Dipole ordering and ionic conductivity in NASICON-Type Na<Subscript>3</Subscript>Cr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> structures

The aspects of structure, dipole ordering, and ionic conductivity of the Na₃Cr₂(PO₄)₃ crystal with the four polymorphic phases (α, α', β, and γ) have been investigated. The features of the α-Na₃Cr₂(PO₄)₃ crystal structure and its dipole ordering and relaxation polarization in the low-temperature α and α' phases have been refined. The occurrence of Na₃Cr₂(PO₄)₃ dipole ordering in the α and α' phases and high ionic conductivity in the β and γ phases is attributed to the structural changes in the rhombohedral [Me₂(PO₄)₃]^–3_3∞ crystal frame upon phase transformations α → α', α' → β, and β → γ. A model for explaining the dipole ordering and ionic conductivity phenomena in Na₃Cr₂(PO₄)₃ is proposed.     

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.     

Ionic conductivity of isostructural crystals of the superionic conductors Li3Fe2(PO4)4 and Li3Sc2(PO4)3

The results of measurements of the ionic conductivity σ in Li₃M₂(PO₄)₃ (M=Fe, Sc) single crystals along various crystallographic directions are analyzed. Possible causes of the different behavior of σ in the isostructural crystals are discussed: a jump of the conductivity in the transition to the superionic phase in Li₃Sc₂(PO₄)₃ and its absence in Li₃Fe₂(PO₄)₃; the existence of a conductivity maximum in different crystallographic directions (along the c axis in Li₃Sc₂(PO₄)₃ and along the a axis in Li₃Fe₂(PO₄)₃). Fiz. Tverd. Tela (St. Petersburg) 39, 83–86 (January 1997)     

Characteristics of the Li<Superscript>+</Superscript>-Ion Conductivity of Li<Subscript>3</Subscript>R<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> Crystals (R = Fe,Sc) in the Superionic State

The characteristics of Li+-ion conductivity σdc of structural γ modifications of Li₃R₂(PO₄)₃ compounds (R = Fe, Sc) existing in the superionic state have been investigated by impedance spectroscopy. The type of structural framework [R₂P₃O₁₂]_∞^3- affects the σdc value and the σdc activation enthalpy in these compounds. The ion transport activation enthalpy in γ-Li₃R₂(PO₄)₃ (ΔH_σ = 0.31 ± 0.03 eV) is lower than in γ-Li₃Fe₂(PO₄)₃ (ΔH_σ = 0.36 ± 0.03 eV). The conductivity of γ-Li₃Fe₂(PO₄)₃ (σ_dc = 0.02 S/cm at 573 K) is twice as high as that of γ-Li₃R₂(PO₄)₃. A decrease in temperature causes a structural transformation of Li₃R₂(PO₄)₃ from the superionic γ modification (space group Pcan) through the intermediate metastable β modification (space group P2₁/n) into the “dielectric” α modification (space group P2₁/n). Upon cooling, σdc for both phosphates decreases by a factor of about 100 at the superionic TSIC transition. In Li₃Fe₂(PO₄)₃ σdc gradually decreases in the temperature range T_SIC = 430–540 K, whereas in Li₃R₂(PO₄)₃ σdc undergoes a jump at T_SIC = 540 ± 25 K. Possible crystallochemical factors responsible for the difference in the σdc and ΔH_σ values and the thermodynamics and kinetics of the superionic transition for Li₃R₂(PO₄)₃ are discussed.     

Electrical properties of heavily doped fluorite-structured BaF2:RF3 (R=La, Pr, Nd, Gd, Tb, Y, Sc) single crystals

The superionic conductivity and dielectric response of heavily doped fluorite-structured Ba_1−xR_xF_2+x (R=La, Pr, Nd, Gd, Tb, Y, Sc; x=0.005–0.45) crystals are reported. The highest ionic conductivity is found for R=Sc and x=0.1. Upon ScF₃ doping, small Sc³⁺ ions rearrange their surroundings, create excessive fluoride interstitial ions and bring about a high ionic conductivity. For each dopant, the concentration dependence of the ionic conductivity is non-linear. A monotonous concentration dependence of the ionic conductivity is found only for La³⁺ doping. Upon doping with Nd³⁺, Gd³⁺, Tb³⁺, Y³⁺ and Sc³⁺ ions, a conductivity maximum is observed at x=0.1–0.2. Upon Pr³⁺ doping, this maximum is split. The influence of defect clustering on the concentration dependence of the conductivity is discussed. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Defects,phase transitions and dynamical disorder in superionic protonic conductors H3OUO2PO4 · 3H2O and CsHSO4

