TGS Crystals Research Articles

Growth and characterization of TGS and DTGS single crystals doped with Pt(II), Pt(IV) and L‐alanine

AbstractThe single crystals of triglycine sulfate doped with bivalent and tetravalent Pt‐ions and with L‐alanine (LADTGS/Pt(II) and LATGS/Pt(IV)) were grown in the ferroelectric phase from low temperature solutions. Using stick seeds the full‐shaped crystals with many growth pyramids have been prepared. The growth rate along the axis c of the doped crystals is much higher than in an undoped one. Morphology, domain structure and P‐E hysteresis loops have been investigated. The effect of the dopant on the growth velocity is explained on the basis of catalytic action of supposed platinum complexes. The first series of the pyroelectric detectors have been prepared from these materials and their fundamental parameters are presented. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dielectric anomalies in the TGS ferroelectric below the critical temperature after exposure to a transverse electric field

The complex permittivity of a TGS crystal, previously exposed to a prolonged constant electric field perpendicular to the ferroelectric axis, has been measured in the temperature region from 85 to 355 K , at frequencies of 10 −1– 10 5 Hz . The results have been compared with the analogous data obtained for the crystal annealed in the paraelectric phase. It has been found that the transverse electric field permanently suppresses the complex permittivity signal between 200 K and the critical temperature 322.2 K .

Thermostimulated Electron Emission in Paraelectric Phase of TGS Crystals with Nickel Admixture

Thermostimulated electron emission from ferroelectric TGS crystal with an admixture of nickel was experimentally detected in the temperature range exceeding Curie point by ∼10 K for the samples heated with rather high rate. An increase of the heating rate q was shown to result in an increase of the emission current in the entire investigated temperature interval. In TGS crystal with nickel admixture the temperature of emission appearance insignificantly depends on the rate q and is close to the case of pure TGS. The temperature of disappearance of emission monotonously increases with q. The interval of pulling of emission to paraphase here is less than for a case of pure TGS heated with the same rate. Peculiar features of emission in the investigated crystal can be explained by the relaxation of screening charges of spontaneous polarization. Less temperature of termination of the emission in TGS crystal with nickel admixture as compared with pure TGS crystal is explained by less Maxwell relaxation time in the doped crystal. The work was made under support of VZ-010 grant intended for creation of Research Educational Center “Wave processes in non-homogeneous and non-linear media” and INTAS grant “Ferroelectrics templated in nanoporous membranes.”

Dielectric Non-Linearity of TGS Crystals with an Admixture of Europium Ions

Introduction of europium ions into TGS crystal was shown to lead in appearance of minima in the dependence of effective dielectric permittivity ϵ eff on the strength of external electric field E. Near Curie point a decrease in the values of ϵ eff in the peaks of considered dependences is less than for TGS crystals with the impurity of chromium ions and is close to similar values for TGS crystals with L, α -alanine. These results can be probably due to the appearance of large elastic stresses in the process of TGS crystal growth with europium ions due to the large radius of these ions. It is confirmed by a high degree of unipolarity and large values of the emission current for all investigated samples.

Kinetics of Electron Emission from Ferroelectrics

Kinetics of thermostimulated emission in ferroelectric crystals is examined. It is shown that in pure TGS crystals characteristic emission relaxation time depends on temperature, and decreases as one approaches the phase-transition point. The relaxation time of emission signals in crystals enriched with impurities is several times less than in pure materials and does not depend on temperature. Thermally stimulated electron emission is experimentally observed in TGS crystals, in a temperature range with upper limits of 10–15 𝒦 above the Curie point. The reason for the occurrence of emission above the Curie point is related to slow relaxation of the screening charges, which persist in the paraelectric phase.

Pyroelectric Properties of AspTGS Crystals

Pyroelectric properties of 10 mole% L-asparagine doped TGS crystals are reported. Doping in the TGS crystal with polar molecules results in modifications in the spontaneous polarization, creation of an internal bias field Eb, which like an external field tends to keep the crystal in single domain state. The dopant concentration in the crystal has a significant influence on spontaneous polarization and the strength of the internal bias field.

Internal bias field in TGS crystals doped with different impurities

The influence of an internal bias field formed by impurity ions of heavy metals and alpha-alanine molecules on the dielectric nonlinearity and emission properties of ferroelectric triglycine sulfate crystals has been studied. It is shown that a role of defects increases near the phase transition, which manifests itself in a decrease in the maximum values of the effective permittivity and an increase in the emission current with increasing impurity concentration.

