Über Ordnungs-Unordnungsphänomene bei Sauerstoff perowskiten vom Typ A2+3B2+M5+2O9

Perovskites of the type A²⁺₃B²⁺M⁵⁺₂O₉, where A²⁺ = Ba, Sr; B²⁺ = Mn, Co, Ni, Zn; M⁵⁺ = Nb, Ta, show order-disorder phenomena. At lower temperatures a thermodynamically unstable disordered cubic perovskite is formed ($\text{1}\text{3}$ formula unit—$\text{AB}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{M}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{O}{}_3$—in the cell), which transforms irreversibly into a 1: 2 ordered high-temperature form with 3L structure (sequence (c)₃). For A²⁺ = Ba this lattice is hexagonal (space group $\text{P}\text{3}\text{m1}$; one formula unit in the cell); with A²⁺ = Sr a triclinic distortion is observed. For Ba₃CoNb₂O₉ a second transformation into a cubic disordered perovskite takes place at 1500°C. This transition is reversible and of the order-disorder type. The vibrational and diffuse reflectance spectra are discussed.     

Observation of X1A1 vinylidene by photoelectron spectroscopy of the C2H2− ion

Photoelectron spectra of the vinylidene anion (C₂H₂^?) show vibrational structure in X¹A₁ vinylidene up 12 kcal/ mol above the vibrational ground state. Analysis yields an EA(C₂H₂$\text{X}$¹ A₁) of 0.47 ± 0.02 eV, and frequencies for the CC stretch and HCH bend. Absence of the ³B₂ state in the photoelectron spectra indicates the ¹A₁-³B₂ splitting in vinylidene is ? 1.7 eV.     

Molecular predissociation dynamics revealed through multiphoton ionisation spectroscopy. I. The C1B1 states of H2O and D2O

The $\text{C}$¹B₁ states of H₂O and D₂O have been observed by means of three photon absorption (four photon ionisation) spectroscopy. Differences between the experimentally observed 3 + 1 multiphoton ionisation spectrum and that predicted by the appropriate asymmetric-top three-photon line-strength theory are attributed to $\text{C}$ state predissociation. Two separate predissociation mechanisms have been identified, one (heterogeneous) relying on a-axis parent molecular rotation to couple the bound B₁ state to an unbound state of A₁ electronic symmetry, the other (homogeneous) involving a second, dissociative excited electronic state of B₁ symmetry. Having established the detailed $\text{C}$ state predissociation dynamics, two photon absorption spectra of H₂O and D₂O ($\text{C}$ ← $\text{X}$) can be predicted accurately: studies of individual quantum-state-selected photofragmentation processes from H₂O($\text{C}$) are proposed.     

Fluorescence quantum yields and cascade-free lifetimes of state selected CO+2, COS+, CS+2 and N2O+ determined by photoelectron—photon coincidence spectroscopy

The details and principles of an apparatus built for measurements of fluorescence quantum yields and cascade-free lifetimes of open-shell cations are reported. These rely on the detection of coincidences between energy selected photoelectrons and undispersed photons. The results of such measurements for CO⁺₂, COS⁺, CS⁺₂ and N₂O⁺ in selected vibrational levels of their excited states are presented. Non-unity fluorescence quantum yields are found for some vibronic levels of CO⁺₂($\text{B}$), COS⁺ ($\text{A}$), N₂OP⁺($\text{A}$) and a non-exponential decay is observed for CS⁺₂($\text{A}$). The data yield the following values for the radiative lifetimes: CO⁺₂($\text{A}$) 124 ± 6 ns, CO⁺₂($\text{B}$) 140 ± 7 ns, COS⁺($\text{A}$) 550 ± 50 ns and N₂O⁺($\text{A}$) 240 ± 12 ns.     

