Vibrational energy exchange between CO(v=1) and 14N2 and CO(v=1) and 15N2XXXured to 60 k in the gas phase

Laser fluorescence technique has been used to measure (VV) exchange rates between CO(v = 1) and ¹⁴N₂ and ¹⁵N₂ XXX At 65 K exchange to ₁₅N₂ has 65 times the higher rate constant. Comparison with data for (VV) exchange in liquid ₂ shows that the isolated binary collision theory does not hold for the case of CO(v = 1) and ¹⁴N₂.     

Vibrational energy transfer between 12C16O and 13C16O

The rate of the energy exchange process ¹²C¹⁶O (v= 1)+ ¹³C¹⁶O (v= 0) ? ¹²C¹⁶O (v= 0) +¹³C¹⁶O (v= 1) + 47.2 cm^?1 has been measured by laser-induced fluorescence, k₂ = (7.2 ± 0.3) × 10⁴ s^?1 Torr^?1 at 293 K. This result is in reasonable agreement with that predicted by theories based on long-range dipole-dipole interactions     

Ferromagnetic ordering in ThSi2 type CeAu0.28Ge1.72

The compound CeAu_0.28Ge_1.72 crystallizes in the ThSi₂ structure type in the tetragonal space group I4₁/amd with lattice parameters a=b=4.2415(6) Å c=14.640(3) Å. CeAu_0.28Ge_1.72 is a polar intermetallic compound having a three-dimensional Ge/Au polyanion sub-network filled with Ce atoms. The magnetic susceptibility data show Curie-Weiss law behavior above 50 K. The compound orders ferromagnetically at ∼8 K with estimated magnetic moment of 2.48 μ_B/Ce. The ferromagnetic ordering is confirmed by the heat capacity data which show a rise at ∼8 K. The electronic specific heat coefficient (γ) value obtained from the paramagnetic temperature range 15-25 K is∼124(5) mJ/ mol K². The entropy change due to the ferromagnetic transition is ∼4.2 J/mol K which is appreciably reduced compared to the value of R ln(2) expected for a crystal-field-split doublet ground state and/or Kondo exchange interactions.     

Kinetic study of the reactions Sn+N2O→SnO+N2

The optical pumping method of alkali molecules by atom—molecule exchange collisions is applied to obtain the magnetic shielding difference σ(Na) — σ(Na₂) = (29 ± 16) × 10^?6 between Na atoms and Na₂ molecules and the scalar nuclear spin—spin coupling constants d_s = (306 ± 30)s^?1 of ²³Na³⁹K.     

Optical dephasing of the S1 ← S0 transition of free-base porphin in an n-decane host studied by photochemical hole-burning: a case of slow exchange

The homogeneous width and frequency of S₁ ← S₀ 0-0 transitions of free-base porphin in site B of n-decane are studied by photochemical hole-burning (T = 1.2–4.2 K). A localized phonon mode of 7 cm^?1 is identified as a phonon sideband and holes burnt into it yield a lifetime of 115 ± 20 ps. The results are consistent with the exchange model for slow exchange.     

Dynamics of [Zn(D2O)6] in [Zn(D2O)6][SiF6] crystal as studied by 1D, 2D spectra and spin-lattice relaxation time of H NMR

The dynamics of [Zn(D₂O)₆]²⁺ in [Zn(D₂O)₆][SiF₆] was investigated by ²H NMR one-dimensional spectra, two-dimensional exchange spectra and spin-lattice relaxation time (T₁). The lineshapes of those spectra and T₁ were dominated by the 180° flip of the water molecules and the reorientation of [Zn(D₂O)₆]²⁺ about the C₃ axis. The variation of lineshape of the one-dimensional spectrum below room temperature can be explained by only the 180° flip of the water molecules. The spectrum at room temperature showed a typical shape due to the rapid 180° flip of water molecules. The change in lineshape of the one-dimensional ²H NMR spectrum is caused by the three-site jump of [Zn(D₂O)₆]²⁺ about its C₃ axis above 333 K. Information of the reorientation of [Zn(D₂O)₆]²⁺ below 333 K could not be obtained from the one-dimensional spectrum and T₁. In this temperature range, the two-dimensional exchange spectrum was effective for analysis of molecular motion. The effects of multiple motions, the 180° flip of the water molecules and the reorientation of [Zn(D₂O)₆]²⁺ about the C₃ axis, on the lineshape of the two-dimensional exchange spectrum were studied using spectral simulation.     