Electron Emission from Irradiated Ferroelectrics

Decrease of thermostimulated exoelectron emission from the crystals of ferroelectric TGS was observed experimentally after X-ray irradiation with E q = 8 keV. Decrease of emission current occurred for all the investigated times of irradiation from 1 minute to 60 minutes. Samples under investigation were placed in series along the direction of X-rays propagation. Some of the samples were also obtained by cleaving of previously irradiated bar of TGS crystal. In both cases an increase of emission current j em was observed with decrease of irradiation dose. Dependence of j em on the linear heating rate of a sample q = dT/dt was studied. Under low rates of heating emission was observed only in ferroelectric phase. For high values of q = 5 K/min pulling of electron emission into paraphase takes place. For all the rates of heating the temperature of terminating of emission in the irradiated TGS is less as compared with pure TGS.

Characterization of phosphoric acid doped TGS single crystals

Single crystals of phosphoric acid (H3PO4) doped triglycine sulfate (TGSP) are grown from aqueous solution containing 10–50mol% of H3PO4. Large sized crystals of dimension (70mm×40mm×25mm) suitable for IR detector applications are obtained with 20mol% of H3PO4. They exhibit elongated domains, and the domain density is higher than in undoped TGS crystals. FT-IR analysis indicates the presence of both zwitter and glycinium ions in the doped TGSP crystals, which is due to the low incorporation of H3PO4 into the TGS crystal lattice. The dielectric properties TGSP crystals measured in the temperature range 27–65°C and in frequency range 102–105Hz show a broad transition, and the peak dielectric constant values are reduced with increasing H3PO4 concentration. Hysteresis loops with high coercive fields are observed up to 20mol% of H3PO4. At a higher concentration, double loops are observed due to the polarization switching inhibition by H3PO4 molecules. Dopant concentration is found to have a marginal influence on the spontaneous polarization of the TGSP crystals.

Growth of thiourea-doped TGS crystals and their characterisation

Thiourea-doped Triglycine sulphate (ThTGS) crystal with three different concentrations of thiourea was grown from aqueous solution by slow cooling technique. The cell parameters were determined from powder X-ray diffraction analysis. A qualitative analysis of the presence of thiourea in doped crystals was estimated by FTIR analysis. Microhardness studies were carried out using Leitz Weitzler hardness tester at room temperature. Dielectric properties of the crystals were studied which showed a shift in the T c when compared to pure TGS crystal. Pyroelectric studies were carried out and the pyroelectric coefficient was found. Piezoelectric studies ( d 33 coefficient) has also been carried out.

Modification of the Dielectric Properties of TGS Crystal Grown Under the Influence of DC Electric Field with Different Strengths

TGS crystals grown under two different dc electric field strengths exhibit modified ferroelectric properties. The modifications show a dependence on the field strength applied during crystal growth. Prominent modifications were observed on crystals grown at high fields (80 kV/cm) than in crystals grown under lower dc electric fields (10 kV/cm). The modified ferroelectric properties stem from the domain structures influenced by the applied field. The domains on the low field grown crystals are more elongated and appear as thin strips. In the case of high dc field grown crystals, some of the domains are tilted under the influence of the field, resulting in the misalignment of the major axis of the domains with respect to the c-axis. The domain structure modifications under the influence of fields lead to an increase in the relaxation times. Reduced dielectric permittivity and spontaneous polarization also illustrate the effect created by the field.

Electron Emission of Ferroelectrics

The connection between main parameters of switching and emission of electrons in ferroelectrics-coercive field and threshold field of emission is investigated. It is shown that temperature, concentration, thickness and frequency dependencies of E th can be explained by the corresponding dependencies of E c . Kinetics of thermostimulated emission in ferroelecric crystals is examined. It is shown that in pure TGS crystal characteristic emission relaxation time depends on temperature and decreases as one approaches the phase-transition point. Thermally stimulated electron emission is experimentally observed in TGS crystals in temperature range with upper limit of 10–15 K above the Curie point. The emission disappearance below the Curie point at low q is explained as the result of full emptying of the electron traps under slow heating. The reason for the occurrence of emission above the Curie point is related to the screening charges and, because of their slow relaxation, persists in the paraelectric phase.

Dielectric Properties of L-Asparagine Doped TGS (Asp-TGS) Crystals

Low frequency dielectric properties of TGS doped with a new dopant L-asparagine and its influence on the dynamics of the TGS system is reported. L-asparagine doped TGS (Asp-TGS) crystals exhibit lower dielectric permittivity compared to pure TGS. The ratio of the Curie constant indicates partial pinning of the domains by the dopants. The tg δ maxima observed in the studied temperature range are explained in terms of the hindered rotations of the glycine groups in the crystal structure.

Scanning Near-Field Raman Microscopy on Ferroelectrics

By combining scanning atomic force microscopy with micro-probed Raman scattering spectroscopy, we have developed a new type of scanning near-field Raman scattering system. A special designed cantilever with aperture of about 150 nm in diameter was used both as an illumination nano-source and a nano-detector of scattering light. Near-field Raman spectra of internal modes in TGS and GMO single crystals were observed at their cleavage surfaces at room temperature. The results are compared with those obtained by a conventional Raman microscope.