Crystal-field calculation of the 3d multiplets of the binuclear Ni(II) monothiocarboxylates

The electronic spectra of the dimers (Ni(RCOS)₂)₂ and (Ni(RCOS)₂)₂ · C₂H₅OH are calculated by crystal-field theory in terms of different polyhedra of coordination around the two Ni²⁺ ions. The necessary radial parameters are taken from other clusters of Ni²⁺ after a detailed study of their intercluster transferability. The calculations are organized in two blocks. In the first one, the Ni²⁺Ni²⁺ interaction is neglected and the clusters Ni^AS₄, Ni^BO₄, and Ni^BO₅ are considered. In the second block, this interaction is explicitly taken into account as a point-charge perturbation produced by one of the two metal ions on the 3d states of the other one. Accordingly, we deal with the Ni^AS₄Ni^B, in which the Ni^B acts as a pointlike ligand of charge q, and with the Ni^BO₄Ni^A and the Ni^BO₅Ni^A, where the Ni^A is the punctual ligand. The calculation shows that the optical spectra and magnetic moments of these dimers can be best reproduced if $\text{q ? 0}$, i.e., when the metal-metal electrostatic interaction is negligible. The agreement with the experiments quickly disappears as q grows. This calculation supports the interpretation of Melson et al. (Inorg. Chem.9, 1116 (1970)) according to which the 3d electronic distribution of the Ni²⁺ ions in these dimers is probably controlled by their nearest neighbors rather than by the metal-metal interaction.     

A classification of metaborate crystals based on Raman spectroscopy

The Raman spectra of crystals built solely of metaborate triangles provide fingerprint identification of three distinct network types. Classified according to increasing cation field strength these are: rings with degenerate intra-annular bonds and D_3h symmetry, distorted rings with alternating intra-annular bonds and C_3h symmetry, as well as chains. The occurrence of each network type has been associated with a characteristic range of cationic field strength. This approach led to the discovery of a hitherto unknown C_3h ring strontium metaborate crystal, with Sr²⁺ cations in 9- or 10-fold coordination to oxygen atoms. The Raman spectra of the mixed cation metaborates Ba₂Ca(B₂O₄)₃ and Ba₂Mg(B₂O₄)₃ confirm their C_3h-ring structure and clearly point to the fact that the synergetic effect of dissimilar cations to the metaborate network cannot be predicted by the additivity of their field strengths.     

C2O(X3Σ−): absolute reaction rates measured by laser induced fluoresence

Absolute rate constants are reported for reactions of C₂O($\text{X}\text{?}$³Σ^?) under pseudo-first-order decay conditions. C₂O is generated by laser photodissociation of C₃O₂ at 266 nm, and detected by dye-laser induced fluorescence on the $\text{A}\text{?}$³Π_i-$\text{X}\text{?}$³Σ^? transition. Rate constants of (433 ± 12), (3.30 ± 0.12) and (1.12 ± 0.05) × 10^?13 cm³ molecule^?1 s^?1 are reported for reactions with NO, O₂ and isobutene. The NO value is approximate due to an apparent dark reaction between NO and C₃O₂. Upper limits of 1 × 10^?14 cm³ molecule^?1 s^?1 are reported for reactions with H₂, CO₂, C₃H₂ and C₂H₄. The C₂O + C₃O₂ reaction does not follow pseudo-first-order decay kinetics. Two explanations are proposed to explain this observation. Results are compared with previous relative rate measurements and are discussed in terms of their relevance to combustion chemistry.     

A contribution to the structural chemistry of A-type rare earth sesquioxides

La₂O₃ and Nd₂O₃ have been annealed at temperatures between 1000 and 1600°C for decontamination. Only products annealed at least at these temperatures appeared to be monophasic. Both X-ray powder and electron single-crystal diffraction revealed the A-type sesquioxide structure. No difference could be found in samples either quenched or slowly cooled to room temperature. Two space groups, $\text{P}\text{3}\text{m1}$ and $\text{P6}{}_3\text{mmc}$, are reported for the A-type structure. X-Ray powder studies seemed to support the latter on the evidence of extinctions. Electron diffraction from single crystals, however, indicated the space group $\text{P}\text{3}\text{m1}$, confirming the so-called Pauling structure, while $\text{P6}{}_3\text{mmc}$ can now be excluded unequivocally.     

