The structure of the glass phase of Rb1−x(ND4)xD2PO4

X-ray scattering techniques have been used to study the diffuse scattering from a single crystal of Rb_1–x(ND₄)_xD₂PO₄ withx=0.65. This system has a structural glass phase at low temperatures resulting from the competing ferroelectric interactions of RbD₂PO₄ and antiferroelectric interactions of (ND₄)D₂PO₄. The diffuse scattering shows a broad peak with a maximum occurring at a wavevector of about 0.3a ^*, and the intensity of these peaks is surprisingly different for wavevectorsq on opposite sides of the Bragg reflections. A model of the D bonding is developed which suggests that the diffuse scattering arises from the interaction between ferroelectric displacements alongc, ferroelectric displacements alongb, and transverse acoustic modes polarized alongb andc. The model accounts for the incommensurate wavevector and, qualitatively, for the intensity of the diffuse scattering around different Bragg reflections. The temperature dependence of the scattering is also measured.     

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)     

Lithium ion conductive glass–ceramics with Li3Fe2(PO4)3 and YAG laser-induced local crystallization in lithium iron phosphate glasses

The glasses with the composition of 37.5Li₂O–(25 − x)Fe₂O₃–xNb₂O₅–37.5P₂O₅ (mol%) (x = 5,10,15) are prepared, and it is found that the addition of Nb₂O₅ is effective for the glass formation in the lithium iron phosphate system. The glass–ceramics consisting of Nasicon-type Li₃Fe₂(PO₄)₃ crystals with an orthorhombic structure are developed through conventional crystallization in an electric furnace, showing electrical conductivities of 3 × 10^− 6 Scm^− 1 at room temperature and the activation energies of 0.48 eV (x = 5) and 0.51 eV (x = 10) for Li⁺ ion conduction in the temperature range of 30–200 °C. A continuous wave Nd:YAG laser (wavelength: 1064 nm) with powers of 0.14–0.30 W and a scanning speed of 10 μm/s is irradiated onto the surface of the glasses, and the formation of Li₃Fe₂(PO₄)₃ crystals is confirmed from XRD analyses and micro-Raman scattering spectra. The crystallization of the precursor glasses is considered as new route for the fabrication of Li₃Fe₂(PO₄)₃ crystals being candidates for use as electrolyte materials in lithium ion secondary batteries.     

Structure and proton conductivity of mechanochemically treated 50CsHSO4·50CsH2PO4

Mixtures of CsHSO₄ and CsH₂PO₄ were mechanochemically treated using a planetary type of ball mill. The changes in structure and proton conductivity of the solid acid compounds with the treatment have been investigated. Cs₃(HSO₄)₂(H₂PO₄) and Cs₅(HSO₄)₃(H₂PO₄)₂ were formed during milling. The mechanochemically treated composite consisting of Cs₃(HSO₄)₂(H₂PO₄) and Cs₅(HSO₄)₃(H₂PO₄)₂ showed higher conductivity than the untreated mixture. In addition, a high temperature phase of Cs₂(HSO₄)(H₂PO₄) was generated from the composite at around 100 °C on heating. Conductivity of the mechanochemically treated composite significantly increased at temperatures around 90 °C on heating. The value becomes 2 × 10^− 3 S cm^− 1 at around 180 °C. On the other hand, no steep decrease is observed on cooling. The activation energies of the mechanically milled sample with high conductivities were estimated to be about 0.3 eV for both heating and cooling processes. The relatively high proton conductivity and a low activation energy for the proton conduction should be ascribed to the presence of the high temperature phase of Cs₂(HSO₄)(H₂PO₄).     

A novel red phosphor BaZn2(PO4)2:Sm3+, R+ (R=Li,Na, K)

BaZn₂(PO₄)₂:Sm³⁺ phosphor was synthesized by a high temperature solid-state reaction in atmosphere. BaZn₂(PO₄)₂:Sm³⁺ phosphor can be efficiently excited by ultraviolet and blue light, and the emission spectrum consists of three emission peaks at 568, 606 and 660 nm. By increasing the Sm³⁺ doped content, the emission intensity of the phosphor can reach the maximum at 0.02 mol Sm³⁺, then the concentration quenching occurs. By introducing the compensator charge R⁺ (R=Li, Na, K) into the BaZn₂(PO₄)₂:Sm³⁺ phosphor, its emission intensity can be enhanced. The Commission International de l'Eclairage (CIE) chromaticity coordinates of Ba_0.96Zn₂(PO₄)₂:0.02Sm³⁺, 0.02K⁺ phosphor were (x=0.623, y=0.361). The results indicate that BaZn₂(PO₄)₂:Sm³⁺, R⁺ (R=Li, Na, K) may be a promising red phosphor for white light-emitting diodes.     

