Domain structure investigations in triglycine sulphate single crystals

The domain structure of pure and irradiated TGS crystals was investigated by electron microscope decoration technique. Changes in the decoration pictures due to irradiation are shown and the way in which small lenticular domains disappear in the spontaneous ageing process is presented. Moreover, the thickness of the domain wall was assessed to be for good TGS crystals, grown in the paraelectric phase, of about 120 Å.     

Domain structure of Na1 − x LixNbO3 crystals

The domain structure of single-crystal and ceramic samples of Na_{1 ? x} Li_xNbO₃ solid solutions (at x ≤ 0.14) in the orthorhombic ferroelectric and antiferroelectric phases at room temperature is investigated by optical and electron microscopies and X-ray diffraction. The characteristic feature of the domain structure is the formation of 90° complexes consisting of laminar domains with a specific orientation relative to the lattice of the initial cubic phase. Consideration is given to the specific features in packing of these complexes and typical configurations of domains in the crystals. Observations revealed that the domain structure can involve 90°, 60°, 120°, and 180° boundaries, as well as (hhl) boundaries of the S type whose orientation depends on the cell distortion and changes with a variation in _x. The indices of these boundaries are determined. The density of 180° boundaries in the ferroelectric phase is very low compared to that of non-90° boundaries.     

Brittle fracture of single crystals and active structure of cleavage surfaces

By the method of vacuum decoration at an electronmicroscopic scale with the use of lowangle electron diffraction the dependence of brittle fracture of alkali halide crystals and of the active structure of cleavage surfaces on the character of ordered distribution of point defects in crystals has been studied. The occurrence of cleavage steps reflects the presence in crystal volume, where brittle fracture took place, of a high degree of periodicity of point defects mainly in one crystallographic direction. The smooth areas of the cleavage surface reflect the existence in these places of a periodic distribution of point defects in two mutually perpendicular crystallographic directions. Coincidence of periods between the cleavage steps with the periods in the arrangement of point defects on smooth areas of the cleavage surface allows the steps to be regarded not only as elements of the geometric microrelief but primarily as electrically active linear elements of the crystal surface, reflecting the linear arrangement of point defects. Together with cleavage steps exhibiting a stable in time activity there arise cleavage steps with an extremely high, but rather shortlived activity being the consequence of the brittle fracture process proper. The electrically active linear elements of the point defect lattice – linear elements of the potential relief of the surface – play a most important role in heterogeneous processes, in particular crystallization processes.     

Criterion for identification of ferroelectric domains in TGS crystals from AFM images

Criteria for identification of actual (dynamic) domains and morphologically similar domain “memory” regions are proposed based on the study of various types of contrast of topographic atomic-force microscopy images of lenslike regions on the polar surface of TGS crystals. Inaccuracy in identification may result in further errors in estimating the parameters of the domain structure. The images of ferroelectric domains in the spreading-resistance mode that indicate directly the presence of conductive properties of the domain walls have been obtained for the first time.     

Electrical Processes Occurring at the Moment of Cleaving NaCl Crystals

The electrical processes occurring on cleaving tempered NaCl crystals have been studied by decoration methods on the scale of electron microscopy. The decorating agent, gold, was evaporated at the moment of performing the cleavage. The obtained decoration pictures differed from those described in the literature. Features similar to the propagation of waves were observed. They begin on cleavage steps and are mostly parallel to 〈100〉. On interaction of the “waves” with the steps or with other “waves” there arise black “threads” distinguished, the same as the wave boundaries, owing to a very high contrast on account of the higher density of decorating particles. The waves and threads are interpreted as microbreakdowns, which have also a two-stage character - stages of wave and thread formation, propagate along definite crystallographic directions, and occur at the boundary of strained sites. The charge sign of the waves is determined from the degree of coalescence of decorating gold particles. The detected waves reflect the occurrence of specific polarization states in local sites of the surface. These states are induced by local electric fields of a very high intensity occurring on cleaving the crystals. These polarization states disappear in 10-15 s after the cleavage is made due to neutralization of the charges. Analogy is drawn between polarization structures in form of waves and stable linear polarization bridges occurring in crystals between oppositely charged point defects and their assemblies.     

