Low Site Symmetry Research Articles

Influence of nanoenvironment on luminescence of Eu 3+activated SnO 2 nanocrystals

The emission intensity of the peak at 612 nm ( 5D 0→ 7F 2) of the Eu 3+ ions activated SnO 2 nanocrystals (doped and coated) is found to be sensitive to the nanoenvironment. We have compared the luminescence efficiencies of the nanocrystals of SnO 2 doped by Eu 2O 3 with those of SnO 2 coated by Eu 2O 3 and we found that the intensities are significantly higher in coated nanocrystals. Furthermore, it is clear from luminescence intensity measurements that Eu 3+ ions occupy low symmetry sites in the Eu 2O 3 coated SnO 2 nanocrystal. The analysis suggests that the radiative relaxation rate is higher in Eu 2O 3 coated SnO 2 nanocrystals than Eu 2O 3 doped SnO 2 nanocrystals due to the asymmetric environment of Eu 3+ ions in coated samples.

Can the low symmetry crystal (ligand) field parameters be considered compatible and reliable?

This paper stems from our recent efforts to clarify the inconsistencies between the crystal-field parameter (CFP) data sets reported in the literature for various low symmetry transition ion–host systems. Prompted by the results of these studies, in this paper we consider the intricate properties of CF Hamiltonians, hitherto not fully understood. The term ‘low symmetry’ here includes the continuous rotational symmetry cases, i.e. hexagonal II, tetragonal II, trigonal II, monoclinic, and triclinic ones, as well as orthorhombic ones. The following fundamental intricate aspects are considered: (1) selection of the axis systems, (2) types of CFPs and their properties, (3) introduction of a new notion of a nominal axis system for the fitted CFP data sets, (4) implications of the Noether's theorem and the algebraic symmetry of CF Hamiltonians, (5) correlation properties among CFP data sets, (6) the rotational degrees of freedom and the reduction of the number of independent CFPs, which are of special importance for triclinic symmetry cases, and (7) extension of the multiple correlated fitting technique, including a quantitative method for CFP data sets comparison. Clarification of these intricate aspects enables us to provide a general framework aimed at achieving an increased compatibility and reliability of CFP data sets for transition ions at low symmetry sites in crystals. This framework may be especially useful for trivalent lanthanides in technologically important low symmetry hosts for which only limited energy-level data are available.

The TiPd2 compound studied by PAC with [formula omitted] and [formula omitted] probes

The intermetallic compound TiPd2 has been investigated with the perturbed angular correlation technique using radioactive 181Hf/181Ta and 111In/111Cd probes. The measurements confirmed an orthorhombic distortion of the C11b structure in the temperature range from 24 to 1023K. However, the temperature dependence of the 111Cd quadrupole parameters demonstrated that above 650K the indium ions reach the lattice site of high symmetry characterized by an axially symmetric electric field gradient (EFG). The situation for the 181Hf/181Ta probes is more complex: we observed two electric field gradients, whose fractions interchange near 700K, possibly due to 181Hf probes switching their positions but still occupying a low-symmetry site.

Europium probe for estimation of site symmetry in glass films, glasses and crystals

The paper is devoted to the memory of Brian Wybourne the pioneer in theory of symmetry and spectra of trivalent rare earths. We evaluate here the influence of site symmetry of the surrounding molecules in glass matrices on europium ion used as a probe. A method by which systematic classification can be performed on descending site symmetry in a large number of solid hosts is based on the ratios of intensities of electric to magnetic dipole transition. Experimental work involves preparation of a number of glass matrices prepared by a sol–gel method, and incorporating trivalent europium. The fluorescence arising from the quintet D state is used to evaluate the site symmetry of the probe europium ion in the prepared matrices, crystals, conventionally prepared glasses, solutions, and complexes incorporated in sol–gel matrices. From the large amount of tabulated data one can see that for totally symmetric sites the factor I(D0–F2)/I(D0–F1) is lower than one, and increases to 10 for systems with very low symmetry sites. The absolute intensities of the transitions depend also on the amount of covalency of Eu with the surrounding ligands.

