Search

A recently developed combinatorial method utilizing angular dependence of evaporation rate was used to create compositional spread thin film libraries of Tris(2-pyridin-2-yl-indolizino[3,4,5-ab] isoindole-C(1), N('))iridium(III) [Ir(pin)(3)] and 4,4(')-N,N(')-dicarbazol-biphenyl (CBP) composite, with the molar fraction of Ir(pin)(3) complex varying in the 0.0003chi(Ir(pin)(3) )0.96 range. Spatially correlated absorbance and photoluminescence (PL) measurements were used to quantify luminescence quenching of photo-excited CBP molecules by the Ir(pin)(3) complex as a function of the average distance between two molecules. It was found that the CBP--Ir(pin)(3) energy transfer proceeds by the Forster mechanism with the Forster radius of 30 A. The CBPxIr(pin)(3) composite has the highest photoluminescence quantum efficiency approximately 0.95, for chi(Ir(pin)(3) )=0.03 and is characterized by a structured green emission (lambda(max)=538 nm) originating from the ligand-centered (pi-pi(*))(3) state of the Ir(pin)(3) complex. On the contrary, the PL spectra of Ir(pin)(3) bulk are characterized by a weak red emission (lambda(max)=673 nm) attributed to the lowest metal-to-ligand charge transfer state. A statistical analysis based on a binomial distribution indicates that the emission from the (pi-pi(*))(3) state is quenched in Ir(pin)(3) molecules that are in a direct contact with each other.

A series of mixed sodalite samples, Na(8)[Al(6)Si(6)O(24)]Br(x).(H(3)O(2))(2-x), with the unit cell stoichiometries varying in the 0 < x

We examine the effects of monoderivatization on the electronic properties of C60. For this we chose the phenyl-C61-octanoic acid cholesteryl ester, [6,6]PCOCr, whose nonlinear optical properties have been investigated in the past. While the optical absorption spectrum of this methano fullerene is similar to that of C60, substantial differences are observed upon doping with potassium. Similarly, the doping-dependent conductivity of the functionalized fullerene shows two maxima as opposed to the single maximum for C60. The experimental observations are consistent with the doping-induced degeneracy removal of the parent C60 LUMO (t1u) orbital, which in potassium-doped methanofullerene splits into two components separated by about 0.5 eV. We provide experimental evidence that the doping of [6,6]PCOCr proceeds, as in C60, with six consecutive reduction (electron transfer) steps, yielding K6[6,6]PCOCr stoichiometry at the end. The transport in partially doped [6,6]PCOCr thin films occurs by thermally activated hopping of the charge carriers with activation energy Ea~=0.25 eV and hopping probability proportional to the number of unpaired electrons in the reduced molecule.

The photoinduced charge transfer between tetracyano-anthraquino-dimethane (TCAQ) derivatives and poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO−PPV) has been studied by photoinduced absorption (PIA) spectroscopy in the VIS and IR spectral region and by light induced electron spin resonance (LESR). Studies on three different TCAQ derivatives with different side chain substitutions are reported. Among them a supermolecule with TCAQ attached to a fullerene have been synthesized to serve as tandem electron acceptor. The photoinduced absorption in the Vis/near-IR range shows a broad plateau around 1.8 eV followed by two peaks at 1.35 and 1.24 eV with an additional broad feature below 0.5 eV for all three acceptors in composite with MDMO−PPV. All PIA features have a power law excitation intensity dependence with an exponent close to 0.5 as expected for bimolecular kinetics. LESR studies show one absorption at a g-factor of 2.0028 with a width (peak-to-peak) of 3.5 G, originating from TCAQ ...

