Degree Of Covalency Research Articles

Luminescent properties of Eu2+ and Ce3+ ions in strontium litho-silicate Li2SrSiO4

Abstract The luminescent properties of Eu 2+ and Ce 3+ ions in Li 2 SrSiO 4 have been studied upon excitation in the 2–20 eV region. Based on the results of luminescent measurements, values of the crystal field splitting and the centroid shift of the Ce 3+ 5d configuration in Li 2 SrSiO 4 were found and compared with those of Ce 3+ ions in some other inorganic compounds. The Eu 2+ ions in Li 2 SrSiO 4 exhibit a broad band emission with a maximum at 576 nm, which is due to the 4f 6 5d→4f 7 transition. It was shown that the long-wavelength position of the Eu 2+ emission in Li 2 SrSiO 4 is caused by the large crystal-field splitting of the Eu 2+ 4f 6 5d configuration and relatively high degree of covalency of the Eu–O bond. The stabilization of Eu 2+ ions in Li 2 SrSiO 4 during the synthesis process requires a strong reducing agent. Two phenomenological approaches to explain the low stability of Eu 2+ in Li 2 SrSiO 4 are also discussed.

Luminescent studies of Dy3+ ion in alkali lead tellurofluoroborate glasses

Dy 3+ -doped alkali lead tellurofluoroborate (RLTB) glasses (R=Li, Na and K) were prepared by melt quenching technique. Judd–Ofelt theory has been used to evaluate the three intensity parameters Ω 2 , Ω 4 and Ω 6 from the experimental oscillator strengths. The photoluminescence spectra obtained by the excitation wavelength of 385 nm show four emission bands at 454, 483, 575 and 665 nm corresponding to the 4 I 15/2 → 6 H 15/2 and 4 F 9/2 → 6 H J/2 ( J =15/2, 13/2 and 11/2) transitions, respectively. The laser characteristic parameters like fullwidth at half maxima (FWHM), stimulated emission cross-sections ( σ e ), optical gain parameters ( σ e × τ exp ) and gain bandwidth parameters ( σ e ×FWHM) were determined. From the visible emission spectra, yellow to blue (Y/B) intensity ratios and chromaticity coordinates were also estimated. The lifetimes of 4 F 9/2 metastable state were also measured and discussed. ► RLTB system is a novel host for optical absorption and luminescence studies. ► KLTB glass exhibits a greater degree of covalency for Dy–O bond. ► Decay curves of 4 F 79/2 state show bi-exponential behaviour in all the glasses. ► Dy 3+ doped RLTB glasses are the good materials for solid state lasers. ► Dy 3+ : RLTB glasses are suitable materials for white light emission.

Polymer complexes. LIII. Supramolecular coordination modes and structural of novel sulphadrug complexes

Novel polymer complexes of copper(II), palladium(II), platinum(II) and cadmium(II) containing homopolymer [4-acrylamido benzene sulphonyl guanidine; (HL)] and various anions (SO 4 2−, CH 3COO −, NO 3 −, Br − or Cl −) have been designed and carried out. Their structures were investigated by elemental analyses, spectral (IR, UV–vis, 1H NMR and ESR) and magnetic moments. The modes of interactions between the ligand and the metals were discussed, where oxygen (of O S O group) and nitrogen atom [of imino nitrogen (NH/N) of the guanidine group] are involved in chelation. The homopolymer shows two types of coordination behaviour. In mononuclear polymer complexes 4 and 6– 10, it acts as a neutral bidentate ligand chelated through the NH and O atoms, whereas in the polymer complexes 1– 3, 5 and 11, monobasic bidentate ligand is coordinated through the –N and –O atoms. The poly-chelates are of 1:1/1:2 (metal–homopolymer) stoichiometry and exhibit four coordination. On the basis of electronic spectral data and magnetic susceptibility measurement square planar geometry has been proposed. The ESR spectral data provided information about their structure on the basis Hamiltonian parameters and degree of covalency. From the electron paramagnetic resonance and spectral data, the orbital reduction factors were calculated.

Synthesis, Characterization, and Reactions of Isolable (β-Diketiminato)Nb(III) Imido Complexes.