Optical study and crystal field analysis of praseodymium doped lanthanum metaborate

Praseodymium doped lanthanum metaborate, La(BO2)3, is synthesized by solution chemistry. The title compound crystallizes in the monoclinic space group I2/c. The ten-fold coordinated rare earth ion is located in the C2 low point symmetry site. The optical and crystal field analyses of Pr3+ in this host lattice are presented. The absorption spectra are recorded from the visible to the far IR domain at 300 and 9 K. The low temperature absorption spectra show only one electronic line for 3H4 → 3P0 transition confirming a unique local environment for the rare earth ion in this metaborate phase. Under various Ar+ laser excitations, the emission of Pr3+ ion is detected from the 3P0 and 1D2 excited levels of the 4f2 ground configuration. A selective excitation in the 1D2 level was also performed. The electrostatic crystal field (cf) effect on the 4f2 configuration has been evaluated on the basis of the cf theory with nine phenomenological crystal field parameters (cfp) in the C2v point symmetry, using 54 (52) experimental cf levels deduced from the absorption and emission spectra. The final simulation is achieved with an r.m.s. of 16.1 (12.6) cm−1.

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 2F5/2→2F7/2 transition appears in the typical for this ion region: about 2000cm−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 enhancement in Eu3+-doped α- and γ-Al2O3 produced by pressure-assisted low-temperature combustion synthesis

Intense red luminescence in Eu3+-doped gamma (γ) and alpha (α) alumina (Al2O3) phosphors obtained by direct and indirect combustion synthesis at low-temperatures is reported. γ and α-(Al1−xEux)2O3 are easily produced by combustion synthesis at 280 °C in the range of x=0.001–0.06 at. %. The well-defined direct synthesis allows europium ions to incorporate into the α or γ alumina lattice in spite of the large size difference between these ions and the aluminum cations in Al2O3. These materials yield a strong fluorescence at room temperature due to f–f transition lines within Eu3+ (4f6) electron emission configuration. Furthermore, from luminescence measurements, it is deduced that Eu3+ occupy low-symmetry sites in the Al2O3 lattice.

Local coordination of Eu(III) in organic/inorganic amine functionalized hybrids

The sol–gel method was used to prepare two families of organic/inorganic hybrids incorporating europium triflate, classed as di-urethanesils and aminosils. A siliceous network to which short polyether chains are covalently bonded through urethane linkages, composes the di-urethanesil host. A siliceous network containing pendant amine terminated propyl chains, forms the aminosils. The xerogels were investigated by photoluminescence, particularly the local interaction between the Eu 3+ ions and the host matrix. The Eu 3+ local coordination was modelled in terms of a local-field perturbation representing the ion’s nearest ligands interaction potential. While for the aminosils the Eu 3+ ions occupy one low-symmetry local site—crystal-field strength of ca. 760.5 cm −1 and 5 D 0 lifetime of 0.6–0.7 ms—two local Eu 3+ environments with distinct point symmetry group, 5 D 0 lifetimes (ca. 0.2–0.3 and 1.4–1.8 ms, respectively) and covalent nature—crystal-field strengths of ca. 540 and 740–780 cm −1, respectively—were identified in the di-urethanesils.

Orthorhombic Ba3[BN2]2: A New Structure Type for an Alkaline Earth Metal Nitrido Borate.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Orthorhombic Ba 3[BN 2] 2: a new structure type for an alkaline earth metal nitrido borate

Yellow, transparent single crystals of Ba 3[BN 2] 2 (P2 12 12 1 (No. 19), a=424.73(2), b=1105.60(4) and c=1475.72(6) pm, Z=4) are obtained by the reaction of Ba 2N and BN in arc-welded Nb ampoules at 1300 K. The title compound crystallizes, despite its familiar 3:2:4 stoichiometry, in a new structure type. The vibrational spectra of orthorhombic Ba 3[BN 2] 2 have been measured and assigned. The IR spectrum reveals strong band splitting arising from the low site symmetry and the presence of two different sets of crystallographically independent [BN 2] 3−groups. The new structure and the vibrational data are compared with those previously reported for the cubic form of Ba 3[BN 2] 2.

