Electronic absorption and vibrational IR and Raman studies of binary phosphate β-K4Ce2P4O15

The binary phosphate K₄Ce₂P₄O₁₅ was prepared in the polycrystalline state in the solid state reaction of cerium oxide and potassium phosphate KPO₃. The phosphate fragment of this compound appears in the form of two PO₄³⁻ and one P₂O₇⁴⁻ anions occupying the sites of low symmetry. Electronic absorption, emission as well as infrared and Raman spectroscopic methods have been applied to characterise the properties and structure of the compound studied. Its electronic spectra agree with the Ce³⁺ ion spectroscopic characteristics. The ²F_5/2→²F_7/2 transition appears in the typical for this ion region: about 2000 cm⁻¹. The multiplet structure of the spectrum suggests the existence of at least two crystallographic different sites of this ion in the unit cell. The absorption bands in the range 25000–45000 cm⁻¹ have been assigned to the 4f¹→5d¹ transitions of the cerium ion and CT transition of the phosphate ligands. The vibrational spectra were discussed on the basis of correlation diagrams and factor group analysis.

The radiation-less mechanism of the return from the excited state to the ground state via CT states in the system studied is proposed.     

Luminescence based on energy transfer in xerogels doped with Ln2−xTbx(WO4)3

This paper presents preparation, optical absorption and photoluminescence properties of luminescent materials consisting of Ln_2−xTb_x(WO₄)₃ [where Ln = Gd(III) or La(III)] incorporated into silica xerogel. Photoluminescence behaviour of the salt in the rigid matrix was studied by the luminescence spectroscopy. The excitation spectra of the system Ln_2−xTb_x(WO₄)₃ show an intense broad band with a maximum placed at about 240 nm. This band is attributed to ligand–metal charge transfer (LMCT) inside the tungstate group. On the other hand, Tb³⁺ ion exhibits its characteristic emission in the material. Owing to energy transfer from the excited tungstate groups to the Tb³⁺ ions the emission intensity is improved. The energy transfer from WO₄²⁻ group to Tb(III) ion is particularly effective for such dopants as Gd_0.4Tb_1.6(WO₄)₃ or La_0.8Tb_1.2(WO₄)₃ incorporated into SiO₂ xerogel. Concentration of the emission quenchers such as water molecules and OH groups was reduced by thermal treatment. The high emission intensity and easy preparation of these systems make them potential candidates for application as luminescent materials.     

Excitons and rare-earth ions in CsCdBr3

CsCdBr₃ has a quasi-linear crystal structure. It consists of covalently bound [CdBr]₆⁴⁻ chains separated by chains of Cs⁺ ions. The trivalent rare-earth (RE) ions substitute for divalent Cd ions forming predominantly pair centers of the type RE³⁺-(Cd vacancy)-RE³⁺. A minority of RE ions forms “single-ion” centers with more distant charge compensation. The electronic structure around the band gap is determined by the [CdBr]₆⁴⁻ octahedra. The lowest excitonic states of the lattice are charge-transfer states of these octahedra. At low temperatures they form self-trapped excitons which become mobile around 80 K.

In addition we find defect-localized excitons at the RE pairs and single ions with slightly modified spectra. There is a strong energy transfer between the RE ions and the defect-localized excitons in both directions with transition times below 10⁻⁸ s. For the cooperative fluorescence transition ¹D₂×¹G₄→³H₄׳H₄ in Pr³⁺: CsCdBr₃ a frequency-modulated vibronic sideband spectrum was found with up to four repetitions of the frequency of the localized optical phonon mode at the ion pair.     

Luminescence properties of La1−χTmχTa7O19

The luminescence properties of Tm³⁺ in La_1−χTm_χTa₇O₁₉ solid solutions were examined systematically. The substitution of Tm³⁺ for La³⁺ was carried out by a decomposition reaction of nitrates involving the corresponding constituents at 1200 °C in air. X-Ray diffraction patterns of the solid solutions indicated that the crystal structure consisted of a network of (La_1−χ³⁺Tm_χ^staggered|3+, Ta⁵⁺)—O²⁻ polyhedra interstratified with a double layer of Ta⁵⁺—O²⁻ polyhedra. According to the excitation and emission spectra, the most intense emission was found near 460 nm and quenched above χ=0.14 in La_1−χTm_χTa₇O₁₉. Also, lifetime results verified that the emission could be assigned not to the transition ¹G₄ å ³H₆, but to the transition ¹D₂ å³H₄. Upon cathode ray excitation some emissions of Tm³⁺ were superimposed by a broad emission due to the clusters of Ta⁵⁺—O²⁻ polyhedra. As a result, a low dimensional arrangement of Tm³⁺ was much more preferable for getting intense emission because it reduced the energy migration between Tm³⁺ ions.     

