Phase relations in the CaAl2O4|CaGa2O4 system

The solid state reactions and the phase relations in the CaAl₂O₄|CaGa₂O₄ system, of which both end-members have the stuffed tridymite structure, were examined by using three kinds of starting materials; A (CaCO₃ + (Al,Ga)₂O₃), B (CaAl₂O₄ + CaGa₂O₄) and C (CaCO₃ + Al₂O₃ + Ga₂O₃). In the starting material B, a very low rate of solid state reaction between CaAl₂O₄ and CaGa₂O₄ was found, which seemed to be due to very slow interchange of Al³⁺ and Ga³⁺ located in tetrahedra of this structure. In order to obtain the probable equilibrium phase relations, it was necessary to use the starting material A. In the present system, a new phase was found in a wide range of composition as a stable phase, which was supposed to have the same structure as so-called metastable phase of CaGa₂O₄ and different array of tetrahedra from either CaAl₂O₄ or CaGa₂O₄|I.     

High-pressure modifications of CaAl2O4 and CaGa2O4

The modifications of CaAl₂O₄ and CaGa₂O₄ with the stuffed tridymite structure were examined under high temperatures (600 ～ 1500 °C) and high pressures (10 ～ 40 kb). Calcium monoaluminate CaAl₂O₄ was found to transform to three kinds of high-pressure modifications. The original CaAl₂O₄ (CA-I) changed to the phase CA-II which had m-CaGa₂O₄ type structure with a different array of tetrahedra in the six-membered rings of tetrahedron. The phase CA-II transformed either to the phase CA-IV with CaFe₂O₄ type structure or to an unknown phase (CA-III) under high pressures. The phase CA-IV was obtained under the pressures above 30 kb and at the temperatures above 1000 °C. Calcium monogallate CaGa₂O₄ transformed to the CaFe₂O₄ type structure above 30kb and 700 °C. No phases such as CA-II and CA-III were found. The structural relations among these modifications were discussed.     

Synthesis of CaAl2O4:Eu,Nd long persistent phosphor by combustion processes and its optical properties

Eu²⁺,Nd³⁺ co-doped calcium aluminate with high brightness and long persistent luminescence was prepared by the combustion method. The luminescent properties of CaAl₂O₄-based luminescent materials have been studied systematically. The phosphor powders were further investigated by X-ray diffractometer (XRD), photoluminescence excitation and emission spectra (PL) and brightness meter. The analytical results indicated that the phase of CaAl₂O₄ was formed when the initiating combustion temperature was 400 °C. The broad band UV excited luminescence of the CaAl₂O₄:Eu²⁺,Nd³⁺ was observed at the blue region (λ_max = 440 nm) due to transitions from the 4f⁶5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. The decay time of the persistence indicated that the persistent luminescence phosphor has bright phosphorescence and maintains a long duration.     

GeO2 dopant induced enhancement of red emission in CaAl12O19:Mn phosphor

In order to search efficient red-emitting phosphors for white LEDs application, CaAl₁₂O₁₉:Mn⁴⁺ phosphors have been prepared by a combustion method assisted with GeO₂ flux. The influence of GeO₂ concentration and annealing temperature on the structure and luminescence intensity for the phosphors has been investigated. The mechanism for luminescence enhancement has been discussed. At GeO₂ doping concentration of 1.5 mol%, the red emission intensity increases by 81% under 330 nm UVA excitation. More isolated luminescence center Mn⁴⁺ ions rather than pairs of Mn⁴⁺-Mn²⁺ ions are formed in the lattice with the introduction of GeO₂ at high temperature oxidation, leading to the enhancement of the red emission. A feasible new way to enhance the red emission in CaAl₁₂O₁₉:Mn⁴⁺ phosphor is obtained.     

Tunable color emitting of CaAl2Si2O8: Eu,Tb phosphors for light emitting diodes based on energy transfer

A series of single-phased CaAl₂Si₂O₈: Eu, Tb phosphors have been synthesized at 1400 °C via a solid state reaction. The emission bands of Eu²⁺ and Eu³⁺ were observed in the air-sintered CaAl₂Si₂O₈: Eu phosphor due to the self-reduction effect. Tb³⁺ ions that typically generated green emission were added in CaAl₂Si₂O₈: Eu phosphor for contributing for a wider-range tunable emission. Energy transfer from Eu²⁺ to Tb³⁺ and the modulation of valence distribution of Eu²⁺/Eu³⁺ that contributes to the tunable color emitting were elucidated. More importantly, a white emission can be obtained by controlling the codoped contents of Li⁺ as well as suppressing the self-reduction degree of Eu. The white light emitting with the color coordinate (0.326, 0.261) was obtained, which indicates that CaAl₂Si₂O₈: Eu, Tb is a promising tunable color phosphor for application in ultraviolet light emitting diodes (UV-LEDs).     

