Diffuse phase transformations in neodymium-doped BaTiO3 ceramics

Phase transitions in BaTiO₃ doped with neodymium have been studied using temperature-capacitance measurements, differential scanning calorimetry, X-ray powder diffraction and electron paramagnetic resonance spectroscopy. The coexistence of a para-electric cubic phase with the ferroelectric tetragonal and orthorhombic phases is noticed within the temperature range corresponding to the stability region of the latter phases in undoped BaTiO₃. The broad maximum in the effective dielectric constant shifts to lower temperature with increase in neodymium content. The heat of transformation measured at the Curie point tends to zero above 3.5at%Nd and the phase transition gradually approaches the second order. These are characteristics of a diffuse phase transformation. Using the quantitative results from the above studies, a T-x topological diagram is constructed for the BaTiO₃-xNd system wherex < 5.5at%. The phase contents vary with processing parameters as well as grain size, and the very existence of more than one phase in a given area of the T-x diagram indicates the metastable thermodynamic equilibrium prevailing in BaTiO₃-Nd ceramics. The inhomogeneous distribution of lattice defects may be the major cause for such a behaviour.     

Measurement of Specific Heat Capacity from 150 to 310 K by Thermal Radiation Calorimetry

A novel method for measurement of the specific heat capacity in the temperature range from 150 to 310 K is described. In order to achieve good temperature homogeneity in a disk-shaped specimen, a cylindrical heater was used in an apparatus based on thermal radiation calorimetry. A mixture of Bi₂O₃ and MgO powders was used for blackening the surfaces of the specimen, the heater, and the inside wall of the chamber. The specific heat capacities of Ni, fused quartz glass, and BaTiO₃ ceramic were measured to test the performance of the calorimeter. Agreement to within 5% of the values published in the literature was obtained for these samples. Thermal hysteresis and anomalies associated with the first-order phase transition in BaTiO₃ were detected in the present experiment.     

Critical examination of heat capacity measurements made on a Quantum Design physical property measurement system

We examine the operation and performance of an automated heat-capacity measurement system manufactured by Quantum Design (QD). QD’s physical properties measurement system (PPMS) employs a thermal-relaxation calorimeter that operates in the temperature range of 1.8-395 K. The accuracy of the PPMS specific-heat data is determined here by comparing data measured on copper and synthetic sapphire samples with standard literature values. The system exhibits an overall accuracy of better than 1% for temperatures between 100 and 300 K, while the accuracy diminishes at lower temperatures. These data confirm that the system operates within the ±5% accuracy specified by QD. Measurements on gold samples with masses of 4.5 and 88 mg indicate that accuracy of ±3% or better can be achieved below 4 K by using samples with heat capacities that are half or greater than the calorimeter addenda heat capacity. The ability of a PPMS calorimeter to accurately measure sharp features in C_p(T) near phase transitions is determined by measuring the specific heat in the vicinity of the first-order antiferromagnetic transition in Sm₂IrIn₈ (T₀=14 K) and the second-order hidden order (HO) transition in URu₂Si₂ (T_N=17 K). While the PPMS measures C_p(T) near the second-order transition accurately, it is unable to do so in the vicinity of the first-order transition. We show that the specific heat near a first-order transition can be determined from the PPMS-measured decay curves by using an alternate analytical approach. This correction is required because the latent heat liberated/absorbed at the transition results in temperature-decay curves that cannot be described by a single relaxation time constant. Lastly, we test the ability of the PPMS to measure the specific heat of Mg¹¹B₂, a superconductor of current interest to many research groups, that has an unusually strong field-dependent specific heat in the mixed state. At the critical temperature the discontinuity in the specific heat is nearly 15% lower than measurements made on the same sample using a semi-adiabatic calorimeter at Lawrence Berkeley National Laboratory.     

Dilatometric and dielectric studies on phase transition of potassium dihydrogen phosphate-boric acid binary system

Linear thermal expansion coefficient, dielectric data and transition temperature of KH₂PO₄-H₃BO₃ binary are reported. The transition temperature obtained from differential scanning calorimetry (DSC) is in concurrence with the dielectric and thermal expansion data. Further, this transition is classified as first order. Thermal expansion coefficients and enthalpy values of the binary are reported and compared with the data of pure KH₂PO₄.     

