Ferroelasticity and ferroelectricity in betaine arsenate

Single crystals of betaine arsenate, an addition compound of the amino acid betaine with H₃AsO₄, were investigated by dielectric, optical, caloric, and X-ray methods. At room temperature betaine arsenate is ferroelastic with two different monoclinic domains. This state seems to disappear at 411 K. Below T_c = 119 K the ferroelastic domains show a ferroelectric behaviour with a spontaneous polarization P_s = 2μC/cm² at T = T_c - 35 deg.     

Microstructural Characteristics of Strontium Magnesium Niobate

Microstructural studies were conducted on the domain boundaries in Sr(Mg_1/3Nb_2/3)O₃ (SMN) complex perovskite compound using X-ray diffractometry and transmission electron microscopy. Both the 1:2 chemical ordering of B-site cations and the tilting of oxygen octahedra were involved in SMN. SMN had a 1:2 ordered monoclinic unit cell, which was distorted by the antiphase tilting of oxygen octahedra. Two types of domain boundaries were found: the antiphase boundaries (APBs), which are not confined crystallographically, and the ferroelastic domain boundaries, which were parallel to the crystallographic planes. SMN had the superlattice reflections of type ±⅙[111] and ±½[111] in the electron diffraction patterns, which cannot be indexed in terms of the 1:2 ordered trigonal phase with only a hexagonal unit cell. The presence of the ferroelastic domains that contained both the 1:2 ordered and the antiphase tilting had been verified by a high-resolution transmission electron microscopy lattice image. The structure of SMN was well explained by a model proposed by other researchers. The formation of the 1:2 ordered domains preceded the ferroelastic domains. Normally, the growth of the ferroelastic domain is not affected by APBs, but it is interrupted by them when the driving force for growth is insufficient, resulting in the stoppage of the domains at APBs.     

In-situ observation of needle domain evolution in barium titanate single crystals

We report in-situ optical observations of the ferroelastic switching in a barium titanate single crystal under compressive stress. Optical micrographs were captured in two regions showing distinct arrangements of domains. Coarsely spaced needle domains in a matrix consisting of a differently oriented large single domain were found to retreat under the application of compressive stress of around 1–2 MPa. However, a comb of closely spaced needle domains was found to be more stable, retreating only slightly under a similar magnitude of applied load. The observations show that the pattern of needle domains influences the ferroelastic switching process and the observed coercive stress can depend strongly on domain arrangement.     

Ferroelastic Toughening in Bismuth Vanadate

Bismuth vanadate has been used as a model system to examine toughening by stress-induced domain motion in a ferroelastic materials. Fracture toughnesses in the ferroelastic and paraelastic states have been compared. Toughening and R -curve behavior have been observed in the ferroelastic state. Single crystals of BiVO₄ show domain nucleation and propagation under compressive loading and domain generation during fracture. The increase in toughening in the ferroelastic state can be explained by a combination of crack deflection and domain wall motion, the latter contributing to R -curve behavior.     

Ferroelectric and ferroelastic phase transitions in (CH3NH3)3Sb2Br9 crystals

(CH₃NH₃)₃Sb₂Br₉ crystals were studied by pyroelectric method in the temperature range 10 - 200 K. Pyroelectric measurements revealed reversal spontaneous polarization below 141 K along the a-axis. The optical observations showed that MABA possesses ferroelastic domains in all low temperature phases.     

Ferroelastic mechanical behaviour of porous La0.6Sr0.4Co0.2Fe0.8O3−δ

The present study investigates the mechanical behaviour of ferroelastic La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ with porosity ranging from 0.9% to 26.1% under uniaxial compression at room temperature. Both dense and porous samples have ferroelastic domains and exhibit ferroelastic mechanical behaviour. The apparent modulus, compressive strength and critical stress of the material dramatically decrease with increasing porosity, exhibiting exponential relationships. Comparison of the mechanical behaviours of samples with similar porosity but different porous structures reveals that the ferroelastic mechanical behaviour depends not only on porosity but also structural factors such as pore size and distribution.     