Fundamentals in ferroelectric transition of TGS

An extended Landau type-model has been used to find the three-terms polynomial expansion of the Gibbs free energy, that describe the second-order phase transition in pure TGS crystal. Temperature dependence of the coefficients was calculated using experimental susceptibilities and polarization, measured on a large temperature range. In a second version of the model, the influence of the external electric field was considered. The third coefficient δ E gets negative values near the Curie point. Gibbs free energy curves G E ( P, T), versus polarization, show peculiar behaviour depending on an external electric field, in the temperature range 3–5°C near the Curie point. The share contribution of the electric field 1.5 kV/cm, exceed 100% at Δ T= T C− T <1.5°C. All curves become asymmetrical and the external barriers of the double minimum potential, for Δ T⩽4°C, show turning points for values smaller than k B T. The central barrier, at Δ T<25°C, is about two orders of magnitude smaller than k B T. The energy corresponding to the internal field E int≈ P S/3 ε 0, is higher or comparable to k B T, only at Δ T⩾10°C. The equation of state E( P, T), which is supposed to represent the hysteresis loop P( E), does not give the right order of magnitude of the measured coercive field. On this ground and according to some other literature data, we conclude that the free energy G( P, T) of TGS is a non-analytical function. Some other models, taking into account short and long-range interaction, have to be considered.

Influence of X-Ray irradiation on the formation of dielectric hysteresis loop for unipolar TGS crystals

Formation of the internal bias field Eb created in TGS crystals by X-ray emission has been studied. The sign and the value of Eb were shown to be dependent on the type of the surface (positive or negative) of unipolar sample which was irradiated by X-rays

Thermostimulated exoemission from KH2PO4 and from TGS crystals

The thermally stimulated exoemission (TSEE) from KDP was observed for the first time after the preliminary polarization and only in the paraelectric phase. The registration of TSEE was performed simultaneously with the thermostimulated depolarization current. The high contribution of adsorbed charges and insignificant participation of pyroelectric or electret charges in TSEE are discussed. This hypothesis has been confirmed experimentally: the total emitted charge after the preliminary polarization of TGS crystals in vacuum was much less than after the preliminary polarization in air

The Effect of D- and DL- Tyrosine Admixture on Dielectric Properties of TGS Crystals

Single crystals of TGS admixtured with L and DL- tyrosine (abbreviated as LTyrTGS and DLTyrTGS) were grown from a solution containing 0.7, 1 and 1.4 wt% of tyrosine relative to pure TGS. Temperature measurements (5 - 340 K) of permittivity l and loss tangent tg i of the samples were performed at selected frequencies of the measuring field varied from 1 kHz to 13 MHz. The values of spontaneous polarization P s , coercive field E c and bias field E b were obtained from the hysteresis loop. With increasing concentration of the dopands in the admixtured crystals, the values of l max and tg i max decreased, while that of the bias field and coercive field increased, and the Curie temperature revealed insignificant changes. The domain structure observations proved a refinement of the domain structure relative to that in pure TGS crystals and a decrease in the time of spontaneous ageing. The actual amount of the admixtures in the crystals was determined by UV spectroscopy.

Temperature Anomalies of Anisotropy Degree of Crystal's Characteristics at Phase Transitions

Investigation of temperature dependences of the optical and dilatative characteristics of TGS, DMAAlS and DMAGaS crystals for three principal directions of optical indicatrix in the temperature ranges including phase transition points T c are presented. For characterization the crystalline anisotropy of physical characteristic the only one value (degree of anisotropy) is introduced. Temperature dependences of the anomalier part of this characteristic for different physical values are characterized by noticeable minima near T c , that can be interpreted as small disordering (rebuilding) the crystal structures during crystal symmetry change in the temperature range near T c .

Structure and Properties of TGS Crystal Grown from the Seed Irradiated with Alpha-Particles

Investigations of the structure and properties of TGS crystals grown from the seed irradiated with f -particles have been performed. Analysis of habitus of the crystal shows asymmetry of the growth rates for the facets relative to the seed along polar axis Y (V 0.10 > V 0.10 ). Diffraction patterns obtained using powder technique and the calculation of parameters for elementary cell unit show an increase of parameter "b". An increase of microhardness at the surface of X, Y and Z cut-offs was observed independent on the distance from the sample to a seed. The obtained distribution of microhardness in the crystal grown from the irradiated seed is related with the character of dislocations distribution along the different directions and specific features of domain structure, and also with the number of domain walls emerging to the surface of the crystal.