Low temperature polarization effects in ice

When an initially unpolarized HF doped ice specimen is warmed at a constant rate in an applied electric field two peaks are observed in the current. The low temperature peak occurs near 100°K and the temperature at which this peak occurs is seen to increase as the HF concentration decreases. The second peak appears to occur randomly in the temperature range 125–135°K. These peaks are also observed if the sample is cooled in an applied electric field and then warmed at a constant rate with the field removed. It is suggested that the first peak is due to a dielectric relaxation process which is governed by the L defects released from the HF molecules. This release of L defects is shown to obey the law of mass action with an activation energy for liberation of an L defect of 0.12±0.06 eV and a dissociation constantk _o ^L≈10²⁹ m^?3. A simple theoretical model of ice is also developed which predicts the current reversal phenomenon observed by Dengelet al. [11] suggesting that it is due to dipole relaxation and not to ferroelectric ordering.     

Intrinsic polyatomic defects in Sc2(WO4)3

The intrinsic formation of polyatomic defects in Sc₂(WO₄)₃-type structures is studied by Mott Littleton calculations and Molecular Dynamics simulations. Defects involving the WO₄²⁻ tetrahedron are found to be energetically favorable when compared to isolated W and O defects. WO₄²⁻ Frenkel and (2Sc³⁺, 3WO₄²⁻) Schottky defects exhibit formation energies of 1.23 eV and 1.97 eV, respectively and therefore may occur as intrinsic defects in Sc₂(WO₄)₃ at elevated temperatures. WO₄²⁻ vacancy and interstitial migration processes have been simulated by classical Molecular Dynamics simulations. The interstitial defect exhibits a nearly 10 times higher mobility (with a migration energy of 0.68 eV), than the vacancy mechanism (with a slightly higher migration energy of 0.74 eV) and thus should dominate the overall ionic conduction. Still both models reproduce the experimental activation energy (0.67 eV) nearly within experimental uncertainty.     

Giant dielectric relaxation in SrTiO3-SrMg1/3Nb2/3O3 and SrTiO3-SrSc1/2Ta1/2O3 solid solutions

Ceramic samples of (1−x)SrTiO₃-xSrMg_1/3Nb_2/3O₃ and (1−x)SrTiO₃-xSrSc_1/2Ta_1/2O₃ were prepared, and their dielectric properties were studied at x=0.005–0.15 and 0.01–0.1, respectively, at frequencies 10 Hz–1 MHz and at temperatures 4.2–350 K. A giant dielectric relaxation was observed in the temperature range 150–300 K, and not so strong but well-developed relaxation was found in the temperature range 20–90 K. The activation energy U and the relaxation time τ₀ were determined to be 0.21–0.3 eV and from 10⁻¹¹ to 10⁻¹² s for the high-temperature relaxation and 0.01–0.02 eV and 10⁻⁸–10⁻¹⁰ s for the low-temperature relaxation, respectively. The additional local charge compensation of the heterovalent impurities Mg²⁺ and Nb⁵⁺ (or Sc³⁺ and Ta⁵⁺) by free charge carriers or the host ion vacancies is suggested to be the underlying physical mechanism of the relaxation phenomena. On the basis of this mechanism, the Maxwell-Wagner model and the model of reorienting dipole centers Mg²⁺ (or Sc³⁺) associated with the oxygen vacancy are proposed to explain the high-temperature relaxation with some arguments in favor of the latter model. The polaron-like model with the Nb⁵⁺-Ti³⁺ center is suggested as the origin of the low-temperature relaxation. The reasons for the absence of ferroelectric phase transitions in the solid solutions under study are also discussed. From Fizika Tverdogo Tela, Vol. 44, No. 11, 2002, pp. 1948–1957. Original English Text Copyright ? 2002 by Lemanov, Sotnikov, Smirnova, Weihnacht. This article was submitted by the authors in English.     