Phase transitions and ionic mobility in hydrogen zirconium phosphates with the NASICON structure, H1±XZr2−XMX(PO4)3·H2O, M = Nb, Y

Phase transitions and the mobility of proton-containing groups in hydrogen zirconium phosphate HZr₂(PO₄)₃·nH₂O with the NASICON structure were studied by X-ray powder diffraction, ¹H, ³¹P NMR, IR spectroscopy and TG analysis. Heating HZr₂(PO₄)₃·H₂O above 420 K results in dehydration and in a rhombohedral-triclinic phase transition. Continued heating to about 490 K results in the thermal activation of cation disordering and phase transition of HZr₂(PO₄)₃ from triclinic to rhombohedral phase. Parameter “a” of HZr₂(PO₄)₃ lattice decreases during the heating. It is shown that oxonium ions in HZr₂(PO₄)₃·H₂O are characterized by high rotation and translation mobility. Rotation mobility of oxonium ions can be increased by the substitution of zirconium by yttrium or niobium.     

Thermal Conductivity during Phase Transitions

Thermal conductivity is a very basic property that determines how fast a material conducts heat, which plays an important and sometimes a dominant role in many fields. However, because materials with phase transitions have been widely used recently, understanding and measuring temperature‐dependent thermal conductivity during phase transitions are important and sometimes even questionable. Here, the thermal transport equation is corrected by including heat absorption due to phase transitions to reveal how a phase transition affects the measured thermal conductivity. In addition to the enhanced heat capacity that is well known, it is found that thermal diffusivity can be abnormally lowered from the true value, which is also dependent on the speed of phase transitions. The extraction of the true thermal conductivity requires removing the contributions from both altered heat capacity and thermal diffusivity during phase transitions, which is well demonstrated in four selected kinds of phase transition materials (Cu₂Se, Cu₂S, Ag₂S, and Ag₂Se) in experiment. This study also explains the lowered abnormal thermal diffusivity during phase transitions in other materials and thus provides a novel strategy to engineer thermal conductivity for various applications.     

Dielectric and ferroelectric properties of (1-x)(Na0.5K0.5)NbO3-xBaTiO3 ceramics

(1-x)(Na_0.5K_0.5)NbO₃-xBaTiO₃ ceramics were prepared by a solid state reaction approach, and their dielectric and ferroelectric properties were evaluated together with the crystal structure. Three phase transitions at T_t1, T_t2 and T_t3 were observed by the combination of DTA analysis and dielectric characterization. These phase transitions corresponded to those of (Na_0.5K_0.5)NbO₃, and they were greatly pulled down by forming solid solution with BaTiO₃. The phase transition around T_t1 was incompletely diffusive and the appearance of diffusiveness of non ferro-paraelectric phase transition was an exception. The hysteresis loops changed their shapes from “square” into “thin square” with increasing x.     

Mechanical loss and Young's modulus associated with phase transitions in barium titanate based ceramics

The mechanical loss (Q ^–1) and Young's modulus (E) were measured as a function of temperature in the range 100 K3 driven electrostatically in flexural vibration mode at resonance frequencies between 2–4 kHz. Q ^–1(T) and E(T) curves show the following three phase transitions in BaTiO₃: rhombohedral to orthorhombic (T _R-O), orthorhombic to tetragonal (T _O-T), and tetragonal to cubic (T _Curie). Each phase transition induces a very narrow mechanical loss peak of Q ^–1 and a very sharp anomaly in the elastic modulus. Moreover, three other mechanical loss peaks are located below the phase transition temperatures in coarse grained specimens and these peaks seem to be associated with the motion of domain walls. These secondary peaks are not observed in fine grained undoped ceramics. The use of dopants such as Co or Nb produces an attenuation of the mechanical loss, a smoothing of elastic modulus anomalies, and a shifting of the phase transition temperatures.     

Single crystal growth and characterization of nearly stoichiometric LiVO2

LiVO₂ undergoes an imperfectly understood orbital ordering transition near 500 K resulting in a loss of magnetic moment below the transition. Studies of the transition have been hampered by a lack of high-quality stoichiometric single crystals. Here we report the growth and basic characterization of large, nearly stoichiometric LiVO₂ single crystals. The crystals were characterized by magnetic susceptibility, electrical resistivity, differential scanning calorimetry, and specific heat measurements over a temperature range from 2 to 650 K. A first-order phase transition with large hysteresis near T_t≈500 K was observed in all measurements. An anisotropy of the order of 100 was observed in the in-plane versus out-of-plane resistivity, and the inferred semiconducting energy gap was 0.18 eV for T<T_t and 0.14 eV for T<T_t. Electron diffraction experiments were performed on LiVO₂ single crystals at temperatures below and above T_t. Superlattice reflections were observed below T_t and disappeared upon heating above the phase transition temperature. Upon cooling below T_t, the supperlattice spots reappeared. Bright field electron micrographs indicate that the crystals develop a roughly hexagonal network of cracks.     

