Dielectric properties of thallium gallium diselenide layered crystal in the incommensurate phase

The dielectric measurements of the layered crystal were studied in temperature range of successive phase transitions. The measurements revealed that the phase transition occurred in 242 K is an incommensurate phase transition. When the sample is annealed at a stabilized temperature in the incommensurate phase, a remarkable memory effect has been observed on cooling run. The mechanism of the memory effect in the incommensurate phase of the semiconducting ferroelectric TlGaSe₂ can be interpreted in the frame of the theory of defect density waves. This theory claims that the memory effect is the result of pinning of the incommensurate structure by the lattice inhomogeneities. With decreasing the annealing temperature the phase transition temperature shifts to lower temperatures gradually. Moreover, the peak intensities also increase gradually. In addition to these effects, the phase transition temperature shifts to lower temperatures with increasing annealing time. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time‐dependent dielectric behaviour in incommensurate TlInS2 crystal

The results of measurements of temperature and time dependencies of the dielectric constant of TlInS₂ in the temperature interval of successive incommensurate and commensurate phase transitions have been presented. Considerable decrease of the dielectric constant in a temperature interval including successive phase transitions was observed after annealing the sample at a fixed temperature in the ferroelectric state. Additionally, it has been revealed that the time dependencies of the real part of dielectric susceptibility and dielectric losses angle tangent of TlInS₂ crystal in the incommensurate phase exhibits an unusual behaviour, which is accompanied by exponential decreasing of the dielectric constant and increasing of dielectric losses angle tangent by time. The observed effects are qualitatively explained using defect‐density‐wave model and phenomenological theory of incommensurate systems containing discommensurations undergoing nucleation and evolution processes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dielectric characteristics of Ce3+ doped Sr0.61Ba0.39Nb2O6 with Cole‐Cole plots technique

In this paper, we have investigated two‐relaxator mechanism and dielectric characteristics of Ce³⁺ doped Sr_0.61Ba_0.39Nb₂O₆ with dielectric spectroscopy measurements. The crystal undergoes a ferroelectric phase transition at 340 K. The temperature dependence of the real and imaginer part of the complex dielectric susceptibility in vicinity of ferroelectric‐paraelectric phase transition has been studied in the frequency region 0.1 kHz–10 MHz. The measurements of the dielectric constant of the real and imaginer parts show strongly frequency dependence. The investigations of the dielectric constant revealed a non‐Debye type dielectric relaxation for Ce⁺³ doped SBN61 by using Cole‐Cole plots. It reveals the coexistence of the two dielectric relaxators in vicinity of the phase transition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth,composition, ferroelectric and magnetic properties of new multiferroic Pb3.3Mn4.8Ni1.1Ti0.56O15.3 single crystals

Multiferroic single crystals in the novel system Pb‐Mn‐Ni‐Ti‐O have been grown by the high temperature solution growth method. At room temperature the crystals are indexed in the hexagonal space group P6₃cm. The dielectric and magnetic properties along with the temperature dependence of the c‐lattice parameter have been studied in the temperature range 2 K ‐ 500 К. The magnetic measurements reveal a paramagnetic to antiferromagnetic phase transition around 48 K. The dielectric permittivity exhibits a maximum at 430 K, indicating ferroelectric to paraelectric phase transition. The temperature dependent Raman and XRD measurements around 430 K reveal an anomaly and abrupt change of the lattice parameter along the z‐axis respectively, thus confirming the ferroelectric‐to‐paraelectric phase transition.     

X‐ray structural phase analysis of CdTe semiconductor annealed in air

X‐ray structural phase analysis of CdTe semiconductor, thermally annealed in air at temperature range 373‐773 K and annealing time 24 hrs, is investigated applying Rietveld method. The results showed that at low temperatures < 523 K, oxygen diffuses into the (1/21/21/2) interstitial sites of the CdTe lattice and its relative occupancy increases with the annealing temperature. For higher temperatures ≥ 523 K, the thermally grown oxide CdTeO₃ phase is developed on expense of CdTe phase. The percentage phase abundance of each phase is determined at each temperature applying a standardless method. The rate of oxidation with temperature is found to be non‐linear.     