Domain structure and properties of triglycine sulfate crystals with profile <Emphasis Type="Italic">D</Emphasis>,<Emphasis Type="Italic">L</Emphasis>-α- and <Emphasis Type="Italic">L</Emphasis>-α-alanine-doped layers

A complex investigation of the domain structure and dielectric properties of triglycine sulfate (TGS) crystals containing profile layers doped with D,L-α-alanine (DLATGS) and L-α-alanine (LATGS) impurities is carried out. The images of the DLATGS and LATGS layers and ferroelectric domains are obtained by piezoelectric force microscopy; the parameters of the domain structure and the degree of unipolarity are determined. It is established that DLATGS layers are multidomain and LATGS stripes are mainly single-domain. The experimental data on the macroscopic dielectric properties of the crystals are compared with the results of a microscopic analysis of the domain structure.     

Study of LiNbO3 single crystals with a regular domain structure by piezoresponse force microscopy

Lithium niobate crystals with a regular domain structure have been studied by piezoresponse force microscopy. The period of regular domains and the domain-wall width (w = 45 nm) have been calculated for Z- and Y-cut crystals based on an analysis of two-dimensional images of the domain-structure piezoresponse. It is shown that for the Y-cut crystal, both positive and negative domain boundaries can be identified when recording the lateral component of piezoresponse.     

Crystal growth and domain structure of triglycine sulphate doped with Pt(II)

The single crystals of triglycine sulphate doped with bivalent Pt-ions (TGS-Pt) were grown in full-shaped form, i.e. with many growth sectors. The domain structure of individual growth sectors was studied by the liquid crystal method. The growth sectors are mostly in a single domain state but spontaneous polarization is reversed in growth sector boundaries, which coincide with charged domain walls. Only the growth sector 001 reveals a blockshaped domain structure, however it differs from usual lenticular domains of the undoped TGS. The concentration of Pt in various growth sectors is different. The growth sectors with the higher concentration of Pt than 3 × 10⁻²% exhibit the unipolarity stable against thermal schocks.     

Reduction of domain wall width in VTE treated near stoichiometric LiNbO3 crystals

LiNbO₃ is a ferroelectric crystal and grows with multi domains. Different domains are separated by boundaries which are known as domain boundaries. Domain walls for congruent and VTE (Vapor Transport Equilibration) treated near stoichiometric lithium niobate samples were visualised in different crystallographic directions using chemical etching technique. The sample is etched in the mixture of HF and HNO₃ (in 1:2 volume ratios) for 10 minutes at boiling temperature. Measured domain wall width was found approximately 15‐20 µm for congruent (CLN) and it reduces to 1‐3 µm for VTE treated near stoichiometric (SLN). Activation energies were also measured by two‐probe method and found to be increasing in stoichiometric sample. This activation energy is related to defect density in the crystals. Activation energy is higher for less defective crystals. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A contribution to the study of the influence of impurities on the growth and some physical properties of TGS single crystals

Single crystals of TGS doped with Ni²⁺, Cu²⁺ and Fe³⁺ have been prepared under identical conditions by the method of temperature decrease (below the phase transition point). Presentation of the influence of impurities on the development of individual crystal faces is based on goniometric measurements. The influence of impurities on the domain structure of TGS is documented by microphotographs of domain structures and by investigation of the variation with time of the average domain width. It is shown that the distribution of impurities (between the liquid and solid phases) is approximately proportional to the concentrations of metal glycino chelates in the solution. From the results concerning the influence of impurities on various physical properties follows that the degree to which the properties are affected depends not only on the impurity concentration but also on the specific intensity of their action which is due to variations in the binding forces in the structure of TGS.     

