Oxide ion conduction mechanism in RE9.33(SiO4)6O2 and Sr2RE8(SiO4)6O2 (RE = La,Nd) from neutron powder diffraction

The oxide ion conduction mechanism was clarified for high purity RE_9.33(SiO₄)₆O₂ and Sr₂RE₈(SiO₄)₆O₂ (RE = La and Nd) by Rietveld and maximum entropy method (MEM) analysis using neutron powder diffraction data collected at room temperature. All the compounds had an apatite-type structure in the space group P6₃/m. Neither site splitting nor interstitial sites of the oxide ion was observed. RE_9.33(SiO₄)₆O₂ had cation vacancies only at the 4f site. In Sr₂RE₈(SiO₄)₆O₂, the 4f sites were fully occupied by strontium and rare earths with a molar ratio of 1 : 1. Also, in RE_9.33(SiO₄)₆O₂, the oxide ion at the hexagonal channel site had a large displacement along the c-axis. This large displacement is induced by cooperative rotation of SiO₄ tetrahedra around rare earths at the 4f site through oxide ion polyhedra around another rare earth at the 6h site. The displacement, enhanced by a vacancy in the 4f site, is directly related to oxide ion conduction in RE_9.33(SiO₄)₆O₂.     

Structural transformations in LiGd9(SiO4)6O2 and Ca2Gd8(SiO4)6O2 crystals containing isolated [SiO4] complexes: Raman spectroscopic study

The vibrational spectra of single crystals of the LiGd₉(SiO₄)₆O₂ and Ca₂Gd₈(SiO₄)₆O₂ oxyapatites have been studied using Raman spectroscopy at room and high temperatures. The spectra of internal and external vibrations in these structures have been resolved. The structural transformation of the LiGd₉(SiO₄)₆O₂ and Ca₂Gd₈(SiO₄)₆O₂ oxyapatites in the processes of melting and crystallization, as well as during rapid quenching of the melt, has been investigated. It has been found that the melting of the LiGd₉(SiO₄)₆O₂ compound has an incongruent character and that new metastable disordered phases are formed during rapid quenching of the Ca₂Gd₈(SiO₄)₆O₂ melt.     

Radiation-induced swelling and amorphization in Ca2Nd8(SiO4)6O2

Specimens of Ca₂Nd₈(SiO₄)₆O₂ were doped with 1.2 wt% ²⁴⁴Cm and the effects of self-radiation damage from alpha decay were determined as a function of cumulative dose. The macroscopic volume of the specimens increased exponentially with dose to a limiting (saturation) value of ～8.0%. The initially crystalline material became completely X-ray amorphous at a dose of 11.7 × 10²⁴ alpha decays/m³. The dissolution rate of the amorphous state was about an order of magnitude higher than the crystalline state. The stored energy of the amorphous state was ～130 J/g. Differential thermal analysis along with isochronal and isothermal-step annealing were used to study the kinetics associated with the thermal recovery of the radiation-induced swelling and amorphization. A single recovery stage associated with recrystallization of the amorphous material was observed and the activation energy was determined to be 3.1±0.2 eV.     

Raman and infrared spectra of doped La8+xSr2−y(SiO4)6O2+δ compounds compared to the ab initio‐obtained spectroscopic characteristics of fully stoichiometric La8Sr2(SiO4)6O2

Vibrational spectroscopy data were used to gain insight into the possible locations of extra oxygen ions introduced into La_8+xSr_2−y(SiO₄)₆O_2+δ compounds to raise their ionic conductivity. Perturbations observed in the Raman and infrared spectra of these compounds with increasing δ were explained by using the ab initio calculation results for the fully stoichiometric (x = y = δ = 0) lattice. This allowed the inference that the extra oxygen ions are incorporated into La O tunnel‐like fragments inherent in the studied structures. Copyright © 2010 John Wiley & Sons, Ltd.     

IR and Raman spectra of Ca8La2(VO4)6O2

The IR and Raman spectra of the oxyapatite Ca₈La₂(VO₄)₆O₂ has been measured and discussed in comparison with those of related species. Data for Ca₈La₂(PO₄)₆O₂ and for a solid solution of composition Ca₈La₂(PO₄)₆O₂ are also reported.     