Oxygen ionic conduction in brownmillerite CaAl0.5Fe0.5O2.5+δ

The oxygen permeability of CaAl_0.5Fe_0.5O_2.5+δ brownmillerite membranes at 1123-1273 K was found to be limited by the bulk ionic conduction, with an activation energy of 170 kJ/mol. The ion transference numbers in air are in the range 2×10⁻³ to 5×10⁻³. The analysis of structural parameters showed that the ionic transport in the CaAl_0.5Fe_0.5O_2.5+δ lattice is essentially along the c axis. The largest ion-migration channels are found in the perovskite-type layers formed by iron-oxygen octahedra, though diffusion in tetrahedral layers of the brownmillerite structure is also possible. Heating up to 700-800 K in air leads to losses of hyperstoichiometric oxygen, accompanied with a drastic expansion and, probably, partial disordering of the CaAl_0.5Fe_0.5O_2.5+δ lattice. The average thermal expansion coefficients of CaAl_0.5Fe_0.5O_2.5+δ ceramics in air are 16.7×10⁻⁶ and 12.6×10⁻⁶ K⁻¹ at 370-850 and 930-1300 K, respectively.     

Luminescent properties of Eu-activated (Sr1−z, Caz)(Al1−y, By)2O4 phosphors for UV LEDs

A series of (Sr_1−z, Ca_z)(Al_1−y, B_y)₂O₄:xEu²⁺ phosphors were synthesized by the sol–gel process and were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and photoluminescence (PL) excitation and emission spectra. The experiment results revealed that the highest intensity of Sr(Al_1.98, B_0.02)O₄:Eu²⁺ phosphor with pure monoclinic SrAl₂O₄ was achieved by annealing at the temperature of 1200 °C and the Eu²⁺ content of 8 mol%. However, when the post-treatment temperature for Sr(Al_1.98, B_0.02)O₄: Eu²⁺ was over 1200 °C, the Sr₄Al₁₄O₂₅ phase appeared as a minor phase, inducing small blue-shift in the emission peak (520–509 nm). Doping higher content of B³⁺ (y = 0.02–0.40) into SrAl₂O₄:Eu²⁺ at 1200 °C resulted in the transformation of phase from SrAl₂O₄ to Sr₄Al₁₄O₂₅ as well as to SrB₂Al₂O₇, which made the emission intensity enhance and the emission shift to a much shorter wavelength region (λ_p = 467 nm). It was found that, instead of purely using Sr atoms, Ca atoms with content of 20–40% could induce the crystal structure of (Sr_1−z, Ca_z)(Al_1−y, B_y)₂O₄:xEu²⁺, which led to SrAl₂O₄ from monoclinic to hexagonal phase. As a result, SrAl₂O₄ solid solution was obtained and then SrAl₂O₄:Eu²⁺ to emit 518 nm green light. At higher Ca content (z > 40%), a new CaAl₂O₄ solid solution was formed and a blue emission of CaAl₂O₄:Eu²⁺ was obtained.     

Solubility of SrAl2O4 in CaAl2O4—a high resolution powder diffraction study

The solid solubility of the SrAl₂O₄-CaAl₂O₄ system has been investigated using high resolution synchrotron powder diffraction methods. Analysis of the patterns shows there is a limited composition range at which single phase samples can be obtained, x<0.25 and x>0.75 for the series Ca_1−xSr_xAl₂O₄. At intermediate compositions up to three phases are observed, two monoclinic and one hexagonal. The temperature dependence of the structures is also described.     

Controllable synthesis and magnetic properties of Fe3O4 and Fe3O4@SiO2 microspheres

We present a systematic study on the preparation, microstructure, and magnetic properties of Fe₃O₄ microspheres and Fe₃O₄@SiO₂ microspheres. Results showed that Fe₃O₄ microspheres’ diameter can be tuned by Fe³⁺ concentration, whereas their average grain size can be tuned by polyethylene glycol (PEG) 2000 dosage or PEG molecular weight. The magnetic saturation value of Fe₃O₄ microspheres was observed to be dependent on their average grain size, but not the sphere diameter. Fe₃O₄@SiO₂ microspheres with different magnetic saturation values were achieved by adjusting shell thickness. Furthermore, the synthesized Fe₃O₄ and Fe₃O₄@SiO₂ microspheres with high and controllable magnetic saturation value endow them with great application potentials.     