We have investigated both the chemical reduction of (BDI)Nb(V) imido complexes (BDI = HC[C(Me)NAr](2); Ar = 2,6-(i)Pr(2)-C(6)H(3)) to the formal Nb(III) oxidation state and the ability of these Nb(III) complexes to behave as two-electron reductants. The reduction of the Nb(V) species was found to depend heavily on the nature of available supporting ligands, but the chemistry of the reduced compounds proceeded cleanly with a number of unsaturated organic reagents. Accordingly, the novel Nb(V) bis(imido) complexes supported by the monoazabutadiene (mad) ligand (mad)Nb(N(t)Bu)(NAr)(L') (L' = py, thf) were formed by either KC(8) reduction of (BDI)Nb(N(t)Bu)Cl(2)(py) in the absence of strong π-acids or by H(2) reduction of the Nb(V) dimethyl complex (BDI)Nb(N(t)Bu)Me(2) in THF. These products are likely formed though an intramolecular, 2 e(-) reductive C-N bond cleavage, as has been observed previously for related Group 4 systems, suggesting that transient Nb(III) intermediates were present in both cases. In the presence of 1,2-bis(dimethylphosphino)ethane (dmpe), KC(8) reduction of (BDI)Nb(N(t)Bu)Cl(2)(py) was arrested at the Nb(IV) oxidation state to give (BDI)Nb(N(t)Bu)Cl(dmpe), which was characterized by solution-state EPR spectroscopy as a Nb-centered paramagnet with strong coupling to the two equivalent phosphorus nuclei (A(iso){(93)Nb} = 120.5×10(-4) cm(-1), A(iso){(31)P} = 31.0×10(-4) cm(-1), g(iso) = 1.9815). When strong π-acids were used to intercept the thermally unstable Nb(III) complex (BDI)Nb(N(t)Bu)(py) prior to reductive cleavage of the ligand C-N bond, the thermally stable Nb(III) species (BDI)Nb(N(t)Bu)(CX)(2)(L″) (X = O, L″ = py; X = NXyl, L″ = CNXyl; Xyl = 2,6-Me(2)-C(6)H(3)) were obtained in good yields. The Nb(III) complexes (BDI)Nb(N(t)Bu)py, (BDI)Nb(N(t)Bu)(CO)(2)(py) and (BDI)Nb(N(t)Bu)(CO)(2) were subsequently investigated for their ability to serve as two-electron reducing reagents for both metal-ligand multiple bond formation and for the reduction of organic π-systems. The reduction of mesityl azide by (BDI)Nb(N(t)Bu)(py) and diphenylsulfoxide by (BDI)Nb(N(t)Bu)(CO)(2) led to the monomeric bis(imido) and dimeric oxo complexes (BDI)Nb(N(t)Bu)(NMes)(py) and [(BDI)Nb(N(t)Bu)](2)(μ2-O)(2), respectively. MeLi addition to (BDI)Nb(N(t)Bu)(CO)(2)(py) resulted in the formation of a Nb-acylate via methide addition to one of the carbonyl carbons. The acylate product was revealed to have a short Nb-C(acylate) bond distance (2.059(4) Å), consistent with multiple Nb-C bond character resulting from Nb(III) back-bonding into the acylate carbon. The interaction of (BDI)Nb(N(t)Bu)(CO)(2) with two equivalents of 4,4'-dichlorobenzophenone resulted in the clean, quantitative formation of the corresponding pinacol coupling product, but introduction of the ketone in 1: 1 molar ratios resulted in mixtures of the pinacol product and the starting material, suggesting that ketone coordination to the Nb(III) complex may be reversible. Relatedly, addition of 1-phenyl-1-propyne to (BDI)Nb(N(t)Bu)(CO)(2) formed a thermally unstable 1: 1 Nb/alkyne complex, as characterized by NMR and IR spectroscopies; reaction of this species with HCl/MeOH yielded a 2: 1 mixture of 1-phenyl-1-propene and the free alkyne, suggesting a high degree of covalency in the Nb-C bonds.

