Dielectric and AC ionic conductivity investigations in K3H(SeO4)2 single crystal

Optical observation under the polarizing microscope and DSC measurements on K₃H(SeO₄)₂ single crystal have been carried out in the temperature range 25-200 °C. It reveals a high-temperature structural phase transition at around 110 °C. The crystal system transformed from monoclinic to trigonal. Electrical impedance measurements of K₃H(SeO₄)₂ were performed as a function of both temperature and frequency. The electrical conduction and dielectric relaxation have been studied. The temperature dependence of electrical conductivity indicates that the sample crystal became a fast ionic conductor in the high-temperature phase. The frequency dependence of conductivity follows the Jonscher's universal dynamic law with the relation σ(ω)=σ(0)+Aωⁿ, where ω is the frequency of the AC field, and n is the exponent. The obtained n values decrease from 1.2 to 0.1 from the room temperature phase to fast ionic phase. The high ionic conductivity in the high-temperature phase is explained by the dynamical disordering of protons between the neighboring SeO₄ groups, which provide more vacant sites in the crystal.     

Dielectric properties of Bismuth Titanate (Bi4Ti3O12) synthesized using solution combustion route

Ferroelectric Bismuth Titanate (Bi₄Ti₃O₁₂) was prepared by solution combustion route with glycine as fuel. The single phase Bismuth Titanate was obtained after calcination at 800 °C, which was confirmed with the help of X-ray diffraction studies and EDS analysis. SEM micrographs of the calcined powders show agglomerated particles, which is typical of combustion synthesis. Behavior of dielectric constant and dielectric loss as a function of temperature of as prepared sample are reported here. Ferroelectric to paraelectric phase transition occurs at the temperature T_c∼650 °C. Impedance studies were made in the frequency range from 1 KHz to 1 MHz. The semicircles observed in the complex impedance diagrams indicate deviation from the Debye behavior. Activation energy of the sample around T_c is found to be ∼0.35 eV and below T_c is ∼0.13 eV, which was calculated using the Arrhenius plots.     

Post-sintering annealing effect on the diffuse phase transition in (Pb1−xBax)(Yb0.5Ta0.5)O3 ceramics

The effect of post sintering annealing on the dielectric response of (Pb_1−xBa_x)(Yb_0.5Ta_0.5)O₃ ceramics in the diffuse phase transition range (x=0.2) has been investigated. The samples are prepared by conventional solid-state reaction method. The samples are sintered at 1300 °C for 2 h and annealed at different temperatures (800, 900 and 1000 °C) for 8 h and at 800 °C for different time durations (8, 12 and 24 h). A significant change in the dielectric response has been observed in all the samples. The dielectric constant increases remarkably and the dielectric loss tangent decreases. The dielectric peaks of the annealed samples are observed to be more diffused with noticeable frequency dispersion compared to the as sintered sample.     

Crystallographic study of the phase transition in (Me4P)2ZnBr4

The compound (Me₄P)₂ZnBr₄, a member of the β-K₂SO₄ structure class, undergoes a phase transition at 84°C from the room temperature space group P12₁/c1 to the parent Pmcn structure. The room temperature structure corresponds to a ferrodistortive transition of B_1g symmetry at the zone center. At room temperature, the compound has lattice constants a=9.501(1), b=16.055(2), c=13.127(2) Å and β=90.43(1)°. For the high temperature phase, the orthorhombic cell has dimensions a=9.466(2), b=16.351(3) and c=13.284(2) Å. The structures consist of two crystallographically independent Me₄P⁺ cations and the ZnBr₄²⁻ anions. In the room temperature phase, all three ionic species show substantial displacement from the mirror plane perpendicular to the a-axis that exists in the high temperature phase, as well as rotations out of that plane. The thermal parameters of the cations are indicative of substantial librational motion. Measurements of lattice parameters have been made at 2-5°C intervals over the temperature range 40-140°C. The changes in the lattice constants appear continuous at T_c (within experimental limits) indicating that the phase transition is likely second-order. The a lattice constant shows an anomalous shortening as T_c is approached. Thermal expansion coefficients are calculated from this data. An application of Landau theory is used to derive the temperature dependencies of spontaneous shear strain and corresponding elastic stiffness constants associated with the primary order parameter.     