The Mössbauer spectrum of synthetic hureaulite: Fe5 2+(H2O)4(PO4H)2(PO4)2

The crystal structure of synthetic ferrous hureaulite, Fe₅ ²⁺ (H₂O)₄(PO₄H)₂(PO₄)₂, was refined from single-crystal X-ray data. It is monoclinic, space group C2/c, with a=17.487(4), b=9.017(2), c=9.338(2) Å, β=96.27(3)°, V=1463.6(6) Å³, Z=4 and D _calc=3.327 g/cm³. This end member of the hureaulite series was crystallized under distinctly acidic conditions, by a method that gives perfect crystals, large enough for X-ray single crystal studies. The main feature of the hureaulite structure is that it has an equal number of normal (PO₄)³⁺ and acid (PO₄H)²⁺ tetradentate groups. These are centered on Fe²⁺ atoms and share corners with edge-linked octahedra, forming pentamer units. The five Fe²⁺ atoms are distributed on three distinct sites in these units. This can be directly observed in the Mössbauer spectrum at 295 K, which contains three doublets whose relative intensities correspond to the 1:2:2 distributions of crystallographic sites.     

Preparation of proton conductive composites with CsHSO4/CsH2PO4 and phosphosilicate gel

New composite materials were prepared using cesium hydrogen sulfate (CsHSO₄) or cesium dihydrogen phosphate (CsH₂PO₄) and phosphosilicate gel (P₂O₅–SiO₂ gel). In X-ray diffraction patterns of these composites, diffraction peaks due to Cs₂H₅(SO₄)₂(PO₄) and CsH₅(PO₄)₂ were observed for CsHSO₄–(P₂O₅–SiO₂ gel) composites and CsH₂PO₄–(P₂O₅–SiO₂ gel) composites, respectively. These composites showed high conductivities in the order of 10^− 3 S cm^− 1 at 150 °C due to melting of Cs₂H₅(SO₄)₂(PO₄) or CsH₅(PO₄)₂ in the composites. In the cooling process, the CsHSO₄–(P₂O₅–SiO₂ gel) composites kept relatively high conductivity to 110 °C where solidification of Cs₂H₅(SO₄)₂(PO₄) occurs, whereas CsH₂PO₄–(P₂O₅–SiO₂ gel) composites showed relatively high conductivity continuously to ambient temperature.     

Vibrational study of the superprotonic phase transition in the mixed crystal Rb2(HSeO4)(H2PO4)

Raman spectra (10–1200 cm⁻¹) of polycrystalline samples of Rb₂(HSeO₄)(H₂PO₄) were studied at temperatures ranging from 300 to 423 K. An assignment of most of the observed bands is proposed. The first‐order phase transition previously detected at 382 K was characterized as: This superionic‐protonic transition is believed to be governed by librations of the HSe/PO₄²⁻ ion and the A OH (A = Se, P) stretching mode. It corresponds to the weakening of  Se(P) O H˙˙˙ H O Se(P) hydrogen bonds and to the melting of the proton sublattice into a quasi‐liquid state in which the protons and the HSe/PO₄²⁻ ions contribute to the unusually high conductivity. The activation energy that was determined from the plot Δν_1/2 versus temperature for the ν (A OH) band has the same order of magnitude as that determined from conductivity measurements. Copyright © 2006 John Wiley & Sons, Ltd.     

Superprotonic solid solutions between CsHSO4 and CsH2PO4

Solid solutions of (CsHSO₄)_1 − x(CsH₂PO₄)_x (x = 0.25-0.75) were synthesized by mechanical milling method over a wide range of compositions. Superprotonic cubic phase was confirmed for all these samples between 293 and 420 K depending on its composition. These superprotonic phases have primitive cubic structure similar to that of CsH₂PO₄. The kinetic stability of the supercooled cubic phase depends both on the composition x and the humidity of surrounding atmosphere. The most stable composition of the cubic phase was found around x = 0.67 and could be maintained for several days even under ambient atmosphere. The ionic conductivities of these superprotonic phases reached 10^− 2–10^− 3 S∙cm^− 1 at 450 K. With increasing x the ionic conductivity at the superprotonic phase decreased continuously associated with the increase of the activation energy. These findings suggest that the average number of the hydrogen bonds between XO₄ (X = S, P) units plays an important role on the stability of the cubic phase and also on the conductivity.     

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.     