Excited state dynamics of C2O(A3Πi)

The fluorescence decay and bimolecular electronic quenching behavior of C₂O ($\text{A}$³Π_i) is reported. C₂O($\text{X}$³Σ^?) is produced by laser photolysis of C₃O₂ at 266 nm and is subsequently excited by a tunable flashlamp pumped dye laser. The fluorescence decay is highly nonexponential and dominated by both short (≈ 15 μs) and long (50–250 μs) decay components. The long-lived emission, itself, is nonexponential. The fluorescence decay is modeled as the sum of three exponential components. The short-lived emission is quenched by C₃O₂ at higher than the gas kinetic rate while the long-lived fluorescence is quenched much less efficiently. Fluorescence quenching measurements are also reported for collisions with Ar, N₂ and O₂.     

Fluorescence spectra of laser-excited YO molecules in a H2O2Ar flame

CW dye laser induced fluorescence emission and thermal emission spectra of YO-molecules in a 1 atm H₂O₂Ar flame of 2430 K were recorded simultaneously. Narrow band laser excitation was applied to four rotational lines in the (1, 1) Q-branch of the A²Π_{$\text{3}\text{2}$}→X²Σ⁺ transition and broadband excitation was applied to several separate Q-branches of the A²Π_{$\text{1}\text{2}$,$\text{3}\text{2}$}→X²Σ⁺ transitions. From the differences between the fluorescence emission spectra and thermal emission spectra, we conclude that collisional de-excitation of an excited vibronic level takes place by vibrational relaxation, decay to the electronic ground state and by intermultiplet transfer in order of increasing probability.     

Bis(dialkylmetall)quadratate der elemente aluminium,gallium und indium

The reaction of squaric acid, H₂C₄O₄, with the trialkyl derivatives of aluminium, gallium and indium (R₃M with R = CH₃, C₂H₅) in a $\text{1}\text{2}$ molar ratio leads to bis(dialkylmetal) squarates. The Ga and In compounds dissociate in water forming R₂M⁺ and C₄O₄^2? ions. From these solutions the squarates crystallize as mono- and dihydrates, respectively. The vibrational spectra (IR and Raman) are discussed.     

The 1-6 weberite Na2Te2O7 and some observations on compounds with the weberite structure

The 1-6 weberite Na₂Te₂O₇ (Imm2, a = 7.233 Å, b = 10.104 Å, c = 7.454 Å) has been prepared by high-pressure synthesis. It is shown that $\text{a}$) (the mean unit-cell dimension per formula unit) of oxide weberites A₂B₂O₇ can be represented as a linear function of the effective ionic radii of A and B. The problem of the true space-group symmetry of weberite is discussed.     

The crystal structure of the 42-Å subcell of a layer structure with approximate composition Ba4Nb2S9

Platy crystals from the products of a mixture 4 Bas : 2 Nb : 5 S reacted at 1000°C have cell constants a = 13.754(3) Å, c = 83.73(2) Å, $\text{R}\text{3}\text{m}$. The reciprocal lattice had a pronounced subcell with dimensions a = 6.877(1) Å, c = 41.84(1) Å, same space group. Three dimensional X-ray diffraction data were collected using monochromatized Mo Kα radiation and of 5051 measured intensities 1892 were considered observed. From the set of observed intensities 611 reflections having all even indices were used to refine the crystal structure of the 42 × 7-Å subcell. The final R = 0.036 and ωR = 0.052 for the 611 observed amplitudes and R = 0.046, ωR = 0.052 for all 711 amplitudes of the subcell. The structure is based on the stacking of hexagonal BaS₃ layers with the sequence DABABDBCBCDCACAD. The D layer denotes a disordered level and occurs at $\text{z = 0,}\text{1}\text{3}$ and $\text{2}\text{3}$. The different letters for the ordered layers are based on the Ba positions in that layer. The Nb ions occupy octahedral interstices and form a unit of three face sharing octahedra parallel to c. The column is terminated above and below by disordered levels. The NbNb distances are 3.22 Å, causing displacement of Nb from the centers of the two outside octahedra. One Ba is in the center of a triangular orthobicupola formed by 12 S atoms. The other Ba is in the center of a hexagon of 6 S with 3 additional S above this layer forming $\text{1}\text{2}$ of a cuboctahedron. The lower half consists of a disordered layer of atoms. The NbS distances are 2.279, 2.433, and 2.683 Å; BaS distances vary between 3.1 and 3.5 Å. The subcell content based on the ordered structure only is Ba₁₂Nb₉S₃₆. The placement of disordered Ba and S at $\text{z = 0,}\text{1}\text{3}$, and $\text{2}\text{3}$ levels of the subcell leads to the unlikely composition Ba_16.5Nb₉S₄₂. The ordered structure most likely has a composition Ba₄Nb₂S₉, z = 36, so that the subcell composition should be Ba₁₈Nb₉S_40.5. The completely ordered structure has not been solved.     