Dielectric relaxations in Tb0.91Yb1.38Bi0.71Fe5O12 ceramics

Dielectric relaxations of Tb_0.91Yb_1.38Bi_0.71Fe₅O₁₂ ceramics were investigated. A Debye-type relaxation was observed in the temperature range of 125-620 K with an activation energy of 0.29 eV. This activation energy agreed well with that of carriers hopping between Fe²⁺ and Fe³⁺, indicating that this relaxation might be a dipolar-type relaxation associated with the hopping carries. A high relaxorlike dielectric peak with a very strong frequency dispersion in the high temperature range of 400-620 K might be originate from the oxygen vacancies related dielectric relaxation.     

Dielectric relaxation in SrTiO<Subscript>3</Subscript>: Mn

Dielectric relaxation (activation energy U≈0.03 eV, relaxation time τ₀≈5×10^?11 s) has been observed in SrTiO₃: Mn solid solutions at low temperatures. It is assumed that the relaxation is related to reorientation of the polarons localized at defects of the {Mn _Ti ²⁺ -O^?} type and that the deviations from classical thermally activated behavior at the lowest temperatures reached are due to the quantum tunneling mode.     

Dielectric relaxation and computer simulation studies of rutile-structured MnF2 crystals

The fluorides of the rutile structure are relatively simple ionic materials with tetragonal symmetry for which the dominant intrinsic defect has not been established. The present experiments involve low-temperature dielectric relaxation measurements on Er³⁺-and Y³⁺-doped MnF₂ single crystals. Unexpectedly, dielectric loss peaks were observed at cryogenic temperatures, involving very low activation energies,E. For both dopants a prominent peak is observed for samples oriented parallel to thec-axis withE 6 meV and in perpendicular orientations withE=37 meV for Er³⁺ and 46 meV for Y³⁺ doping. Such lowE-values are probably too small to be controlled by lattice migration of a defect. Rather, we expect that they are due to a very low symmetry configuration created when the ions near the defect move off symmetry to a more stable configuration. Computer simulation calculations have been carried out which are much improved over early studies of this system in terms of the code used and the F-F interatomic potentials. The results show that the energy per defect for the anion Frenkel (1.53 eV) is lower than that of the Schottky (1.99 eV). It was also shown that the fluorine interstitial, F_i, adopts a split-interstitial form. This defect associates strongly with trivalent dopants Er and Y to produce a low symmetry dipolar structure with the necessary off-symmetry configuration to explain the experimental findings. Since there is no alternative way to explain these low temperature relaxations in terms of impurities associated with Mn vacancies, as would be required by the Schottky model, we conclude that these experiments serve to establish the nature of the intrinsic defect in MnF₂ as anion Frenkel.     

Microstructural interpretation of conductivity and dielectric response of Ce<Subscript>0.9</Subscript>Eu<Subscript>0.1</Subscript>O<Subscript>1.95</Subscript> oxygen ion conductors

This work focuses on the structure property co-relation study of Eu³⁺-doped ceria nanomaterials prepared through citrate auto-ignition process and sintered at three different temperatures. The microstructure and dielectric properties were found to be affected by the sintering temperature. The particle size was found to play a major role to the migration of charge carriers in the samples. The dielectric constant has been found to control the formation of dopant-vacancy interaction though columbic interaction in defect pair (Eu′_Ce – V_o ^??)^? and neutral trimers (Eu′_Ce – V_o ^?? – Eu′_Ce). The sample sintered at 800 °C shows the lowest value of lattice parameter due to the highest value of dopant-vacancy interaction. The migration energy for oxygen vacancy conduction was found to increase with particle size that reduces the ionic conductivity values. The rate of hopping was found to decrease due to blocking of charge-carrier diffusion due to the growth of particle.     