Luminescence study of lanthanide(III) ions in non-aqueous solutions containing azide ions

Luminescence lifetime and intensity measurements of lanthanide(III) ions, Ln³⁺ (Eu, Gd and Tb), in non-aqueous solutions containing azide ions (N₃⁻) have been performed to study the quenching effect of N₃⁻ using time-resolved laser-induced luminescence spectroscopy and conventional luminescence spectroscopy. The luminescence Stern–Volmer quenching constants K_sv^Φ and azide non-radiative decay constants k_N3 of the excited Eu(III) ion in the presence of N₃⁻ were measured in N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylformamide (NMF), formamide (FA) and methanol (MeOH). The obtained quenching effect of the Ln³⁺ luminescence by azide in the non-aqueous solvents shows the order: Gd>Eu>Tb, which does not obey the energy gap law, ΔE, between the emitting and the ground state of the Ln³⁺ ion (Gd>Tb>Eu). The anomaly in the quenching pattern observed in the case of Eu³⁺ can be attributed to its partial reduction to Eu²⁺, additionally to the energy transfer from excited state of Eu³⁺ to N₃⁻. The values of quenching rate constants k_N3measured for Eu³⁺ in various solutions depend on the acceptor number (AN) of the solvents used.     

Effect of Zn and Li codoping ions on nanosized Gd2O3:Eu phosphor

Nanosized Gd_1.92−x−yZn_xLi_yEu_0.08O_3−δ phosphor was fabricated by combustion synthesis. The effect of Zn²⁺ and Li⁺ ions on the crystallization behavior, morphology, and luminescence property of Gd₂O₃:Eu³⁺ was investigated. The results indicated that incorporation of Zn²⁺ and Li⁺ ions into Gd₂O₃:Eu³⁺ nanoparticles (NPs) could lead to a remarkable increase of photoluminescence or X-ray excited luminescence, and the intensity at 612 nm was increased by a factor of 7.1 or 21.5 in comparison with that of undoped sample. The enhanced luminescence was regarded as the results of the creation of oxygen vacancies due to the Gd³⁺ sites occupied by Li⁺ ions, the alteration of the crystal field surrounding the activator Eu³⁺ ions owing to the incorporation Zn²⁺ ions into interstitial sites, and the flux effect of Zn²⁺ and Li⁺ ions. The Zn- and Li-codoped Gd₂O₃:Eu³⁺ phosphor with highly enhanced luminescence is very encouraging for applications in high-resolution display devices.     

HCO3-和SO42-对Cu点蚀行为的影响

在不同浓度配比的HCO₃^-和SO₄^2-混合溶液中,利用循环极化电化学测试方法和SEM,对Cu工作电极的循环极化行为和点蚀表面形貌进行了系统的研究.结果表明,在高电位范围的循环极化实验中,Cu的点蚀行为可分为活性溶解型点蚀和钝化膜破裂型点蚀;随SO₄^2-浓度的升高Cu点蚀的敏感性增大.由于HCO₃^-与SO₄^2-的协同作用,随HCO₂^-浓度升高点蚀敏感性呈先增大后减小的规律.在钝化膜破裂型点蚀中,SO₄^2-提高Cu点蚀的诱发能力;HCO₃^-降低Cu点蚀的诱发能力.2种离子对点蚀自修复能力的影响无明显规律.     