The optical absorption and emission spectra of Cs2NaSmCl6

Optical absorption and emission spectra are reported for single crystals of the cubic elpasolite Cs₂NaSmCl₆. The variable temperature spectra obtained at high resolution are assigned using energies and relative intensities. Transitions from the ground level, ⁶H_{$\text{5}\text{2}$} to cystal fi levels of ⁶H_{$\text{7}\text{2}$-$\text{15}\text{2}$}, ⁶F_{$\text{1}\text{2}$-$\text{11}\text{2}$}, ⁴G_{$\text{5}\text{2}$-$\text{9}\text{2}$}, ⁴F_{$\text{3}\text{2}$,$\text{5}\text{2}$}, ⁴I_{$\text{9}\text{2}$}, and ⁶P_{$\text{3}\text{2}$, $\text{5}\text{2}$} are located and characterized. Intensity calculations are reported for magnetic dipole allowed transitions. The dominance of vibronic intensity in ⁶H_{$\text{5}\text{2}$}→⁶F _{$\text{1}\text{2}$-$\text{9}\text{2}$} and ⁶P_{$\text{3}\text{2}$, $\text{5}\text{2}$} transitions is accounted for qualitatively through the ligand polarization model involving quadrupole metal (Sm³⁺)-ligand (Cl^?) interaction mechanisms. The E_u″(⁶H_{$\text{5}\text{2}$})→E′(⁶H_{$\text{1}\text{2}$}) E_u′(⁶F_{$\text{1}\text{2}$}) no-phonon transition is postulated to be pure electric quadrupole allowed. The ground state magnetic moment is determined to be very small from magnetic circular dichroism (MCD) spectra.This study has led to the assignment of nearly all of the crystal field levels in the visible and IR region for Cs₂NaSmCl₆. A total of 27 such levels were identified, 17 from no-phonon transitions and the rest from vibronic transitions. The magnetic dipole intensity calculated using intermediate coupling O_h wavefunctions along with a crystal field analysis of the splitting pattern was used in the assignment of the levels. Vibronic bands were observed for all transitions and their vibrational symmetries were tentatively assigned. MCD data were used to determine the magnet moment of the ground state.     

Infrared and Raman spectra of lithium trihorate: Vibrational assignments and a correlation with its nonlinear optical activity

Infrared and polarized single crystal Raman spectra of lithium triborate (LBO; LiB₃O₅) are reported and analysed. All four (A₁, A₂, B₁ and B₂) symmetry species of the C_2v, point group isomorphic to the C_2v⁹(Pna2₁) space group are Raman active in distinct crystal polarization experiments. A complete set of symmetry assignments based on a factor group analysis is presented. The internal and external vibrations are separated on the basis of frequency and the observation that the magnitude of the correlation splitting for the internal modes is very small. A comparison of the internal frequencies of various borate species suggests a correlation between the B-O stretching frequency and the nonlinear optical efficiency of the material.     