EuIII‐Doping of Lamellar Bilayer and Amorphous Mono‐Amide Cross‐Linked Alkyl/Siloxane Hybrids

AbstractTwo structurally different but chemically similar families of alkyl/siloxane mono‐amidosil hosts, represented by m‐A(x) [where x = 14 or 8 represents the number of CH2 groups of the pendant alkyl chains directly bonded to the carbonyl group of the amide cross‐link] have been doped with a wide range of concentrations of Eu(CF3SO3)3. Mono‐amidosils m‐A(x)nEu(CF3SO3)3 with n ≥ 10 (where n is the molar ratio of carbonyl groups per Eu3+ ion) have been analyzed. The m‐A(8)nEu(CF3SO3)3 mono‐amidosils are transparent and amorphous films, in which the alkyl chains adopt gauche conformations. In contrast, the m‐A(14)nEu(CF3SO3)3 mono‐amidosils are solid powders; here the lamellar bilayer hierarchical structure of m‐A(14) coexists with a new lamellar phase in which the Eu3+ ions are bonded to carbonyl oxygen atoms of the amide groups. At n = 10 the hydrogen‐bonded associations formed are highly ordered and considerably stronger than those found in the less concentrated hybrids and in the nondoped matrices. “Free” and weakly coordinated triflate ions occur in all the mono‐amidosil samples. The hybrids are white light emitters (maximum quantum yield: 0.08 ± 0.01), presenting a broad emission band in the blue/purplish‐blue spectral region (ascribed to the hybrid host) superimposed on the 5D0→7F0–4 Eu3+ intra‐4f6 transitions. Two Eu3+ local coordination sites (named A and B) have been discerned in both systems. Site A is attributed to weakly coordinated Eu3+/CF3SO3– ion pairs, whereas site B involves Eu3+ coordination to the oxygen atoms of the C=O groups, of the CF3SO3– ions and of the water molecules. For site B, the long‐range order of the hybrid host induces distinct features in the energy of the 5D0→7F0–4 transitions, the 5D0 lifetime and the degree of covalency of the Eu3+–first‐ligand bonds.

Electrospinning preparation and luminescence properties of Eu(TTA) 3phen/polystyrene composite nanofibers

Efficient luminescent composite nanofibers, composed of polystyrene (PS, M w=250000) and europium complex Eu(TTA) 3phen (TTA=2-thenoyltrifluoroacetone, phen=1,10-phenanthroline) with diameters ranging from 350 nm to 700 nm, were prepared by electrospinning and characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), fluorescence spectroscopy, and thermogravimetric analysis (TG). The room-temperature fluorescence spectra of the composite nanofibers were composed of the typical Eu 3+ ion red emission, assigned to the transitions between the first excited state ( 5D 0) and the multiplet ( 7F 0–4). Owing to the incorporation of the europium complex into the PS fiber matrix and the subsequent distortion of the symmetry around the lanthanide ions by the capping PS, the polarization of the Eu 3+ ions was enhanced, which increased the probability for electronic dipole allowed transitions. The monochromaticity ( 5D 0→ 7F 2/ 5D 0→ 7F 1) around the Eu 3+ ions was also efficiently improved. Judd-Ofelt intensity parameters (Ω 2 and Ω 4) were determined from the emission spectra based on the 5D 0→ 7F 2 and 5D 0→ 7F 4 electronic transitions, respectively. The results showed that the Ω 2 values of the composite nanofibers were apparently higher than that of the pure complex, indicating an increased covalency degree in the europium first coordination shell due to the modification of PS matrix. The modification of the polymer matrix also resulted in much higher thermal stability of the composite fibers than that of the pure complex.