Electrical conductivity and dielectric properties of potassium sulfamate single crystals

Single crystals of potassium sulfamate are grown by the method of slow evaporation at constant temperature. AC electrical conductivity of potassium sulfamate is measured in the temperature range 300–430 K and in the frequency region between 100 Hz and 3 MHz along the a, b and c‐axes. Complex impedance spectroscopy is used to investigate the frequency response of the electrical properties of the potassium sulfamate single crystal. Temperature variation of AC conductivity and dielectric measurements show a slope change around 345 K for both heating and cooling run and this anomaly is attributed as phase transition, which is well supported by the DSC measurements. Value of loss tangent in the temperature region 330–400 K is found to be very low. Activation energies for the conduction process are calculated along the a, b and c‐axes. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Thermal Annealing on Structural Properties,Morphologies and Electrical Properties of TiO2 Thin Films Grown by MOCVD

TiO₂ thin films, were deposited on Si(100) and Si(111) substrates by metalorganic chemical vapor deposition at 500 °C, and have been annealed for 2 min, 30 min and 10 hours at the temperature from 600 °C to 900 °C, in oxygen and air flow, respectively. XRD and atomic force microscopy characterized the structural properties and surface morphologies of the films. As‐deposited films show anatase polycrystalline structure with a surface morphology of regular rectangled grains with distinct boundaries. Rutile phase formed for films annealed above 600 °C, and pure rutile polycrystalline films with (110) orientation can be obtained after annealing under adequate conditions. Rutile annealed films exhibit a surface morphology of equiaxed grains without distinct boundaries. The effects of substrate orientation, annealing time and atmosphere on the structure and surface morphology of films have also been studied. Capacitance‐Voltage measurements have been performed for films deposited on Si(100) before and after annealing. The dielectric properties of TiO₂ films were greatly improved by thermal annealing above 600 °C in oxygen.     

Flux growth and low temperature dielectric relaxation in piezoelectric Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals

Single crystals of Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT 91/9) have been grown by flux method after modifications in temperature profile, flux ratio and addition of excess ZnO/B₂O₃ which resulted in enhanced perovskite yield (more than 95%). Only a few crystals showed the presence of pyrochlore phase/variation in composition. A comparative characterization of these crystals were carried out in respect of piezoelectric charge coefficient d₃₃, dielectric constant, ac conductivity and hysteresis loop after cutting and poling the crystals along [001] direction. The total activation energy for conduction has been found to increase with Ti‐content in the sample. The effect of ZnO on growth behavior has been analyzed. A detailed analysis of PZNT (91:9) has been carried out at low temperature in respect of the various thermodynamic parameters related to the dielectric relaxation mechanism, like optical dielectric constant, static dielectric constant, free energy of activation for dipole relaxation, enthalpy of activation and relaxation time, have been calculated in the vicinity of transition temperature in the lower temperature region. The activation energy for relaxation at ‐10 and ‐49 °C have been found to be 0.09 and 0.02 eV respectively. The results were analyzed and a detailed dielectric analysis and low temperature relaxation behavior of PZNT crystals were interpreted. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural,optical and dielectric studies on K1‐x(NH4)xH2PO4 mixed crystals grown from z‐cut seeds

Mixed crystals of K_1‐x(NH₄)_xH₂PO₄(KADP) were grown from KDP (KH₂PO₄) dominated mixed solutions with varying molar proportion of ADP (NH₄H₂PO₄) addition. It was found that, as the increase of ADP molar concentration, the growth rate along z‐axis of KADP crystal decreased rapidly. The structure of KADP crystals was investigated by powder XRD and the lattice parameter was calculated. The results showed that the lattice parameter c of KADP crystal increased with the molar concentration of ADP. The optical homogeneity of grown KADP crystals was determined with a differential phase‐shifting interferometry. Frequency dependences of the dielectric constant and dielectric loss of KADP crystals were measured at room temperature (290 K). The dielectric constants of KADP crystals were almost invariant with the increase of frequency. In the region of 10²∼10⁴Hz, the values of the dielectric loss reduced with the increase of frequency. The piezo‐resonance coupling effect still exists in KADP crystals at room temperature, but shifted to low frequency band. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