Synthesis and electrical characterisation of the apatite-type oxide ion conductors Nd9.33+xSi6-yGayO26+z

Apatite-type oxides have been attracting interest as a new class of oxide ion conductors. In this paper we examine the effect of Ga doping on the conductivity of the apatite silicate system, Nd_9.33+xSi₆O_26+3x/2 and compare the results to those reported for similar doping studies in La_9.33+xSi₆O_26+3x/2. The highest conductivities are observed for samples containing oxygen excess, which is in agreement with previous reports that interstitial oxide ions are important for high oxide ion conduction in these materials. For oxygen stoichiometric materials, i.e. Nd_9.33+x/3Si_6-xGa_xO₂₆, the Ga doping results in a significant increase in activation energy and a consequent lowering of the low temperature conductivity. This is contrary to results previously reported for the La containing analogues, which showed an enhancement of conductivity on Ga doping up to x=1.5. Possible explanations for the differences between the two systems are discussed. Paper presented at the Patras Conference on Solid State Ionics — Transport Properites, Patras, Greece, Sept. 14 – 18, 2004.     

Dispersion of the Г 6 (1) -Г 6 (2) Nd crystal field excitation in Nd2CuO4

We performed inelastic neutron scattering experiments to determine the dispersion of the Г ₆ ⁽¹⁾ -Г ₆ ⁽²⁾ crystal field excitation in Nd₂CuO₄. Our results can be very well described within the random phase approximation model. This allows a direct determination of the exchange coupling constants between the Nd ions. The superexchange interactions mediated by the oxygen and copper-oxygen layers are determined to be significantly stronger than the in-plane exchange between the Nd ions.     

Synthesis and conductivities of the apatite-type systems, La9.33+xSi6−yMyO26+z (M=Co, Fe, Mn) and La8Mn2Si6O26

Apatite-type oxides of formula (La/Sr)_10−xSi₆O_26+y have been attracting significant interest recently, because of their high oxide ion conductivity. In this paper we report the synthesis and conductivities of phases based on doping La_9.33Si₆O₂₆ with Co, Fe, Mn on the Si site, according to the formula La_9.33+x/3Si_6−xM_xO₂₆ (M=Co, Fe, Mn). Substitution limits observed were x≤1.5 (Co), x≤1.25 (Fe), x≤0.5 (Mn). Higher Mn levels could be achieved by substituting onto the La site, with it being possible to prepare the phase La₈Mn₂Si₆O₂₆. The highest conductivities were observed for the Co doped samples, although investigations into the dependence of conductivity on p(O₂) (0.2–10⁻⁵ atm.) indicated that the conductivity was dominated by the electronic component in these cases. In contrast, the conductivities for the Fe and Mn doped samples were mainly ionic in the same p(O₂) range. Experiments into varying the oxygen content of these doped phases indicated that increasing the oxygen content above the nominally stoichiometric O₂₆ appears to increase the oxide ion conductivity. Preliminary studies of the reactivity of the electrolyte La_9.33Si₆O₂₆ with potential SOFC cathode materials (La_1−xSr_xMO₃; M=Co, Fe, Mn) suggests that reaction can occur at high temperatures leading to the incorporation of the transition metal into the apatite electrolyte. However, the fact that these doped phases exhibit high conductivities suggests that this may limit any problems caused by such a reaction at the electrolyte-electrode interface. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Synthesis and superconductivity of a new 1222-type T1-based layered cuprate (T1, Nb) Sr2(Nd,Ce)2Cu2O z

Synthesis and superconductivity of a new 1222-type layered cuprate (Tl_1–x Nb _x ) Sr₂(Nd_1–y Ce _y )₂Cu₂O _z have been studied. The structure of this cuprate is directly related to that of Nb-1222 NbSr₂(Nd, Ce)₂Cu₂O _z with tetragonal body-center lattice. Partial substitution of Tl for Nb in Nb-1222 phase improves its superconductivity. (Tl_1–x Nb _x ) Sr₂(Nd_0.75Ce_0.25)₂Cu₂O _z samples prepared by the typical procedure exhibit superconductivity withT _c of 30–40 K. Effects of Tl and Nb on superconductivity of this cuprate are briefly discussed.     

Crystal structure and optical spectra of LiLa9(SiO4)6O2 crystals activated with Er3+

LiLa₉(SiO₄)₆O₂ (LLS) crystals activated with Er³⁺ have been grown by the flux growth method. Their apatite structure has been characterized by single-crystal X-ray diffraction. The absorption and emission spectra have been measured in the visible and NIR regions. They are affected by a significant inhomogeneous broadening making these crystals interesting active media for tunable solid-state laser operation.     