Combustion synthesis and luminescence properties of Dy 3+-doped MgO nanocrystals

MgO nanocrystals doped with Dy 3+ have been synthesized by a combustion method. The synthesized sample is characterized by X-ray diffraction, transmission electron micrograph, Fourier transform infrared, and photoluminescence spectroscopy. The as-prepared MgO nanocrystals appear to be single cubic crystalline phase and the diameter is in the range of 20–25 nm. The hypersensitive transition ( 4F 9/2→ 6H 13/2 of Dy 3+) emission is prominent in the emission spectra resulting from the low-symmetry local site at which Dy 3+ ions locate. In addition, the dependence of the luminescence intensity on Dy 3+ concentration is also discussed.

Adsorption geometry of Cu( [formula omitted])-(12 × 2)-14Xe

Low-energy electron diffraction was used to study the adsorption geometry of Xe on Cu(1 1 0). At low temperatures, a higher-order commensurate (12 × 2) structure can be formed that consists of rows of Xe atoms aligned along the direction of the rows on Cu(1 1 0), and having a spacing between the rows equal to the Cu row spacing. A dynamical LEED study of this structure at 20 K indicates that the rows of Xe atoms are localized on top of the Cu substrate rows. The lateral registry of the Xe rows along the Cu rows includes 2 Xe atoms per (12 × 2) unit cell in atop positions above Cu atoms, with the others in lower-symmetry sites. The average Xe–Cu interlayer spacing is 3.3 ± 0.1 Å and there is a small degree (0.1 Å) of vertical rumple in both the overlayer and the top substrate layer. The substrate surface with adsorbed Xe has essentially the same relaxation as the clean surface. The analysis of the adatom vibrations in this structure indicates that the isotropically-averaged rms vibration amplitude is 0.2 Å for Xe, while that of the top-layer Cu atoms is 0.04 Å.

Crystal field and microscopic spin Hamiltonians approach including spin–spin and spin–other-orbit interactions for d 2 and d 8 ions at low symmetry C3 symmetry sites: V 3+ in Al 2O 3

A new module has been developed within the CFA/MSH computer package, which is applicable for d 2 and d 8 ions at sites of trigonal symmetry type I ( C 3 v , D 3, D 3 d ) and type II ( C 3, C 3 i ), including the ‘imaginary’ CF term. For the first time the spin–spin (SS) and spin–other-orbit (SOO) interactions have also been included in the Hamiltonian. This module enables to study the contributions to the energy levels and the spin Hamiltonian parameters, i.e. zero-field splitting D and g-factors: g ∥ and g ⊥. The contributions arising from the spin–orbit (SO), SS, and SOO interaction as well as those due to the low symmetry CF effects induced by the distortion angle ϕ, which describes the difference between C 3 and C 3 v symmetry, can be studied. As an application of the new module, calculations have been carried out for V 3+(3d 2) ions in α-Al 2O 3 crystal, taking into account for the first time the SS and SOO interactions, and the low symmetry CF effects. The results show that (i) the contributions from the SS and SOO interactions to the energy levels are larger for free V 3+ ions than those for V 3+ ions in α-Al 2O 3 crystal, (ii) both the contributions to the SH parameters and the energy levels arising from the SOO interaction are larger than those arising from the SS interaction, (iii) the contributions due to the low symmetry CF effects induced by the distortion angle ϕ are in general significant, (iv) D and g ⊥ are sensitive to the distortion angle ϕ, whereas g ∥ is insensitive to ϕ, and (v) the influence of the lattice distortions on the spectroscopic properties of V 3+ ion in α-Al 2O 3 is pronounced. It appears important for similar ion-crystal cases to consider the lattice distortions in detailed calculations, which take into account the relevant contributions from the SO, SS and SOO interactions. A good agreement between the theoretical and experimental results has been obtained.