Synthesis, Spectroscopic and Magnetic Properties of the Co2(OH)(PO4)1–x(AsO4)x [0 ≤ x ≤ 1] Solid Solution

AbstractThe Co2(OH)(PO4)1–x(AsO4)x [0 ≤ x ≤ 1] solid solution was prepared by hydrothermal synthesis and, polycrystalline samples characterized by X‐ray powder diffraction and spectroscopic measurements. The cell parameters of the isostructural phosphate–arsenate phases follow Vegard's law in the whole range of composition. The IR spectra are characteristic of three distinct features corresponding principally to the vibrations of both the hydroxide and tetrahedral (XO4)3– (X = P, As) groups together with the evolution of the intensity of stretching [νas (X–O)] vibration modes corresponding to the PO4 and AsO4 tetrahedra in the solid solution. Diffuse reflectance data show bands belonging to the octahedral and trigonal bipyramidal geometries of the Co2+ ions with a high degree of covalence in the Co–O bonds of the arsenate phases. Magnetization measurements of Co2(OH)(PO4)1–x(AsO4)x [0 ≤ x ≤ 1] show the existence of antiferromagnetic interactions with the presence of a ferromagnetic component below the ordering temperature. TN decreases from 71 to 19 K as the arsenate amount is increased. ZFC‐FC curves show irreversibility just below TN for Co2(OH)(PO4)1–x(AsO4)x [x = 0.1–0.75]. The magnetic behavior of Co2(OH)(PO4)0.1(AsO4)0.9 is completely different from the rest of the hydroxyphosphate–arsenate members and similar to that observed in the undoped arsenate compound where an incommensurate magnetic phase is observed.

Tetrahedral and Square Planar Ni[(SPR2)2N]2 complexes, R = Ph & iPr Revisited: Experimental and Theoretical Analysis of Interconversion Pathways, Structural Preferences, and Spin Delocalization

Sulfur-containing mono- or bidentate types of ligands, usually form square planar Ni((II))S(4) complexes. However, it has already been established that the bidentate L(-) dithioimidodiphosphinato ligands, [R(2)P(S)NP(S)R'(2)](-), R, and R' = aryl or alkyl, can afford both tetrahedral and square planar, NiS(4)-containing, homoleptic Ni(R,R')L(2) complexes, owing to an apparent structural flexibility, which has not, so far, been probed. In this work, the literature tetrahedral Ni[R(2)P(S)NP(S)R(2)](2) complexes, R = Ph (Ni(Ph,Ph)L(2), 1(Td)) and R = (i)Pr (Ni(iPr,iPr)L(2), 2) as well as the newly synthesized Ni[(i)Pr(2)P(S)NP(S)Ph(2)](2) complex (Ni(iPr,Ph)L(2), 3), have been studied by UV-vis, IR, and (31)P NMR spectroscopy. Complex 3 was shown by X-ray crystallography to be square planar, and magnetic studies confirmed that it is diamagnetic in the solid state. However, it becomes paramagnetic in solution, as it shows a similar UV-vis spectrum to one of the tetrahedral 1(Td) and 2 complexes. The crystal structure of the potassium salt of the asymmetric ligand, [(i)Pr(2)P(S)NP(S)Ph(2)]K, has also been determined and compared to those of the protonated (i)Pr(2)P(S)NHP(S)Ph(2) ligand and complex 3. All three, 1(Td), 2, and 3, Ni(R,R')L(2) complexes show strong paramagnetic effects in their solution (31)P NMR spectra. The magnetic properties of paramagnetic complexes 1 and 2 in the solid state were investigated on oriented crystals, and their analysis afforded remarkably small values of the spin-orbit coupling constant (lambda) and orbital reduction factor (k) parameters, implying significant delocalization of unpaired electronic density toward the ligands. The above experimental findings are combined with data from standard density functional theory and correlated multiconfiguration ab initio theoretical methods, in an effort to investigate the interplay between the square planar and tetrahedral geometries of the NiS(4) core, the mechanistic pathway for the spin-state interconversion, the degree of covalency of the Ni-S bonds, and the distribution of the spin density in this type of system. The analysis provides justification for the structural flexibility of such ligands, affording Ni(R,R')L(2) complexes with variable metallacycle conformation and NiS(4) core geometries. Of particular importance are the large zero-field splitting values estimated by both experimental and theoretical means, which have not, as yet, been verified by direct methods, such as electron paramagnetic resonance spectroscopy. The findings of our work confirm earlier observations on the feasibility of synthesizing either tetrahedral or square planar NiS(4) complexes containing the same type of ligands. They can also form the basis of investigating structure-properties relationships in other NiS(4)-containing systems.

