Comparative study of the spectroscopic properties of Cr-doped LiAlO2 and LiGaO2

A detailed spectroscopic study of the optical characteristics of the tetrahedrally coordinated Cr⁴⁺ ion in LiAlO₂ and LiGaO₂ is given. From absorption and excitation measurements the crystal field parameter Dq and the Racah parameter B were determined to be Dq=1065 cm⁻¹, B=450 cm⁻¹, and Dq/B=2.4 for LiAlO₂ and Dq=1055 cm⁻¹, B=428 cm⁻¹, and Dq/B=2.5 for LiGaO₂. For the Racah parameter C only a lower limit can be given, i.e. 2417 cm⁻¹ for LiAlO₂ and 2667 cm⁻¹ for LiGaO₂. Due to the strong crystal field splitting — caused by the low site symmetry — the ³B(³T₂) crystal field component is the metastable and thus the emitting level. In the low-temperature absorption and emission spectra the expected three spin–orbit components of the ³B level are found at 8273, 8296, and 8300 cm⁻¹ for Cr⁴⁺:LiAlO₂ and 8610, 8623, and 8632 cm⁻¹ for Cr⁴⁺:LiGaO₂. The emission lifetime of Cr⁴⁺ in LiAlO₂ is 95 μs at 10 K and single exponential. In Mg-codoped LiAlO₂ and in LiGaO₂ the Cr⁴⁺ decay is double exponential. In Cr,Mg:LiAlO₂ two centers can be clearly distinguished, while in Cr:LiGaO₂ a variety of centers are observed, probably due to different charge compensation processes between Li, Ga, and Cr. The quantum efficiencies at room temperature are 42% for Cr:LiAlO₂ and 23% for Cr:LiGaO₂. Already at low temperature nonradiative decay processes occur. The temperature dependence of the lifetimes were analyzed with the model of Struck and Fonger. Excited state absorption measurements indicate that in the spectral region of the emission the excited state absorption cross-section is larger than the stimulated emission cross-section. Therefore laser oscillation is unlikely in these systems.     

Vibrational spectroscopic study of the arsenate mineral strashimirite Cu8(AsO4)4(OH)4·5H2O—Relationship to other basic copper arsenates

The basic copper arsenate mineral strashimirite Cu₈(AsO₄)₄(OH)₄·5H₂O from two different localities has been studied by Raman spectroscopy and complemented by infrared spectroscopy. Two strashimirite mineral samples were obtained from the Czech (sample A) and Slovak (sample B) Republics. Two Raman bands for sample A are identified at 839 and 856 cm⁻¹ and for sample B at 843 and 891 cm⁻¹ are assigned to the ν₁ (AsO₄³⁻) symmetric and the ν₃ (AsO₄³⁻) antisymmetric stretching modes, respectively. The broad band for sample A centred upon 500 cm⁻¹, resolved into component bands at 467, 497, 526 and 554 cm⁻¹ and for sample B at 507 and 560 cm⁻¹ include bands which are attributable to the ν₄ (AsO₄³⁻) bending mode. In the Raman spectra, two bands (sample A) at 337 and 393 cm⁻¹ and at 343 and 374 cm⁻¹ for sample B are attributed to the ν₂ (AsO₄³⁻) bending mode. The Raman spectrum of strashimirite sample A shows three resolved bands at 3450, 3488 and 3585 cm⁻¹. The first two bands are attributed to water stretching vibrations whereas the band at 3585 cm⁻¹ to OH stretching vibrations of the hydroxyl units. Two bands (3497 and 3444 cm⁻¹) are observed in the Raman spectrum of B. A comparison is made of the Raman spectrum of strashimirite with the Raman spectra of other selected basic copper arsenates including olivenite, cornwallite, cornubite and clinoclase.     

The B1 ← A1 absorption system of NO2 in Xe matrices: evidence for Renner—Teller interaction

A tentative vibrational assignment of the ²B₁ ← ²A₁ absorption system of NO₂ in solid Xe is reported. About 65 bands were analysed, yielding normal vibration energies of ν₁ = 1230, ν₂ = 450 and ν₃ = 2040 cm⁻¹. The electronic transition energy can be estimated to be T₀₁₀ = 14160 cm⁻¹ (14220 cm⁻¹ for the gaseous phase). These observations are in good agreement with predictions made using ab initio calculations. Evidence for Renner—Teller interaction is documented by a systematic staggering of frequency intervals between successive bands in the ν₂ progression of the state.     