Thermal expansion in (La0.62Pb0.38)MnO3: Possible evidence for a coupling between elastic and magnetic exchange forces

Using single-crystal, automated diffractometer techniques, the linear coefficient of thermal expansion has been determined for La_0.62Pb_0.38MnO₃ from 298 to 627 K. The linear coefficient of thermal expansion is observed to undergo a change from 7.2 × 10^?5 Å/K for T < T_c to 10.8 × 10^?5 Å/K for T > T_c. It is concluded that while the rhombohedral distortion in the (La, Pb)MnO₃ system can be understood qualitatively on the basis of ionic size and polarizability considerations alone, the quantitative systematics of the distortion parameters and the change in the linear thermal expansion coefficient at T_c indicate a significant coupling between the elastic and magnetic exchange forces.     

Synthesis, structure and magnetic properties of Na6Co2O6

Na₆Co₂O₆ was synthesized via the azide/nitrate route by reaction between NaN₃, NaNO₃ and Co₃O₄. Stoichiometric mixtures of the starting materials were heated in a special regime up to 500°C and annealed at this temperature for 50 h in silver crucibles. Single crystals have been grown by subsequent annealing of the reaction product at 500°C for 500 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X-ray analysis of the crystal structure (, Z=1, a=5.7345(3), b=5.8903(3), c=6.3503(3) Å, α=64.538(2), β=89.279(2), γ=85.233(2)°, 1006 independent reflections, R₁=8.34% (all data)), cobalt is tetrahedrally coordinated by oxygen. Each two CoO₄ tetrahedra are linked through a common edge forming Co₂O₆^6- anions. Cobalt ions within the dimers, being in a high spin state (S=2), are ferromagnetically coupled (J=17 cm^-1). An intercluster spin exchange (zJ′=−4.8 cm^-1) plays a significant role below 150 K and leads to an antiferromagnetically ordered state below 30 K. Heat capacity exhibits a λ-type anomaly at this temperature and yields a value of 19.5 J/mol K for the transition entropy, which is in good agreement with the theoretical value calculated for the ordering of the ferromagnetic-coupled dimers. In order to construct a model for the spin interactions in Na₆Co₂O₆, the magnetic properties of Na₅CoO₄ have been measured. This compound features isolated CoO₄ tetrahedra and shows a Curie-Weiss behavior (μ=5.14 μ_B, Θ=−20 K) down to 15 K. An antiferromagmetic ordering is observed in this compound below 10 K.     

Theoretical study of intra- and inter-chain magnetic interactions in [Ni(chxn)2Br]Br2

The intra- and the inter-chain magnetic interactions in [Ni(chxn)₂Br]Br₂, which is one of the typical one-dimensional (1-D) MX complexes are examined by using an unrestricted hybrid DFT (UB3LYP) method. Calculated effective exchange integral (J) value along the 1-D chain is 2J_Intra = −4016 K and is close to an experimental result (−3600 K). On the other hand, a very weak anti-ferromagnetic inter-chain interaction through Br⁻ ions is observed. The value is estimated to be 2J_Inter = −2 to −6 K. In addition to the J values, transfer integral (t), on-site Coulomb repulsion (U) and charge transfer energy (E^CT) values along the 1-D chain are also estimated to be 0.46, 2.46 and 0.6 eV, respectively.     

