Pyroelectric and dielectric properties of some light rare-earth orthochromites

The measurement of pyroelectic coefficient (p) and dielectric constant (K) of rare-earth orthochromites RCrO₃, where R=praseodymium, neodymium, samarium, europium and gadolinium, have been reported in the temperature range 300 to 450 K. Pyroelectric data show that praseodymium and neodymium orthochromites do not have ferroelectric phase in this temperature range, whereas samarium, europium and gadolinium orthochromites are ferroelectric. The dielectric data support the above conclusion. Spontaneous polarization (P _s) for samarium, europium and gadolinium have also been evaluated. The maximum values forP _s are 4.6 × 10^–3 C m^–2 for SmCrO₃, 0.42 C m^–2 for EuCrO₃ and 0.72 C m^–2 for GdCrO₃.     

Microwave dielectric properties of xMgTiO3-(1 − x)(Na1/2Ln1/2)TiO3 ceramics

Na_1/2Ln_1/2)TiO₃> ceramic has a high relative dielectric constant and a positive temperature coefficient of resonant frequency (_f) (where Ln represents a lanthanide: La⁺³, Pr⁺³, Nd⁺³ and Sm⁺³). On the other hand, MgTiO₃ ceramic has a high Qf value and a negative temperature coefficient. We have investigated the microwave dielectric properties of MgTiO₃-(1 – x)(Na_1/2 Ln_1/2) TiO₃. In this system, there are no indications of a solid-solution or a secondary phase. There are mixed phases only with MgTiO₃ and Na_1/2 Ln_1/2)TiO₃ phases. Its dielectric characteristics (Q ^*f, temperature coefficient and dielectric constant) are intermediate between (Na_1/2 Ln_1/2) TiO₃ and MgTiO₃ and are predictable by the logarithmic mixing rule. The temperature coefficient of dielectric ceramic compositions approximates to zero at each Ln = La, x = 0.9, Ln = Pr, x = 0.87, and Ln = Nd, x = 0.84. At this time, there are Q ^*f values in the range of 55 000 to 28 000 GHz and relative dielectric constants in the range of 22 to 25.     

Self-propagating high-temperature synthesis of Lu<Subscript>2</Subscript>O<Subscript>3</Subscript> powders for optical ceramics

A technique has been developed for the self-propagating high-temperature synthesis of lutetium oxide (Lu₂O₃) powders using citric acid, glycine, and lutetium acetate as fuels. We have carried out thermodynamic analysis of synthesis conditions and examined the effect of the nature of the fuel on the properties of the resultant powders. Using vacuum sintering at a temperature of 1780°C and powders prepared with glycine as a fuel and containing 25 mol % yttrium oxide and 5 mol % lanthanum oxide as sintering aids, we have obtained transparent lutetium oxide-based ceramics.     

Observations of an ordered Pd7Eu phase in Pd-Eu alloys

The Pd-Eu alloys containing 6.1 to 14.3 at% Eu have been studied by metallographic examinations, X-ray diffraction, electron microscopic observations and electrical resistance measurements. The results for the annealed alloys indicate the presence of an ordered phase in the form of a Pd₇Eu superlattice, which is similar to the order found in Pd₇R (R=cerium, gadolinium, dysprosium, yttrium or samarium) previously. The order-disorder transition of the ordered Pd₇Eu phase in the alloys between Pd-10 and 16.6 (Pd₅Eu) at% europium alloys is accompanied with a peritectoid reaction:-Pd + Pd₅Eu Pd₇Eu at 850 ± 10K. The ordering temperatures for Pd-6.1 and 7.5at % Eu alloys are lower than the peritectoid reaction temperature, and they correspond to the solid solubility limit of europium in the-Pd phase.     