Lanthanide absorption spectral probe in titania nanoparticles synthesized by ultrasonic assistant sol–gel method

Titania nanoparticles were obtained by an ultrasonic assistant sol–gel method. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermal analysis. The optical absorption of the samples has been measured by photoacoustic (PA) spectroscopy, which is powerful for detecting small amount of strongly scattering materials. The structural variations of the sample during the phase transitions were firstly studied using Nd 3+ as an absorption spectral probe. The PA results show that the TiO 2 gel heated at 50 °C is basically amorphous and still contains abundant trapped water and ethanol, which makes the environment around Nd 3+ similar with that of its aqueous ion. For the sample calcined at higher temperature, the f–f transitions of Nd 3+ exhibit a continuous red shift along with the gel-to-anatase transformation, indicating an increase of the “degree of covalency” for Nd 3+ bonding. For the sample calcined at 1100 °C, however, the f–f transitions of Nd 3+ show blue shifts and the hypersensitive transition intensities of Nd 3+ decrease, indicating an increase of ionicity for Nd 3+ bonding. This can be attributed to the segregation of Nd 3+ ions to the external surface, forming Nd 4Ti 9O 24 during the anatase-to-rutile transition.

Study of erbium (III) doped titanium dioxide nanoparticles by photoacoustic spectroscopy

Nanocrystalline titania as photocatalyst has attracted considerable attention for its potential use in environmental cleaning. Recently, lanthanide ions doped titania samples have been shown to increase the photocatalytic efficiency of selected reactions. In this work, TiO 2 nanoparticles doped with Er 3+ were prepared via an ultrasonic assisted sol-gel method. The optical properties of the samples were determined by photoacoustic (PA) spectroscopy. It was found that the absorption edge shifted to lower wavelength when the particle size fell into nanometer range, which is the evidence of a quantum size effect. The nature of ligand bonding to Er 3+ and the structural properties were investigated with the PA absorptions of the f-f transitions of Er 3+, which are sensitive to the local environment. The results showed that TiO 2 gel heated at 70 °C still contained abundant trapped water and ethanol, and the environment around Er 3+ was similar to the one in the aqueous ion. The “degree of covalency” for Er 3+ bonding increased continuously during the gel-to-anatase transition. For the sample calcined at 1100 °C, however, the f-f transitions of Er 3+ showed blue shifts and the intensity of the hypersensitive transition decreased, indicating an increase of ionicity in the Er 3+ bonding. This can be attributed to the segregation of Er 3+ ions to the external surface, forming Er 2Ti 2O 7 during the anatase-to-rutile transition, which was confirmed with XRD and TEM measurements.

Metal Sulfide in a C82Fullerene Cage: A New Form of Endohedral Clusterfullerenes

The row of endohedral fullerenes is extended by a new type of sulfur-containing clusterfullerenes: the metal sulfide (M(2)S) has been stabilized within a fullerene cage for the first time. The new sulfur-containing clusterfullerenes M(2)S@C(82)-C(3v)(8) have been isolated for a variety of metals (M = Sc, Y, Dy, and Lu). The UV-vis-NIR, electrochemical, and FTIR spectroscopic characterization and extended DFT calculations point to a close similarity of the M(2)S@C(82) cage isomeric and electronic structure to that of the carbide clusterfullerenes M(2)C(2)@C(2n). The bonding in M(2)S@C(82) is studied in detail by molecular orbital analysis as well as with the use of quantum theory of atom-in-molecules (QTAIM) and electron localization function (ELF) approaches. The metal sulfide cluster formally transfers four electrons to the carbon cage, and metal-sulfur and metal-carbon cage bonds with a high degree of covalency are formed. Molecular dynamics simulations show that Sc(2)S cluster exhibits an almost free rotation around the C(3) axis of the carbon cage, resulting thus in a single line (45)Sc NMR spectrum.

Mössbauer study of compounds of Cu3 − x Fe x SnS4 and Cu2Fe1 −x Zn x SnS4 systems

A Mossbauer study of the structural and charge states of 57Fe and 119Sn atoms in the compounds of Cu3 −xFexSnS4 and Cu2Fe1 − xZnxSnS4 systems was performed. It was shown that the iron atoms in the compounds of both systems were in the divalent and trivalent states occupying the tetrahedral positions of the structure. The character of the changes of the degree of covalency of the Fe2+-S, Fe3+-S and Sn4+-S bonds during the isomorphic substitution in the systems was established.