Structural and impedance properties of Ca3Nb2O8 ceramics

Polycrystalline sample of Ca₃Nb₂O₈ was prepared by a high-temperature solid-state reaction technique. X-ray diffraction (XRD) analysis confirms the formation of single-phase compound of hexagonal (rhombohedral) crystal structure at room temperature. Scanning electron micrograph of the material showed uniform grain distribution on the surface of the sample. Detailed studies of dielectric properties of the compound, studied in a wide frequency range (10²-10⁶ Hz) at different temperatures (25-500 °C), exhibit a dielectric anomaly suggesting phase transition of ferroelectric-paraelectric and structural type at 300 °C. Electrical properties of the material were analyzed using a complex impedance technique. The Nyquists plot showed the presence of bulk effect in the material in the studied temperature range. Studies of electrical conductivity over a wide temperature range suggest that the compound has negative temperature coefficient of resistance behavior.     

Crystal structure and dielectric properties of ordered perovskites Ba2BiSbO6 and BaSrBiSbO6

Neutron powder diffraction studies showed that the ordered perovskites Ba₂BiSbO₆ (BBS) and BaSrBiSbO₆ (BSBS) crystallize in a rhombohedral structure with the space group R3¯. The room-temperature lattice parameters are a=6.0351(2) Å; α=60.202(1)° and a=5.9809(2) Å; α=60.045(2)°, respectively. BBS exhibits a dielectric anomaly near room temperature which may be related to structural transition from the R3¯ to low-temperature monoclinic I2/m symmetry. BSBS shows a dielectric anomaly near 723 K which coincides with a phase transition from the rhombohedral to cubic (Fm3¯m) structure. In contrast to BBS, BSBS does not undergo structural transition below room temperature.     

Dielectric relaxation in single crystal NH4H2PO4 in the high-temperature regime

The dispersion curves of the dielectric response in single crystal NH₄H₂PO₄ were obtained in the radio frequency range and below the high-temperature transition at T_p−160 °C. The results reveal dielectric relaxation at low frequency, which is about 10⁵ Hz at 70 °C, and it shifts to higher frequencies (∼3×10⁶ Hz) as the temperature increases. The relaxation frequency was determined from the peak obtained in the imaginary part of the permittivity as well as from the derivative of the real part of the permittivity. The activation energy E_a=0.55 eV, obtained from the relaxation frequency is very close to that derived from the dc conductivity. We suggest that this dielectric relaxation could be due to the proton jump and phosphate reorientation that cause distortion and change the local lattice polarizability inducing dipoles like      

Electrical properties of high Curie temperature (1−x)Pb(In1/2Nb1/2)O3-xPbTiO3 single crystals grown by the solution Bridgman technique

0.65Pb(In_1/2Nb_1/2)O₃-0.35PbTiO₃ (PINT65/35) (starting composition) single crystals were grown successfully through the solution Bridgman technique using PbO flux and PMNT67/33 seed crystals. Because of the composition variation, the final composition of achievable crystals is in a range of 0.32-0.34 with the corresponding T_c range of 265-269 °C. The (001) plates of as-grown PINT66/34 single crystals show high Curie temperature (T_c=269 °C) and rhombohedral-tetragonal phase transition temperature (T_rt=134 °C). Besides, good electrical properties with high dielectric constant (ε>3000), low dielectric loss (tan δ∼1.2%), high piezoelectric constant (d₃₃∼2000 pC/N) and large electromechanical coupling factor (k_t≈59%) at room temperature have been obtained on the (001) plates. The sound velocity, acoustic impedance and other piezoelectric parameters were also measured on the (001) plates in this study, which provide us more detailed information about PINT66/34 single crystals.     