Luminescence of Bismuth‐Activated Ceramics of Yttrium and Scandium Oxides

The luminescence spectra of Y₂O₃:Bi and Sc₂O₃:Bi ceramics have been investigated. The spectra have been resolved into elementary components by the Alentsev–Fock method. It has been established that the luminescence is attributed to emission centers of three types, two of which are due to the replacement of Y³⁺ (or Sc³⁺) by Bi³⁺ at the nodes of the crystal lattice of Y₂O₃ (or Sc₂O₃) with the point symmetry C ₂ and C _3i . The emission center Bi³⁺ in the position C_3i leads to the appearance of blue luminescence with maxima at 3.03 eV for Y₂O₃:Bi and at 3.05 eV for Sc₂O₃:Bi; this luminescence is attributed to the transition ³ P ₁–¹ S ₀. The emission center Bi³⁺ in the position C ₂ initiates green luminescence (which is also related to the ³ P ₁–¹ S ₀ transition in Bi³⁺) with a maximum in the region of 2.40 eV in Y₂O₃:Bi and in the region of 2.46 eV in Sc₂O₃:Bi. The red luminescence band with maxima at 1.85 eV in Y₂O₃:Bi and at 1.95 eV in Sc₂O₃:Bi is related to the presence of structural defects.     

Relaxation studies of bulk samarium doped ceria electrolyte

The nanocrystalline samarium doped ceria powder of phase Ce_0.8Sm_0.2O_1.9 was pelletized and sintered at about 1,623 K for the study of relaxation process. The impedance measurements were done within the frequencies from 1 Hz to 1 MHz for wide temperature range of 529–673 K. The temperature-dependent relaxation frequency found to be shifted towards higher frequency region from 17.26 to 707.96 KHz and the corresponding relaxation time for oxygen ions transport was varied from 9.22 to 0.22 μS. The temperature dependent conductivity and relaxation frequency revealed Arrhenius nature, showing activation energies 1.18 and 1.01 eV, respectively. This difference was attributed to pronounced interference due to the grain boundaries. The effect of sintering on microstructure of the sample was studied by using SEM and X-ray diffractometer (XRD). The structural and phase stability study was done by high-temperature (HT)-XRD. The thermal expansion coefficient of the samarium doped ceria powder determined from HT-XRD was found to be 13.9 × 10⁻⁶ K⁻¹.     

Synthesis and dielectric characteristics of La0.5Bi0.5MnO3 ceramics

La_0.5Bi_0.5MnO₃ ceramics with a single phase were prepared by a solid-state reaction method, and their dielectric properties were characterized. Two dielectric relaxations with a giant dielectric constant were identified in the temperature range from 125 to 350 K. The electron hopping between Mn³⁺ and Mn⁴⁺ was found to be the origin of the dielectric relaxation at low temperatures (125–200 K) with an activation energy of 0.18 eV. The high temperature (200–350 K) dielectric relaxation can be attributed to the conduction.     

Fe1+ and charge transfer in ZnS. analysis of source experiments results

The evolution with the temperature of the relative Fe¹⁺ transient state population in ZnS is analyzed with the coherent relaxation model. The evolution of the Fe¹⁺ life time is compatible with an activation energy of 0.24 eV. The quadrupolar coupling observed at low temperature may be due to the population out of thermal equilibrium of an electronic excited state of Fe¹⁺. These results are in agreement with optical measurements on impurities in semiconductors.     

Na+-ion conductivity of double phosphate Na3Sc2(PO4)3 in the region of the β-γ transition

The Na⁺-ion conductivity σ of double phosphate Na₃Sc₂(PO₄)₃ in the region of the β-γ transition has been studied using impedance spectroscopy (1–10⁶ Hz). The polycrystalline sample of Na₃Sc₂(PO₄)₃ has been prepared by solid-phase synthesis and ceramic technology. It has been found that, upon the β-γ transition, the conductivity σ of Na₃Sc₂(PO₄)₃ suffers a ～1.5-fold jump at 470 ± 2 K upon heating and a ～2.5-fold jump at 430 ± 4 K upon cooling (the temperature hysteresis of the jump in σ is 40 K). For double sodium-scandium phosphate γ-Na₃Sc₂(PO₄)₃ in the superionic state, σ attains 0.07 S/cm at 700 K and the ion transport activation enthalpy is 0.42 ± 0.02 eV.     