Blue-shifted hydrogen-bonded complexes. II. CH3F(HF)1 n 3 and CH2F2(HF)1 n 3

The equilibrium structures, binding energies, and vibrational spectra of the clusters CH₃F(HF)_{1 n 3} and CH₂F₂(HF)_{1 n 3} have been investigated with the aid of large-scale ab initio calculations performed at the Møller–Plesset second-order level. In all complexes, a strong C–FH–F halogen–hydrogen bond is formed. For the cases n = 2 and n = 3, blue-shifting C–HF–H hydrogen bonds are formed additionally. Blue shifts are, however, encountered for all C–H stretching vibrations of the fluoromethanes in all complexes, whether they take part in a hydrogen bond or not, in particular also for n = 1. For the case n = 3, blue shifts of the ν(C–H) stretching vibrational modes larger than 50 cm⁻¹ are predicted. As with the previously treated case of CHF₃(HF)_{1 n 3} complexes (A. Karpfen, E. S. Kryachko, J. Phys. Chem. A 107 (2003) 9724), the typical blue-shifting properties are to a large degree determined by the presence of a strong C–FH–F halogen–hydrogen bond. Therefore, the term blue-shifted appears more appropriate for this class of complexes. Stretching the C–F bond of a fluoromethane by forming a halogen–hydrogen bond causes a shortening of all C–H bonds. The shortening of the C–H bonds is proportional to the stretching of the C–F bond.     

A1 → E emission of 1-phosphapropyne cation, CH3CP

The ²A₁ → ²E emission spectrum of CH₃CP⁺ in the gas phase has been observed in the 530–590 nm region. The cations were produced by electron impact on either an effusive beam or seeded helium supersonic free jet or CH₃CP. Two pairs of spin-orbit separated bands are identified: O⁰₀, ^O_O and 2^O₁, ^O₁. The derived constants are (in cm⁻¹): T₀=18656(1), a″_O=−85(2) and ν″₂=1503(2).     

The crystal and magnetic structures of Ca2NdRuO6, Ca2HoRuO6, and Sr2ErRuO6

The crystal structure of Sr₂ErRuO₆ has been refined from neutron powder diffraction data collected at room temperature; space group P2₁/n, A = 5.7626(2), B = 5.7681(2), C = 8.1489(2) Å, β = 90.19(1)°. The structure is that of a distorted perovskite with a 1:1 ordered arrangement of Ru⁵⁺ and Er³⁺ over the 6-coordinate sites. Data collected at 4.2 K show the presence of long range antiferromagnetic order involving both Ru⁵⁺ and Er³⁺. The temperature dependence of the sublattice magnetizations is described. The crystal structure of Ca₂NdRuO₆ is also that of a distored perovskite (P2₁/n, A = 5.5564(1), B = 5.8296(1), C = 8.0085(1) β = 90.19(1)°. The β = 90.07(1)°) with a random distribution of Ca²⁺ and Nd³⁺ on the A site and a 1:1 ordered arrangement of Ca²⁺ and Ru⁵⁺ on the 6-coordinate B sites. The Ru⁵⁺ sublattice is antiferromagnetic at 4.2 K but there is no evidence for magnetic ordering of the Nd³⁺ ions. Ca₂HoRuO₆ is also a distorted perovskite (P2₁/n, A = 5.4991(1), B = 5.7725(1), C = 7.9381(2), β = 90.18(1)° at 4.2 K) with a cation distribution best represented as Ca_1.46Ho_0.54[Ca_0.54Ho_0.46Ru]O₆. There is no ordering among the Ca³⁺ or Ho³⁺ ions on either the A or the B sites, but the Ca/Ho ions form a 1:1 ordered arrangement with Ru⁵⁺ on the B sites. At 4.2 K the Ru⁵⁺ ions adopt a Type I antiferromagnetic arrangement but there is no evidence of long range magnetic ordering among the Ho³⁺ ions.     

Electronic structure and reactivity of Mg+(H2O)n cluster ions

Electronically excited states of magnesium-water cluster ions, Mg⁺(H₂O) _n ,n=1–5, are studied by photodissociation after mass selection. The observed photodissociation spectra are assigned to the²P–²S type transitions localized on the Mg⁺ ion with the aid of ab initio CI calculations. In addition to evaporation of water molecules, photoinduced intracluster reaction to produce MgOH⁺(H₂O) _n is found to occur efficiently, with pronounced size dependence. The intriguing features observed in the mass spectrum of nascent cluster ions are discussed in relation to the stepwise solvation of this reaction.     