A study of ferroelectric domain patterns in orthorhobic KNbO3 with optical microscopy

Ferroelectiric domain patterns in natural surface layers of Czochralski as-grown crystals are very complicated. The inner domain patterns are comparatively simple and consist of a regular arrays of so-called thin lens-like 90° domains. Only few inner 60° and 120° domains crossing the 90° domain arrays are observed. The inversion or 180° domains are shape-like separated islands. The 60° microdomains are easily introduced into the surface layers during the specimen processing. There are some elastic interactions between the different kinds of ferroelectric domain walls, the orientations of which are in good coincidence with the theory proposed by Janovec.     

On the incorporation mechanism and distribution mode of impurities in potassium acid phthalate crystals grown from aqueous solutions

Mechanism of impurity structure formation in crystals grown from aqueous solutions has been studied on the example of potassium acid phtalate (abbreviated hereafter as KAP) single crystals. Gold decoration technique at an electronmicroscopic scale has been applied to the study of the distribution of uncontrolled impurities on KAP cleavage face (010) after 10, 20 and 30 days of growth, taking into consideration different growth rates in 〈001〉 and 〈001 〈 directions. A technique for visualization of impurities in water, based on the adsorption of these impurities by the surface of amorphous film of nitrocellulose (parlodion) and the vacuum decoration with gold of these impurities, has been developed. Differences in the impurity structure of KAP regions located in 〈001〉 and 〈001〉 directions from the seed have been established. In 〈001〉 direction after 20 days of growth impurity assemblies 0.1—0.4 μm in size are revealed, and in 〈001〉 direction heterogeneous impurity structure is revealed only after 30 days of growth. The real (impurity) structure of KAP outside impurity assemblies is quite homogeneous and is the same throughout the whole crystal volume, the impurities incorporating mainly into complex active centres. From comparison of the changes in time of the impurity structure of water used for crystallization solutions and the impurity structure of KAP crystals a conclusion is made that the impurity structure of crystals is “programmed” in the impurity structure of crystallization solutions which regularly changes with time, i. e. impurities from different kinds of assemblies which are selectively adsorbed by the growing crystal faces. The role of the adjacent to the growing face interfacial layers which control the growth rate and have a complex impurity structure is stressed.     

Dielectric properties of triglycine sulfate crystals with defects at low and infralow frequencies

Dielectric responses of several crystals in ultraweak measuring fields at low and infralow frequencies are compared, namely, of nominally pure, Cr-and Lα-alanine-doped triglycine sulfate (TGS) crystals and TGS + Cr³⁺ crystals irradiated with X-rays. It is shown that dopant-induced bias fields give rise to crystal unipolarity, suppress the domain contribution to their dielectric response, and diffuse the phase transition. It is established that X-ray irradiation of the crystals results in “radiation annealing” of TGS + Cr³⁺ crystals, which increases their permittivity and diminishes diffusion of the phase transition.     

Internal bias field in triglycine sulphate crystals with <Emphasis Type="Italic">L</Emphasis>-, α-alanine grown at negative temperatures

The dielectric and pyroelectric properties of triglycine sulphate (TGS) crystals with L, α-alanine impurities grown at negative temperatures have been investigated. It is shown that a lower impurity concentration (2 mol % in solution) in this temperature range leads to the formation of internal bias fields of the same order of magnitude (∼800 V/cm) as for TGS crystals grown at T ⩽ 50°C but with an L, α-alanine concentration of 20 mol % in solution.     

Vapour diffusion growth and characterisation of fully deuterated triglycine sulphate (ND2CD2COOD)3D2SO4

A new vial-in-vial vapour diffusion method for growing single crystals of fully deuterated triglycine sulphate (TGS) has been developed. Single crystals of hydrogenous TGS were also grown for comparison purposes. The crystals have been characterised using x-ray diffraction and differential scanning calorimetry. The phase transition temperature was 334.0±0.5 K for fully deuterated TGS compared to 322.3±0.3 K for hydrogenous TGS. These values compare well with the expected T_C.     

