Twin Domain Boundaries Research Articles

Observation of domain boundaries in a TbNi2B2C single crystal

The domain structure of TbNi2B2C was studied by the finest (⋍0 nm) magnetic-particle decoration below 13 K. The twin domain boundaries caused by magnetoelastic stresses were observed in the {110} planes.

TEM study of the formation of twins in PMP-processed YBCO bulks with submicron Y 2BaCuO 5 particles

YBCO bulks with submicron Y 2BaCuO 5 (211) particles have been prepared under oxygen partial pressures of 1 and 0.21 atm, respectively, by the powder melting process (PMP) technique. The microstructures of the as-grown samples and the oxygenated samples have been characterized by transmission electron microscopy and their oxygen contents have also been measured. No twins could be found in the as-grown sample prepared under P(O 2)=0.21 atm. On the other hand, twins were observed to nucleate at the 211/123 interface and subgrain boundaries and extend into the interior of a YBCO matrix in the as-grown sample produced under P(O 2)=1 atm. The oxygen contents were 6.63 and 6.76 for the two samples, respectively. High densities of twins with an average width of 60–80 nm were found in the oxygenated samples prepared both under P(O 2)=1 atm and under P(O 2)=0.21 atm. Moreover, a kind of twin domain boundary structure has been observed. The formation mechanisms of the high density of twins and the twin domain boundary structure have been discussed in this paper.

Domain structures in epitaxial Pb(Zr 0.68,Ti 0.32)O 3 thin films

Domain structures and their volume fractions in (0 0 1)-, (1 0 1)- and (1 1 1)-oriented epitaxial Pb(Zr 0.6,Ti 0.4)O 3 (PZT) films grown on (0 0 1), (1 1 0) and (1 1 1) SrTiO 3 (STO) single crystal substrates, respectively, were characterized by using a high resolution X-ray diffraction reciprocal space mapping (RSM) technique. The results of RSM revealed the presence of (1 0 1 ̄ ) and (1 1 1 ̄ ) orientation as well as (1 1 0) and (1 1 1) orientations for the films grown on (1 1 0) and (1 1 1) STO substrates, respectively, although (0 0 1)-oriented film had (0 0 1)-oriented simple domain structure. These multiple domain configurations seemed to be introduced by {1 0 0} and {1 0 1} twin domain boundaries. The RSM analysis also showed that the relative volume fractions of (1 0 1 ̄ ) and (1 1 1 ̄ ) orientations were 22% and 52% against (1 1 0) and (1 1 1) orientations, respectively.

Microstructure of bulk PMP YBCO with submicrometre Y2BaCuO5particles

The microstructure of bulk YBCO processed by the powder melting processusing a submicrometre Y2BaCuO5 precursor in combination with the addition of CeO2was characterized by scanning electron microscopy, transmissionelectron microscopy and high resolution electron microscopy. The mean diameterof the Y2BaCuO5 precursor was less than 500 nm. As a result, themean diameter of the Y2BaCuO5 particles entrapped in the textured bulk YBCO isabout 500 nm. Two types of Y2BaCuO5 were found within the bulk YBCO: onewith one or two nanometric Y2O3 cores; the other, however, hasno cores. The refinement mechanism for Y2BaCuO5 particles related to theemployment of a submicrometre Y2BaCuO5 precursor and the minor addition ofCeO2 is discussed. In addition, a high density of twins with an averagespacing of about 60-80 nm was observed. Moreover, a twin domain boundarystructure where domains with different orientation twins meet one another wasfound. The formation mechanisms for this high density of twins and the twindomain boundary structure are also discussed.

