Spontaneous Lattice Strain Research Articles

Role of correlated disorder on structural stability and functional properties in [formula omitted

In perovskites, correlated disorder arises due to chemical substitution and often stimulates the functional properties of these materials. Powder neutron diffraction measurements have been used in conjunction with electrical measurements to investigate the effect of correlated disorder in (1−x)NaNbO3−(x)BaTiO3 (NNBTx) solid solution by partially substituting the Ba site with Ca. The effect of correlated disorder on competition and cooperation between polar and antiferrodistortive mode has been investigated. Enhanced dielectric constant (by three times) is observed compared to NNBTx. Emergence/vanishing of diffraction peaks along with change in the intensities is observed in powder neutron diffraction data of (1−x)NaNbO3−x(Ba0.7Ca0.3)TiO3 (NNBCTx). Analysis of this data suggests a structural phase transition with composition change (x), while the phase stability of functional ferroelectric phase at room temperature is extended to lower temperatures. It has also been found that there is a gradual weakening of antiferrodistortive mode (R4+) and presence of spontaneous lattice strain with increasing x. The structural parameters of low-temperature orthorhombic phase of NNBCT05 are modeled in the framework of Grüneisen approximation with a Debye model for the internal energy.

Dielectric and ferroic properties of metal halide perovskites

Halide perovskite semiconductors and solar cells respond to electric fields in a way that varies across time and length scales. We discuss the microscopic processes that give rise to the macroscopic polarization of these materials, ranging from the optical and vibrational response to the transport of ions and electrons. The strong frequency dependence of the dielectric permittivity can be understood by separating the static dielectric constant into its constituents, including the orientational polarization due to rotating dipoles, which connects theory with experimental observations. The controversial issue of ferroelectricity is addressed, where we highlight recent progress in materials and domain characterization but emphasize the challenge associated with isolating spontaneous lattice polarization from other processes such as charged defect formation and transport. We conclude that CH3NH3PbI3 exhibits many features characteristic of a ferroelastic electret, where a spontaneous lattice strain is coupled to long-lived metastable polarization states.

Electrostrain in excess of 1% in polycrystalline piezoelectrics.

Piezoelectric actuators transform electrical energy into mechanical energy, and because of their compactness, quick response time and accurate displacement, they are sought after in many applications. Polycrystalline piezoelectric ceramics are technologically more appealing than single crystals due to their simpler and less expensive processing, but have yet to display electrostrain values that exceed 1%. Here we report a material design strategy wherein the efficient switching of ferroelectric-ferroelastic domains by an electric field is exploited to achieve a high electrostrain value of 1.3% in a pseudo-ternary ferroelectric alloy system, BiFeO3-PbTiO3-LaFeO3. Detailed structural investigations reveal that this electrostrain is associated with a combination of several factors: a large spontaneous lattice strain of the piezoelectric phase, domain miniaturization, a low-symmetry ferroelectric phase and a very large reverse switching of the non-180° domains. This insight for the design of a new class of polycrystalline piezoceramics with high electrostrains may be useful to develop alternatives to costly single-crystal actuators.

Crystallographic Structure Analysis of a Ti-Ta Thin Film Materials Library Fabricated by Combinatorial Magnetron Sputtering.

Ti-Ta thin films exhibit properties that are of interest for applications as microactuators and as biomedical implants. A Ti-Ta thin film materials library was deposited at T = 25 °C by magnetron sputtering employing the combinatorial approach, which led to a compositional range of Ti87Ta13 to Ti14Ta86. Subsequent high-throughput characterization methods permitted a quick and comprehensive study of the crystallographic, microstructural, and morphological properties, which strongly depend on the chemical composition. SEM investigation revealed a columnar morphology having pyramidal, sharp tips with coarser columns in the Ti-rich and finer columns in the Ta-rich region. By grazing incidence X-ray diffraction four phases were identified, from Ta-lean to Ta-rich: ω phase, α″ martensite, β phase, and a tetragonal Ta-rich phase (Ta(tetr)). The crystal structure and microstructure were analyzed by Rietveld refinement and clear trends could be determined as a function of Ta-content. The lattice correspondences between β as the parent phase and α″ and ω as derivative phases were expressed in matrix form. The β ⇌ α″ phase transition shows a discontinuity at the composition where the martensitic transformation temperatures fall below room temperature (between 34 and 38 at. % Ta) rendering it first order and confirming its martensitic nature. A short study of the α″ martensite employing the Landau theory is included for a mathematical quantification of the spontaneous lattice strain at room temperature (ϵ̂max = 22.4(6) % for pure Ti). Martensitic properties of Ti-Ta are beneficial for the development of high-temperature actuators with actuation response at transformation temperatures higher than 100 °C.