In‐situ investigation of the temperature dependent structural phase transition in CuInSe2 by synchrotron radiation

The temperature dependent structural phase transition from the tetragonal chalcopyrite like structure to the cubic sphalerite like structure in CuInSe₂ was investigated by in‐situ high temperature synchrotron radiation X‐ray diffraction. The data were collected in 1K steps during heating and cooling cycles (rate 38 K/h). The Rietveld analysis of the diffractograms led us to determine the temperature dependence of the lattice parameters, including the tetragonal deformation, |1‐η|, and distortion |u‐¼| (η=c/2a, a and c are the tetragonal lattice constant; u is the anion x‐coordinate). The thermal expansion coefficients α_a and α_c of the tetragonal lattice constant which are related to the linear thermal expansion coefficient α_L were obtained, as were α_a of the cubic lattice constant, also α_u and α_η. The transition temperature is clearly identified via a strong anomaly in α_L. The temperature dependence of the anion position parameter was found to be rather weak, nearly α_u∼0, whereas α_η increases slightly. However, both increase strongly when approaching to within 10 K of the transition temperature (the critical region) and |1‐η| as well as |u‐¼| go to zero with |T‐T_trans|^0.2 approaching the phase transition. The cation occupancy values, derived from the Rietveld analysis, remain constant below the critical region. Close to the transition temperature, the number of electrons at the Cu site increases with a dercrease in the number of electrons at the In site with increasing temperature, indicating a Cu‐In anti site occupancy, which is assumed to be the driving force of the phase transition. At the transition temperature 67% of Cu⁺ were found to occupy the Me1 site with a corresponding 67% of In³⁺ at the Me2 site. Although full disorder is reached with 50%, this level seems to be high enough that the phase transition takes place. The order parameter of the phase transition, goes with |T‐T_trans|^β to zero with the critical exponent β=0.35(7) which is in good agreement to the critical exponent β=0.332 calculated for order‐disorder transitions according to the Ising model. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dielectric relaxation and morphologic properties of CaCu3Ti4O12 doped with GeO2

CaCu₃Ti₄O₁₂ (CCTO) is a material with giant dielectric constant, presenting good stability over a wide temperature and frequency ranges. The preparation method and doping has a great influence on the microstructure and dielectric properties of this material. In this work, doping CCTO with 2–10 wt% GeO₂ has been shown to increase the dielectric constant. We studied the prepared samples by X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy. X-ray diffraction shows the presence of nanocristals. Grains and grain boundaries compositions have been observed by scanning electron microscopy with energy dispersive X-ray spectrometry mapping. Impedance spectroscopy measurements, in the frequency range from 75 kHz to 30 MHz, and temperature from 250 to 325 K, have been performed. The data were analyzed using the Cole–Cole model of dielectric relaxation.     

Decrease in dielectric loss of CaCu3Ti4O12 by the addition of TeO2

CaCu₃Ti₄O₁₂ (CCTO) has challenged for the last few years the scientific community due to its large dielectric constant, which is almost temperature and frequency independent, from 100 K to 400 K and from 1 kHz to 1 MHz, respectively. This makes the material desirable for many electronic applications. However, the dissipation factor is very large, with tan δ values, at room temperature and 1 kHz, higher than 0.1.In our work we report how the addition of TeO₂ lowers the dielectric loss and, although there is a decrease of dielectric constant of doped samples relatively to the undoped one, high dielectric constant values are still being reached. The sample of doped CCTO with 1.5% of TeO₂ by weight, presents, at room temperature and 60 kHz, a large dielectric constant, over 3000, and a dissipation factor around 0.09, which represents a decrease on tan δ over 30% relatively to the CCTO undoped sample. Two relaxation processes were identified for all the samples, one at MHz region and the other one at low frequency region (< 1 kHz). DC bias voltage was applied up to 40 V and a strong dc bias influence on the low frequency region was observed both at dielectric and impedance responses of the undoped sample, which was much weaker than the dc bias effects on the 4% Te doped sample.Dielectric measurements will be discussed and correlated with the samples' microstructure, supported on internal barrier layer capacitance (IBLC) and surface barrier layer capacitance (SBLC) models.     