Crystallographic phase transition, electrical and magnetic properties of stoichiometric La(Mn,Rh)O3

Temperature-induced phase transitions, electrical conductivity, and some magnetic properties of stoichiometric LaMn_1−xRh_xO₃ (0.1≤x≤0.5) have been studied. The phase transitions of LaMn_0.9Rh_0.1O₃ were investigated by high-temperature X-ray powder diffractometry and orthorhombic-pseudocubic-rhombohedral transitions were observed. The electrical conductivity measurement shows semiconducting property above room temperature. Breaking of slope in the temperature dependence of the conductivity bring the evidence in support of the phase transition for LaMn_0.9Rh_0.1O₃. At low temperature, the compounds show ferromagnetic behavior but the ferromagnetic intensity increases with the increase of the oxygen nonstoichiometry.     

Phase transition and PTCR effect in erbium doped BT ceramics

In this work the dielectric behaviour and main features of the phase transition of BaTiO₃ and Ba_0.99Er_0.01TiO₃ ceramics were carefully investigated. The temperature and frequency dependences of the dielectric properties of erbium doped BaTiO₃ ceramics were measured in the 25–225 °C and 100 Hz to 10 MHz ranges, respectively. From this study, a dielectric anomaly in the ferroelectric–paraelectric phase transition of the Ba_0.99Er_0.01TiO₃ ceramic was observed. The features of the samples phase transition were analysed by using Curie–Weiss, Santos–Eiras’ and order parameter local phenomenological models. In the BaTiO₃ system, all models showed a normal phase transition, while was not possible to establish the character of the phase transition in the Ba_0.99Er_0.01TiO₃ system. The relaxation parameters of conductive processes for the study ferroelectric materials, analysed in the time domain, did not show any influence on the ferroelectric–paraelectric phase transition. Finally, it was demonstrated that the anomaly observed on the phase transition of the erbium doped BaTiO₃ ceramics is associated with the processes that results in the PTCR effect.     

Study of glass-nanocomposite and glass–ceramic containing ferroelectric phase

Transparent glass nanocomposite in the pseudo binary system (100 − x) Li₂B₄O₇–xBaTiO₃ with x = 0 and 60 (in mol%) were prepared. Amorphous and glassy characteristics of the as-prepared samples were established via X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC) respectively. The precipitated BaTiO₃ nanocrystal phase embedded in the glass sample at x = 60 mol% was identified by transmission electron microscopic (TEM). The optical transmission bands at 598 and 660 nm were assigned to Ti³⁺ ions in tetragonal distorted octahedral sites. The precipitated Li₂B₄O₇, BaTi(BO₃)₂ and BaTiO₃ nanocrystallites phases with heat-treatment at 923 K for 6 h (HT923) in glass–ceramic were identified by XRD, TEM and infrared absorption spectroscopy. The as-prepared at x = 60 mol% and the HT923 samples exhibit broad dielectric anomalies in the vicinity of the ferroelectric-to-paraelectric transition temperature. The results demonstrate that the method presented may be an effective way to fabricate ferroelectric host and development of multifunctional ferroelectrics.     

Dielectric properties,thermal expansion and heat capacity of (1 − x)Na0.5Bi0.5TiO3–xBaTiO3 single crystals (x = 0, 0.02, 0.025, 0.0325 and 0.05)

High-quality and large-size lead-free (1 − x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ single crystals (x = 0, 0.025, 0.0325 and 0.05) were grown using Czochralski method. The obtained samples were of pure perovskite structure with rhombohedral symmetry at room temperature. Thermal expansion, heat capacity, ferroelectric and dielectric properties were measured in a wide temperature range. The broad anomalies observed in thermal expansion and heat capacity were corresponded to structural, ferroelectric and dielectric anomalies, related to temperature features of polar regions and formation of a long-range order ferroelectric phase. The Burns temperature was found to increase with increasing BaTiO₃ content. At low-frequency (100 Hz–100 kHz) the samples showed diffuse phase transitions. The obtained results were discussed in terms of local electric and strain fields caused by a difference in ionic radii between (Na,Bi) and Ba ions.     

Effect of fluxing additive on sintering temperature, microstructure and properties of BaTiO3

Various fluxing materials are added to technical ceramics in an attempt to lower their sintering temperatures and make their processing economical. The effect of 0·3?wt% Li₂CO₃ addition on the phase, microstructure, phase transition temperatures and dielectric properties of BaTiO₃ was investigated in the present study. The addition of 0·3?wt% Li₂CO₃ was observed to lower the optimum sintering temperature by ??200°C with no second phase formation and cause a five-fold reduction in grain size. Rhombohedral-to-orthorhombic and tetragonal-to-cubic phase transitions at the expected temperatures were evident from the Raman spectra, but the orthorhombic-to-tetragonal phase transition was not clearly discernible. The persistence of various phase(s) at higher temperatures in the flux-added materials indicated that the phase transitions occurred relatively slowly. A decrease in dielectric constant of Li₂O-added BaTiO₃ in comparison to pure BaTiO₃ may be due to the diminished dielectric polarizability of Li^?+? in comparison to Ba^2?+?.     