Proton NMR relaxation study of the CsHSO4 solid acid system

At 141 °C the solid acid CsHSO₄ is known to undergo transition to a superprotonic phase that is characterized by dramatic (several-order-of-magnitude) increases in hydrogen ion conductivity. Proton NMR spin-spin relaxation time T₂ measurements reported here for CsHSO₄ also reveal substantial increases (factors of 20-30) in the vicinity of the transition temperature. In the temperature range just below the transition (70-136 °C), T₂ increases by a factor of order 10 relative to the rigid-lattice regime, suggesting motional narrowing of the NMR resonance line. In the regime of motional narrowing, the activation energy barrier to diffusion is 0.40 eV, as determined from the present T₂ results. NMR spin-lattice relaxation T₁ measurements also show behavior consistent with transition to a regime of rapid hydrogen motion. In particular, proton T₁'s decrease with temperature (from 80 to 120 °C), and then drop sharply near the transition temperature. Above the transition temperature, T₁ exhibits a minimum in which the correlation time is found to be ∼2 ns.     

High temperature structural studies of SrRhO3

High resolution X-ray powder diffraction studies have shown SrRhO₃ to transform from an orthorhombic Pnma structure at room temperature through an intermediate Imma phase to a tetragonal I4/mcm structure near 800 °C. The orthorhombic Imma phase exists over a very limited temperature range, of less than 20°. The diffraction data suggests the Pnma to Imma transition is continuous and demonstrates that the Imma-I4/mcm transition is first order.     

Immiscibility in the NiFe2O4-NiCr2O4 spinel binary

The solid solution behavior of the Ni(Fe_1−nCr_n)₂O₄ spinel binary is investigated in the temperature range 400-1200 °C. Non-ideal solution behavior, as exhibited by non-linear changes in lattice parameter with changes in n, is observed in a series of single-phase solids air-cooled from 1200 °C. Air-annealing for 1 year at 600 °C resulted in partial phase separation in a spinel binary having n=0.5. Spinel crystals grown from NiO, Fe₂O₃ and Cr₂O₃ reactants, mixed to give NiCrFeO₄, by Ostwald ripening in a molten salt solvent, exhibited single-phase stability down to about 750 °C (the estimated consolute solution temperature, T_cs). A solvus exists below T_cs. The solvus becomes increasingly asymmetric at lower temperatures and extrapolates to n values of 0.2 and 0.7 at 300 °C. The extrapolated solvus is shown to be consistent with that predicted using a primitive regular solution model in which free energies of mixing are determined entirely from changes in configurational entropy at room temperature.     

Phase transition, structural and electrical properties of Pb(Zn1/3Nb2/3)O3-doped Pb(Ni1/3Nb2/3)O3-PbTiO3 ceramics prepared by solid-state reaction method

Phase pure perovskite (1−x−y)Pb(Ni_1/3Nb_2/3)O₃-xPb(Zn_1/3Nb_2/3)O₃-yPbTiO₃ (PNN-PZN-PT) ferroelectric ceramics were prepared by conventional solid-state reaction method via a B-site oxide mixing route. The PNN-PZN-PT ceramics sintered at the optimized condition of 1185 °C for 2 h exhibit high relative density and rather homogenous microstructure. With the increase of PbTiO₃ (PT) content, crystal structure and electrical properties of the synthesized PNN-PZN-PT ceramics exhibit successive phase transformation. A morphotropic phase boundary (MPB) is supposed to form in (0.9−x)PNN-0.1PZN-xPT at a region of x=32-36 mol% confirmed by X-ray diffraction (XRD) measurement and dielectric measurement. The MPB composition can be pictured as providing a “bridge” connecting rhombohedral ferroelectric (FE) phase and tetragonal one since crystal structure of the MPB composition is similar to both the rhombohedral and tetragonal lattices. Dielectric response of the sintered PNN-PZN-PT ceramics also exhibits successive phase-transition character. 0.64PNN-0.1PZN-0.26PT exhibits broad, diffused and frequency dependent dielectric peaks indicating a character of diffused FE-paraelectric (PE) phase transition of relaxor ferroelectrics and 0.40PNN-0.1PZN-0.50PT exhibits narrow, sharp and frequency independent dielectric peaks indicating a character of first-order FE-PE phase transition of normal ferroelectrics. The FE-PE phase transition of 0.56PNN-0.1PZN-0.34PT is nearly first-order with some diffused character, which also exhibits the largest value of piezoelectric constant d₃₃ of 462pC/N.     