Surprises in the structural chemistry of zeolites

High-resolution electron microscopy, apart from strikingly confirming the correctness of the X-ray-based models for the skeletal structure of the aluminosilicate frameworks of zeolites, points to the existence of new families of ordered, crystalline microporous solids (e.g., with composition A_xB_xC_m?xO_2m · nH₂O, where A is an exchangeable monovalent cation, B is Al or Ga, C is Si or Ge, and x, m, n are integers.) It also reveals crystalline imperfections and unexpected superlattice structures in A-type and faujasitic zeolites, and the nature of the intergrowths in, for example, $\text{ZSM-5}\text{ZSM-11}$ materials. The short-range order of Si and Al within the aluminosilicate framework may be directly explored by magic-angle-spinning NMR (MASNMR) employing ²⁹Si and ²⁷Al nuclei. This technique probes the site symmetry and environment of these atoms. Al in tetrahedral as well as in octahedral sites may be readily identified and so may the populations of groups such as Si(OAl)₄, Si(OAl)₃, (OSi), etc., so that new information is obtained pertaining to Si, Al ordering in a variety of zeolitic solids.     

Microwave-optical double resonance in molecular ions. Co+

Microwave—optical double resonance signals have been detected in a mass-selected ion-beam spectrometer for ¹²C¹⁶O⁺. With the optical excitation of fluorescence from the R₁ ($\text{1}\text{2}$) line of the (0,0) band of the A²Π_{$\text{3}\text{2}$} ← X²∑ transition of 20350.6 cm^?1, the microwave resonances occurred at 118101.8 ± 0.2 MHz and at 117694 ± 2 MHz corresponding to the N= 1, J = $\text{3}\text{2}$ ← N = 0, J = $\text{1}\text{2}$ and the N = 1, J = $\text{1}\text{2}$ ← N = 0, J = $\text{1}\text{2}$ transitions.     

A2Σ+ → X2Πi emission spectra of the phosphaethyne cations HCP+ and DCP+

Electron impact excited $\text{A}$²Σ⁺ → $\text{X}$²Π_i emission spectra of HCP⁺ and DCP⁺ have been observed. The spectra consist of short progressions in ν″₃. The 0 0⁰0 → 0 0¹0 bands have been studied under high resolution and rotational analyses carried out. Some of the more important derived constants are (in cm^?1) HCP⁺; ν^″₃ = 1150(10), A^″₀ = -146.97(3), B^″₀ = 0.6224(16), B^′₀ = 0.6690(17); DCP⁺; ν^″₃ = 1110(10), A^″₀ = -146.71(1), B^″₀ = 0.5284(2), B^′₀ = 0.5682(2).     

Isothermal decomposition of ternary oxide AxByOz on an isobar—Stability of perovskite ABO3 (A = La,Sm, Dy; B = Mn,Fe) in a reducing atmosphere

The isothermal decomposition of any ternary oxide A_xB_yO_z on liberation of n moles of oxygen at a constant pressure is found to be driven by the mixing entropy ΔS_m = ?nRln P_O2 of the total entropy change ΔS = ΔS° + ΔS_m. The stability of A_xB_yO_z towards isothermal decomposition into a biphasic solid mixture is derived from the equilibrium condition $\text{ΔG1 = 0}$ as functions of standard changes ΔH° and ΔS°. Assuming ΔS° = 44n and calculating ΔH° in terms of lattice energies U(ABO₃) and U(A₂O₃), the stability of perovskites is given as a function of the ionic radius of the A³⁺ ion. The calculated stability agrees well with that observed. The effect of electronic entropy change ΔS_e on ΔS° is demonstrated for AFeO₃ (A = La, Sm, Dy).     

Single-crystal growth and crystal structure refinement of CuAlO2

Single crystals of the delafossite-type compound CuAlO₂ were grown from Al₂O₃Cu₂O melt by a slow-cooling method from 1200°C. Three types were found in as-grown crystals (single crystals, short-columnar twin crystals with concave angles, and laminar twin crystals). The twinning form is similar to the spinel-type twin. CuAlO₂ is rhombohedral, $\text{R}\text{3}\text{m, a = 2.8604(7), c = 16.953(5)}\text{A}\text{?}$, Z = 3, Dx = 5.12 g/cm³ and Dm = 5.06 g/cm³. The crystal structure of CuAlO₂ was analyzed by means of single-crystal X-ray diffraction with a conventional R value = 0.038. The value of the U₁₁ component of the thermal parameter of Cu⁺ was twice as large as U₃₃.