Flux growth of LiNdP4O12 single crystals

LiNdP₄O₁₂ single crystals were grown by slow-cooling a Li₂OP₂O₅ flux. Crystal morphology and macroscopic crystal quality depend on the solution composition in the ternary system Li₂ONd₂O₃P₂O₅. Under the favorable growth conditions, platy crystals of 15 × 10 × 1 mm³ were obtained.     

Structure refinement, infrared and Raman spectra of KDyP4O12

Crystals of KDyP₄O₁₂ have been grown by the flux technique and characterized by single-crystal X-ray diffraction. KDyP₄O₁₂ crystallizes in the monoclinic C2/c space group with lattice parameters: a=7.8158(3), b=12.3401(5), c=10.4382(3) Å, β=111.053°(2), V=939.6(4) Å³, Z=4. The crystal structure has been refined yielding a final R(F²)=0.034 and R_w(F²)=0.082 for 902 independent reflections (F_o²≥2σ(F_o²)). The structure of KDyP₄O₁₂ consists of DyO₈ polyhedra and cyclotetraphosphate P₄O₁₂ groups sharing oxygen atoms to form a three-dimensional framework, delimiting intersecting tunnels in which the potassium ion is located. Each K⁺ ion is bonded to 10 oxygen atoms. The energies of the vibrational modes of the crystal were obtained from measurements of the infrared and Raman spectra.     

Ionic conductivities of Na2Ti3O7, K2Ti4O9 and their related materials

Ionic conductivities were measured on the polycrystalline samples of layered titanates, Na₂Ti₃O₇ and K₂Ti₄O₉, and their derivatives. The activation energies and the prefactors of the conductions were 0.70 eV and 7.9 × 10 (Ωcm)^?1K for Na₂Ti₃O₇ and 0.81 eB and 1.1 × 10³ (Ωcm)^?1K for K₂Ti₄O₉. A small amount of Nb₂O₅ was doped to these titanates substituting TiO₂. Remarkable enhancements of ionic conductivities were observed with the doping. A new metastable phase, Li₂Ti₃O₇, was prepared by ion-exchange of Na₂Ti₃O₇ and its ionic conductivity was measured.     

Comparative analysis of crystal field effects and optical spectroscopy of six-coordinated Mn ion in the Y2Ti2O7 and Y2Sn2O7 pyrochlores

The electronic energy levels of the six-coordinated Mn⁴⁺ ion in the pyrochlores Y₂B₂O₇ (B = Sn⁴⁺, Ti⁴⁺) have been computed using the exchange charge model of crystal field theory. The calculated Mn⁴⁺ energy levels and their trigonal splitting are in good agreement with the experimental spectra. The calculated crystal field parameters show that the higher crystal field strength in Y₂Sn₂O₇ arises from an increased orbital overlap effect between the Mn⁴⁺ ion and the nearest oxygen ions, which are located at the 48f crystallographic position of the pyrochlore lattice. This increased overlap in Y₂Sn₂O₇ occurs despite the fact that the Mn⁴⁺-O²⁻ bond distance in Y₂Sn₂O₇ is longer than in Y₂Ti₂O₇ and is attributed to a lack of hybridization (covalent bonding) between the filled 2p orbital of oxygen ion occupying the 48f site of the pyrochlore lattice and the filled Sn⁴⁺ 4d¹⁰ orbital. The low temperature emission spectrum of Mn⁴⁺ activated Y₂Sn₂O₇ is analyzed in terms of a weak zero phonon line (R-line) with accompanying vibrational side bands.     

Synthesis of SrAl2O4 from a mixed-metal citrate precursor

A mixed-metal citrate precursor method was used to synthesize SrAl₂O₄. The effects of the pH of the starting solutions and the molar ratio of citric acid to total metal cations concentration (CA/M) on the formation of SrAl₂O₄ were studied. DTA, TG, FT-IR, XRD and field emission scanning electron microscopy (FESEM) were used to characterize the precursors and the derived oxide powders. XRD analysis showed that single-phase SrAl₂O₄ was synthesized from CA/M = 2 precursors at a temperature of 900 °C for 2 h, without the formation of any intermediate phase.     

Neutron diffraction studies of NiCoP4O12 and NiZnP4O12

The title compounds belong to a group of isostructural $\text{M}$₂P₄O₁₂ tetrametaphosphates with monoclinic ($\text{C}$2/$\text{c}$) symmetry. The two structures have been profile-refined by means of the Rietveld technique on the basis of neutron powder diffraction data. The three-dimensional framework is built up of tetrametaphosphate anions and two distinct metal-oxygen octahedra, (M1)O₆ and (M2)O₆. The latter octahedron is somewhat larger and more distorted than (M1)O₆. The results show that in both solid solutions nickel preferentially enters the M1 sites, a tendency also observed in olivine and sarcopside, which contain $\text{M}$O₆ octahedra similar in shape and symmetry to those in $\text{M}$₂P₄O₁₂.     