Properties of ytterbium and neodymium doped alkali metal yttrium double phosphates of the M3Y1−xLnx(PO4)2 type

The spectroscopic behaviour of the Nd³⁺ and Yb³⁺ doped alkaline metal yttrium double phosphates, M₃Y_1−xLn_x(PO₄)₂ (M=Na, Rb; x=0.01–0.3) were studied for both powder and single crystal samples. The high resolution absorption and emission spectra were measured in the visible and IR regions. Spectral changes with the Nd³⁺ and Yb³⁺ concentration were interpreted. The absorption strengths of the 4f–4f transitions were analysed and used to assess the structural modifications of the two double phosphates. Based on the 4 K absorption spectra the number of metal sites occupied by the dopants was investigated.

Strong emission from Na₃Y_1−xNd_x(PO₄)₂ involving the ⁴F_3/2→⁴I_9/2,⁴I_11/2,⁴I_13/2,⁴I_15/2 transitions were observed whereas the corresponding emission from the rubidium phosphate was presumably quenched by multiphonon processes due to the water molecules absorbed in the channel-like structure.

The IR spectra were used to assign the vibronic components of the electronic transitions. The Yb³⁺ emission bands were broadened depending on the Yb³⁺ concentration (1–10 mol%). The tentative energy level scheme of the ground and excited ²F_J (J=7/2, 5/2) levels was described.     

Scintillation and luminescence properties of Ce-activated K3Lu(PO4)2

K₃Lu(PO₄)₂:Ce single crystals have been studied under gamma, X-ray, VUV, and UV excitation. For all of the excitation forms, the luminescence of these materials is dominated by the d–f emission bands of Ce³⁺. The shape and position of these bands, however, depends on the form of the excitation and on temperature due to crystallographic structural phase changes and multiple types of Lu³⁺ ions sites in the which the Ce³⁺ ion can substitute for Lu. The highly efficient and fast scintillation of these materials is based on radiative recombination of electron-hole pairs via Ce ions, and the scintillation characteristics identify K₃Lu(PO₄)₂:Ce as a promising fast and efficient scintillator.     

High-pressure synthesis and crystal structures of two new polyphosphides, NaP5 and CeP5

Two novel polyphosphides, NaP₅ and CeP₅, were prepared in a BN crucible by the reaction of elemental components under a high pressure of 3 GPa at 800–950 °C. The X-ray structural analysis showed that NaP₅ crystallizes in an orthorhombic space group Pnma with a=10.993(2) Å, b=6.524(1) Å, c=6.903(1) Å, Z=4 and CeP₅ in the monoclinic group P2₁/m with a=4.9143(5) Å, b=9.6226(8) Å, c=5.5152(4) Å, β=104.303(6)°, Z=2. The crystal structure of NaP₅ consists of a three-dimensional framework _∞³[P₅]¹⁻ constructed by P---P bonds among four crystallographically inequivalent phosphorus sites, with large channels hosting the sodium cations, while CeP₅ is a layered compound containing _∞²[P₅]³⁻ polyanionic layers that are separated by Ce³⁺ ions. NaP₅ exhibits the diamagnetic behavior, while the temperature-dependent magnetic susceptibility of CeP₅ essentially follows the Curie–Weiss law.     

Optical study of praseodymium dicarboxylate [Pr(H2O)]2[O2C(CH2)3CO2]3.4H2O

Praseodymium dicarboxylate, [Pr(H₂O)]₂[O₂C(CH₂)₃CO₂]₃.4H₂O]–glutarate, Pr[glut], is synthesized by hydrothermal techniques. The title compound crystallizes in the monoclinic space group C2/c (No. 15). The rare earth cation is coordinated by nine oxygen atoms, eight oxygen atoms from the carboxylate groups and one from the water molecule. The local symmetry of Pr site is low, C_s. The absorption spectra of Pr[glut] are recorded from the visible to the far IR domain at 300, 77 and 9 K. Under various Ar⁺ laser excitations no emission is detected from ³P₀ and ¹D₂ excited levels of Pr³⁺ ion. In the low temperature absorption spectra only one electronic line is recorded for ³H₄→³P₀ transition. It confirms a unique local environment for the rare earth ion in Pr[glut]. The utility of the ‘barycenter curves’ in the attribution of electronic lines is demonstrated. Energy level scheme of 36 Stark components is deduced from the absorption spectra. The parametric calculation was performed on the whole 4f² (Pr³⁺) configuration with the starting set of crystal field parameters obtained previously for the Eu³⁺ ion in the isostructural compound. Eight free ion and nine phenomenological crystal field parameters in C_2v symmetry reproduce quite well several electronic levels of Pr³⁺ ion experimentally observed in Pr[glut]. A good r.m.s. standard deviation of 14.8 cm⁻¹ is obtained.     