Specific properties of the hydrogen bond and protons appear to be responsible for the formation of the quasi-liquid state of conducting ions and the resulting superionic behaviour. This state is reached by successive phase transitions involving the mobile species and their interactions with a more or less rigid framework. H₃OUO₂PO₄ · 3H₂O (HUP) and CsHSO₄ can be considered as models of wet and dry superionic conduction, respectively. Interactions between static effects and dynamical disorder as well as the coupling between sublattices are discussed in relation to results obtained by calorimetry, impedance spectroscopy (up to 10 GHz), vibrational spectroscopy and quasi-elastic neutron scattering.     

Glass transition (Tg) and dielectric loss in Na3PO4–Pb3(PO4)2–BiPO4 phosphate glasses

Thermal and dielectric loss properties of Na₃PO₄-Pb₃(PO₄BiPO₄ (Na₂O-PbO-Bi₂O₃-P₂O₅) phosphate glasses, have been studied by the differential scanning calorimetry (DSC) and electrical factor loss (tgδ) measurements. Experiments have been carried out from ambient temperature to 500°C and show a strong influence of sodium ions on T_g and tgδ.     

Dielectric properties of superionic lead fluoride

Recent experimental studies show that the dielectric constant of lead fluoride increases rapidly as the superionic transition temperatureT _c is approached. In this work the static dielectric constant (ε) is calculated theoretically for low and high temperature limits. Assuming conduction to take place through a percolation mechanism,ε is found to diverge at the threshold temperatureT _th where ionic conduction starts.     

Optical and electrical conduction mechanisms of [N(CH3)3H]CdCl3

The X-ray powder diffraction patterns shows that at room temperature [N(CH₃)₃H]CdCl₃ crystallizes in the orthorhombic system with the Pbnm space group. The analysis of the data revealed the existence of optical allowed direct transition mechanisms with the band gap energy equal to 5.3 eV. The temperature dependences of the real part of dielectric permittivity show a relaxation process at high temperature that can be explained by the reorientational motion of alkyl chains. The alternative current (AC) electrical conduction in compound is governed by three processes, which can be attributed to several models: the correlated barrier hopping (CBH) model in phases I and II, the non-overlapping small polaron tunneling (NSPT) model in phases III and IV.     

Frequency dependence of ionic conductivity and dielectric relaxation studies in Na3H(SO4)2 single crystal

Electrical impedance measurements of Na₃H(SO₄)₂ were performed as a function of both temperature and frequency. The electrical conductivity and dielectric relaxation have been evaluated. The temperature dependence of electrical conductivity reveals that the sample crystals transformed to the fast ionic state in the high temperature phase. The dynamical disordering of hydrogen and sodium atoms and the orientation of SO₄ tetrahedra results in fast ionic conductivity. In addition to the proton conduction, the possibility of a Na⁺ contribution to the conductivity in the high temperature phase is proposed. The frequency dependence of AC conductivity is proportional to ω^s. The value of the exponent, s, lies between 0.85 and 0.46 in the room temperature phase, whereas it remains almost constant, 0.6, in the high-temperature phase. The dielectric dispersion is examined using the modulus formalism. An Arrhenius-type behavior is observed when the crystal undergoes the structural phase transition.     

Sodium superionic conductor sputter-deposited coatings

Thin sodium superionic conductor (Nasicon) coatings are deposited on rotating substrates by co-sputtering in the reactive mode of a Zr-Si and a Na₃PO₄ target. The influence of the discharge current and of the target-to-substrate distance is investigated owing to the targeted Na₃Zr₂Si₂PO₁₂ stoichiometry. A thermo-structural analysis shows that the amorphous as-deposited coating of convenient composition crystallises around 700 °C in the monoclinic structure. Electrical measurements performed at room temperature after various annealing treatments indicate a ionic conductivity of about 2·10⁻³ S·cm⁻¹, consistent with that of bulk Nasicon. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Phase transitions in the Na3PO4-BiPO4 system