Short- and long-range ordering in the ilmenite-hematite solid solution

Magnetic and cation ordering in the ilmenite–hematite solid solution (FeTiO3–Fe2O3) has been studied using in-situ high-temperature time-of-flight neutron powder diffraction. Synthetic samples containing 60 and 65% FeTiO3 (ilm60 and ilm65, respectively) were heated under vacuum up to 1000 °C and their magnetic structure, crystal structure and cation distribution were determined via Rietveld refinement. The quenched starting materials display diffuse superlattice reflections, indicative of short-range cation order. The short-range ordered structure is interpreted with the aid of statistical simulations to be a fine-scale alternation of ordered and antiordered ilmenite-like twin domains, separated by hematite-like twin-domain boundaries (TDBs). Peak width analysis demonstrates that the twin domains have a pronounced shape anisotropy, with average lengths of 20 ± 1 and 60 ± 2 A along the c-axis, and 100 ± 9 and 100 ± 4 A along [0&1macr;1]* in ilm60 and ilm65, respectively. Long-range order increases initially by a process of domain coarsening as the quenched samples are heated below the cation order-disorder temperature, Tod. The degree of order then decreases as they are heated through the transition. This leads to a kinetic relaxation behaviour, in which the observed rate of ordering is determined by the balance between the rate of coarsening and the rate of disordering within the domains. A phenomenological kinetic model is developed, which provides an excellent description of the observed behaviour in both samples. Once long-range order has been established, the equilibrium degree of order as a function of temperature is well described by a modified Bragg-Williams model, yielding values of Tod=830 ± 20 °C and 911 ± 20 °C for ilm60 and ilm65, respectively. Analysis of the magnetic scattering and spontaneous strain demonstrates that short-range magnetic order remains at temperatures well above the bulk Curie temperatures (Tc=178 °C and 143 °C in ilm60 and ilm65, respectively). This indicates significant magnetic heterogeneity in the samples, which may be related to the presence of a high density of Fe-enriched TDBs in the quenched material.

Real-time observation of twin boundary motion in organic crystal (TMTSF)2X

Ferroelastic domain boundary can be regarded as a type of twin domain boundary. A twin deformation is induced in a conducting organic crystal (TMTSF)2X when the shear stress is applied. TMTSF denotes tetramethyle-tetraselenafulvalene and × does an anion such as CIO4 + and PF6 +. The boundary can move easily along one direction under the small shear stress of about 1 kPa. We could successfully observe the motion of the isolated twin boundary in real time and investigate control parameters of its motion. The motion of the twin boundary is intermittent even under a constant stress. The boundary can move under stress above a small threshold. Temperature dependence of the local velocity strongly depends on the applied stress: when the temperature is lowered, the velocity increases under the small stress while decreases under the large stress. The local velocity depends not only on external stress and temperature but also on the waiting time before the onset of motion.

Structure and twinning of tetragonal Ca3Mn2Ge3O12garnet

Single crystals of tetragonal Ca 3 Mn 2 Ge 3 O 12 garnet (space group symmetry I 4 1 /a , a = 12.3098(7) A, c = 12.3277(9) A) were characterized by X-ray diffraction and transmission electron microscopy. Their structure is topologically isosymmetric to tetragonal high-pressure garnets such as majorite, displaying the same two distinct types of macroscopic twin mechanisms. Twinning occurs as pseudo-merohedral ferroelastic twin lamellae with preferred orientation of the twin-domain boundaries parallel to {101} tet , whereas merohedral ferrobielastic twin domains occur without any orientational preference. The crystal structure was determined from single-crystal X-ray diffraction data of a crystal fragment completely free of pseudo-merohedral twin domains. It shows two different Jahn-Teller distorted MnO 6 octahedra, with a different orientation of the axis of polyhedral elongation. The ordering scheme of these Jahn-Teller distorted octahedra follows in an alternating pattern the densest rod packing, and the cooperative effect of the electronically induced octahedral distortion was found to be responsible for the cubic-to-tetragonal symmetry breaking in Ca 3 Mn 2 Ge 3 O 12 garnet. The extent of polyhedral distortion indicates a partially dynamic character of the Jahn-Teller effect. The distortion of the tetrahedral T2 sites controls the cooperative effect of the lattice strain induced by the Jahn-Teller distortion of the Mn 3+ octahedra and thus is responsible for the overall lower symmetry.