Cooling rate dependence of the lattice parameters of (TMTSF)2ClO4

We report a diffractometric determination of the thermal dependence of the triclinic lattice parameters of (TMTSF)₂ClO₄. Low temperature measurements show that c and γ differ significantly in the quenched (Q) and relaxed (R) states. Tight binding calculations of the band structure of the organic sublattice, neglecting the ClO₄⁻ potential, show that the variations of γ between the Q and R states do not affect appreciably the shape of the Fermi surface (FS). We thus propose that the nesting properties of the FS are destroyed below the anion ordering transition by the b^*/2 component of the ClO₄⁻ Coulomb potential.     

Ca15(CBN)6(C2)2O- a new cubic structure type containing CBN-anions together with acetylide and oxo anions

Ca₁₅(CBN)₆(C₂)₂O contains CBN⁴⁻ anions as well as C₂⁻ units and isolated O²⁻ anions. The compound is obtained by heating a stoichiometric mixture of CaO, C and BN with an excess of Ca in sealed niobium ampoules to 1270 K. It crystallizes in the space group Ia d with a = 1656.84(9) pm. Preparation, crystal structure, NMR and IR-spectroscopic properties are discussed.     

Low temperature thermoluminescence properties of Eu and R doped CaAl2O4

The thermoluminescence (TL) of calcium aluminate persistent luminescence materials doped with Eu²⁺ and co-doped with R³⁺ ions were studied between 20 and 325 K. The basic material, CaAl₂O₄:Eu²⁺, showed three TL bands between 150 and 300 K in the glow curve. Changing the R³⁺ co-dopant to the adjacent element in the rare earth series affects significantly the thermoluminescence intensity and even the position of the glow curve maximum. The physical effect seemed to be removing the traps since the La³⁺, Y³⁺, and Lu³⁺ ions suppressed the thermoluminescence. The Pr³⁺, Ho³⁺, and Dy³⁺ co-doping enhanced mainly the low temperature traps and these materials have an intense but relatively short persistent luminescence at room temperature. The Nd³⁺ and Tm³⁺ ions enhanced the TL bands close to room temperature and are thus the most suitable co-dopants to induce intense and long persistent luminescence. The quenching of the thermoluminescence by Sm³⁺ was concluded to be due to the presence of Sm²⁺ that removes totally the traps.     

Electrical properties of magnesium sodium hydrogen monophosphate: MgNa3H(PO4)2

Magnesium sodium hydrogen monophosphate, Na₃MgH(PO₄)₂, crystallizes in the triclinic cell . The crystal morphology is related to the synthesis temperature and the evaporation rate. Samples were characterized through X-ray diffraction and chemical analysis, examined by IR and Raman vibrational spectroscopy and impedance and modulus spectroscopy techniques. The conductivity relaxation parameters associated with some H^· conduction have been determined from an analysis of the M^″/M_max^″ spectrum measured in a wide temperature range. Transport properties in this material appear to be due to an H^· ion hopping mechanism.     

Complexation, luminescence and energy transfer of Ln(III) ions with phenylphosphonic acid

Spectral characterisation of the Ln(III) ion complexes with phenylphosphonic acid (PPA) was made and the mode of Ln(III) ions coordination was established. From the absorption spectra the forces of the Nd³⁺ oscillator were obtained, the intensity of emission and luminescence lifetimes of Eu³⁺ and Tb³⁺ were measured. The Stern–Volmer constants K_sv^φ and K_sv^τ determined indicated the presence of associated forms of complexes. The quantum yield of luminescence of Eu³⁺ and Tb³⁺ complexes was found using Ru(bpy)₃²⁺as a standard. The complexes were characterised by quantum yields (Φ=0.4 and 0.008 for Tb and Eu ions). The solid state Eu³⁺ complex was studied by luminescence spectroscopy, IR and thermogravimetry. The complex was found thermally stable and with no water molecules in its structure.     