Lanthanide oxides: Thermochemical approach to hydration

The behaviour toward hydration of a series of Ln₂O₃ has been studied as a function of water pressure and temperature by means of temperature programmed decomposition measurements. Thermochemical calculations have pointed out the structural change associated in going from ytterbium to lutetium in Ln(OH)₃ to be responsible for the apparent anomalous behaviour of Lu₂O₃. On this basis generalizations about the expected behaviour along the 4f series have been considered taking into account the simple ionic model for solid lanthanide compounds.     

The influence of zinc oxide–cerium oxide nanoparticles on the structural characteristics and electrical properties of polyvinyl alcohol films

With the objective to investigate the influence of zinc oxide–cerium oxide (ZnO–Ce₂O₃) nanoparticles on the electrical properties of polyvinyl alcohol (PVA), PVA/ZnO–Ce₂O₃ nanocomposite films were prepared by solution intercalation method with different weight percentage viz., 0.5, 1.0, and 2.0?wt% of ZnO–Ce₂O₃ nanoparticles. The fabricated nanocomposites were characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The effect of ZnO–Ce₂O₃ nanoparticles on the dielectric constant (ε′), dielectric loss (ε″), electric modulus (M′ and M″), ac conductivity (σ _ac), and dielectric loss tangent (tan δ) over a range of frequencies at room temperature of PVA nanocomposites have been studied. FT-IR, XRD, and DSC analysis indicates the nature of ZnO–Ce₂O₃ nanoparticles interaction with the PVA matrix. The morphological behavior of the nanocomposites has been performed using scanning electron microscopy (SEM). The dielectric behaviors such as dielectric constant (ε′) and dielectric loss (ε″) increases with increase in nanoparticle concentration, but decreases with increase in frequency. But, the electric modulus (M′) increases with increase in frequency. Dielectric loss tangent (tan δ) decreases with increase in filler content at lower frequency, but at higher frequencies the tan δ increases with increase in nanoparticles content. AC conductivity (σ _ac) of PVA/ZnO–Ce₂O₃ nanocomposites increases with increasing frequency following the universal dielectric response law.     

Physical properties of some rare earth tellurite glasses

Mossbauer and IR spectra as well as the electrical conductivity have been measured to give an idea about the structure and the electrical properties of some rare earth tellurite glasses containing Fe₂O₃. The glasses denoted [1 – (2x + 0.05)] TeO₂ ·xFe₂O₃ · (x + 0.05) Ln₂O₃, wherex = 0.0 and 0.05 and Ln = lanthanum, neodymium, samarium, europium or gadolinium, were prepared by fusing a mixture of their respective reagent grade oxides in a platinum crucible at 800° C for one hour. The Mossbauer parameters such as isomer shift, quadruple splitting and line width were found to be a function of the polarizing power (charge/radius) of the rare earth cations. The Mossbauer parameters were not affected by the heat treatment of the glass samples. Both of the Te-O-Ln and Te-O-Fe stretching vibrations were obtained from the IR results which indicate that the rare earth oxides and iron oxide are partially covalent. The electrical resistivity was measured as a function of temperature from 293 to 520 K. Both the electrical resistivity and the activation energy were found to be a function of the atomic number (Z) of the rare earth cations. The results were interpreted on the basis of the electronic structure of the glass.     

Effect of simultaneous substitution of La and Mn on dielectric behavior of barium titanate ceramic

A few compositions in the valence compensated system Ba_1−x La _x Ti_1−x Mn _x O₃ were synthesized by solid-state ceramic method to study the effect of co-doping lanthanum and manganese in equimolar amounts on dielectric behavior of BaTiO₃. Compositions with x ≤ 0.10 have shown solid solution formation. Compositions with x ≤ 0.05 are found to have tetragonal structure at room temperature while composition with x = 0.10 is cubic. Plots of relative dielectric constant, ε_r versus temperature for composition with x = 0.01 show dielectric anomalies around 376 ± 2, 264 ± 2 and 179 ± 2 K which correspond to cubic to tetragonal (T _C–T), tetragonal to orthorhombic (T _T–O) and orthorhombic to rhombohedral (T _O–R) transition, respectively, similar to BaTiO₃. Curie temperature has been found to decrease with increasing concentration of lanthanum and manganese simultaneously in barium titanate. The broadening in the dielectric peak at cubic to tetragonal (T _C–T) transition temperature increases with increasing x. For x = 0.10, only one anomaly at 100 K is observed in its ε_r versus T plots. The observation of this single anomaly may be due to pinching effect of the substitutions on the three phase transitions.     