Atomic structure of Sr0.75Ba0.25Nb2O6 single crystal and composition-structure-property relationin (Sr,Ba)Nb2O6 solid solutions

The structure of the Sr_0.75Ba_0.25Nb₂O₆ single crystal has been investigated by x-ray diffraction. The occupancies of the Ba and Sr sites in two structural channels are determined. It is found that these sites are split in the large (pentagonal) channel. A qualitative correlation is revealed between the smearing of the phase transition and the displacement of the Sr atom from the m symmetry plane in the pentagonal channel at different [Sr]/[Ba] ratios. The degree of acentricity of the NbO₆ octahedra is analyzed as a function of the [Sr]/[Ba] ratio.     

Impurity effect on the Raman spectra of Sr0.6Ba0.4Nb2O6

Impurity effect on the rare earth ion doped Sr_0.6Ba_0.4Nb₂O₆(SBN40) was studied at room temperature. Doping the rare earth ions of Pr³⁺ or Nd³⁺ changed the Raman profile: spectral broadening, central frequency shifts and relative intensity decreasing. Two reasons are considered according to the ferroelectric and optical properties of the rare earth ion doped SBN40: impurity-induced crystal disorder and the crystal structure change. SBN40 is the general disorder crystal and the disorder which is enhanced by doping the rare earth ion is especially strong along the x?y plane of the material compared with that along the polar C-axis.     

Conductivity and water uptake of Sr4(Sr2Nb2)O11·nH2O and Sr4(Sr2Ta2)O11·nH2O

The hydrated oxygen deficient complex perovskite-related materials Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O were studied at high water vapour pressures over a large temperature range by electrical conductivity measurements, thermogravimetry (TG), and X-ray powder diffraction (XRPD). In humid atmospheres both materials are known to exhibit protonic conductivity below dehydration temperatures, with peak-shaped maxima at about 500 °C. In this work we show that the peaks expand to plateaus of high conductivity from 500 to 700 °C at a water vapour pressure of 1 atm. However, in situ synchrotron XRPD of Sr₄(Sr₂Nb₂)O₁₁·nH₂O as a function of temperature shows that these observations are in fact coincident with melting and dehydration of a secondary phase Sr(OH)₂. The stability of Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O in humid atmospheres is thus insufficient, causing decomposition into perovskites with lower Sr content and SrO/Sr(OH)₂ secondary phases. This, in turn, rationalizes the observation of peaks and plateaus in the conductivity of these materials.     

Growth,spectral and laser properties of Nd3+:Na2La4(WO4)7 crystal

A Nd³⁺:Na₂La₄(WO₄)₇ crystal with dimensions of ? 17 × 30 mm³ was grown by the Czochralski method. The thermal expansion coefficients of Nd³⁺:Na₂La₄(WO₄)₇ crystal are 1.32 × 10^?5 K^?1 along c-axis and 1.23 × 10^?5 K^?1 along a-axis, respectively. The spectroscopic characteristics of Nd³⁺:Na₂La₄(WO₄)₇ crystal were investigated. The Judd–Ofelt theory was applied to calculate the spectral parameters. The absorption cross sections at 805 nm are 2.17 × 10^?20 cm² with a full width at half maximum (FWHM) of 15 nm for π-polarization, and 2.29 × 10^?20 cm² with a FWHM of 14 nm for σ-polarization. The emission cross sections are 3.19 × 10^?20 cm² for σ-polarization and 2.67 × 10^?20 cm² for π-polarization at 1,064 nm. The fluorescence quantum efficiency is 67 %. The quasi-cw laser of Nd³⁺:Na₂La₄(WO₄)₇ crystal was performed. The maximum output power is 80 mW. The slope efficiency is 7.12 %. The results suggest Nd³⁺:Na₂La₄(WO₄)₇ crystal as a promising laser crystal fit for laser diode pumping.     