Flat epitaxial ferromagnetic CoFe2O4 films on buffered Si(001)

Ferromagnetic films of spinel CoFe₂O₄ have been grown epitaxially on Si(001) using CeO₂/YSZ double buffer layers. The heterostructures were built in a single process by pulsed laser deposition with real-time control by reflection high-energy electron diffraction. YSZ and CeO₂ grow cube-on-cube on Si(001) and CoFe₂O₄ grows with (111) out-of-plane orientation, presenting four in-plane crystal domains. The interface with the buffer layers is smooth and the CoFe₂O₄ surface is atomically flat, with roughness below 0.3 nm. The films are ferromagnetic with saturation magnetization around 300 emu/cm³. The properties signal that CoFe₂O₄ is a good candidate for monolithic devices based on ferromagnetic insulating spinels.     

Electrochemical extraction of lithium from LiMn2O4

Lithium has been removed electrochemically at 15 μA/cm² from LiMn₂O₄ (spinel) to yield single phase Li_1?xMn₂O₄ for 0 < × ? 0.60. The electrochemical curve suggests that beyond x = 0.60 an electrochemical process other than lithium extraction occurs. Powder X-ray-diffraction spectra indicate that during the extraction process the [Mn₂]O₄ framework of the spinel structure remains intact. Previous results have shown that 1.2 Li⁺ ions can also be inserted into LiMn₂O₄, which suggests that lithium may be cycled in and out of the [Mn₂]O₄ framework of the spinel structure over a wide range of x, at least from Li_0.4Mn₂O₄ to Li₂Mn₂O₄. Discussion of the mechanism of formation of λ-MnO₂ in an acidic environment is extended.     

Crystal habits of LiMn2O4 and their influence on the electrochemical performance

Crystal habits of LiMn₂O₄ prepared through a sol-gel method using different starting materials (metal acetates and metal nitrates) are studied using a crystal shape algorithm. Density functional theory (DFT) as implemented in VASP is employed to study the thermodynamic stabilities and the electronic structure of the different hkl planes of LiMn₂O₄, as identified by the crystal shape algorithm. The crystal habit of lithium manganate prepared through the metal acetate route, LiMn₂O₄ (A), seems to possess a higher thermodynamic stability compared to the metal nitrate route viz. LiMn₂O₄ (N). Electrochemical cycling measurements show that the capacity retention in LiMn₂O₄ (A) is better than LiMn₂O₄ (N) at low (C/10) as well as at higher (5C) rates.     

New pnictides with Ce2O2S-type structure

The compounds EuMn₂P₂, EuMn₂As₂, EuMn₂Sb₂, YbMn₂As₂, and YbMn₂Sb₂ were prepared for the first time. Their crystal structure corresponds to that of Ce₂O₂S (ordered “anti-”La₂O₃ or CaAl₂Si₂-type structure). Their lattice constants are reported. Attempts to prepare isotypic compounds with typically three-valent rare earth metals have failed so far. The new compounds can be rationalized as normal valence compounds with the expected oxidation numbers e.g. Eu²⁺Mn²⁺Mn²⁺P^3?P^3?.     

Ce<Superscript>3+</Superscript> and Eu<Superscript>2+</Superscript> luminescence in calcium and strontium aluminates

Compound CaAl₄O₇ (CA4), SrAl₄O₇ (SA4), CaAl₁₂O₁₉ (CA12) and SrAl₁₂O₁₉ (SA12) have been synthesized by using single step combustion method. The phosphors have been characterized by XRD, SEM and PL techniques. Both CA4 and SA4 possess monoclinic crystal structure whereas CA12 and SA12 possess hexagonal structure. Effects of crystal symmetry on the emission spectrum have been studied by doping the samples with Ce³⁺ and Eu²⁺ ions. The luminescence properties of Ce³⁺ and Eu²⁺ in these hosts is discussed on the basis of their covalent character and the crystal field splitting of the d-orbital of dopant ions. The spectroscopic properties, crystal field splitting, centroid shift, red shift and stokes shift have been studied. Spectroscopic properties of Eu²⁺ ions have been accurately predicted from those of Ce³⁺ ions in the same host. Most importantly experimental results were matched excellently with the calculated results. The preferential substitution of Ce³⁺ and Eu²⁺ at different Ca²⁺, Sr²⁺ crystallographic sites have been discussed. The dependence of emission wavelengths of Ce³⁺ and Eu²⁺ on the local symmetry of different crystallographic sites was also studied by using Van Uitert’s empirical relation. Differences in the emission spectrum of these samples have been observed despite their similar crystal structures and space group. Possible reasons have been discussed.