A mathematical model for analysis of sequentially coupled crystallization–melting differential scanning calorimetry peaks and the use of the model for assessing the crystallization resistance of tellurite glasses

Differential scanning calorimetry (DSC) characterization of tellurite glasses doped with lanthanum oxide, which improves their crystallization resistance, has revealed a phase transformation specific to such glasses, in which partial crystallization of a sample is followed by melting of the crystals formed. The experimentally observed dependence of the decrease of crystallization–melting peaks across a series of disperse samples of (TeO₂)_0.72(WO₃)_0.24(La₂O₃)_0.04 glass with increasing particle size upon extrapolation to the size of a bulk sample has been used to assess the crystallization resistance of tellurite glasses for optical applications. The assessment technique comprises DSC characterization of particle-size-classified glass samples and the use of a mathematical model for obtaining the degree of crystallization as a function of temperature and time, α(T, t) through analysis of nonisothermal DSC peaks representing a partial glass crystallization process passing into melting. The crystallization resistance of glass is estimated by extrapolating the maximum α values as a function of particle size to a preform size. Tested for (TeO₂)_0.72(WO₃)_0.24(La₂O₃)_0.04 glass, the technique offers the possibility of selecting preforms for producing fibers from compositionally new, chemically pure tellurite glasses at a given phase purity level.     

X-ray absorption study of rare earth ions in Sr2MgSi2O7:Eu,R persistent luminescence materials

The valence of the europium dopant and selected rare earth co-dopants (Ce³⁺, Dy³⁺, and Yb³⁺) in the Sr₂MgSi₂O₇:Eu²⁺,R³⁺ persistent luminescence materials were studied by room temperature XANES measurements. The results indicated the co-existence of both divalent and trivalent europium in all the studied materials. The relative amount of Eu³⁺ was observed to increase upon increasing exposure to X-rays, as expected by the persistent luminescence mechanism. This suggests a simultaneous filling of oxygen vacancies initially created by the reducing preparation conditions. For the Dy and Yb co-dopants, only trivalent species were observed. On the other hand, traces of tetravalent cerium were present in the Eu,Ce co-doped materials.     

Application of lanthanum to pseudo-boehmite and γ-Al2O3

Elements such as lanthanum, zirconium, thorium and cerium are known to raise the thermostability of -Al₂O₃. Among these elements lanthanum is the most effective. Three procedures have been used to apply lanthanum homogeneously to the surface of pseudo-boehmite and -Al₂O₃, namely, incipient wetness impregnation, deposition-precipitation from a homogeneous solution, and specific adsorption of a La(EDTA)-complex. The latter procedure results in supports covered homogeneously with lanthanum. The minimum amount of lanthanum required to render the catalyst supports thermostable, depends on the procedure to apply the lanthanum. The specific adsorption procedure offers the possibility to obtain thermostable supports with loadings as low as 0.5 wt % La.High-resolution electron microscopy was used to assess the mechanism of the stabilization by application of foreign metal ions in amounts less than a monolayer. It appeared that addition of foreign metal ions brings about crystallization of the amorphous surface layer of -Al₂O₃.     