Ferroelastic Domain Switching as a Toughening Mechanism in Tetragonal Zirconia

X-ray diffraction data are presented in support of the existence of ferroelasticity in tetragonal zirconia. Reorientation of ferroelastic domains by externally applied stress is proposed as a toughening mechanism. Toughening by this mechanism can occur in addition to transformation toughening and also explains high toughness of some zirconias which exhibit no transformation to the monoclinic form. Zirconia ceramics toughened by the ferroelastic mechanism have the potential to retain high toughness at elevated temperature unlike transformation toughening.     

Microstructure Characterization of the (1−x)La2/3TiO3·xLaAlO3 System

Microstructural characterizations on the (1− x )La_2/3TiO₃· x LaAlO₃ (LTLA) system were conducted using transmission electron microscopy. The presence of La₂Ti₂O₇ and La₄Ti₉O₂₄ phases in pure La_2/3TiO₃ is confirmed by the electron diffraction pattern. When x = 0.1, the ordering due to the A-site vacancies could be confirmed by the presence of antiphase boundaries (APBs) and return ½(100) superlattice reflection. As x increases, the ordering decreases and finally disappears when x = 0.6. The tilting of oxygen octahedra could be demonstrated by the presence of the ferroelastic domains in the matrix and return ½(111) and return ½(110) superlattice reflections in selected area electron diffraction patterns. In pure LaAlO₃, only the antiphase tilting of oxygen octahedra is present due to the presence of return ½(111) superlattice reflection. In the LTLA system of x = 0.1, both the antiphase and in-phase tiltings of the oxygen octahedra are involved; however, in the range of x from 0.3 to 0.9, the antiphase tilting of oxygen octahedra has appeared. The growth of the ferroelastic domains is influenced by the APBs in the matrix.     

Observation of the ferroelastic domain structure near the phase transition point

Temperature and time dependences of the ferroelastic domain structure were observed near the ferroelastic phase transition temperature in [N(CH₃)₄]₂ZnBr₄ single crystal. It was, found that the domain structure reaches a metastable state within about 20sec after quenching. The domain size increases with decreasing temperature.     

Stress–Strain Behavior During Compression of Polycrystalline La1−xCaxCoO3 Ceramics

Mechanical stress–strain behavior of LaCoO₃, La_0.8Ca_0.2CoO₃, and La_0.7Ca_0.3CoO₃ was studied under compression at 25° and 300°C. A hysteresis in the stress–strain relationship due to reorientation of ferroelastic domains (deformation twins) was observed, and a remanent strain is measured after unloading. The cohersive stress, defined as the maximum in effective elastic compliance during first loading, increases with substitution of Ca for La and decreases with increasing temperature. Domain reorientation was confirmed by X-ray diffraction of surfaces parallel and perpendicular to the loading direction. LaCoO₃ can be regarded as a soft ferroelastic material while the 30% Ca-substituted material is a hard ferroelastic. The hysteresis of the stress–strain relationship was clearly dependent on both composition and temperature.     

Thermal conductivity of Gd2(MoO4)3 crystal from 0.5 to 80 K

Thermal conductivity of the improper ferroelastic Gd₂(MoO₄)₃ crystal has been studied in the temperature range from 0.5 to 80 K by the steady-state heat-flow method. The ferroelastic domain walls were found to be strong centers of phonon scattering. At temperatures lower than 40 K, the thermal conductivity of the singledomain samples was by almost one order of magnitude greater than that of the multidomain ones.     