The Electrocaloric Effect in BaTiO<Subscript>3</Subscript>–SrTiO<Subscript>3</Subscript> Solid Solution

We have studied the electrocaloric effect in 0.68BaTiO₃–0.32SrTiO₃ solid solution, as well as the dielectric, pyroelectric, and piezoelectric properties of this compound in the vicinity of the first-order phase transition in a bias electric field. The dielectric and electromechanical contributions to the pyroelectric and electrocaloric effects are discussed.     

Effect of BaTiO3 additive on the electrical properties of Na0.50Bi0.50TiO3 lead free ceramics

The near morphotropic phase boundary (MPB) compositions of lead-free piezoelectric ceramics based on sodium bismuth titanate (Na_0.50Bi_0.50TiO₃: NBT) and barium titanate (BaTiO₃: BT) were carefully investigated by conventional high temperature mixed-oxide method. All the ceramics exhibit single phase rhombohedral symmetry. The frequency (100 Hz to 1 MHz) and temperature (Room temperature–500 °C) dependence of impedance spectroscopy of (1 − x)Na_0.50Bi_0.50TiO3–xBaTiO₃ (x = 0.0, 0.06, 0.07 and 0.08) ceramics were investigated by impedance analyzer. The frequency explicit plots of Z″ versus frequency at various temperatures show peaks in the higher temperature range (>400 °C). The compounds show dielectric relaxation, which is found to be of non-Debye type and the relaxation frequency shifted to higher side with increase in temperature. The activation energy values obtained for different BT content suggest that the electrical conduction in NBT is mainly due to the mobility of the ionized oxygen defects.     

High-temperature dehydration behavior and protonic conductivity of RbH2PO4 in humid atmosphere

The high-temperature (HT) properties of RbH₂PO₄ have been investigated here by several methods. Two anomalies at T_p (∼109 °C) and T^′p (∼276 °C) in differential scanning calorimetry (DSC) measurement are due to structural transition from tetragonal (phase III) to monoclinic (phase II) and monoclinic to an unidentified phase I, respectively. The initial dehydration event in RbH₂PO₄ occurs at ∼250 °C, leading to the formation of dimer crust (Rb₂H₂P₂O₇) on the external surface of crystal particles which decelerates the further dehydration process. The conductivity measurement was performed under a highly humidified N₂ condition PH₂O≈0.56 atm to suppress its dehydration. It revealed two reversible superprotonic phase transition at T_p and T^′p. For the one at T^′p, the conductivity increases sharply by ∼2 orders of magnitude and the high-conductivity phase I was stable till melting. However, the other one at T_p shows a relatively small jump in conductivity.     

Synthesis and Properties of NdMCr2O5 (M = Na, K, Cs) and NdMgCr2O5.5 Chromites

NdMCr₂O₅ (M = Na, K, Cs) and NdMgCr₂O_5.5 are prepared by solid-state reactions between appropriate oxides and carbonates and are shown to have a tetragonal structure. The heat capacity of these chromites, measured from 298.15 to 673 K, exhibits sharp changes attributable to second-order phase transitions. The C _p ⁰(T) data are represented by quadratic best fit equations. The electrical resistivity of the chromites is measured between 303 and 493 K. The results attest to semiconducting behavior of the materials in certain temperature ranges.     

Hydrogen adsorption on alkali metal surfaces: Wetting,prewetting and triple-point wetting

We report the results of a quartz microbalance study of the multilayer adsorption of H₂ and D₂ on sodium and on rubidium. On sodium H₂ and D₂ display the well known phenomenon of triple-point wetting. Interestingly low temperature adsorption isotherms of H₂ on the evaporated Na surface show sharp layering transitions. On rubidium a weaker substrate H₂ and D₂ undergo a wetting transition in the liquid phase followed by sharp presetting transitions away from liquid-vapor coexistence. The prewetting phase diagram of H₂ on Rb has been quantitatively mapped out.     

High temperature structural phase transitions in SrSnO3 perovskite

Three high temperature structural phase transitions have been identified in the perovskite-structured phase SrSnO₃ using differential scanning calorimetry and dilatometry, and have subsequently been structurally characterised using high-resolution neutron powder diffraction. Between 298 and 905 K, SrSnO₃ is orthorhombic, space group Pmcn before undergoing a continuous phase transition to a second orthorhombic phase in space group Incn. At 1062 K there is a first order phase transition to a tetragonal phase in space group I4/mcm before finally transforming to the aristotype phase at 1295 K. Using the magnitude of the anti-phase tilt as a measure of the order parameter for the transition from I4/mcm to suggests this transition is tricritical in nature. Crystal structures are reported for the three hettotype phases.