Structural perspective on the anomalous weak-field piezoelectric response at the polymorphic phase boundaries of (Ba,Ca)(Ti,M)O3 lead-free piezoelectrics ( M=Zr , Sn, Hf)

Although, as part of a general phenomenon, the piezoelectric response of $\mathrm{Ba}(\mathrm{T}{\mathrm{i}}_{1\ensuremath{-}y}{M}_{y}){\mathrm{O}}_{3}$ ($M=\mathrm{Zr}$, Sn, Hf) increases in the vicinity of the orthorhombic ($Amm2$)-tetragonal ($P4mm$) and orthorhombic ($Amm2$)-rhombohedral ($R3m$) polymorphic phase boundaries, experiments in the last few years have shown that the same phase boundaries show significantly enhanced weak-field piezoproperties in the Ca-modified variants of these ferroelectric alloys, i.e., $(\mathrm{Ba},\mathrm{Ca})(\mathrm{Ti},M){\mathrm{O}}_{3}$. So far there is a lack of clarity with regard to the unique feature(s) which Ca modification brings about that enables this significant enhancement. Here, we examine this issue from a structural standpoint with $M=\mathrm{Sn}$ as a case study. We carried out a comprehensive comparative structural, ferroelectric, and piezoelectric analysis of the $Amm2$ phase in the immediate vicinity of the $P4mm\text{\ensuremath{-}}Amm2$ phase boundaries of (i) Ca-modified $\mathrm{Ba}(\mathrm{Ti},\mathrm{Sn}){\mathrm{O}}_{3}$, as per the nominal formula $(1\ensuremath{-}x)\mathrm{BaT}{\mathrm{i}}_{0.88}\mathrm{S}{\mathrm{n}}_{0.12}{\mathrm{O}}_{3}\text{\ensuremath{-}}(x)\mathrm{B}{\mathrm{a}}_{0.7}\mathrm{C}{\mathrm{a}}_{0.3}\mathrm{Ti}{\mathrm{O}}_{3}$ and (ii) without Ca modification, i.e., $\mathrm{Ba}(\mathrm{T}{\mathrm{i}}_{1\ensuremath{-}y}\mathrm{S}{\mathrm{n}}_{y}){\mathrm{O}}_{3}$. We found that the spontaneous lattice strain of the $Amm2$ phase is noticeably smaller in the Ca-modified counterpart. Interestingly, this happens along with an improved spontaneous polarization by enhancing the covalent character of the Ti-O bond. Our study suggests that the unique role of Ca modification lies in its ability to induce these seemingly contrasting features (reduction in spontaneous lattice strain but increase in polarization).

Direct Access to the Order Parameter: Parameterized Symmetry Modes and Rigid Body Movements as a Function of Temperature

The first order phase transition of CsFeO2 was investigated using synchrotron powder diffraction data as a function of temperature. Two alternative approaches were used to describe the deviation of the framework crystal structure relative to the high-symmetry parent structure: symmetry (a.k.a. distortion) modes and polyhedral-tilt parameters. In both cases, the relevant parameters were refined as a function of temperature using the method of parametric Rietveld refinement. We demonstrate a semi-automated and generally applicable method for the determination of spontaneous lattice strain variations, order parameters and power-law exponents as derived from Landau theory.

Magnetoelectric coupling in the cubic ferrimagnetCu2OSeO3

We have investigated the magnetoelectric coupling in the lone pair containing piezoelectric ferrimagnet Cu2OSeO3. Significant magnetocapacitance develops in the magnetically ordered state (TC = 60 K). We find critical behavior near TC and a divergence near the metamagnetic transition at 500 Oe. High-resolution X-ray and neutron powder diffraction measurements show that Cu2OSeO3 is metrically cubic down to 10 K but that the ferrimagnetic ordering reduces the symmetry to rhombohedral R3. The metric cubic lattice dimensions exclude a magnetoelectric coupling mechanism involving spontaneous lattice strain, and this is unique among magnetoelectric and multiferroic materials.