Laser induced photodissociation of H+2 and D+2 ions

Photodissociation of H⁺₂ and D⁺₂ has been observed in a crossed beam experiment. A laser used as photon source. The ion and laser beam cross each other inside the laser cavity. The momentum spectra of the resulting H⁺ or D⁺ fragments are recorded with a mass spectrometer. From the spectra the excess kinetic energy is calculated. These values agree with the theoretically expected ones within the experimental error. From the measured intensity distribution the relative population for several vibrational states in the primary ion beam is calculated. Our values deviate from the usual assumed Franck-Condon pattern as well as from the values reported by Dunn. The angular dependence of the fragments is also measured. This dependence indicates a polarization of the primary beam perpendicular to its direction.     

Syntheses, crystal structures and spectroscopic properties of KEu[AsS4], K3Dy[AsS4]2 and Rb4Nd0.67[AsS4]2

Three rare earth compounds, KEu[AsS₄] (1), K₃Dy[AsS₄]₂ (2), and Rb₄Nd_0.67[AsS₄]₂ (3) have been synthesized employing the molten flux method. The reactions of A₂S₃ (A = K, Rb), Ln (Ln = Eu, Dy, Nd), As₂S₃, S were accomplished at 600 °C for 96 h in evacuated fused silica ampoules. Crystal data for these compounds are: 1, monoclinic, space group P2₁/m (no. 11), a = 6.7276(7) Å, b = 6.7190(5) Å, c = 8.6947(9) Å, β = 107.287(12)°, Z = 2; 2, monoclinic, space group C2/c (no. 15), a = 10.3381(7) Å, b = 18.7439(12) Å, c = 8.8185(6) Å, β = 117.060(7)°, Z = 4; 3, orthorhombic, space group Ibam (no. 72), a = 18.7333(15) Å, b = 9.1461(5) Å, c = 10.2060(6) Å, Z = 4. 1 is a two-dimensional structure with ²_∞[Eu(AsS₄)]⁻ layers separated by potassium cations. Within each layer, distorted bicapped trigonal [EuS₈] prisms are linked through distorted [AsS₄]³⁻ tetrahedra. Each Eu²⁺ cation is coordinated by two [AsS₄]³⁻ units by edge-sharing and bonded to further two [AsS₄]³⁻ units by corner-sharing. Compound 2 contains a one-dimensional structure with ¹_∞[Dy(AsS₄)₂]³⁻ chains separated by potassium cations. Within each chain, distorted bicapped trigonal prisms of [DyS₈] are linked by slightly distorted [AsS₄]³⁻ tetrahedra. Each Dy³⁺ ion is surrounded by four [AsS₄]³⁻ moieties in an edge-sharing fashion. For compound 3 also a one-dimensional structure with ¹_∞[Nd₀.₆₇(AsS₄)₂]⁴⁻ chains is observed. But the Nd position is only partially occupied and overall every third Nd atom is missing along the chain. This cuts the infinite chains into short dimers containing two bridging [As₄]³⁻ units and four terminal [AsS₄]³⁻ groups. 1 is characterized with UV/vis diffuse reflectance spectroscopy, IR, and Raman spectra.     

Crystal structure and Mössbauer spectroscopy of Y2SrFeCuO6.5, a double layer perovskite intergrowth phase

The crystal structure of Y₂SrFeCuO_6.5 was determined from single-crystal X-ray and neutron powder diffraction studies. M_r = 488.81, orthorhombic, Ibam, a = 5.4036(8)[5.4149(1)] Å, b = 10.702(1)[10.7244(1)] Å, c = 20.250(2)[20.2799(2)] Å; values in square brackets are neutron data. V = 1171.0(4), Z = 8, D_x = 5.544 g cm⁻³, λ = 0.71069 Å, μ = 345.1 cm⁻¹, R = 0.048 for 567 observed reflections. The Fe/Cu atoms occupy randomly the approximate center of oxygen pyramids. The pyramids share the apical oxygen and articulate laterally by corner sharing of oxygen to form a double pyramidal layer perpendicular to c. The pyramidal slabs are separated by double layers of Y that are in 7-fold coordination to oxygen, forming a defect fluorite unit. Mössbauer spectra indicate a unique iron environment and magnetic ordering at about 265 K. The paramagnetic phase coexists with the magnetic phase over an approximate temperature range 300-263 K, characteristic of magnetic ordering in 2-D magnetic structures. The isomer shift, 0.26, and quadrupole splitting, 0.56 mm sec⁻¹, are consistent with Fe³⁺ in 5-fold coordination and H_int values also indicate classic high spin Fe³⁺. The average Y---O bond length is 2.331(6) Å and Sr is in a dodecahedral environment in which, however, two oxygen atoms at the corners of the cube are missing. The average Sr---O bond length is 2.793(10) Å. The structure is derived from the Ruddlesden-Popper phase Sr_n+1Ti_nO_3n+1 with n = 2.     