Synthesis and crystal structure of double Ln(III) malonates with Co(NH3) 6 3+ in the outer sphere

Double Ln(III) malonates of two different compositions crystallize from malonate solutions containing [Co(NH₃)₆]³⁺ ions. Lanthanides of the beginning of the series form compounds of the composition [Co(NH₃)₆][Ln(mal)₂]₃·6H₂O (I) (Ln = La, Ce, Pr, Nd; mal = C₃H₂O^2?), and those of the end of the series form compounds of the composition [Co(NH₃)₆]₂[Ln₃(mal)₇(Hmal)(H₂O)₄]·nH₂O (II) (Ln = Tb, Ho, Er, Tm). Structure I is based on trimeric anionic complexes [Ln₃(mal)₆]^3? linked with each other to form a branched 3D network with [Co(NH₃)₆]³⁺ cations and water molecules accommodated in large voids. The coordination mode of malonate ions in I with the coordination capacity equal to 5 was unknown previously for lanthanide malonate compounds. The Ln(1) atom has the maximum possible for malonate compounds coordination number (CN) 12, and the Ln(2) atom has CN 9. The structure of II consists of anionic chains [Ln₃(mal)₇(Hmal)(H₂O)₄] _n ^3? between which the [Co(NH₃)6]₃₊ cations and water molecules are arranged. One independent malonate ion in the structure is coordinated in the bidentate chelate fashion to the Ln(1) atom, and the other independent chelate-bridging ligand is coordinated in the bidentate fashion to the Ln(2) atom and in the monodentate fashion to the Ln(1) atom. As a result, tetrameric fragments linked in anionic chains are formed in the structure of II. The Ln(1) and Ln(2) atoms have CN 8.     

Preparation, characterization and dielectric properties of Sr5LnTi3Ta7O30 (Ln=La, Nd) ceramics

Sr₅LnTi₃Ta₇O₃₀ (Ln=La, Nd) ceramics were prepared by the conventional high temperature solid state reaction route. The sintered samples were characterized by X-ray diffraction and scanning electron microscopy methods. SLTT and SNTT belong to paraelectric phases of filled tetragonal TB structure at room temperature. SLTT and SNTT ceramics show high dielectric constant of 131 and 116 together with low dielectric loss 0.008 and 0.0036 at 1 MHz, respectively. In comparison with Ba-based ceramics with TB structure, the temperature coefficients of the dielectric constant () are significantly reduced.     

The preparation of some 17O-enriched oxides

Magnesium oxide, lanthanum (III) oxide, cerium (IV) oxide and europium (III) oxide have been prepared with approximately 10 mole % ¹⁷O-enrichment by direct reaction between ¹⁷O-enriched oxygen and the metals. Apparatus for producing ¹⁷O-enriched oxygen by electrolysis from enriched water is described, as are the techniques for its storage, manipulation and reaction.     

Heat capacity and thermodynamic functions of LaVO<Subscript>4</Subscript> and LuVO<Subscript>4</Subscript> from 7 to 345 K

The heat capacities of lanthanum and lutetium orthovanadates have been measured at temperatures from 7 to 345 K using an adiabatic calorimeter. No anomalies have been detected in the heat capacity data. The thermodynamic functions (C _p ⁰(T), S ⁰(T), and H ⁰(T) − H ⁰(0)) of the two compounds have been calculated in the temperature range studied, and their Debye characteristic temperatures have been estimated.     

Effect of gadolinium content on the thermal volume change of Ba4Na2Nb10O30 − Ba3NaGdNb10O30 solid solutions at ferroelectric phase transition temperature

For sintered samples of Ba₃₊ x Na_1+xGd_1–xNb₁₀O₃₀ (1 x 0), the effect of the addition of gadolinium to barium sodium niobate (BNN,x = 1, tungsten bronze type) was examined by differential scanning calorimetry (DSC) and X-ray powder diffractometry. The volume change (i.e. especially the change ofc-axis length) at the ferroelectric phase transition temperature (T _c< 590 dg C) decreased with the increasing gadolinium content. This result suggests that the addition of gadolinium or other lanthanide elements to BNN is possibly effective to obtain an uncracked single crystal with the Czochralski technique. The DSC indicator method is useful to determine the relative magnitude of volume change atT _c for the same type of sample series, a method which is much easier than the dilatometer method. The relation between the DSC peak area and the volume change of unit cell atT _c is discussed from the thermodynamic viewpoint.     