On the investigation of magnetic critical phenomena in ferromagnets by μSR and PAC

On the basis of current theoretical views on the critical phenomena in isotropic Heisenberg ferromagnets the power temperature behavior Λ=c(τ)λ₀τ^-w has been derived for the muon spin relaxation rate Λ as π-T _c ⁻¹ (T-T _c ) → 0⁺. It is shown that the crossover from an exchange critical regime to a dipolar one is accompanied not only with the change in the critical exponentw in the above law, but also with the reduction of the coefficientc(π). A comparison with the temperature behaviour of the inverse nuclear relaxation timet _R ⁻¹ measured in the PAC experiment is carried out.     

Vibrational spectroscopic study of lithium intercalation into LiTi2(PO4)3

Ex situ vibrational spectra are recorded during the first discharge of LiTi₂(PO₄)₃. Spectral changes are consistent with a two-phase model for the electrochemical insertion of Li⁺ ions. Differences in the frequencies and relative intensities of the LiTi₂(PO₄)₃ and Li₃Ti₂(PO₄)₃ bands are due to changes in the effective force constants, dipole moment derivatives, and polarizability derivatives as Li⁺ is inserted into LiTi₂(PO₄)₃. The intramolecular PO₄³⁻ bending modes (ν₂ and ν₄) are found to be more sensitive to Li⁺ insertion than the intramolecular PO₄³⁻ stretching modes (ν₁ and ν₃). This is because ν₂ and ν₄ are less localized than ν₁ or ν₃ and are more susceptible to small structural changes in the unit cell. Furthermore, a band at 487 cm^− 1 appears in the infrared spectrum of Li₃Ti₂(PO₄)₃. This band is assigned as a Li⁺ ion cage mode and is due to Li⁺ ions that occupy the M(3) and M′(3) sites in the Li₃Ti₂(PO₄)₃ structure. A small degree of band broadening is also detected in the vibrational spectra when Li⁺ ions are inserted, which might indicate some disordering in the cathode material.     

Synthesis, structure, and dielectric properties of KH2PO4 fractal aggregates

Macroscopic fractal aggregates of KH₂PH₄ (KDP) measuring up to 500 μm have been obtained. The fractal structure forms as a result of the precipitation of KDP particles from a supersaturated aqueous solution in the presence of a temperature gradient followed by a diffusioncontrolled mechanism of aggregation. The electron-microscopic analysis performed has shown that the fractals are formed predominantly from crystallites of the tetragonal modification measuring ∼1 μm. The dielectric constant (ɛ) of fractal KH₂PO₄ has been measured in the temperature range 80–300 K. A characteristic anomaly has been discovered on the ɛ(T) curve in the vicinity of 122 K, which attests to a ferroelectric phase transition. The absolute value of ɛ is significantly smaller than the components ɛ ₁₁ and ɛ ₃₃ for KH₂PO₄. Fiz. Tverd. Tela (St. Petersburg) 41, 2059–2061 (November 1999)     

Hydrothermal synthesis, controlled microstructure, and photoluminescence of hydrated Zn3(PO4)2: Eu3+ nanorods and nanoparticles

In this article, Zn₃(PO₄)₂: Eu³⁺ nanorods and nanoparticles have been prepared by the hydrothermal method. The optimum pH value has been found at the range of 3–8 for the preparation of orthorhombic Zn₃(PO₄)₂: Eu³⁺, whose morphologies are affected by the pH value. At the same temperature for hydrothermal reaction, the product presents nanorods at pH 4, while it shows nanoparticles at pH 6. Furthermore, the influences of the hydrothermal reaction temperature on the morphology and microstructure have also been investigated, suggesting that the morphology and microstructure cannot be changed with the hydrothermal temperature at the same pH value. Finally, the photoluminescence of Eu³⁺ on Zn₃(PO₄) nanorod/nanoparticle have been studied, both of which present the characteristic emission lines of Eu³⁺ and the ⁵D₀–⁷F₁ transition corresponds the strongest emission. This indicates that Eu³⁺ occupied the inversion center in Zn₃(PO₄) host.     

Influence of the Structure of the MEu(PO3)4 Crystals on Their Luminescent Properties

The influence of the M⁺ cations of alkali metals on the luminescent properties of the Meu(PO₃)₄ crystals, where M represents Li, Na, K, Rb, and Cs, is considered. The nonlinear dependences of the spectral position of the barycenters in the J components of the ⁷ F term of Eu³⁺ on the ionic radius R(M⁺), which are individual in the series of crystals of chain and cyclic structure, have been revealed. It is shown that the liability of the ligands of the europium complex in MeEu(PO₃)₄ to the polarizing action of the cations of alkali metals is manifested as an intermediate dependence of the luminescence lifetime of Eu³⁺ on R(M⁺).     