Thermal and physical properties of hollandite-type K1.3Mn8O16 and (K,H3O)xMn8O16

Two kinds of samples of cryptomelane: synthetic single crystals of K_1.33Mn₈O₁₆ (A) and (K,H₃O)_xMn₈O₁₆ powder prepared by aqueous chemistry (B) were studied by thermogravimetry, magnetic, and electrical (dc and ac) measurements. B loses water at 100–185°C. A and B are decomposed in the range 460–610°C into Mn₂O₃ in air and MnO, under vacuum. They are antiferromagnetic with T_N 18 K (A), 11 K (B). A is a semiconductor with σ(300 K) ～ 3 Ω^?1 m^?1 and E_σ = 0.38 eV. The ac measurements did not reveal any significant contribution of ionic conduction up to 740 K.     

Neutron powder diffraction and magnetic measurements on TlMnI3 and TlFeI3

TlMnI₃ and TlFeI₃ are isostructural with NH₄CdCl₃. TlMnI₃ has a spiral structure which can be described with an incommensurable vector k, in the direction of the b1 axis of length 0.3614(5)b1. The spins lie in the (0 0 1) plane. TlMnI₃ exhibits antiferromagnetic behavior with a Néel temperature of 6.0(2) K. The exchange interaction was calculated to be $\text{zJ}\text{k}\text{= ?1.6}\text{K}\text{, z}$ being the number of nearest neighbors. Discontinuities in the magnetization are found for both the [1 0 0] and [0 1 0] directions at fields H^a_SF = 30.1(2) kOe and H^b_SF = 14.1(2) kOe. The magnetic structure of TlFeI₃ consists of puckered ferromagnetic (1 0 0) planes, which are coupled antiferromagnetically. The magnetic moments are parallel to the b axis. The Néel temperature is 21.5(3) K. $\text{zJ}\text{k}$ was found to be ?10(1) K with g = 2.68 and s = 2. The magnetic structures found for TlMnI₃ and TlFeI₃ are derived taking into account inter- and intra-double-chain interactions via two I^? ions.     

Defects and Luminescence Behavior of Cubic F23 [(NH4(18-Crown-6))4MnX4] [TlX4]2 (X=Cl, Br) Crystals

Luminescence from [(NH₄(18-Crown-6))₄MnBr₄][TlBr₄]₂ (1), [(NH₄(18-Crown-6))₄MnCl₄][TlCl₄]₂ (2), [(NH₄(18-Crown-6))₂MnBr₄] (3), and [(NH₄(18-Crown-6))₂MnCl₄] (4) was studied in search of new insights regarding crystal defects in 2. Emission from 3 and 4 is normal Mn²⁺(⁴T₁(⁴G)→⁶A₁); that of 2 (λ_max≈520 nm at ca. 300 K and 560 nm at 77 K) is unusual and temperature dependent. Thermal barriers (kJ/mol, assignment): green emission of 1 and 2, T<150 K (1-2, NH⁺₄ rotations), 150<T<250 K (7-14, energy migration among [MnX₄]²⁻), 250<T<300 K (26-35, rotations of 18-Crown-6)); yellow emission of 2: T;<250 K (7-8, energy migration among [MnX₄]²⁻), T>250 K (29 kJ/mol, defect-to-Mn²⁺(⁴T₁(⁴G)) back energy transfer). Crystal data for 4: Space group P2₁/c; Z=4; a=20.173(1) Å; b=9.0144(8) Å; c=20.821(1) Å; β=98.782(5)°; V=3741.9(8) ų; R_w=0.059; R=0.054.     