Adsorption of chlorofluorocarbons in nanoporous solids; a combined powder neutron diffraction and computational study of CFCl3 in NaY zeoliteCCDC reference number 201879.

The crystal structure of NaY (Na54Si138Al54O384) + CFCl3 has been determined at 20 K by Rietveld analysis of powder neutron diffraction data in space group Fdm. The sodium cations were located preferentially in sites II, which are fully-occupied and also in sites I, which are half-occupied. A new cation position in the β-cages was detected around x = 0.125, y = 0.125, z = 0.031. Important cation migration is observed upon adsorption of CFCl3, as shown by the absence of cations in sites I′, known to be a favorable cation site in bare NaY, together with the detection of this new site in the β-cages. Part of extraframework cations remained undetected, that were assumed to be located in the supercages in low symmetry sites. The CFCl3 molecules are found in the 12-ring windows, where they accomodate Cl⋯Ozeolite and F⋯Ozeolite van der Waals interactions together with Cl⋯Na(II) electrostatic interactions. (N,V,T) Monte Carlo simulations were performed in order to elucidate the role of extraframework cations in the adsorption of CFCl3 in NaY. They show that the presence of Na ions in the 12-ring window (sites III′) has to be taken into account in order to get adsorption geometries consistent with that obtained from neutron diffraction.

A transferable representation of the induced multipoles in ionic crystals

Electronic structure calculations of the induced dipole and quadrupole moments on a fluoride ion at low symmetry sites in a model crystalline environments are described. The results are used to characterize the short range contributions to the induced multipoles on the F− ion that arise from overlap between the wavefunctions of the ions. These are represented by general functions of the positions of the surrounding ions, suitable for use in a computationally tractable simulation model. The present calculations are designed to test the transferability to other classes of materials of this representation which, to date, largely has been deduced from calculations on distorted alkali halide crystals. First, the induced multipoles in mixed crystals of the alkali halides are considered, in order to check that they are predicted reliably by combining the representations deduced for the corresponding pure materials. Second, polarization effects in the alkaline earth fluorides MgF2, and CaF2, in a locally distorted fluorite crystal structure are examined. It is found that the material-specific parameters in the representation are related through simple functions of the ionic radii. This relationship holds between MgF2, and CaF2, in the fluorite structure, but also extends to the alkali fluorides previously studied. In order to illustrate the significance of polarization effects, the polarization model for CaF2 is combined with an ab initio pair potential for the repulsion and dispersion interactions and used in a molecular dynamics simulation. Inclusion of the polarization term improves greatly the calculated phonon frequencies in the crystal.

Synthesis, Crystal Structure, and Oxide Ion Conductivity in Bi4.6Ca1.1VO10.5.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Vibrational behavior of the SO stretches in compounds with kröhnkite-type chains Na 2Me(SeO 4) 2·2H 2O with matrix-isolated SO 42− and Me′ 2+ guest ions (Me = Mn, Co, Ni, Cu, Zn, Cd)