Phase relations in the K<Subscript>2</Subscript>MoO<Subscript>4</Subscript>–Ln<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript>–Zr(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> (Ln = La–Lu,Y) systems

We have studied phase relations in the K₂MoO₄–Ln₂(MoO₄)₃–Zr(MoO₄)₂ (Ln = La–Lu, Y) systems by the method of “intersecting cuts,” identified pseudobinary joins in their composition triangles, and constructed their phase compatibility diagrams. The systems have been shown to contain new ternary molybdates with the general formula K₅LnZr(MoO₄)₆ (Ln = Dy–Lu and Y). The thermal characteristics of the synthesized compounds have been studied by differential scanning calorimetry in the temperature range 25–700°C. The new ternary molybdates crystallize in a trigonal structure (sp. gr. R\(\bar 3\)c, Z = 6).     

Physico-chemical properties of Bismuth nitrate filled PVA–LiClO<Subscript>4</Subscript> polymer composites for energy storage applications

A series of polymer composites were prepared by the addition of Bismuth nitrate (Bi(NO₃)₃) and Lithium perchlorate (LiClO₄) to poly(vinyl alcohol) (PVA) polymer matrix by solvent casting method. The microstructures and surface morphology were systematically characterized by FTIR spectroscopy, X-ray diffraction, AFM and SEM analysis. The DSC thermal studies exhibit the transformations in the composites due to the effect of Bi(NO₃)₃. The mechanical strength, ac and dc conducting properties were studied. The variation of ac conductivity of the composites with frequency follows Jonscher’s universal power law and found to be increased with increasing temperature. The sample containing 10 wt% Bi(NO₃)₃ exhibits good mechanical strength, high conductivity and dielectric constant at room temperature. The variation of conducting properties of 10 wt% Bi(NO₃)₃ filled composite with temperature has been explained based on hopping mechanism. The frequency exponent (s) have been well fitted with the proposed correlation equation of the barrier hopping model. The transport parameters are estimated using Rice and Roth model. The activation energy is calculated for the sample PVA–LiClO₄ matrix filled with 10 wt% Bi(NO₃)₃ using Arrhenius equation.     

Synthesis and characterization of PLZT (10/65/35)

Lead zirconate titanate (PZT) powder of composition Pb(Zr_0.65Ti_0.35)O₃ was modified by incorporation of 10% lanthanum as a supervalent cation by the process of evaporative decomposition of aqueous solutions of lead nitrate, lanthanum nitrate, zirconyl nitrate and tetra-isopropyl titanate. X-ray diffraction patterns and infrared spectra of the compound calcined at different temperatures indicate that formation of rhombohedral PLZT is possible at 550°C for 1 H. The room-temperature dielectric constant measured at 1 kHz was found to be 991. From the study of dielectric constant as a function of temperature,T _c was found to be at 85°C. SEM study of the compound has shown grain-size homogeneity and quality of the compound. All these results suggest that this PLZT powder may be suitable for device applications.     

Large electromechanical anisotropic modified lead titanate ceramics

Modified lead titanate (PT) oxalates were prepared by mixing an ethanol solution with (calcium, lanthanum, neodymium, samarium or gadolinium) and titanium with an ethanol solution of oxalic acid containing lead, and (cobalt and tungsten) or manganese by blending. The resulting complex oxalate, after washing with ethanol for several times, was an amorphous powder with a very fine particle size. The amorphous coprecipitate gave, on calcining, a high sinterable modified lead titanate powder of compositions (Pb_0.76Ca_0.24) ((Co_0.5W_0.5)004Ti_0.96)O₃ or (Pb_0.88La_0.08) (Ti_0.98Mn_0.02)O₃ which, on sintering, produced highly densified bodies (98% theoretical density) at relatively low temperatures (1100 and 1150°C for Ca-PT and Ln-PT, respectively). Linear thermal expansion measurements, microstructural development of the modified lead titanates, and the influence of grain size onc/a ratio have also been studied.