Electrical study and dielectric relaxation behavior in?nanocrystalline Ce0.85Gd0.15O2?δ material at intermediate temperatures

The nanocrystalline material of 15 mol% Gd-doped ceria (Ce_0.85Gd_0.15O_2−δ ) was prepared by citrate auto ignition method. The electrical study and dielectric relaxation technique were applied to investigate the ionic transport process in this nanocrystalline material with an average grain size of 13 nm and the dynamic relaxation parameters are deduced in the temperature range of 300–600°C. The ionic transference number in the material is found to be 0.85 at 500°C at ambient conditions. The oxygen ionic conduction in the nanocrystalline Ce_0.85Gd_0.15O_2−δ material follows the hopping mechanism. The grain boundary relaxation is found to be associated with migration of charge carriers. The frequency spectra of modulus M″ exhibited a dielectric relaxation peak corresponding to defect associates (Gd-V_{o^{\blacksquare \blacksquare}})^{_\blacksquare}(\mathrm{Gd}\mbox{-}\mathrm{V}_{\mathrm{o}}^{_{_{{\blacksquare\,\blacksquare}}}})^{_{_{{\blacksquare}}}}. The material exhibits very low values of migration energy and association energy of the oxygen vacancies in the long-range motion, i.e., 0.84 and 0.07 eV, respectively.     

Dipolar studies in CaF2 with Ce3+

Dielectric relaxation in CaF₂ doped with various amounts of Ce³⁺ (0·01 to 1·0 mol%) was measured. The value of the activation energy for orientation of the dipoles {Ce³⁺-F^? interstitial} was determined to be H = (0·46 ± 0·01) eV. The frequency factor was found to have the value τ_o = (5 ± 1) × 10^?15 sec, giving for the vibrational frequency of the interstitial the value ν_o = (5 ± 1) × 10¹³ sec^?1.The number of dipoles contributing to the dielectric loss peak was determined to be between 10¹⁷ and 8 × 10¹⁷ cm^?3 for the different doping amounts of Ce³⁺. Optical absorption measurements showed the existence of large aggregate bands. We could verify that there exists a second-order reaction of aggregation, which is responsible for the non-linearity found between optical absorption at 305 nm and the nominal concentration of Ce³⁺ in the samples. On the other hand, if we assume that the centers which contribute to optical absorption at 305 nm are those also responsible for the relaxation peak, we find that the number contributing to each process is not the same. We can define an interaction radius R as the minimum separation between two dipoles allowing them to contribute to the relaxation peak. From our experimental data R ? 3·8 × 10^?7 cm.     

Luminescence of Donor‐Acceptor Pairs in Compounds of Yttrium and Scandium Oxides

The luminescence spectra of Sc₂O₃, Y₂O₃, and Y₂GeO₅ ceramics and thin films exposed to laser and cathode excitation were investigated. The investigation of the properties of longwave luminescence bands in Sc₂O₃ with maxima at 2.65, 2.35, and 2.05 eV, in Y₂O₃ with maxima at 2.60, 2.35, and 2.10 eV, and also in Y₂GeO₅ with maxima at 2.55, 2.25, and 2.00 eV point to the fact that they are caused by radiative recombination of the excited donoracceptor pairs Sc³⁺ (or Y³⁺)O^2–.     

Conductivity enhancement in fluorite-structured Ba1−xLaxF2+x solid solutions

The ionic conductivity of single crystals of the fluorite-structured solid solutions Ba_1?xLa_xF_2+x(10^?3 <×<0.45) has been studied as a function of temperature and composition in the range 300–900 K. Three regions can be discerned in the concentration dependence of the ionic conductivity: a dilute concentration region (x<10^?3), where classic relations between solute content and ionic conductivity hold; an intermediate concentration region (10^?3<x?5×10^?2), where large changes occur in the conductivity activation enthalpy and the magnitude of the conductivity; and a concentrated solid solution region (x?5×10^?2) characterized by enhanced ionic motion. In the dilute region the migration enthalpy for interstitial fluoride ions is determined to be 0.714 eV, while a value of 0.39 eV is found for the (La_BaF_i)^X association enthalpy. The defect chemistry in the intermediate concentration region is shown to be controlled by a superlinear increase of the concentration of mobile defects, while in the concentrated solid solution region a composition-independent amount of ≈1 mole% of interstitial fluoride ions with enhanced mobility, carry the current.     