The infrared spectra of related compounds with kröhnkite-type chains Na 2Me(SeO 4) 2·2H 2O (Me=Mn, Co, Ni, Cu, Zn, Cd) containing matrix-isolated SO 4 2− guest ions are reported and discussed with respect to the SO stretching modes ν 3 and ν 1. Due to the low site symmetry C 1 of the SO 4 2− guest ions three bands for ν 3 and one band for ν 1 are seen in all spectra. When SO 4 2− guest ions are incorporated in the triclinic Na 2Zn(SeO 4) 2·2H 2O, Na 2Co(SeO 4) 2·2H 2O and Na 2Ni(SeO 4) 2·2H 2O host lattices the ν 3 stretching region resembles a higher local symmetry (A 1⊕E) of the SO 4 2− guest ions than the crystallographic one (i.e. Δ ν ab>Δ ν bc instead of Δ ν ab≈Δ ν bc, a, b and c being three ν 3 components). Hence, the ratio between Δ ν ab and Δ ν bc additionally to Δ ν max (the difference between the highest and the lowest wavenumbered SO stretching modes) has to be taken into account when the SO 4 2− guest ion distortions are considered (the higher the ratio Δ ν ab/Δ ν bc is, the weaker the distortion is). Both the site group splittings of the triplet component ν 3 (Δ ν ac) and Δ ν max values are an adequate measure for the SO 4 2− guest ion distortion when the guest ions are incorporated in the monoclinic Na 2Cu(SeO 4) 2·2H 2O, Na 2Mn(SeO 4) 2·2H 2O and Na 2Cd(SeO 4) 2·2H 2O (i.e. Δ ν ab≈Δ ν bc). In addition to the local potential at the lattice site of the host lattice, the metal environment of the SO 4 2− guest ions reflects on the guest ion distortions. A correlation between the SO 4 2− guest ion distortions and the metal electronic configurations has been found and discussed. Me′ 2+ guest ions incorporated additionally to the SO 4 2− guest ions in the selenate lattices do not influence practically on the vibrational behavior of the SO stretches.

Nuclear Quadrupole Coupling of 17O and 33S in Ionic Solids: Invalidation of the Sternheimer Model by Short-Range Corrections

Electronic structure calculations of electric field gradients (EFG) at the nuclei of the anions F-, O2-, and S2- for distorted octahedral environments representing the condensed phases of LiF, MgO, and MgS show dominant short-range corrections to the Sternheimer model. In this model, the EFG at the nucleus is proportional to that calculated from charges external to the ion with the constant of proportionality reflecting screening by the anion electron density. The short-range corrections suppress and even reverse the Sternheimer predictions and act in such a way as to damp the site specificity of the nuclear quadrupole coupling constants (NQCC) observed in magnetic resonance experiments. In contrast, the Sternheimer model is shown to describe accurately the calculated EFG at the nucleus of the Na+ cation. The physical origin of the short-range corrections is connected to the well-characterized short-range contributions to the induced dipoles and quadrupoles on ions at low-symmetry sites in the condensed p...

Synthesis, Crystal Structure, and Oxide Ion Conductivity in Bi4.6Ca1.1VO10.5

A novel bismuth calcium vanadate, Bi 4.6 Ca 1.1 VO 10.5 , has been synthesized. Its crystal structure has been determined by synchrotron X-ray diffraction. The compound crystallizes in orthorhombic space group Pnnm, with cell parameters a = 11.930(2) A, b = 16.030(3) A, and c = 11.500(2) A. The crystal structure can be viewed as consisting of fluorite-like slabs of edge-sharing OA4 (A = Bi, Ca) tetrahedra that extend along the c axis of the crystal and interspersed disordered VO 4 groups. Bi and Ca atoms undergo site disorder, with Bi preferentially occupying locations with low site symmetry and irregular coordination environments. Variable temperature powder diffraction measurements reveal a change in the thermal expansion coefficient of Bi 4.6 Ca 1.1 VO 10.5 above 600 °C and this coincides with the change in conductivity as revealed by a.c. impedance measurements.

Constructing accurate potential energy surfaces for a diatomic molecule interacting with a solid surface: H2+Pt(111) and H2+Cu(100)

By applying a corrugation-reducing procedure we have interpolated the six-dimensional (6D) potential energy surfaces for the H2/Pt(111) and H2/Cu(100) systems from data obtained by density functional theory (DFT) calculations. We have compared interpolated values with a large number of DFT results not used in the basis for the interpolation and we have obtained an average error below 20 meV and a maximum error of about 30 meV in the regions important for dissociative adsorption. Near the surface the corrugation-reducing procedure gives excellent results using only data from high-symmetry sites. However, we show that to reach the above mentioned accuracy level far from the surface, it is necessary to include information from at least one low-symmetry site. Care has been taken to demonstrate the quality of the interpolation along all degrees of freedom in different regions of the configuration space. The strengths of the method are shown together with the aspects requiring careful handling. A comparison with a direct interpolation method is also made.