Thermal properties of the pyrochlore, Y2Ti2O7

The thermal conductivity and heat capacity of high-purity single crystals of yttrium titanate, Y₂Ti₂O_7, have been determined over the temperature range 2 K?T?300 K. The experimental heat capacity is in very good agreement with an analysis based on three acoustic modes per unit cell (with the Debye characteristic temperature, θ_D, of ca. 970 K) and an assignment of the remaining 63 optic modes, as well as a correction for C_p−C_v. From the integrated heat capacity data, the enthalpy and entropy relative to absolute zero, are, respectively, H(T=298.15 K)−H₀=34.69 kJ mol⁻¹ and S(T=298.15 K)−S₀=211.2 J K⁻¹ mol⁻¹. The thermal conductivity shows a peak at ca. θ_D/50, characteristic of a highly purified crystal in which the phonon mean free path is about 10 μm in the defect/boundary low-temperature limit. The room-temperature thermal conductivity of Y₂Ti₂O₇ is 2.8 W m⁻¹ K⁻¹, close to the calculated theoretical thermal conductivity, κ_min, for fully coupled phonons at high temperatures.     

NMR study of hindered rotation in solid (ND4)2GeF6

Nuclear spin-lattice relaxation times T₁ for deuterons and ¹⁹F nuclei in polycrystalline (ND₄)₂GeF₆ were measured by the pulse method at 8 MHz between 40 K and 300 K and between 4 K and 400 K, respectively. Correlation times and activation energies for the reorientational motions of ND₄⁺ and GeF₆^2? ions were calculated from the measured T₁ values.     

Ionic conductivity in Na+, K+, and Ag+ β″-alumina

This paper presents measurements of the ionic conductivity in single crystals of β″-alumina (0.84 M₂O · 0.67 MgO · 5.2 Al₂O₃, M = Na, K, Ag). Single crystals of sodium β″-alumina were grown from a melt of Na₂O, MgO, and Al₂O₃ at 1660 to 1730°C. Selected crystals were converted to the other isomorphs by ion exchange. The conductivity of sodium β″-alumina varies from 0.18 to 0.01 (ohm · cm)^?1 at 25°C depending upon crystal growth conditions. Potassium β″-alumina has the unusually high room temperature conductivity of 0.13 (ohm · cm)^?1. Silver β″-alumina has a slightly lower conductivity, 4 × 10^?3 (ohm · cm)^?1 at 25°C. The activation energies of sodium and potassium β″-alumina decrease with increasing temperature, while that of silver β″-alumina is constant from ?80 to 450°C.     

Structure and properties of a novel cobaltate La0.30CoO2

The layered cobaltate La_0.30CoO₂ was prepared from Na_xCoO₂ precursor by a solid-state ionic exchange and was characterized by means of X-ray and neutron diffraction, magnetic, thermal and electric transport measurements. The compound consists of hexagonal sheets of edge-sharing CoO₆ octahedra interleaved by lanthanum monolayers. Compared to Na⁺ in the parent system, the La³⁺ ions occupy only one-third of available sites, forming a 2-dimensional superstructure. The deviation from the ideal stoichiometry La_1/3CoO₂ introduces extra hole carriers into the diamagnetic LS Co³⁺ matrix making the sample Pauli paramagnetic. The temperature dependence of the electrical conductivity in La_0.30CoO₂ follows Mott's T^−1/3 law up to about 400 K, which is in contrast with the standard metallic behavior in the Na⁺ homolog possessing the same formal doping. The experiments are complemented by electronic structure calculations for La_0.30CoO₂ and related Na_xCoO₂ systems.     

Synthesis, structure and magnetic properties of the new mixed-valence vanadate Na2SrV3O9

The new complex oxide Na₂SrV₃O₉ was synthesized and investigated by means of X-ray diffraction, electron microscopy and magnetic susceptibility measurements. This oxide has a monoclinic unit cell with parameters a=5.416(1) Å, b=15.040(3) Å, c=10.051(2) Å, β=97.03(3)°, space group P2₁/c and Z=4. The crystal structure of Na₂SrV₃O₉, as determined from X-ray single-crystal data, is built up from isolated chains formed by square V⁴⁺O₅ pyramids. Neighboring pyramids are linked by two bridging V⁵⁺O₄ tetrahedra sharing a corner with each pyramid. The Na and Sr atoms are situated between the chains. Electron diffraction and HREM investigations confirmed the crystal structure. The temperature dependence of the susceptibility indicates low-dimensional magnetic behavior with a sizeable strength of the magnetic intra-chain exchange J of the order of 80 K, which is very likely due to superexchange through the two VO₄ tetrahedra linking the magnetic V⁴⁺ cations.     