Thermal and mechanical properties of LaNbO4-based ceramics

Thermal and mechanical properties of polycrystalline La_1−xA_xNbO₄ (x = 0, 0.005, 0.02 and A = Ca, Sr and Ba) are reported. The materials possess a ferroelastic to paraelastic phase transition close to 500 °C, and the linear thermal expansion is significantly lower (8.6 ± 0.5 × 10⁻⁶ °C⁻¹) for the paraelastic phase compared to the ferroelastic phase (15 ± 3 × 10⁻⁶ °C⁻¹). The hardness was significantly higher for acceptor doped materials (6 GPa) compared to pure LaNbO₄ (3 GPa) due to a significantly smaller average grain size. The fracture toughness of La_0.98Sr_0.02NbO₄, measured by single edge V-notched beam method, was 1.7 ± 0.2 MPa m^1/2 independent of temperature up to 600 °C. The ferroelastic properties of the materials were confirmed by non-linear relationships between stress and strain during compression/decompression, a remnant strain after decompression and the presence of ferroelastic domains. The mechanical properties of LaNbO₄-based materials are discussed with focus on ferroelasticity, microcracking due to crystallographic anisotropy and pinning of ferroelastic domain boundaries.     

Ferroelasticity of t-Zirconia: I, High-Voltage Electron Microscopy Studies of the Microstructure in Polydomain Tetragonal Zirconia

The microstructure of polydomain tetragonal zirconia ( t '-ZrO₂), i.e., a ZrO₂ modification exhibiting ferroelastic behavior, is studied by high-voltage electron microscopy. This material consists of three domain variants of the tetragonal phase with their c-axes nearly orthogonal to each other. Always two variants of these platelike domains are alternately arranged, forming elongated regular colonies. Hence, in both variants the common habit plane of the domains is a {110} twin plane. The colonies are of columnar shape with a 111 longitudinal axis. They are bound by {110} planes, too, which are twin planes for the domains in the contiguous colonies. Owing to their particular structure and the helical arrangement of the adjoining colonies, the material remains coherent and pseudocubic over large macroscopic regions, although it is formed by different tetragonal domains.     

The thermo-mechanical properties and ferroelastic phase transition of RENbO4 (RE = Y,La, Nd,Sm, Gd,Dy, Yb) ceramics

In this work, RENbO₄ (RE = Y, La, Nd, Sm, Gd, Dy, Yb) ceramics with low density, low Young's modulus, low thermal conductivity, and high thermal expansion have been systematically investigated, the excellent thermo-mechanical properties indicate that RENbO₄ ceramics possess the potential as the new generation of thermal barrier coatings (TBCs) materials. X-ray diffraction and Raman spectroscopy phase structure identification reveal that all dense bulk specimens obtained by high-temperature solid-state reaction belonged to the monoclinic (m) phase with C12/c1 space group. The ferroelastic domains are detected in the specimens, revealing the ferroelastic transformation between tetragonal (t) and monoclinic (m) phases of RENbO₄ ceramics. The Young's modulus and hardness of the RENbO₄ ceramics measured by the NanoBlitz 3D nanoindentation method are discussed in details, and the lower Young's modulus (60-170 GPa) and higher hardness (the maximum value reaches 11.48 GPa) indicating that higher resistance of RENbO₄ ceramics to failure and damage. Lower thermal conductivity (1.42-2.21 W [m k]⁻¹ at 500°C-900°C) and lower density (5.330-7.400 g/cm³) than other typical TBCs materials give RENbO₄ ceramics the unique advantage of being new TBCs materials. Meanwhile, the thermal expansion coefficients of RENbO₄ ceramics reach 9.8-11.6 × 10⁻⁶ k⁻¹ and are comparable or higher than other typical TBCs materials. According to the first-order derivative of the thermal expansion rate, the temperature of the ferroelastic transformation of RENbO₄ ceramics can be observed.     