Study of YBa2Cu3O7—δ High-Tc Superconductor Single Crystals by Electron Microscopy

Microstructures of three types of YBa/sub 2/Cu/sub 3/O/sub 7- delta / single crystals having strong differences in their j/sub c/ levels have been studied by means of electron microscopy. In order to find any existing relation between the microstructures and their j/sub c/ values, histograms of the frequency of twin spacings were obtained from SEM as well as TEM measurements. These histograms show that the finest and most regular twin domain texture (twin spacing 130+or-80 nm) may be correlated with the highest j/sub c/ values. The single crystals with the highest j/sub c/ values have stacking faults (due to double Cu-O chain layers) homogeneously distributed in an otherwise rather perfect matrix of 123-phase. These stacking defects have a limited extension of approximately 80 nm, while in the other types of crystals, they have an extension approximately 1 mu m. These results suggest that relatively higher j/sub c/ values are associated with certain characteristic microstructural features related to twin domain boundaries as well as to stacking faults.

Nanometer scale optical studies of twin domains and defects in lanthanum aluminate crystals

A near-field scanning optical microscope (NSOM) in transmission geometry was used to study optical characteristics of twin domains, domain boundaries, and other defects in lanthanum aluminate (LaAlO3) single crystals. Large optical contrast between domains was observed using linearly polarized NSOM light. This domain contrast can be changed and even reversed by varying the direction of the polarization axis with respect to the local crystallographic axis. We further demonstrate that the contrast depends critically on the numerical aperture of the collection objective lens, enabling us to differentiate between possible contrast mechanisms. In addition, microscopic stress birefringence associated with the domain boundaries was observed under crossed-polar conditions. Submicron circular optical features on LaAlO3 are attributed to surface terminations of dislocations.

Electron microscopy study of domain structure due to phase transitions in natural perovskite

Transmission electron microscopy on natural calcium metatitanate perovskite (dysanalyte) reveals the following twin laws in the orthorhombic (space group Pbnm) phase: reflection twins on the {110} and {112} planes, and 90° rotation twins about the [001] axis (referred to as [001]90° twin). Single crystals that were heattreated and quenched from above 1585 K exhibit a dramatic change in domain structure compared with the starting material and specimens quenched from T < 1470 K. Mutually perpendicular {110} and [001]90° twins are observed throughout the crystal, forming a cross-hatched domain texture. 1/2[001] antiphase domains, which are very rarely observed in the starting material, also become dominant in the crystal. This change in domain structure is interpreted as due to a structural phase transition in perovskite at a temperature below 1585 K. From the point symmetry elements that describe the twin laws and the translational elements that relate the antiphase domains, the most likely phase near 1585 K is tetragonal with space group P4/mbm. These results are consistent with high-temperature powder X-ray diffraction study. On the other hand, density of the {112} twins is increased significantly in the crystal quenched from 1673 K. Twin domains are either bound by mutually perpendicular {110} and (001) walls, or by {112} walls with {110} twin domains within the polygonal {112} domains. Both twin density variation and domain morphology suggest that the crystal may be cubic at this temperature. Microstructure of a single crystal deformed at 1273 K and 3.5 GPa (within the orthorhombic stability field) is morphologically quite distinct from that of the heat-treated specimens. Dislocations dominate the microstructure and often interact with twin domain boundaries.

New phase nucleus appearance on a twin boundary, which is with an invaribe velocity or oscillating

The occurrence and dynamics of a nucleus of a new phase on a moving twin boundary are described. The phase diagram of a nucleus on a moving twin boundary is obtained. It is shown that the nucleus can arise on the moving twin boundary only if its velocity v is smaller than a certain value v~. The existence of two kinds of low-symmetry phase trails (unstable and metastable) is described. The friction force, resulting from a trail and acting on a twin boundary is shown to arise. It is of a viscous type in the case of an unstable trail, and of a « dry » friction force type in the case of metastable trail. The oscillation of the twin boundary with the nucleus in the extemal periodic field, conjugated to the order parameter is described. The phase transition (PT) process was first considered by Frenkel in the framework of liquid- vapor transformation. According to Frenkel's mechanism the heterophase fluctuations give rise to nuclei of, say, vapor phase on the background of the superheated liquid phase. Being at nonequilibrium, this nucleus either disappears (if the size of the nucleus is smaller than the critical value), or rapidly increases unlimitedly (in the opposite case) and thus the crystal transforms into the vapor phase (Il. Frenkel's mechanism was generalized for solids, taking into account the elastic properties and anisotropy and making it possible to describe some phenomena of PTS in crystals (2). However, another mechanism of PTS in crystals has attracted considerable interest of late. Real crystals always contain defects : dislocations and dislocation clusters, cracks, inclusions, twin (domain) boundaries and so on. Even the perfect monocrystal has at least a dislocation of growth as well as edges, botli of which can be regarded as defects in the sense considered below. These inhomogeneities change the local value of a transition temperature. Thus the nucleus of a new phase must appear in the vicinity of the defect under a temperature different from that of a bulk PT. This nucleus is stable in contrast to Frenkel's nucleus. It is stabilized by the inhomogeneity. Stable nuclei were observed experimentally in various crystals under PTS of different nature in the vicinity of defects of different types.