Upper limits for the rate constant for the reaction Br + H2O2 → HBr + HO2

Upper limits for the rate constant for the reaction Br + H₂O₂ → HBr + HO₂ have been measured over the temperature range 298 to 417 K in a discharge flow, system using a mass spectrometer as a detector. Results are K₁< 1.5 × 10^?15 cm³ s^?1 at 298 K and K₁< 3.0 × 10^?15 cm³ s^?1 at 417 K, respectively. The implication to Stratospheric chemistry is discus     

Interactions magnétiques dans des groupements binucléaires [Fe2O7]8− au sein de Na8Fe2O7

The magnetic interaction in the structural units [Fe₂O₇]^8?, built of two corner-sharing FeO₄ tetrahedra, in Na₈Fe₂O₇ ($\text{Na}{}_2\text{O}\text{Fe}{}_2\text{O}{}_3\text{=}\text{4}\text{1}$) has been studied by magnetic susceptibility measurements (4.2–500 K). An exchange integral $\text{J}\text{K}{}_{\mathrm{<mtext>B</mtext>}}$ of ?37 K is obtained by comparison of the experimental values and the calculated ones using a Heisenberg-Dirac-Van Vleck-type Hamiltonian ? = $\text{?2J}\text{S}\text{?}{}_1\text{S}\text{?}{}_2$. The hypothesis of magnetically isolated [Fe₂O₇]^8? groups is corroborated by Mössbauer spectroscopy between 1.5 and 77 K. The susceptibility measurements of the solid solutions Na₈Fe_2?xM_xO₇ (M = Al, Ga; 0 ≤ x ≤ 0.2 for Al; 0 ≤ x ≤ 2 for Ga) leads to the same conclusion of the existence of isolated Fe³⁺Fe³⁺ pairs in Na₈Fe₂O₇. The type of substitution of Fe by Al or Ga is determined; homonuclear Fe³⁺Fe³⁺ and M³⁺M³⁺ pairs and heteronuclear Fe³⁺M³⁺ pairs are formed.     

Interpretation of the optical dephasing time and lineshape function in the S0 → T1 exciton transitions of 1,4-dibromonaphthalene

The lineshape function for the S₀ → T₁ absorption in 1,4-dibromonaphthalene (DBN) is analyzed in terms of exchange theory. It is shown that the dominant optical dephasing mechanism for the electric dipole transition to the k = 0 state in the band results from the absorption and emission of a low energy optic phonon. This process dephases the optical absorption because of frequency differences of the phonon in the ground and excited state. In addition, it is shown how to extract the energy of the phonon responsible for dephasing, the phonon absorption rate, and the lifetime in the phonon promoted state from the data. The analysis of the data for DBN shows that very little dephasing of the optical transition occurs before ≈ 15 K but from 15 K to ≈ 40 K the singlet-triplet transitions to site I (20192 cm^?1) and site II (20245 cm^?1) are dephased by absorption and emission of an ≈ 38 cm^?1 and 45 cm^?1 phonon respectively. The phonon absorption rates by the k = 0 state in the exciton band are similar for both sites being 5 × 10⁶ s^?1 and 3 × 10⁵ s^?1 at 4 K and 7 × 10¹¹ s^?1 and 4 × 10¹¹ s^?1 at 30 K for site I and II respectively. Finally, the lifetimes in the phonon promoted state for sites I and II are 0.23 and 0.28 ps over the range 15–40 K.