Synthesis, band structure, and optical properties of Ba2ZnV2O8

A novel compound Ba₂ZnV₂O₈ has been synthesized in high temperature solution reaction and its crystal structure has been characterized by means of single crystal X-ray diffraction analysis. It crystallizes in monoclinic system and belongs to space group P2₁/c with a=7.9050(16), b=16.149(3), , β=90.49(3). It builds up from 1-D branchy chains of [ZnV₂O₈⁴⁻]_∞, and the Ba²⁺ cations are located in the space among these chains. The IR spectrum, ultraviolet-visible diffuse reflection integral spectrum and fluorescent spectra of this compound have been investigated. The calculated results of energy band structure by the density functional theory method show that the solid-state compound of Ba₂ZnV₂O₈ is an insulator with direct band gap of 3.48 eV. The calculated total and partial density of states indicate that the top valence bands are contributions from the mixings of O-2p, V-3d, and Zn-3d states and low conduction bands mostly originate from unoccupied antibonding states between the V-3d and O-2p states. The V-O bonds are mostly covalence characters and Zn-O bonds are mostly ionic interactions, and the ionic interaction strength is stronger between the Ba-O than between the Zn-O. The refractive index of n_x, n_y, and n_z is estimated to be 1.7453, 1.7469, and 1.7126, respectively, at wavelength of 1060 nm for Ba₂ZnV₂O₈ crystal.     

Evidence for charge exchange between N and N2(A Σu) in a low-temperature nitrogen plasma

The emission from the first negative system, N₂⁺(B ²Σ⁺_u)→N₂⁺(X ²Σ⁺_g)+hν, is studied in the flowing nitrogen afterglow of a DC arc plasma. Investigation of the spectrum shows overpopulation of the vibrational levels 6 and 7 of the excited molecular ion, N₂⁺(B ²Σ⁺_u). Selective excitation of these levels is explained by a charge exchange reaction between atomic ions in the ground state and metastable molecules in the N₂(A ³Σ⁺_u) state. The emitted intensity of the first negative system is shown to be linear with electron density n_e for n_e>2×10¹⁶ m⁻³, a higher-order dependence exists below this value. This is consistent with population of N₂⁺(B ²Σ⁺_u) by atomic ions, N⁺.     

Electronic spectra of jet-cooled tropolone(-OD). vibrational analysis for the à B2- A1 transition

The fluorescence excitation spectrum and the single vibronic level fluorescence spectra from the vibronic levels in the à ¹B₂ state of tropolone(-OD) have been measured in a supersonic free jet. Some low frequency fundamentals in the ¹A₁ and à ¹B₂ states have been determined. A tunneling doublet separation has been measured to be 11 cm⁻¹ for the in-plane ring deformation ν′₁₄(a₁) mode, which is significantly larger than 2 cm⁻¹ for the vibrationless state.     

Vanadium(II) alkyls. Synthesis and X-ray crystal structures of trans-VMe2(dmpe)2 and cis-V(CH2SiMe3) 2(dmpe)2