Order-disorder transition in Na3Bi(PO4)2

Na₃Bi(PO₄)₂ exhibits several phase transitions at about 575, 820 and 905°C. The structure was determined at ambient temperature (α-form) and above the first transition (β-form). The α-form cell is monoclinic with a = 19.86(1), b = 5.353(6), c = 13.96(3) Å, β = 110.64(7)°, Z = 8, space group P2₁/ _c ; the structure was solved from 3769 independent reflections to an R value, calculated on intensities, of 0.069. The β-form cell is orthorhombic with a = 18.71(3), b = 7.18(2), c = 5.429(7) Å, Z = 4, space group Pnam; the structure was solved to an R value, calculated on structure factors, of 0.055 using intensities of 858 unique reflections measured on a single crystal at 650°C. Both structures are related to that of glaserite. At high temperature, one of the PO₄ tetrahedra is statistically disordered over two positions related by the m-mirror. Below the transition, ordering of this ion leads to a unit cell of lower symmetry. At the transition, two individuals grow on the two sides of the m-mirror which disappears; thus, at ambient temperature, the crystals are systematically twinned. Above the second transition, the unit cell is hexagonal.     

Elastic properties of monoclinic telluric acid ammonium phosphate,Te(OH)6·2NH4H2PO4·(NH4)2HPO4, near the paraelectric-ferroelectric transition

The elastic constants of Te(OH)₆·2NH₄H₂PO₄·(NH₄)₂HPO₄, TAAP, point symmetrym, have been measured by ultrasonic resonance methods passing through the paraelectric-ferroelectric transition at ca. 320 K. In the range between 273 and 340 K the elasticity tensor exhibits only a slight anisotropy. No discontinuity of the elastic constants is observed. However, some temperature derivatives of the elastic constants show slight anomalies within the range 310 to 325 K. The strongest effect occurs with the longitudinal elastic resistancec ₂₂. The thermal expansion which varies only slightly between 263 and 340 K, is highly anisotropic in contrast to the thermoelastic behaviour. A strong negative thermal expansion is observed in a direction within the mirror plane, ca. 45° apart from the direction of spontaneous polarization.This effect is not accompanied by a corresponding thermoelastic anomaly. The interactions connected with the transition are of the totally symmetric type. Like many other properties the elastic and thermoelastic behaviour of TAAP resembles that of triglycine sulphate (TGS). Larger differences between TAAP and TGS are found in the pressure dependence of various properties. For example the pressure dependence of the transition temperatureT is negative for TAAP (–3.8 K/kbar) and positive for TGS (3.9 K/kbar).Dedicated to Prof. Dr. H.E. Müser on the occasion of his 60th birthday     

Paramagnetic resonance of the (Cr5+O4)H2 Halperin-Varma center in NH4H2AsO4 and KH2AsO4

The electron paramagnetic resonance (EPR) spectrum of Cr⁵⁺(3d ¹,S=1/2) in undeuterated and 58%-deuterated NH₄H₂AsO₄ was investigated. The EPRg-value tensors in the paraelectric and antiferroelectric phases are almost the same as those observed at high and low temperatures in KH₂PO₄ and KH₂AsO₄. This shows that the (Cr⁵⁺O₄)H₂ complex is the same in all crystals, i.e., a wave function ofd _x ²–y ² character coupled to two lateral protons which reorient among the four possible Slater configurations in the paraelectric phase. The remarkable isotope shift of the local dynamic reorientational slowing-down observed in KH₂PO₄ and KH₂AsO₄ in approximate proportion to the shift of the bulk ferroelectric transition temperaturesT _c ^F , and the antiferroelectricT _c ^AF of NH₄H₂AsO₄, is analyzed qualitatively. It is shown to result mainly from the isotope effect of the short-range interactionJ _sr via protons and deuterons for the impurity and for the bulk. Q-band data of the (Cr⁵⁺O₄)H₂ center in 75%-deuterated KH₂AsO₄ are also reported. An averaged high- and nonaveraged low-temperature EPR spectrum is observed in a temperature range of 250 to 290 K. The intensity ratio of the two follows an exp 2(T—T)/ law over four orders of magnitude atT=266 to 273 K and=5.3 to 6.1 K depending on the orientation of the external magnetic field. This novel result in motional narrowing is analyzed analytically to be compatible with a distributionP _A of ₀(T, T, ) of half width, in reorientation times withE=0.23 ±0.02 eV, , probably resulting from the statistical occupation of deuterium atoms among the O—–H–O bridges. Comparison with a theory of Binder definitely proves the extrinsic slowing-down and thus Halperin-Varma type character. In the range of temperatures investigated no local freeze-out has been detected.