High pressure spectroscopy of Pr in LiNbO3

Absorption, emission, and luminescence excitation spectra of the LiNbO₃ crystal doped with 0.5% Pr³⁺ and 0.8% Yb³⁺ are presented. Additionally the photoluminescence spectra at high pressure have been measured. Hydrostatic pressures up to 135 kbar were applied with a diamond anvil cell. Absorption of the Pr³⁺:LiNbO₃ crystal is characterized by the strong threshold at about 400 nm, related to the band-to band-transitions and the sharp structure in the visible region attributed to the transitions to ³P_J and ¹D₂ levels of Pr³⁺ ion. After the 488 nm excitation the yellow emission related to the ¹D₂→³H_J transition of Pr³⁺ have been observed when the ³P₀ emission has not been detected. The excitation spectra of the ¹D₂ luminescence consist of the sharp lines related to the ³H₄→³P_J (J=0, 1, 2) transitions and two broad bands peaked at 340 and 400 nm related probably to the bound exciton. The ¹D₂→³H_J emission shifts with pressure toward the lower energies with the rate of −2.4 cm⁻¹ kbar⁻¹. Additionally, for higher pressures the ¹D₂ emission is considerably quenched. This is explained as being due to the decrease of the energy of the bound exciton with pressure which results in the higher nonradiative depopulation rate of the ¹D₂ state.     

Green up-conversion and infrared emission spectra of U in LaCl3

A strong green emission of U³⁺ ions in LaCl₃ under red excitation has been observed for the first time. The main anti-Stokes fluorescence arises from the U³⁺, ²K_15/2 state (around 550 nm). Theoretical analysis and decaytime measurement of the induced fluorescence show that there are two mechanisms for the up-conversion. The first one consists in excited state absorption and the other one is due to energy transfer between two U³⁺ ions in the ⁴F_9/2 state. In addition a detailed study of infrared emission spectra of LaCl₃:U³⁺ has been carried out and lifetimes of main fluorescent levels were measured at low temperature. From this study three new experimental levels have been assigned.     

Spectral properties of Ho:GdVO4 single crystal

Absorption and emission spectral properties of GdVO₄ single crystal doped with Ho³⁺ ions were investigated at room temperature. Polarized absorption cross section is calculated in the range of 400–2200 nm. Results were analyzed and parameters were calculated based on Judd-Ofelt theory, the emission spectrum shows that the emission intensity around the wavelength of 546 nm associated with transition ⁵S₂ → ⁵I₈ is much stronger than other bands in the observed range and potentially enable the green light output around this emission band in this crystal.     

Zur Kenntnis eines gemischtvalenten Lanthanoxovanadats: La11VV3O26

Single crystals of La₁₁V⁴⁺V₃⁵⁺O₂₆ were prepared by high temperature reactions in an N₂/H₂ mixture above the melting point of the initial oxides V₂O₅–La₂O₃. X-ray investigations of the dark blue crystals reveal triclinic symmetry, space group with = 7.088 Å, β = 10.213 Å, χ = 10.250 Å, = 89.59°, β = 71.10°, τ = 70.00°, Z = 1. The lanthanum-rich compound exhibits a new structure type characterized by a complicated La₁₁O₂₆^19- network with incorporated V⁴⁺/V⁵⁺ ions. The VO₄ tetrahedra are isolated from each other and occupied with V⁴⁺ and V⁵⁺ in a statistical manner.

Résumé

Einkristalle von La₁₁V⁴⁺V₃⁵⁺O₂₆ wurden durch Hochtemperaturreaktionen unter N₂/H₂-Mischungen oberhalb des Schmelzpunktes von V₂O₅-La₂O₃ dargestellt. Die röntgenographische Untersuchung der tiefblauen Kristalle führte zu trikliner Symmetrie, Raumgruppe mit = 7,088 Å, β = 10,213 Å, χ = 10,250 Å = 89,59°, β = 71,10°, τ=70,00°, Z = 1. Die lanthanreiche Verbindung bildet einen neuen Strukturtyp und zeichnet sich durch ein kompliziertes La₁₁O₂₆^19- Gerüst aus, in welches V⁴⁺/V⁵⁺-Ionen eingelagert sind. Die gebildeten VO₄-Tetraeder treten zueinander isoliert auf und sind statistisch mit V⁴⁺ und V⁵⁺ besetzt.