Abstract

The Na₃PO₄-BiPO₄ system has been investigated by X-ray diffraction and D.T.A. It features two definite compounds: Na₃Bi(PO₄)₂ and Na₃Bi₅(PO₄)₆, the first one being polymorphic. It crystallizes in two orthorhombic and two hexagonal forms below melting at 1025°C. The H.T. phase has the glaserite structure, and the other ones are related. Na₃Bi₅(PO₄)₆ is stable only at 680 ? t ? 820°C, but can easily be quenched. It has a non-centrosymmetric cubic structure (S.H.G.) of the eulytite type, and so potential piezoelectric applications can be expected. Na₃PO₄ and Na₃Bi(PO₄)₂ display two extensive ranges of solid solutions with the replacement mechanism 3Na⁺ → Bi³⁺ in the formula Na_3-3x Bi _x PO₄, respectively with 0 ? x ? 0.29 and 0.5 ? x ? 0.62 over 950°C.     

Spray-drying and fabrication of superionic conducting sodium rare-earth silicates

Fine precursor powders of particle diameter of 0.50–7 μm and high surface area (9 m²/g) for superionic conducting Na₅RESi₄O₁₂ (RE=Gd, Y) were successfully produced by spray-drying and an optimised calcination procedure. Using these powders, dense sinters (>98% of theoretical) with ?(NGS) or $\text{?(}\text{NGS}\text{)=1.0}$ (the proposed parameter for purity) were obtained under normal sintering conditions. 300°C ionic resistivities were calculated from ac impedance and had values of 5.6 Ω cm with an activation energy for conduction of 4.5 kcal/mol for Na₅GdSi₄O₁₂ and 7.7 Ω cm and 5.9 kcal/mol for Na₅YSi₄O₁₂. These values were dependent on the ?(NGS) or ?(NYS).     

Study on Ion-dynamics in solids by hole burning spectroscopy

The importance of hole burning spectroscopy for studying elementary migration processes of ions in superionic conductors is being discussed using two examples. In β″-alumina, the potential energy for the conduction of ions along the conduction path, as obtained from the analysis of the results of hole burning spectroscopy, showed that there are two different bare-potentials along the ionic conduction path. One of them is higher than the activation energy for ionic conductivity. This result gives direct evidence that an ionic interaction among the conducting ions plays an important role for ion migration in β″-alumina. In YSZ with various concentration of Y₂O₃, we have observed persistent holes in Pr³⁺ ions doped samples. Their disordered nature will be discussed using the bandwidth considerations. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Electrical properties and conduction mechanism in the sodium nickel diphosphate

The Na_3.6Ni_2.2(P₂O₇)₂ compound was obtained by the conventional solid-state reaction. The sample was characterized by X-ray powder diffraction and vibrational and impedance spectroscopy. The AC electrical conductivity and the dielectric relaxation properties of this compound have been investigated by means of impedance spectroscopy measurements over a wide range of frequencies and temperatures, 209 kHz–1 MHz and 564–729 K, respectively. Dielectric data were analyzed using complex electrical modulus M* at various temperatures. The peak positions ω _m of M″ spectra shift toward higher frequencies with increase in temperature. The AC conductivity data fulfill the power law. Application of the correlated barrier hopping model revealed that the ionic conduction takes place by single-polaron and bipolaron hopping processes.     

离子导体在直流偏压作用下的介电特性

在我们以前的工作中,发现α-LilO₃在c向直流偏压作用下表观介电常数增加。并把这一现象归结为由离子导电所产生。本文进一步研究了离子导体KLiSO₄和NaCl单晶在直流偏压下的介电特性,也观察到了表观介电常数改变的现象,证实了上述结论的正确性。我们也研究了离子导体的这一特性与温度、频率和样品四周气氛的关系,讨论了它发生的条件。 关键词：     

The cation ordering in iron-containing zinc and magnesium orthophosphates determined from Mössbauer spectroscopy

The cation distribution between five- and six-coordinated sites in the isotypic solid solutions γ (Zn, Fe)₃(PO₄)₂ and (Mg, Fe)₃(PO₄)₂ has been determined by means of Mössbauer spectroscopy. Iron is found to be strongly ordered into the six-coordinated sites of γ-(Zn, Fe)₃(PO₄)₂ thus stabilizing this phase with respect to α-Zn₃(PO₄)₂, while (Mg, Fe)₃(PO₄)₂; shows a roughly random cation distribution. Distortion of the coordination polyhedra around iron is suggested to limit the solid solution range of iron in the γ-Zn₃(PO₄)₂ structure type. Isomer shifts for Fe²⁺ in the five-coordinated site are observed to be $\text{1.12±0.01}\text{mm}\text{s}$ at room temperature and $\text{1.24±0.02}\text{mm}\text{s}$ at 77 K.     