Ferroelastic and Plastic Deformation of t'-Zirconia Single Crystals

Polydomain tetragonal ( t' ) zirconia was deformed in compression along a 〈100〉 orientation at various temperatures between 500° and 1400°C. The stress-strain curves showed a plateau corresponding to ferroelastic deformation, followed by plastic deformation at a higher yield stress level. In both ranges, the strain-rate sensitivity of the stress was measured by stress-relaxation tests. The microstructure of the tetragonal domains after ferroelastic deformation and the dislocation substructure were studied by transmission electron microscopy in a high-voltage electron microscope. As expected, ferroelastic deformation suppressed the tetragonal variant with its c -axis parallel to the loading direction. The dislocation structure consisted of intersecting dislocations on different slip systems with strongly bowed-out segments. The microprocesses of deformation are discussed here by comparing the deformation data with those of cubic zirconia deformed in the same orientation and based on the observed microstructure. The particular microstructure of t' zirconia seems to prevent recovery, so that the high flow stress of ∼700 MPa is preserved up to 1400°C.     

Relaxational dielectric anomaly in the ferroelectric phase of K2ZnCl4

The temperature dependence of dielectric constant of K₂ZnCl₄ was measured at 10, 20, 40 and 100 kHz in the temperature range between 120 and 250 K. A relaxational dielectric anomaly was observed near 210 K for the first time. It is ascribed to domain wall pinning induced by ferroelastic micro-domains which begin to form around 200 K and behave as defects. The dielectric anomaly disappeared after several thermal cycles of heating and cooling. This may be due to the remnant ferroelastic matrix in the ferroelectric phase.     

Ferroelasticity of t'-Zirconia: II, In situ Straining in a High-Voltage Electron Microscope

The ferroelastic deformation of t '-ZrO₂, the microstructure of which was described in detail in Part I, was investigated by in situ deformation experiments in the high-voltage electron microscope at 1150°C. During the experiments those two domain variants with their c-axes perpendicular to the [010] tensile direction were transformed into the third one with its c-axis parallel to the tensile direction. The subsequent 'switching' of the domains inside the colonies proceeds much faster than the penetration of the transformation front into a neighboring colony. Therefore, the transformed region, exhibiting a unique tetragonal structure and containing residual defects, preferentially expands into the longitudinal directions of the colonies. The transformation of single domains proceeds instantaneously within the time resolution of the video tape recording.     

Two Types of Domain Boundaries in Lanthanum Magnesium Niobate

Microstructural studies of the domain boundaries in the complex perovskite compound lanthanum magnesium niobate (La[Mg_2/3Nb_1/3]O₃, LMN) were conducted using transmission electron microscopy. Both the 1:1 chemical ordering of B-site cations and the tilting of oxygen octahedra affected the domain boundaries. Two types of domain boundaries were observed. In addition to the presence of antiphase boundaries, which were insensitive to the crystallographic planes, ferroelastic domain boundaries that were caused by the phase transition due to the tilting of oxygen octahedra also were present. In some grains, only one type of oxygen tilting was present, which resulted in a single domain in one grain. Two or three domains were observed in a grain where the walls were parallel to the {110} plane. Many domains also were observed in a grain that had boundaries whose linear characteristics were gradually reduced.     

Ferroelastic phase transitions in some high-temperature superconductors with perovskite-like structure

The behaviour of low-frequency shear modulus of the La_2xSr_xCuO₄ compound with x = 0 and 0.15 in the vicinity of improper ferroelastic phase transition has been studied by the method of inverse torsion pendulum.     

Short Crack R-Curves in Ferroelectric and Electrostrictive PLZT

This work describes the measurement of R -curve behavior in ferroelectric ceramics using four-point bend specimens with controlled semielliptical surface cracks. The results are compared for two compositions of lead lanthanum zirconate titanate. One exhibits ferroelastic behavior, the other electrostrictive linear elastic behavior. R -curves are measured in the crack length regime of 0.1 to 0.8 mm. The ferroelastic composition displays a toughness increase from 0.5 to 1.2 MPa·m^1/2. The linear elastic composition displays a flat R -curve. The R -curve behavior is attributed to ferroelastic toughening.