Synthesis,structural and ferroelectric properties of barium titanate based glass-ceramic nano-composites

Novel glass-ceramic nano-composites based on BaTiO₃–V₂O₅–Bi₂O₃ glasses were prepared via heat-treatment at crystallization temperature (T_cr). The nanostructural and ferroelectric properties were investigated by differential scanning calorimeter (DSC), transmission electron microscope (TEM), X-ray diffraction (XRD) and dielectric measurements, respectively. It was shown by XRD and TEM studies that appropriate heat-treatment glasses can turn into glass-ceramic nano-composites consisting of crystallites smaller than 30 nm embedded in the glassy matrix. The observed dielectric constant of heat‐treated samples, showed average broad peak around 324 K. The average Curie temperature (T_c) observed around 324 K in the present system. This observation of these glass-ceramic nano-composites can be utilized to control T_c by proper adjustment of BaTiO₃ content. This result, therefore, supports dipolar orientation and phase transition in the glass-ceramic samples dispersed with nanocrystalline BaTiO₃. Such a feature can be used for industrial applications where the shape of the application article is a necessary requirement such as integrated capacitor.     

Study on the ferroelectric phase transition in potassium lithium niobate (KLN)

The ferroelectric phase of potassium lithium niobate K₃Li_2–xNb_5+xO_15+2x (KLN) in the range of 0.15 < x < 0.5 is a very promising material for the second harmonic generation (SHG) in the blue visible region (∼410 nm). The ferroelectric phase transition was shown to occur between 400 and 500°C depending on the composition of the KLN phase. In this study several analysis techniques were used to investigate the phase transition on ferroelectric (x = 0.3) KLN samples. The temperature‐dependent measurements of the relative dielectric constant ε₃₃ provided a phase transition temperature of about 470°C. In our DTA experiments, a small but reproducible thermal effect at the phase transition in KLN was indicated. The temperature‐dependent birefringence measurement technique, applied the first time on KLN, shows a second order behaviour at a temperature of 467 °C. However, this phase transition is accompanied by a small thermal effect. The DSC analysis for the other KLN composition (x = 0.5) provided a phase transition temperature of 514 °C. The appearance of a phase transition in the paraelectric KLN phase (Nb content higher than 55 mol%) was also studied. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dielectric relaxation in ferroelectric TlInS2 layered crystals within metastable chaotic state

The results of investigations low frequency dielectric relaxation in layered ferroelectric TlInS₂ crystals are presented. The measurements were performed in the temperature range of 180‐230 K and in the frequency range of 5 kHz–1 MHz. Two different relaxation processes were observed in mentioned temperature interval. The crystal has “slow” and “fast” relaxation mechanisms in low and high frequency region, respectively. The presence of two different relaxation mechanisms in TlInS₂ is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterization of 4‐Aminopyridinium‐4‐nitro phenolate single crystals