Mössbauer study of isomorphous substitutions in Cu2Fe1-xCuxSnS4 and Cu2Fe1-xZnxSnS4 series

The investigation of isomorphous substitutions in minerals of stannite group was carried out for two series Cu2FeSnS4 – Cu3SnS4 (stannite – kuramite) and Cu2FeSnS4 – Cu2ZnSnS4 (stannite – kesterite) by 57Fe and 119Sn Mössbauer spectroscopy. The calculation of the lattice contribution to the electric field gradient tensor at the 57Fe nuclei and an estimation of the quadrupole shift for the determination of Fe3+ atom positions in the structure were carried out. The charge and structural states of Fe and Sn atoms in compounds of the two systems were determined. The schemes of isomorphous substitutions were found for both series in all range of iron concentrations. The changes in the covalence degree of the bonds Fe2+ − S, Fe3+ − S and Sn4+ − S in the process of the substitution were found.

Structural studies of Ba1-xLaxTiO3 using X-ray absorption near-edge spectroscopy

La-doped barium titanate (Ba1-xLaxTiO3) is an important electronic ceramic material used in a wide variety of applications including in multilayer capacitor, transducers, ferroelectric thin film memories, and positive temperature coefficient resistor (PTCR). In this work, we investigated the changes of the local atomic structure around barium and titanium ions of Ba1-xLaxTiO3 compounds prepared by solid state reaction using X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The X-ray Absorption Near-Edge Structure (XANES) spectra at the Ti K-edge (4966 eV) and Ba LIII-edge (5247 eV) were analyzed. Our findings suggest that La3+ doping can modify the degree of covalency of the Ti-O bonds, but cause no change in the BaTiO3 unit cell volume and the atomic coordinate around Ti4+, Ba2+ions.

Atomic and electronic structure of the YBa2Cu3O7/SrTiO3 interface from first principles

The atomic structure, adhesion energy, and bonding nature of the YBa2Cu3O7(001)/SrTiO3(001) interface are systematically investigated from first principles. A total of 48 candidate interface geometries, which involve two SrTiO3 terminations, six YBa2Cu3O7 terminations, and four highly symmetric stacking sequences, were considered. Although the SrTiO3 substrate has a negligible effect on the atomic structure of optimal TiO2-terminated interface, its electronic influence is significant: the electronic states of interfacial O of YBa2Cu3O7 resemble those of O in bulk SrTiO3 rather than bulk YBa2Cu3O7. Consequently, the interfacial O of YBa2Cu3O7 can be reasonably regarded as a natural extension of substrate layer across the interface. However, the effect of substrate on the optimal SrO-terminated interface is twofold characterized atomically by straightening the originally zigzag interfacial YBa2Cu3O7 layer and electronically by altering noticeably the density of states projected on this layer. In addition, the substrate is found to affect the interface chemistry via tuning effectively the first monolayer of the YBa2Cu3O7 film.The preferred geometries (i.e., having the largest adhesion energy) are those that maintain the substrate perovskite stacking across the interface. By applying several methods of analysis, we thoroughly characterized electronic structure and determined that interfacial bonding is mainly ionic, yet maintains a small degree of covalency.

Bonding in Endohedral Metallofullerenes as Studied by Quantum Theory of Atoms in Molecules