Intermediate-temperature polymorphic phase transition in KH2PO4: A synchrotron X-ray diffraction study

We have used synchrotron X-ray diffraction to investigate the structural and chemical changes undergone by polycrystalline KH₂PO₄ (KDP) upon heating within the 30-250 °C temperature interval. Our data show evidence of a polymorphic transition at T∼190 °C from the room-temperature tetragonal KDP phase to a new intermediate-temperature monoclinic KDP modification (spacegroup P2₁/m and lattice parameters a=7.590, b=6.209, c=4.530 Å, and β=107.36°). The monoclinic RDP polymorph remains stable upon further heating to 235 °C, and is isomorphic to its RbH₂PO₄ and CsH₂PO₄ counterparts.     

Role of the TiO6 octahedra on the ferroelectric and piezoelectric behaviour of the poled PbMg1/3Nb2/3O3-xPbTiO3 (PMN-PT) single crystal and textured ceramic

Field cooling (FC) poled/unpoled PMN-29%PT single crystal and room temperature (RT) poled/unpoled PMN-34.5%PT textured ceramic were investigated between ∼0 and 300 °C by thermal expansion, dielectric and Raman spectroscopy. New phase transitions are evidenced at 40, 91 and 180 °C in the case of FC PMN-29%PT as well as at 70 and 200 °C for RT PMN-34.5%PT and their order is discussed. The physical properties of the textured ceramics are rather similar to the ones observed for the single crystals that make them low-cost alternative for a wide range of applications. However, the temperatures and character of the phase transitions strongly depend on the kind of the poling conditions. Temperature dependences of the Raman line parameters show that the NbO₆ octahedra remain stable during temperature increase, while TiO₆ ones evolve quasi-continuously. The step transitions of the Pb²⁺ ion sublattice are evidenced. This suggests that the TiO₆ and Pb²⁺ sublattices are especially coupled. The role of the TiO₆ clusters on the structural phase transitions and dielectric properties of the PbMg_1/3Nb_2/3O₃-xPbTiO₃ (PMN-PT) system is discussed. The presence of the Raman modes above the maximum dielectric permittivity reveals that the local symmetry is lower than the cubic one (Pm3m). The decrease of the Raman line intensities vs. temperature indicates precisely the continuous evolution of the local symmetry towards the cubic one. The temperature evolution of the Rayleigh wing parameters appears sensitive to the phase transitions’ presence.     

Phase diagram of the relaxor (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 investigated by dielectric and Raman spectroscopies

Lead zinc niobate-lead titanate[(1−x)Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃] (PZN-PT) crystals with x=4.5% and x=12% have been investigated using dielectric and Raman measurements over a range of temperatures. Above room temperature, dielectric measurements show that both compositions exhibit structural phase transitions according to the phase diagram proposed by Kuwata et al. [Ferroelectrics 387 (1981) 579]. Below room temperature, an anomaly at around 180 K for the x=12% sample is observed, suggesting another phase transition. Raman measurements are used to study all phase transitions.     

Study of phase transition in ZrO2 doped with Pb3O4

Electrical conductivity of ZrO₂ doped with Pb₃O₄ has been measured at different temperatures for different molar ratios (x=0, 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06). The conductivity increases due to migration of vacancies, created by doping. The conductivity increases with increase in temperature till 180 °C and thereby decreases due to collapse of the fluorite framework. A second rise in conductivity at higher temperatures beyond 500-618 °C is due to phase transition of ZrO₂. DTA and X-ray powder diffraction were carried out for confirming doping effect and transition in ZrO₂.The addition of Pb₃O₄ to ZrO₂ shifted the phase transition of ZrO₂ due to the interaction between Pb₃O₄ and ZrO₂.     