A Mössbauer effect study of the electronic and structural properties of Fe9(PO4)O8

Fe₉(PO₄)O₈ is a mixed valence compound with both layers of (FeO)₆, which are similar to those in stoichiometric wüstite, FeO, and layers of Fe₃PO₆, which are similar to those found in anhydrous iron(III) phosphate, FePO₄. A detailed Mössbauer effect study between 232 and 850 K of the electronic and structural properties of Fe₉(PO₄)O₈ has been undertaken for comparison purposes and to study any valence averaging electron delocalization or exchange that may be present. An earlier single crystal X-ray study has revealed that Fe₉(PO₄)O₈ crystallizes with five distinct iron sites in the ratio of 1∶1∶2∶4∶1. The differently distorted octahedral Fe(1), Fe(3), and Fe(4) sites contain divalent iron, the tetrahedral Fe(5) site contains divalent iron, and the octahedral Fe(2) site contains trivalent iron. Because of the variety of iron sites, the paramagnetic Mössbauer spectra of Fe₉(PO₄)O₈ are complex and exhibit many partially resolved lines. The logarithm of the Mössbauer spectral absorption area and the median isomer shift vary linearly with temperature and yield an effective Mössbauer temperature of 300 K for Fe₉(PO₄)O₈. The temperature dependence of the median isomer shift indicates electron delocalization into an unspecified conduction band above 630 K. The differing site degeneracies, site symmetries, and site valencies make it possible to fit the Mössbauer spectra of Fe₉(PO₄)O₈ with two different models, both of which yield a realistic temperature dependence of the hyperfine parameters, but which lead to different conclusions about the presence of valence averaging electron exchange. Hence, the Mössbauer spectra can not, unequivocally, demonstrate the presence of valence averaging in Fe₉(PO₄)O₈. However, the spectra do indicate the presence of structural changes, both above and below 295 K, which are consistent with a monoclinic space group as suggested by the presence of the weak superlattice reflections reported earlier. The relative component spectral areas indicate, in agreement with the relative Wigner-Seitz cell volumes, that the iron(III) on the Fe(2) site has a relatively low recoil-free fraction, whereas the six-coordinate iron(II) on the Fe(1) site has a relatively high recoil-free fraction.     

Studies of structure, thermal, and electrical properties for Cs 5 H 3 (SO 4 ) 4 crystal

Electrical conductivity, differential scanning calorimetry, and X-ray diffraction measurements were performed on a pentacesium trihydrogen tetrasulfate, Cs₅H₃(SO₄)₄, crystal. The transition entropy at a superionic phase transition and the activation energy of proton migrations in the superionic phase were determined to be 58.2 J K⁻¹ mol⁻¹ and 0.48 eV, respectively. The crystal structure of Cs₅H₃(SO₄)₄ at room temperature was refined. The electrical conduction in Cs₅H₃(SO₄)₄ was discussed with the refined structure.     

Towards a thin films electrochromic device using NASICON electrolyte

The optimisation of the morphology of WO₃ thin films allowed a more efficient electrochromic colouring using Na⁺ ions than H⁺ ones. Therefore, sodium superionic conductor (Na₃Zr₂Si₂PO₁₂, NASICON) films may be used as electrolyte in inorganic electrochromic devices. In this paper, the structure, chemical composition, morphology and electrochromic properties of WO₃, ZnO:Al and Na₃Zr₂Si₂PO₁₂ thin films were studied to develop a novel type of electrochromic device. WO₃, ZnO:Al and Na₃Zr₂Si₂PO₁₂ thin films were deposited using reactive magnetron sputtering of tungsten, zinc and aluminium and Zr–Si and Na₃PO₄ targets, respectively. For transparent conductive oxide coatings, a correlation was established between the deposition parametres and the film’s structure, transmittance and electrical resistivity. Classical sputtering methods were not suitable for the deposition of NASICON films on large surface with homogenous composition. On the other hand, the use of high-frequency pulsed direct current generators allowed the deposition of amorphous films that crystallised after thermal annealing upon 700 °C in the Na₃Zr₂Si₂PO₁₂ structure. Amorphous films exhibited ionic conductivity close to 2 × 10⁻³ S cm⁻¹. Finally, preliminary results related to the electrochromic performance of NASICON, WO₃ and indium tin oxide devices were given. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, France, Sept. 9–15, 2007.