Synthesis and ionic conduction of apatite-type materials

Apatite is a mineral with general formula M₁₀(XO₄)₆Z₂, where M are metallic elements such as Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ln³⁺ etc.; X=P, V, S, Si, Ge, Re, Cr etc; Z=F⁻, Cl⁻, I⁻, OH⁻, O²⁻, S²⁻ etc. Some materials with apatite structure (S.G. P6₃/m) exhibit quite high cationic (Li⁺, H⁺ etc.) and/or anionic (F⁻, Cl⁻ etc.) conduction. Recently, it was reported that some rare earth silicates, e.g., La₁₀(SiO₄)₆O₃, exhibit quite high oxide-ion conductivity. In this paper, we discuss chemical composition, structure, synthetic procedure and ionic conduction of apatite-type materials. Recent improvements are briefly reviewed. High ionic conductivity has been observed for both cation deficient, oxygen stoichiometric La_9.33(SiO₄)₆O₂ and cation stoichiometric, oxygen excess La₁₀(SiO₄)₆O₃ compositions. Grain boundary conductivity is usually low, which tends to dominate the impedance response. The resistance, particularly the grain boundary resistance is also found to depend on pO₂. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Synthesis and luminescence properties of green phosphors Ca6Sr4(Si2O7)3Cl2:Eu2+ for white light emitting diodes

Divalent europium-activated chlorosilicate Ca₆Sr₄(Si₂O₇)₃Cl₂:Eu²⁺ phosphors were synthesized by a conventional solid-state reaction under reductive atmosphere. These phosphors can be efficiently excited by UV–visible light from 320 to 420 nm, which matches that of a near UV-emitting InGaN chip. Under the 360 nm excitation, Ca₆Sr_3.97(Si₂O₇)₃Cl₂:0.03Eu²⁺ phosphor shows a strong and broad emission centering at 515 nm, which is attributed to the 5d→4f transition of Eu²⁺ ion. The mechanism of concentration quenching was determined to be the dipole–dipole interaction and the critical energy-transfer distance of Eu²⁺ was calculated as 3.31 nm. The CIE chromaticity coordinates of Ca₆Sr_3.96(Si₂O₇)₃Cl₂:0.03Eu²⁺ phosphor are (0.127, 0.770) according to the emission spectrum. It can be expected that Ca₆Sr₄(Si₂O₇)₃Cl₂:Eu²⁺ phosphor is a promising candidate as the green component for near-ultraviolet InGaN-based white LED.     

Spectroscopic investigation of rare-earth chromium borates RCr3(BO3)4 (R = Nd, Sm)

The optical absorption spectra of single crystals of neodymium and samarium chromium borates are studied. A magnetic phase transition in NdCr₃(BO₃)₄ (at T _c = 8 ± 1 K) and SmCr₃(BO₃)₄ (at T _c = 5 ± 1 K) is observed. The effective magnetic field acting on the Nd³⁺ ion from the side of the ordered magnetic subsystem of chromium is estimated.     

CW laser action in acentric NdAl3(BO3)4 and KNdP4O12

We report low-threshold, room-temperature, cw lasing in two new high-Nd-concentration materials, NdAl₃(BO₃)₄ and KNdP₄O₁₂, the first ones with acentric space groups. For both materials, thresholds below 1 mW and slope power conversion efficiencies exceeding 20% have been measured. Fluorescence spectra and lifetimes have been measured in Nd_xGd_1?xAl₃(BO₃)₄ and Nd_xGd_1?xP₄O₁₂.     

Colossal magnetoresistance in layered manganite Nd2−2x Sr1+2x Mn2O7 (0 ≤ x ≤ 0.5)

The layered manganite Nd_2−2x Sr_1+2x Mn₂O₇, with x varying between 0 and 0.5, has been synthesized using solid-state reaction method. We have found that Nd_2−2x Sr_1+2x Mn₂O₇ do not form the single-phase layered compound with A₃B₂O₇ structure. Instead, mixtures of various phases, such as, orthorhombic perovskite, i.e., Nd_1−z Sr _z MnO₃, layered manganite and unreacted starting compounds, have been obtained. Except for x=0.4, which is found to be an antiferromagnetic insulator, all other x values yielded metal-insulator transition and ferromagnetic ordering.     

Theoretical investigation of the optical spectra and local lattice structure for Mn5+ in a Sr10(VO4)6F2 crystal

In this paper, the relationships between the optical spectra and local lattice structure for Mn⁵⁺ in a Sr₁₀(VO₄)₆F₂ crystal are established by the crystal- and ligand-field theory. The effect of spin–orbital coupling between the central 3d² ions and ligand ions has been considered in the full energy matrix. Using the matrix and superposition model formula, we have calculated the optical spectra and local lattice structure parameters of Mn⁵⁺ in Sr₁₀(VO₄)₆F₂ with a C_3v system. The calculated results are in good agreement with the observed values. In addition, the trigonal compressed distortions of the (MnO₄)^3? centers in Sr₁₀(VO₄)₆F₂ crystals are also obtained from the calculations.