Treatment of the vanadium(II) tetrahydroborate complex trans-V(η¹-BH₄)₂(dmpe)₂ with (trimethylsilyl) methyllithium gives the new vanadium(II) alkyl cis-V(CH₂SiMe₃)₂(dmpe)₂, where dmpe is the chelating diphosphine 1,2-bis(dimethylphosphino)ethane. Interestingly, this complex could not be prepared from the chloride starting material VCl₂(dmpe)₂. The CH₂SiMe₃ complex has a magnetic moment of 3.8 μ_B, and has been characterized by ¹H NMR and EPR spectroscopy. The cis geometry of the CH₂SiMe₃ complex is somewhat unexpected, but in fact the structure can be rationalized on steric grounds. The X-ray crystal structure of cis-V(CH₂SiMe₃)₂(dmpe)₂ is described along with that of the related vanadium(II) alkyl complex trans-VMe₂(dmpe)₂. Comparisons of the bond distances and angles for VMe₂(dmpe) ₂, V---C = 2.310(5) Å, V---P = 2.455(5) Å, and P---V---P = 83.5(2)° with those of V(CH₂SiMe₃)₂(dmpe)₂, V---C = 2.253(3) Å, V---P = 2.551(1) Å, and P ---V---P = 79.37(3)° show differences due to the differing trans influences of alkyl and phosphine ligands, and due to steric crowding in latter molecule. The V---P bond distances also suggest that metal-phosphorus π-back bonding is important in these early transition metal systems. Crystal data for VMe₂(dmpe)₂ at 25°C: space group P2₁/n, with a = 9.041(1) Å, b = 12.815(2) Å, c = 9.905(2) Å, β = 93.20(1)°, V = 1145.8(5) ų, Z = 2, R_F = 0.106, and R_wF =0.127 for 74 variables and 728 data for which I 2.58 σ(I); crystal data for V(CH₂SiMe₃)₂(dmpe)₂ at −75°C: space group C2/c, with a = 9.652(4) Å, b = 17.958(5) Å, c = 18.524(4) Å, β = 102.07(3)°, V= 3140(3) ų, Z = 4, R_F = 0.033, and R_wF = 0.032 for 231 variables and 1946 data for which I 2.58 σ(I).     

A density functional theory study on boundary of “superreduced” transition metal carbonyl anions [M(CO)n](M=Cr, n=5, 4, 3, z=2, 4, 6; M=Mn, n=5, 4, 3, z=1, 3, 5; M=Fe, n=4, 3, 2, z=2, 4, 6; M=Co, n=4, 3, 2, z=1, 3, 5)

A nonlocal density functional theory (DFT) method has been applied to the calculations on optimized geometry, Mulliken atomic net charges and interatomic Mulliken bond orders as well as total bonding energies (E) in the binary transition metal carbonyl anions with different reduced states [M(CO)_n]^z− (M=Cr, n=5, 4, 3, z=2, 4, 6; M=Mn, n=5, 4, 3, z=1, 3, 5; M=Fe, n=4, 3, 2, z=2, 4, 6; M=Co, n=4, 3, 2, z=1, 3, 5). For comparison of relative stability, a relative stabilization energy D is defined as D=E([M(CO)_n]^z−)−nE(CO). The calculated C–O distances are lengthened monotonously with the increase of the anionic charge, but the M–C distances are significantly lengthened only in the higher reduced states. The relative stabilization energy calculated is a considerable negative value in the lower reduced states, but a larger positive value in the higher reduced states. The DFT calculations show that with the increase of the anionic charge, the Mulliken net charges on the M, C, and O atoms all increase, however, an excess of the anionic charge is mainly located at the central metal atom. The calculated C–O Mulliken bond orders decrease consistently with the increase of the anionic charge, but the M–C bond orders exhibit an irregular behavior. However, the total bond orders calculated clearly explain the higher reduced states to be considerably unstable. From analysis of the calculated results, it is deduced that the stability of the binary transition metal carbonyl anions [M(CO)_n]^z− studied are associated with the coordination number n and the anionic charge z, further, it is possible for the anions studied to be stable if n≥z, conversely, it is impossible when n<z.     

