressure morphotropic phase boundary in pure lead titanate to ambient pressure. We also find that complex microstructures or compositions are not necessary to obtain strong piezoelectricity. This opens the door to the possible discovery of high-performance, pure-compound electromechanical materials, which could greatly decrease costs and expand the utility of piezoelectric materials.

Origin of morphotropic phase boundaries in ferroelectrics

A previously unreported ferroelectric phase has been discovered in a highly homogeneous sample of PbZr0.52Ti0.48O3 by high-resolution synchrotron x-ray powder diffraction measurements. At ambient temperature the sample has tetragonal symmetry (at=4.037 A, ct=4.138 A), and transforms below ∼250 K into a phase which, unexpectedly, has monoclinic symmetry (am=5.717 A, bm=5.703 A, cm=4.143 A, β=90.53°, at 20 K). The intensity data strongly indicate that the polar axis lies in the monoclinic ac plane close to the pseudocubic [111] direction, which would be an example of the species m3m(12)A2Fm predicted on symmetry grounds by Shuvalov.

A monoclinic ferroelectric phase in the pb(zr1-xtix)o3 solid solution

A summary of the work recently carried out on the morphotropic phase boundary (MPB) of PZT is presented. By means of x-ray powder diffraction on ceramic samples of excellent quality, the MPB has been successfully characterized by changing temperature in a series of closely spaced compositions. As a result, an unexpected monoclinic phase has been found to exist in between the well-known tetragonal and rhombohedral PZT phases. A detailed structural analysis, together with the investigation of the field effect in this region of compositions, have led to an important advance in understanding the mechanisms responsible for the physical properties of PZT as well as other piezoelectric materials with similar morphotropic phase boundaries.

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The monoclinic phase in PZT: new light on morphotropic phase boundaries

Dielectric losses behaviour under frequency and a.c. field amplitude variation in TGSe crystals

The Curie temperature and the ideal thermal hysteresis of Pb1−y/2(Zr1−xTix)1−yNbyO3 (0.03 ⩽x⩽0.04, 0.02⩽y⩽0.05) ferroelectric ceramics have been investigated as a function of composition at the FRH–PC phase transition. Experimental results are analyzed using the generalized effective field approach and taking into account the composition dependence of the dimensionless parameters g and h corresponding to the second and third terms in the generalized effective field expansion. With this approach we can describe reasonably well the composition dependence of TC(x,y) at the FRH–PC transition and thermal hysteresis ΔT(x,y) in the narrow range of compositions investigated. The effects of the oxygen octahedra tilt in the dielectric constant at the FRL–FRH transition have also been investigated as a function of composition.

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Dielectric characterization of the phase transitions in Pb1-y/2(Zr1-xTix)1-yNbyO3(0.03≤x≤0.04, 0.02≤y≤0.05)

The dielectric constant and losses of good quality ceramic samples of Nb-doped (1 wt % ${\mathrm{Nb}}_{2}$${\mathrm{O}}_{5}$) ${\mathrm{PbZr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ti}}_{\mathit{x}}$${\mathrm{O}}_{3}$ about the ferroelectric-paraelectric transition for 0.03\ensuremath{\le}x\ensuremath{\le}0.09 have been measured with the aim of investigating the behavior of the thermal hysteresis with composition and the possible existence of tricritical points in this mixed system. The trend of the data, analyzed in terms of a generalized effective-field theory (${\mathit{E}}_{\mathrm{eff}}$=E+\ensuremath{\beta}P+\ensuremath{\gamma}${\mathit{P}}^{3}$+...) indicates that two tricritical points, at x\ensuremath{\simeq}0.26 and x\ensuremath{\simeq}0.51, are implied by the observed behavior at lower x.

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Julio A. Gonzalo

Papers

A monoclinic ferroelectric phase in the pb(zr1-xtix)o3 solid solution

The monoclinic phase in PZT: new light on morphotropic phase boundaries

Dielectric losses behaviour under frequency and a.c. field amplitude variation in TGSe crystals

Dielectric characterization of the phase transitions in Pb1-y/2(Zr1-xTix)1-yNbyO3(0.03≤x≤0.04, 0.02≤y≤0.05)

Composition dependence of the ferroelectric-paraelectric transition in the mixed system PbZr1-xTixO3.