Intergrain flux-pinning in relation to structural phase transformation and tweed formation in YBa 2(Cu 1- xFe x) 3O 7- y and NdBa 2(Cu 1- xFe x) 3O 7- y

Fe-doped superconductors REBa 2(Cu 1- x Fe x ) 3O 7- y (RE=Y, Nd; 0≤x≤0.1) were investigated by conventional and synchrotron X-ray diffraction, thermogravimetric analysis and AC susceptibility measurements. The transition from orthorhombic to macroscopically tetragonal symmetry is analogous for both systems and occurs at x c≈0.030 for YBa 2(Cu 1- x Fe x ) 3O 7- y and at x c≈0.021 for NdBa 2(Cu 1- x Fe x ) 3O 7- y . The spontaneous strain ϵ=2( b- a)/( a+ b) shows tricritical-like scaling as ϵ∼| x- x c| 1 4 , and tweed formation is indicated by diffraction-line-broadening effects. For YBa 2(Cu 1- x Fe x ) 3O 7- y the onset temperature T c of superconductivity is almost constant below x=0.03 (i.e., in the orthorhombic regime) and falls linearly for x > 0.03 in the tweed regime, extrapolating to zero near x=0.2. Samples which were treated twice in flowing O 2 at 400°C for 10 h show a significantly steeper linear decrease of T c with x in the tweed regime. Contrary to the Y-compound, NdBa 2(Cu 1- x Fe x ) 3O 7- y shows no plateau of T c in the orthorhombic regime, and T c drops rapidly with dopant concentration x. The low-field AC susceptibility of each sintered sample is strongly dependent upon applied AC RMS and superimposed DC bias fields, owing to the weak-link network of the superconducting grains. Critical transport current densities, J 0, and pinning force densities α J for the intergranular vortices were estimated from AC susceptibility measurements. Both quantities are strongly suppressed by Fe-doping and the substitution of Nd ions for Y ions. A distinct change of the slope dα J /d x at the critical composition x c for both the YBCO and the NdBCO series indicates a softening of the free energy gradient between superconducting grains and non-superconducting intergrain material due to the formation of twin-domain boundaries in the grains.

A model for the local structure of twin domains and domain boundaries in doped YBa2 ( Cu1− xMx) 3Oy superconductors

The structural transformations of the orthorhombic YBa 2Cu 3O 7 phase under influence of impurity M atoms are considered. The local structure of twin domains and domain boundaries is analyzed for the case when M is substituted for Cu in Cu1 sites and the impurity induced “teragonal” structure appears. In the structural transformations the important role of the additional oxygen captured by the impurity atoms is verified. A model of the new orthorhombic phase for the impurity containing YBa 2(CuM) 3O 7 superconductors is contructed. In this phase half of the Cu (1) sites has five-fold oxygen coordinations and can be occupied by the impurity atoms. The conditions for coexistence of the new phase with the known YBa 2Cu 3O 7, YBa 2Cu 3O 6.5 and YBa 2Cu 3O 6 phases are considered in both domains and domain boundaries. A structural model of the YBa 2(CuM) 3O y crystal explaining many experimental data is suggested.

Micro-twin and antiphase domain boundaries in the orthorhombic phase of La 2− x (Sr, Ba) xCuO 4

We have studied the intricate microstructure of La 2− x (Sr, Ba) x CuO 4 at temperatures where the samples exhibit orthorhombic symmetry. We show that, in addition to the twin boundaries, antiphase domain boundaries also play a key role in the formation of the microstructural network. Antiphase boundaries which have not been reported previously in this system, become a predominant feature particularly in the areas where the regular twin domain boundaries are not present. We also show that the intricate microstructure can be fully understood by considering the possible configurations between the twin and antiphase boundaries which arise from the tilting of the CuO 6 octahedra.