Emission spectra of Cs2NaTbCl6 and Cs2NaYCl6: Tb3+

The emission spectra of microcrystalline Cs₂NaTbCl₆ and Cs₂Na(Y_0.99Tb_0.01)Cl₆ have been measured at room temperature and at 77 K. The crystal structures of these compounds are face-centered cubic and the terbium (III) ions lie at sites of octahedral (O_h) symmetry surrounded by six chloride ions. Emission is observed from both the ⁵D₃ and ⁵D₄ excited states of Tb³⁺. Assignments have been made for nearly all of the magnetic-dipole transitions split out of the Tb³⁺⁷F₆, ⁷F₅, ⁷F₄, ⁷F₃, ⁷F₂, ⁷F₁ ← ⁵D₄ and ⁷F₄, ⁷F₂ ← ⁵D₃ transitions. These assignments are based on the calculated transition energies and relative magnetic-dipole strengths and intensities obtained from a weak-field crystal-field analysis of octahedral TbCl₆^3? units. Magnetic-dipole lines dominate the spectra for transitions of ΔJ = ±1 free-ion parentage, whereas both magnetic-dipole lines and vibronically induced electric-dipole lines contribute significantly to the emission intensities of the ΔJ = 0, ±2 transitions. The crystal-field sub-levels of both ⁵D₃ and ⁵D₄ appear to reach a Boltzmann thermal equilibrium prior to emission. Emission from ⁵D₃ is partially quenched in going from low temperature to high temperature and in going from Cs₂NaYCl₆: Tb³⁺ (1%) to Cs₂NaTbCl₆.This study has led to the identification and assignment of nearly all of the pure magnetic-dipole transitions split out of the Tb³⁺⁷F₆, ⁷F₅, ⁷F₄, ⁷F₃, ⁷F₂, ⁷F₁ ← ⁵D₄ and ⁷F₄, ⁷F₂ ← ⁵D₃ transitions in crystal-line Cs₂NaTbCl₆. The assignments were based on calculated transition energies and relative magnetic-dipole strengths (and intensities) obtained from a (weak-field) crystal-field analysis of octahedral (O_h) TbCl₆^3? clusters. Excellent agreement between the calculated and observed relative intensities of the magnetic-dipole lines was achieved by assuming a Boltzmann equilibrated set of crystal-field sub-levels for both the ⁵D₄ and ⁵D₃ emitting states. Furthermore, the experimental results suggest that ⁵D₄ ← ⁵D₃ relaxation is temperature-dependent.The energy levels calculated and displayed in table 1 appear to be qualitatively correct and are in semiquantitative agreement with the emission results (as interpreted in section 4). Calculated and observed transition energies for the assigned magnetic-dipole transitions generally agree to within 0.2%.One of the most remarkable features of the emission spectra obtained on Cs₂NaTbCl₆ is the absence of any vibrational structure in the ΔJ = ± 1 transitions (⁷F₆, ⁷F₃ ← ⁵D₄ and ⁷F₄, ⁷F₂ ← ⁵D₃), and the presence of extensive vibrational structure in the ΔJ = O, ±2 transitions (⁷F₆, ⁷F₄, ⁷F₂ ← ⁵D₄). If other than OO vibronic transitions do contribute to the ΔJ = ±1 emissions, their intensities must be at least two or three orders-of-magnitude weaker than the OO magnetic-dipole lines. Vibronically induced electric-dipole transitions appear, however, to make substantial contributions to the ⁷F₆, ⁷F₄, ⁷F₂ ← ⁵D₄ emission spectra. A clear-cut theoretical explanation for the absence of vibrational structure in the ΔJ = ±1 transitions is not readily apparent. We are presently examining this problem in greater detail.     

Syntheses and Structure Determination of Nine-Coordinate (NH4)3[TbIII(nta)2(H2O)]·4H2O and Eight-Coordinate (NH4)3[ErIII(nta)2] Complexes

Syntheses and structure determination of Tb^III and Er^III complexes with nitrilotriacetic acids (nta) are reported. Their crystal and molecular structures, molecular formulas, and compositions were determined by single-crystal X-ray structure analyses and elementary analyses, respectively. The crystal of the (NH₄)₃[Tb^III(nta)₂(H₂O)]·4H₂O complex belongs to the monoclinic crystal system and C2/c space group. Crystal data are as follows: a = 16.357(8) Å, b = 8.552(4) Å, c = 17.390(9) Å, β = 104.748(7)°, V = 2352.6(19) ų, Z = 4, Mr = 675.32, D_c = 1.932 g·cm⁻³, μ = 3.112 mm⁻¹, and F(000) = 1368. The final R and Rw are 0.0220 and 0.0494 for 2357 (I > 2σ(I)) unique reflections, R and Rw are 0.0266 and 0.0510 for all 5613 reflections, respectively. The Tb^IIIN₂O₇ moiety in the [Tb^III(nta)₂(H₂O)]³⁻ complex anion has a pseudo-monocapped square antiprismatic nine-coordinate structure, in which the eight coordinate atoms (two N and six O) are from two nta ligands and the water molecule is coordinated to the central Tb^III ion directly as the ninth coordinate atom. The crystal of the (NH₄)₃[Er^III(nta)₂] complex belongs to the trigonal crystal system and R-3^c space group. Crystal data are as follows: a = 7.9181(16) Å, b = 7.9181(16) Å, c = 54.27(2) Å, γ = 120°, V = 2946.7(14) ų, Z = 6, Mr = 597.61, D _c = 2.021 g·cm⁻³, μ = 4.345 mm⁻¹, and F(000) = 1770. The final R and Rw are 0.0295 and 0.0673 for 677 (I > 2σ(I)) unique reflections, R and Rw are 0.0366 and 0.0700 for all 4827 reflections, respectively. The Er^IIIN₂O₆ part in the [Er^III(nta)₂]³⁻ complex anion is an eight-coordinate structure with a pseudo-dicapped octahedron, in which the eight coordinate atoms (two N and six O) are from two nta ligands.Original Russian Text Copyright © 2004 by J. Wang, X. D. Zhang, Y. Wang, Y. Zhang, Z. R. Liu, J. Tong, and P. L. Kang__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 6, pp. 1067–1075, November–December, 2004.     