Disorder processes in A3+B3+O3 compounds: implications for radiation tolerance

Intrinsic defect processes were compared for an extensive range of A³⁺B³⁺O₃ compounds with A cation radii from Sc³⁺ to La³⁺ and B cation radii from Al³⁺ to In³⁺. Crossovers between cation antisite, Schottky and oxygen Frenkel disorder are predicted as a function of cation radius. The radiation tolerance of these compounds was discussed in the light of their relative defect process energies. On this basis there should be a wide variation in radiation tolerance. In particular, only a limited compositional range, coincident with the bixbyite structure, will exhibit the high tolerance previously observed in materials with fluorite and fluorite-related structures. Materials with the rhombohedral or orthorhombic perovskite structures will be significantly less tolerant.     

Oxygen vacancy-transition metal-ion impurity association in SrTiO3

The progressive reduction of SrTiO₃:Mn has been monitored by EPR. The results show the conversion of Mn⁴⁺ to Mn²⁺ and Mn²⁺ to Mn²⁺-V₀ by oxygen-vacancy capture. The latter defect association takes place with the copious oxygen vacancies produced during high-temperature reduction between 1100 - 1300 K. At still higher reduction temperatures, other defects are formed. Analysis of the temperature dependence of the defect association gives a value of 0.7 eV for the enthalpy of association.     

Ionic conductivity and point defects in pure and doped MnF2 and MgF2 single crystals

Conductivity, σ, of MnF₂ and MgF₂ single crystals, pure and doped (with Li⁺, Na⁺, Y³⁺, Gd³⁺), has been measured, from room temperature to 500°C. Further, some crystals were contaminated with O^2? as an additional impurity. These tetragonal (rutile structure) crystals both behave like typical ionic conductors. Of particular interest is the existence of a large anisotropy, σ being largest when measured parallel to the c-axis. Study of the conductivity isotherms and anisotropy as functions of impurity concentration allows identification of the conduction mechanism in terms of the migration of two mobile defects: the fiuorine-ion vacancy, V_F, and interstitial, F_i. A value of 1.44 eV was obtained for the enthalpy of formation of the intrinsic anion Frenkel defect, 0.80 eV for the migration enthalpy of a V_F and 0.88 eV for an F₁ in MnF₂ parallel to the c-axis. Similar values were obtained for MgF₂. This work shows that more information about point defects can be obtained from conductivity measurements in non-cubic cyrstals than in cubic ionic crystals, because of the additional information from conductivity anisotropy.     

Stationary inverted Lyman populations and free-free and bound-free emission of lower-energy state hydride ion formed by an exothermic catalytic reaction of atomic hydrogen and certain group I catalysts

Rb⁺ to Rb²⁺ and 2K⁺ to K + K²⁺ each provide a reaction with a net enthalpy equal to the potential energy of atomic hydrogen. The presence of these gaseous ions with thermally dissociated hydrogen formed a plasma having strong VUV emission with a stationary inverted Lyman population. Significant Balmer α line broadening of 18 and 9 eV was observed from a rt-plasma of hydrogen with KNO₃, and RbNO₃, respectively, compared to 3 eV from a hydrogen microwave plasma. The reaction was exothermic since excess power of about 20 mW/cc was measured by Calvet calorimetry. We propose an energetic catalytic reaction involving a resonance energy transfer between hydrogen atoms and Rb⁺ or 2K⁺ to form a very stable novel hydride ion. Its predicted binding energy of 3.0471 eV with the fine structure was observed at 4071 Å, and its predicted bound-free hyperfine structure lines matched those observed for about 40 lines to within.01 percent. Characteristic emission from each catalyst was observed. This catalytic reaction may pump a CW HI laser.