High dielectric constant and frozen macroscopic polarization in dense nanocrystallineBaTiO3ceramics

Theoretical models for small ferroelectric particles predict a progressive decrease of the Curie temperature, spontaneous lattice strain, and polarization until the critical size corresponding to transition to the cubic phase and disappearance of ferroelectricity is reached. In contrast, the behavior of nanocrystalline $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ ceramics with a grain size of $\ensuremath{\approx}30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ is dominated by extrinsic effects related to the grain boundaries which mask the expected downscaling of properties. While the noncubic crystal structure, the high dielectric constant $(\ensuremath{\approx}1600)$ and the variation of permittivity with temperature suggest a ferroelectric behavior, very slim, and nearly linear polarization hysteresis loops are observed. Evidence for the existence of a ferroelectric domain structure with domains extending over several grains and of polarization switching at local scale is given by piezoresponse force microscopy. The suppression of macroscopic ferroelectric hysteresis and switching originates from a frozen domain structure stable under an external field owing to the effects exerted by the grain boundaries, such as the clamping of the domain walls and the hindrance of polarization switching. Furthermore, the depolarization field originated by the low-permittivity nonferroelectric grain boundaries can cause a significant reduction of polarization. If the grain size is small enough, the ceramic is expected to undergo a ``phase transition'' to a polar phase with nonswitchable polarization. The $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ ceramics with grain size of $30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ investigated in the present study are deemed to be close to this transition.

High Resolution Neutron Powder Diffraction Studies of the Ferroelastic Phase Transition in LaNbO4

ABSTRACTAnalysis of high resolution neutron powder diffraction data of ferroelastic LaNbO4 indicates a monoclinic to tetragonal structural phase transition that is accompanied by anomalous line broadening. The origins of this line broadening have been investigated using a novel modification of the Rietveld technique. The results show that strain broadening is associated with the ferroelastic phase transition both above and below the transition temperature. This anomalous ‘microstrain’ is well-described by a second rank strain tensor, the principal axes of which are collinear with the spontaneous lattice strain. This phenomenon may be a general feature of proper ferroelastic phase transitions.

Ferroelastic effect in RbFeF 4 and CsFeF 4

Orthorhombic CsFeF 4, with spontaneous lattice strain at room temperature of 3.21×10 −4, is ferroelastic. A uniaxial compressive stress of 22 MNm −2 applied along [001] interchanges the a- and c-axes. The ferroelastic atomic displacements in isostructural RbFeF 4 at room temperature are 0.37 Å for Rb, 0.07 Å for F(1) interchanging with F(2), and 0.10 Å for F(3). At 4.5°K, these displacements are estimated as 0.45, 0.30 and 0.11 A. The Fe atom remains invariant under ferroelastic reversal.

Ferroelastic effect in lanthanum orthoferrite

Orthorhombic LaFeO 3, with spontaneous lattice strain ( b - a)/( b + a) = 2.38 × 10 −4, is shown by X-ray methods to be ferroelastic. The highly twinned as-grown crystals are detwinned by applying a compressive stress of about 2 MNm −2 along a combined a, b direction. The [010] direction in a single crystal is reproducibly transformable into the [100] direction under uniaxial stress. The resulting ferroelastic transformation is caused by atomic displacements of 0.23 Å for La, 0.48 Å for O I and 0.79 A for O II, the Fe atom remaining invariant at an inversion center. Coupling is expected between the ferroelasticity and the dominant antiferromagnetic spin orientation, but not with the weak ferromagnetism.

An X-Ray Method for Accurate Determination of Lattice Strains of Crystals

A simple x-ray strainmeter is described which enables us to determine small lattice strains of crystals with the sensitivity of 2×10−6 even in the region of 45° of Bragg angle. The principal feature of this apparatus lies in the large camera radius of 2630 mm. The strainmeter permits not only determination of the thermal expansion at 1°C but also measurement of piezoelectric constants of minute crystals where conventional resonance-antiresonance methods cannot be used. The practical application of this apparatus for detection of small spontaneous lattice strain, say 3′40″ of pure shear xy of Fe–I-boracite, which has recently been found to be ferroelectric in its low temperature form, is demonstrated.