Organic optical material 4‐Aminopyridinium‐4‐nitro phenolate (4AP4NP) has been synthesized, and single crystals of size 20 x 14 x 6 mm³ have been grown from acetone solvent at room temperature by solvent evaporation technique. The grown crystals have been characterized by X‐ray diffraction to determine the cell parameters, and by FT‐IR technique to confirm the formation of the expected compound. The crystal belongs to monoclinic crystal system with space group P2₁/a.The structural perfection of the grown crystals has been analyzed by high‐resolution X‐ray diffraction (HRXRD) rocking curve measurements. The thermal stability of the compound has been determined by TG‐DTA curves. The transmittance of 4AP4NP has been used to determine the refractive index n; the extinction coefficient K and both the real ε_r and imaginary ε_i components of the dielectric constant as functions of photon energy. The optical band gap of 4AP4NP is 2.4 eV. The dielectric and mechanical behavior of the specimen was also studied. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and Electrical Properties of Cd Doped InSe Thin Films

Polycrystalline Cd doped InSe thin films were obtained by thermal co‐evaporation of alpha‐In₂Se₃ lumps and Cd onto glass substrates at a temperature of 150°C. The films were annealed at 150°C and 200°C. The films were found to contain around 46% In, 47% Se and 7% Cd in weight. The films exhibited p‐type conductivity. The results of conductivity measurements have revealed that thermionic emission and variable range hopping are the two dominant conduction mechanisms, in the temperature ranges of 320‐160 K and 150‐40 K respectively. It was observed that above 240 K mobility is limited by the scattering at the grain boundaries. As the temperature decreases, thermal lattice scattering followed by the ionized impurity scattering dominate as the two main mechanisms controlling the mobility. Acceptor to donor concentration ratio was found to be slightly increasing due to annealing.     

Photoelectrical properties of crystalline titanium dioxide thin films after thermo‐annealing

This paper reports the photoelectrical properties of sol gel derived titanium dioxide (TiO₂) thin films annealed at different temperatures (425‐900°C). The structure of the as‐grown film was found to be amorphous and it transforms to crystalline upon annealing. The trap levels are studied by thermally stimulated current (TSC) measurements. A single trap level with activation energy of 1.5 eV was identified. The steady state and transient photocurrent was measured and the results are discussed on the basis of structural transformation. The photocurrent was found to be maximum for the films annealed at 425°C and further it decreases with annealing at higher temperatures. The photoconduction parameters such as carrier lifetime, lifetime decay constant and photosensitivity were calculated and the results are discussed as a function of annealing temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic and electric field characterization of La2CoMnO6 crystals doped with Pb

Crystals of La₂CoMnO₆ doped with Pb were successfully grown by the high temperature solution growth method and their magnetic and transport properties were studied. The examined crystal is found to have predominantly ordered Co²⁺/Mn⁴⁺ structure with randomly distributed Mn³⁺ substituting Co²⁺. A relaxor‐like temperature dependence of the dielectric constant with relaxation maximum is established in the temperature interval 180–210 K. On the base of the dc‐conductivity data, it is assumed that the charge transport in the interval 180–350 K is governed by small‐polaron hopping, whose onset coincides with the Curie temperature.     

Influence of the substrate temperature on the structural,optical, and electrical properties of tin selenide thin films deposited by thermal evaporation method

Thin films of tin selenide (SnSe) were deposited on sodalime glass substrates, which were held at different temperatures in the range of 350‐550 K, from the pulverized compound material using thermal evaporation method. The effect of substrate temperature (T_s) on the structural, morphological, optical, and electrical properties of the films were investigated using x‐ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission measurements, and Hall‐effect characterization techniques. The temperature dependence of the resistance of the films was also studied in the temperature range of 80‐330 K. The XRD spectra and the SEM image analyses suggest that the polycrystalline thin films having uniform distribution of grains along the (111) diffraction plane was obtained at all T_s. With the increase of T_s the intensity of the diffraction peaks increased and well‐resolved peaks at 550 K, substrate temperature, were obtained. The analysis of the data of the optical transmission spectra suggests that the films had energy band gap in the range of 1.38‐1.18 eV. Hall‐effect measurements revealed the resistivity of films in the range 112‐20 Ω cm for films deposited at different T_s. The activation energy for films deposited at different T_s was in the range of 0.14 eV‐0.28 eV as derived from the analysis of the data of low‐temperature resistivity measurements. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)