Metal-cage and intracluster bonding was studied in detail by quantum theory of atoms in molecules (QTAIM) for the four major classes of endohedral metallofullerenes (EMFs), including monometallofullerenes Ca@C(72), La@C(72), M@C(82) (M=Ca, Sc, Y, La), dimetallofullerenes Sc(2)@C(76), Y(2)@C(82), Y(2)@C(79)N, La(2)@C(78), La(2)@C(80), metal nitride clusterfullerenes Sc(3)N@C(2n) (2n=68, 70, 78, 80), Y(3)N@C(2n) (2n=78, 80, 82, 84, 86, 88), La(3)N@C(2n) (2n=88, 92, 96), metal carbide clusterfullerenes Sc(2)C(2)@C(68), Sc(2)C(2)@C(82), Sc(2)C(2)@C(84), Ti(2)C(2)@C(78), Y(2)C(2)@C(82), Sc(3)C(2)@C(80), as well as Sc(3)CH@C(80) and Sc(4)O(x)@C(80) (x=2, 3), that is, 42 EMF molecules and ions in total. Analysis of the delocalization indices and bond critical point (BCP) indicators such as G(bcp)/rho(bcp), H(bcp)/rho(bcp), and |V(bcp)|/G(bcp), revealed that all types of bonding in EMFs exhibit a high degree of covalency, and the ionic model is reasonable only for the Ca-based EMFs. Metal-metal bonds with negative values of the electron-density Laplacian were found in Y(2)@C(82), Y(2)@C(79)N, Sc(4)O(2)@C(80), and anionic forms of La(2)@C(80). A delocalized nature of the metal-cage bonding results in a topological instability of the electron density in EMFs with an unpredictable number of metal-cage bond paths and large elipticity values.

Synthesis, structural characterization, thermal and electrochemical studies of mixed ligand Cu(II) complexes containing 2-phenyl-3-(benzylamino)-1,2-dihydroquinazoline-4-(3H)-one and bidentate N-donor ligands

Some mixed ligand Cu(II) complexes of the type [Cu( L)( en)X 2] ( 1a–3a), [Cu( L)( en)](ClO 4) 2 ( 4a), [Cu( L)( phen)X 2] ( 1b–3b) and [Cu( L)( phen)](ClO 4) 2 ( 4b) [where L = 2-phenyl-3-(benzylamino)-1,2-dihydroquinazoline-4-(3H)-one; en = ethylenediamine; phen = 1,10-phenanthroline; X = Cl −, N 3 − and NCS −] have been prepared. The complexes were characterized on the basis of elemental analysis, molar conductance, magnetic moment, IR, UV–vis, mass, ESR and thermal studies. On the basis of electronic spectral data and magnetic susceptibility measurement octahedral geometry has been proposed for 1a–3a and 1b–3b and square-planer geometry for 4a and 4b. The ESR spectral data of complexes provided information about their structure on the basis of Hamiltonian parameters and degree of covalency. The electrochemical behaviour of mixed ligand Cu(II) complexes was studied which showed that complexes of phen appear at more positive potential as compared to those for corresponding en complexes.

Comparative study on energetic distortions of SO 42− ions matrix-isolated in compounds with kröhnkite-type chains, K 2Me(CrO 4) 2·2H 2O and Na 2Me(SeO 4) 2·2H 2O ( Me = Mg, Co, Ni, Zn, Cd)