Magnetic properties of xNi0.53Cu0.12Zn0.35Fe1.88O4+(1−x)BaTiO3 nanocomposites

The nanocrystalline Ni_0.53Cu_0.12Zn_0.35Fe_1.88O₄ and BaTiO₃ powders were prepared using Microwave-Hydrothermal (M-H) method at 160 °C/45 min. The as synthesized powders were characterized using the X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). The size of the powders that were synthesized using M-H system was found to be ∼30 and ∼50 nm for ferrite phase and ferroelectric phases, respectively. The powders were densified using microwave sintering method at 900 °C/30 min. The ferrite and ferroelectric phases were observed from XRD and morphology of the composites was observed with the Scanning Electron Microscope (SEM).The magnetic hysteresis loops were recorded using the Vibrating Sample Magnetometer (VSM).The frequency dependence of real (μ′) and imaginary (μ″) parts of permeability was measured in the range of 1 MHz-1.8 GHz. The permeability decreases with an increase of BaTiO₃ content at 1 MHz. The transition temperature (T_C) of ferrite was found to be 245 °C. The T_C of composite materials decreases with an increase in BaTiO₃ content.     

Phase evolution and microwave dielectric properties of Bi3SbO7 ceramic

The Bi₃SbO₇ ceramic was prepared by the solid state reaction method and its phase evolution at different temperatures was studied. Low temperature phase α-Bi₃SbO₇ was formed at about 890 °C and it started to transform to high temperature phase β-Bi₃SbO₇ at about 960 °C. Microwave dielectric constants of α-Bi₃SbO₇ ceramic and β-Bi₃SbO₇ ceramic were 43.2 and 37.6, Qf value were 2080 and 5080 GHz, respectively. TCF of α-Bi₃SbO₇ ceramic was near zero and TCF of β-Bi₃SbO₇ ceramic was about −120 ppm/°C. The Bi₃SbO₇ ceramic is a promising candidate for low temperature co-fired ceramic (LTCC) technology due to its large dielectric constant, low dielectric loss at microwave region, low sintering temperature and simple composition.     

Structure, phase transitions and molecular dynamics in 4-methylpyridinium tetrachloroantimonate(III), [4-CH3C5H4NH][SbCl4]

Crystal structure of the 4-methylpyridinium tetrachloroantimonate(III), [4-CH₃C₅H₄NH][SbCl₄], has been determined at 240 K by X-ray diffraction as monoclinic, space group, P2₁/n, Z=8. Differential scanning calorimetry and dilatometric studies indicate the presence of two reversible phase transitions of first order type, at 335/339 and 233/289 K (cooling/heating) with ΔS=0.68 and 2.2 J mol⁻¹ K⁻¹, respectively. Crystal dynamics is discussed on the basis of the temperature dependence of the ¹H NMR spin-lattice relaxation time T₁ and infrared spectroscopic studies. The low temperature phase transition at 233 K of an order-disorder type is interpreted in terms of a change in the motional state of the 4-methylpyridinium cations. The phase transition at 335 K, probably of a displacive type, is characterised by a complex mechanism involving the dynamics of both the cationic and anionic sublattice. The ¹H NMR studies show that the low temperature phase III is characterised only by the dynamics of the CH₃ groups.     

Diffuse phase transition in Ba5NdTi3−xZrxNb7O30 ferroelectric ceramics

Polycrystalline samples of Ba₅NdTi_3−xZr_xNb₇O₃₀ (x=0, 1, 2, 3) compounds of the tungsten-bronze (TB) structural family were prepared by a high-temperature solid-state reaction technique. X-ray study of the compounds shows the formation of single phase compounds in the orthorhombic crystal system at room temperature. Detailed studies of the dielectric properties (dielectric constant and loss tangent) as a function of temperature (−50 to 350°C) at four different frequencies, 1, 10, 100 and 1000 kHz show relaxor behavior and diffuse phase transition of the compounds. Study of temperature dependence of resistivity shows that these compounds have negative temperature coefficients of resistance (NTCR).