Twin-domain size and bulk oxygen in-diffusion kinetics of YBa 2Cu 3O 6+x studied by neutron powder diffraction and gas volumetry

We report experimental results of twin-domain size and bulk oxygen in-diffusion kinetics of YBa 2Cu 3O 6+ x , which supplement a previous and simultaneous study of the structural phase diagram and oxygen equilibrium partial pressure. Analysis of neutron powder diffraction peak broadening show features which are identified to result from temperature independent twin-domain formation in to different orthorhombic phases with domain sizes and 250 and 350Å, respectively. The oxygen in-diffusion flow shows simple relaxation type behaviour J=J 0 exp( −t τ ) despite a rather broad particle size distribution. At higher temperatures, τ is activated with activation energies 0.55 and 0.25 eV in the tetragonal and orthorhombic phases, respectively. Comparison between twin-domain sizes and bulk oxygen in-diffusion time constants indicates that the twin-domain boundaries may contribute to the effective bulk oxygen in-diffusion. All our results may be interpreted in terms of the 2D ASYNNNI model description of the oxygen basal plane ordering, and they suggest that recent first principles interaction parameters should be modified.

High Resolution Imaging of Twin and Antiphase Domain Boundaries in Perovskite KNbO3 Thin Films.

AbstractPerovskite KNbO 3 thin films were heteroepitaxially deposited onto (100) MgO substrates. Twin domains with tetrahedral shape were typically observed. These tetrahedrons were bounded by three {211} twin planes. High resolution transmission electron microscopy (HRTEM) was employed to examine these {211} twin boundaries. Surface steps of {110}&lt;110&gt; type generated by dislocation slip were present on the MgO substrate. The tetrahedral twin domains originate on surface steps, then grow with a stacking fault relationship to the matrix. The strain fields of dislocations near the stacking faults slightly rotate the tetrahedral twin nuclei. This small degree of misalignment between the matrix and the twin domain result in some of the twin boundaries having amorphous boundary regions rather than coherent interfaces.Order-disorder antiphase domains (APD's) were directly imaged with HRTEM. Ultra small APD's ranging from 10 to 30 atomic spacings were observed. The origin of these APD's was either the surface steps of {100} &lt;1 10&gt; type on MgO substrates or the random nucleation of anions at either of two equivalent sites on the MgO substrate.

TRANSMISSION ELECTRON MICROSCOPY STUDY OF TWIN DOMAIN STRUCTURES IN YBa2Cu3O7−x SUPERCONDUCTOR

The study of the twin domain structure of YBa2Cu3O7−x superconductor synthesized by coprecipitation method has been carried out by using the transmission electron microscopy. It was found that the twin domain boundary was parallel to the (110) lattice planes. The twin domain is designated as a 90° one. The same twin structure exhibited different characteristics of spot splitting in its selected area eletron diffraction patterns (SADP) when it was irradiated in different incidence directions. The experimental results were satisfactorily consistent with the theoretical calculation. The mechanism of the formation of 90° twin domain has also been discussed. The phase transformation from tetragonal to orthorhombic taking place in YBa2Cu3O7−x ceramics during cooling is considered to be of the “Martensite”.

The nature of ordering and ordering defects in dolomite

The apparent lack of transformation-induced domains and stacking disorder defects in natural dolomites is considered in light of the different types of order in the dolomite structure. Experimentally produced twin domain boundaries and basal stacking defects are documented in a dolomite using high-resolution electron microscopy and electron diffraction techniques. Results reveal that upon cation ordering to form the dolomite structure, a twin domain is favored over an antiphase domain. The twin domain boundaries closely resemble antiphase boundaries (APB's) and are in contrast for superlattice reflections. However, background contrast within domains is shown to be different when imaged using certain fundamental reflections. The results allow speculation about the nature of ordering at low temperatures. Observations of twin domain boundaries in samples annealed at different temperatures allow estimation of the critical ordering temperature at 1,100°–1,150° C in stoichiometric dolomite.