Analysis of the 4800-Å absorption band of Cs2 by the classical method

The broad absorption band in Cs₂ having peak intensity near 4800 Å is analyzed through computational simulation of the experimental spectrum using the classical method. The absorption, which terminates in a weak satellite at 5223 Å, can be interpreted in terms of a single transition from the ground state (R_e = 4.65 Å, ω_e = 42 cm⁻¹) to an upper state having T_e = 20 470 cm⁻¹, ω_e = 33 cm⁻¹ and R_e = 5.28 Å. The absolute absorption strength is roughly consistent with published lifetime data, but its reliability is limited by thermodynamic uncertainties stemming from the remaining uncertainty in the Cs₂ ground state dissociation enegy. The paper includes a summary of diatomic radiation relations pertinent to the analysis of low-resolution spectra, and a brief discussion of the reduced potential method applied to the alkali dimer ground states.     

Structure determination from powder diffraction data and thermal behaviour of layered lead nitrate oxalate hydrate, Pb2(NO3)2(C2O4).2H2O

The mixed lead nitrate oxalate, Pb₂(NO₃)₂(C₂O₄).2H₂O, has been obtained in a polycrystalline form in the course of a study on precursors of nanocrystalline PZT-type oxides. Its crystal structure has been solved from powder diffraction data collected using a monochromatic radiation from a conventional X-ray source. The symmetry is monoclinic, space group P2₁/c (No. 14), the cell dimensions are a=10.623(2) Å, b=7.9559(9) Å, c=6.1932(5) Å, β=104.49(1)° and Z=4. The structure consists of a stacking of complex double sheets parallel to (1 0 0), forming layers held together by hydrogen bonds. The sheets result from the condensation of PbO₁₀ polyhedra, in which the oxalate and nitrate groups, as well as water molecules, play a major role. The structure is discussed in terms of Pb---O distances, polyhedra shape and lead coordination, with emphasis on the dimensional polymerisation role of water molecules. The thermal behaviour of this layered compound is carefully described from temperature-dependent powder diffraction and thermogravimetric measurements. The enthalpy, Δ_rH=232(3) kJ mol⁻¹, and entropy, Δ_rS=532(8) J K⁻¹ mol⁻¹, of the dehydration reaction have been determined. The high value of Δ_rH demonstrates that the water molecules are strongly bonded in the structure. The complex decomposition proceeds through the crystallisation and decomposition of Pb(NO₃)₂(C₂O₄) into Pb(NO₃)₂ and PbC₂O₄, and, finally, various lead oxides.     

Collisional fragmentation of Ar@C60

The fragmentation patterns resulting from collisions between (Ar@C₆₀)⁺ or (Ar@C₆₀)⁻ ions and H₂, He, CH₄, Ne, Ar and Kr target gases have been measured. The ion-source material Ar@C₆₀ was synthesized by heating C₆₀ under 3000 atm of argon gas, leading to a 10⁻³ concentration of endohedral fullerenes. The fragmentation spectra (charged molecules only) are dominated by positive ions both when positive or negative endohedrals break up. Endohedral fragment ions Ar@C_n⁺ (48n60) as well as all carbon fragments are observed. For collisions involving (Ar@C₆₀)⁻, ejection of the Ar atom together with two electrons, without permanently damaging the fullerene cage, is a prominent reaction channel, indicating that a ‘window' or a deformation in the form of e.g. a large hole, through which the argon atoms can exit, is opened during the collision.