Investigation of X‐ray Satellite Reflections Attributed to Antiphase Boundaries in Epitaxial (Ga,In)P Layers grown on (001) GaAs Substrates

We report on the analysis of additional X‐ray reflections that probably arise from antiphase domain boundaries within (Ga,In)P/(001) GaAs heteroepitaxial layers. Due to the preferred cation ordering along the crystallographic directions [1‐1 1] and [‐1 1 1] which belong to the [110] zone the original sphalerite‐type structure of (Ga,In)P changes into a CuPt‐like of the cation sublattice. This ordering phenomenon causes a loss of symmetry, i.e. the cubic structure is converted into a rhombohedral one. The antiphase boundaries between ordered domains are assumed to behave similar to lattice planes at X‐ray diffraction. Therefore, additional reflections may occur spatially neighboured to the [001] direction. The presented results of X‐ray experiments are discussed in relation to TEM experiments published in the literature in order to explain the origin of the satellite reflections. In the case of the investigated samples (grown on GaAs substrates misoriented 2° towards the azimuthal [010] direction) the APBs run preferentially in directions tilted up to angles of 20° with respect to growth direction. A preferential occurrence of satellite reflections in <13 2 1> directions was observed coinciding with {13 2 1} "lattice planes" whose normals enclose the same angle to the [001] growth direction as the normals of the average planes characterized by APBs. The appearence of the phenomenon in other directions that are also spatially neighboured to the <13 2 1> directions was determined on the basis of the shift of the reflection positions due to tilting the sample around an axis geometrically included in the scattering plane.     

Low-angle boundary investigation in silicon single crystals

Low-angle boundaries in Si single crystals grown from the melt in [111], [100], [112], [110], [118] and [115] directions were investigated by chemical etching, copper decoration technique and X-ray topography. — LAB of four types were found in planes parallel to the crystal growth axes. Rearrangement of dislocations from slip bands into LAB was observed in heavily-doped Si crystals. The origin of LAB in melt grown Si crystals is discussed. It is shown that these boundaries are well interpreted in terms of dislocation alignment formation in the thermal stress field of the growing crystal.     

掺杂TGS晶体的生长及热释电性能研究

选择重稀土离子Dy3 为掺杂阳离子,DL-丙氨酸与L-谷氨酸部分取代甘氨酸分子,生长了不同掺杂配比的TGS晶体。生长和测试实验表明,掺杂TGS晶体较纯TGS晶体易于生长。将掺杂晶体生长溶液的pH值控制在1~4,可改变掺杂晶体的结晶习性。用ICP发射光谱测试了掺杂晶体中稀土元素的含量,用X射线粉末衍射法测定了晶格参数,结果表明:元素已进入晶体,晶格参数稍有变化,但掺杂晶体的对称性仍为C2-2。通过测量掺杂晶体的电滞回线,得到了内偏压场,还测量了各样品的热释电系数、自发极化强度,作了温度曲线,并分析了各掺杂剂对提高热释电性能和锁定极化的作用。结果表明:是有应用前景的热释电材料。     

Study of the domain structure of GASH single crystals and its correlation with their dielectric and switching behaviour

The domain structure topography of GASH single crystals is investigated by electron microscope decoration technique. A large variety of domain shapes was found in “young” crystals immediately after growth, whereas after one or more years of ageing the domain structure became coarse. The domain structure of the samples is related with their dielectric and switching behaviour.     

Effect of Doping and Irradiation on Optical Parameters of Triglycine Sulphate Single Crystals

Crystals of triglycine sulphate (TGS) doped with orthonitroaniline (ONA) irradiated with different doses of gamma-radiation were used to investigate gamma-radiation effect on electrical and optical parameters. The absorption coefficient and the reflectance were measured and hence the extinction coefficient, the optical refractive index and the dielectric constants (e_r′, e_r″) of unirradiated and irradiated TGS crystals were calculated. Values of the allowed indirect optical energy gap E_g^opt. of TGS doped with ONA were calculated as a function of gamma-dose. Value of E_g^opt. decreases from 4.72 eV to 4.25 eV with increasing gamma-doses from 0 up to 3 Mrad. The values of the static dielectric constant e_r(0) and the effective electrical conductivity sigma₀ at room temperature and their dependence on gamma-dose were also calculated.