Infrared spectra of compounds with kröhnkite-type infinite octahedral–tetrahedral chains, K 2 Me(CrO 4) 2·2H 2O and Na 2 Me(SeO 4) 2·2H 2O ( Me = Mg, Co, Ni, Zn, Cd), as well as infrared spectra of the title double salts containing matrix-isolated SO 4 2− guest ions are presented and discussed in the regions of the X–O stretching modes. The SO 4 2− guest ions matrix-isolated in selenate and chromate matrices exhibit four infrared bands corresponding to the four site-group components of the stretching modes in good agreement with the low site symmetry of the host ions ( C 1 site symmetry). The values of Δν 3 (site-group splitting) and Δν max (the difference between the highest and the lowest wavenumbered components of the stretching modes) are used as an adequate measure for the extent of energetic distortion of the matrix-isolated SO 4 2− guest ions. The influence of different crystal-chemical parameters ( Me 2+–O XO 3 bond strengths, sizes of the Me 2+ and Me + ions, electronic configurations of the Me 2+ ions, hydrogen bond strengths, and unit-cell volumes of the host compounds) on the extent of energetic distortion of both the host SeO 4 2− and CrO 4 2− ions, and the SO 4 2− guest ions is analyzed. Correlations between the values of Δν 3 and Δν max of the guest ions and both the degree of covalency of the respective Me 2+–O XO 3 bonds and the electronic configurations of the Me 2+ ions have been found and will be discussed. For example, the energetic distortion of SO 4 2− ions included in the chromate lattices decreases in the order Zn > Cd > Mg as a result of the decreasing covalency of the respective Me 2+–O bonds in the same order (Δν 3 have values of 73, 58 and 36 cm −1, respectively). Furthermore, the values of Δν 3 and Δν max are larger when the metal ions have CFSE ≠ 0 (crystal field stabilization energy, Co 2+, Ni 2+). These cations are more resistant to angular deformations of the MeO 6 octahedra (i.e. changes in the O–Me–O bond angles), thus facilitating the extent of distortion of the matrix-isolated SO 4 2− ions as compared to those having CFSE = 0 (Mg 2+, Zn 2+ and Cd 2+). For example, Δν 3 and Δν max of SO 4 2− ions matrix-isolated in K 2Zn(CrO 4) 2·2H 2O have values of 73 and 163 cm −1, and 116 and 207 cm −1 in Na 2Zn(SeO 4) 2·2H 2O, whereas in the respective nickel lattices Δν 3 and Δν max have values of 88 and 173 cm −1 (K 2Ni(CrO 4) 2·2H 2O) and 127 and 212 cm −1 (Na 2Ni(SeO 4) 2·2H 2O). The SO 4 2− guest ions included in selenate matrices, Na 2 Me(SeO 4) 2·2H 2O, are remarkably much distorted than in chromate ones, K 2 Me(CrO 4) 2·2H 2O, as deduced from the values of Δν 3 and Δν max owing to a stronger static field caused by the smaller Na + ions as compared to that caused by the larger K + ions. The smaller unit-cell volumes of the selenate host compounds, i.e. the higher repulsion potential at the lattice sites at which the guest ions are situated additionally favor the extent of energetic distortion of the sulfate guest ions in the selenate matrices.

Microwave dielectric properties of ATiO 3 (A = Ni, Mg, Co, Mn) ceramics

Relationships between structural characteristics and microwave dielectric properties of ATiO 3 (A = Ni, Mg, Co, Mn) with ilmenite structure were investigated. The oxygen octahedral distortion was dependent on the type of A-site ions which affected to the temperature coefficient of the resonant frequency ( TCF) of ATiO 3 ceramics. The quality factor ( Qf) of ATiO 3 (A = Mn, Ni, Co) specimens was appreciably lowered than that of MgTiO 3 specimens due to the degree of covalency of cation–oxygen ion bond and the ordering of A-site ions. Also, the dielectric constant ( K) was dependent on the electronic oxide polarizabilities of ATiO 3 ceramics.

Magnetic structure of greigite (Fe3S4) probed by neutron powder diffraction and polarized neutron diffraction

We have investigated greigite (Fe3S4) using a combination of neutron powder diffraction and polarized neutron diffraction to give the first unambiguous assignment of its magnetic structure. Our results confirm that greigite has a collinear ferrimagnetic structure with antiferromagnetic coupling between the tetrahedral (A) and octahedral (B) sites. Our analysis also indicates that greigite does not have a significant vacancy concentration or spin canting. High‐resolution neutron powder diffraction results enable determination of sublattice magnetizations of the A and B sites. At room temperature, the average magnetic moments on the two sites are almost the same (∼3.0 μB), with a net magnetic moment of ∼3.0 μB per formula unit (fu). The magnetic moment of the B sites decreases slightly between 10 K and room temperature, while the A site moment is relatively stable as a function of temperature; this indicates that greigite is probably an R‐type ferrimagnet. At 10 K, the average magnetic moments of the A and B sites are 3.0 μB and 3.25 μB, respectively. Neutron diffraction measurements, coupled with magnetic measurements, on our pure synthetic greigite samples indicate that the saturation magnetization of greigite is lower than that of magnetite (Fe3O4). It is proposed that the lower magnetic moment in greigite (saturation magnetization is ∼59 A m2 kg−1) compared to magnetite is probably caused by an increased degree of covalency between iron and sulfur compared to oxygen ligands or by greater delocalization of the 3d electrons in greigite.