N. Ulman, S. G. Boxer, Science 1997, 275, 651. c) W. Knoll, Annu. Rev. Phys. Chem. 1998, 49, 569. [3] Crystallization of Nucleic Acids and Proteins: A Practical Approach (Eds: A. Ducruix, R. Giege), Oxford University Press, Oxford 1992. [4] S. D. Durbin, G. Feher, Annu. Rev. Phys. Chem. 1996, 47, 171. [5] O. D. Velev, E. W. Kaler, A. M. Lenhoff, unpublished. [6] See for example: a) D. E. Koppel, in Fast Methods in Physical Biochemistry and Cell Biology (Eds: R. I. Sha'afi, S. M. Fernandez), Elsevier, New York 1983, pp. 339±367. b) M. Edidin, in Mobility and Proximity in Biological Membranes (Eds: S. Damjanovich, J. SzoE llosi, L. Tron, M. Edidin), CRC Press, Boca Raton, FL 1994, pp. 109±135. [7] a) C. V. Kumar, A. Buranaprapuk, G. J. Opiteck, M. B. Moyer, S. Jockusch, N. J. Turro, Proc. Natl. Acad. Sci. USA 1998, 95, 10 361. b) C. V. Kumar, A. Buranaprapuk, J. Am. Chem Soc. 1999, 121, 4262. [8] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press, New York 1983, pp. 341±381. [9] I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, New York 1971, pp. 383±385. [10] See for example: a) A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, S. Chu, Opt. Lett. 1986, 11, 288. b) S. Chu, Science 1991, 253, 861. c) D. G. Grier, Curr. Opin. Colloid Interface Sci. 1997, 2, 264. d) A. Ashkin, Proc. Natl. Acad. Sci. USA 1997, 94, 4853. [11] a) M. Ishikawa, H. Misawa, N. Kitamura, H. Masuhara, Chem. Lett. 1993, 481. b) J. Hotta, K. Sasaki, H. Masuhara, Y. Morishima, J. Phys. Chem. B 1998, 102, 7687. c) T. A. Smith, J. Hotta, K. Sasaki, H. Masuhara, Y. Itoh, J. Phys. Chem. B 1999, 103, 1660.

We report amplified spontaneous emission (ASE) from a cylindrical capillary structure comprised of a poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) film deposited on the inside wall of a glass capillary tube. When pumped with a stripe-shaped pulsed laser beam focused onto the capillary edge and parallel to its axis, a collimated and circular-shaped emission emerges from the capillary. The threshold pump power for ASE is about 6 kW/cm 2 . In the capillary configuration, the emission is unpolarized with spectral linewidth of 8 nm, centered at 638 nm. The glass capillary seems to play an important role in focusing the excitation beam, in waveguiding the emitted radiation, and in protecting the polymer from photo-oxidation.

The structure of sodium electrosodalite (SES), Na{sub 8}(AlSiO{sub 4}){sub 6}, has been determined at 20 K using synchrotron powder diffraction. Subsequently the electron density was calculated through a periodic unrestricted Hartree-Fock approach and analyzed by topological methods. The F center is found to manifest itself as a maximum in the electron density at a non-nuclear position. Thus it possesses a separate identity and behaves quantum mechanically as an open system, bounded by a surface of local zero flux in the gradient vector field of the electron density. Different basis sets have been considered, and the introduction of a basis set capable of describing the F center leads to a large drop in the total energy. The F center contains almost solely unpaired electron density which is loosely bound and exhibits a very low kinetic energy density. Calculations on both a ferromagnetic and an antiferromagnetic phase have been performed and the total electron densities in the two phases are found to be very similar, with the alternating ordering of the spin density being the only difference between the two phases. The electron localization function has been introduced for an open-shell system and has been used to illustrate the magnetic phase transition.more » {copyright} {ital 1999} {ital The American Physical Society}« less

Evidence for a bcc lattice of $F$ centers in sodium-electro-sodalite (SES), synthesized by exposing dehydrated sodalite to sodium vapor, is presented. A high electron spin density in SES produces isotropic contact shifts in nuclear magnetic resonance (NMR) spectra of the framework nuclei whose magnitude is a discrete function of local electron density. Strong exchange coupling between unpaired electrons gives rise to an antiferromagnetic phase transition in SES at ${T}_{N}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}48\ifmmode\pm\else\textpm\fi{}2\mathrm{K}$, providing the first example of an $s\ensuremath{-}\mathrm{electron}$ antiferromagnet.

Buckminsterfullerene, C{sub 60}, has unique molecular structure which leads to unusual physical and chemical properties. Using a surface forces apparatus, the static adhesion and dynamic friction forces between two smooth crystalline surfaces was measured. The results and analysis suggest that C{sub 60} molecules interact more as macroscopic particles than as conventional molecules. 39 refs., 6 figs.

THE electronic and geometric properties of C-60 can be perturbed by replacing one or more carbon atoms of the fullerene skeleton with an atom of a different element. Exchange of one carbon atom with nitrogen, a trivalent atom with a lone pair of electrons, produces the azafullerene radical C59N; which is isoelectronic with the C-60 radical anion. The process is slightly similar to doping silicon with phosphorus(1). We have previously described the synthesis of the azafullerene dimer(2); here we report the bulk preparation of the simplest azafullerene, C59HN. The electronic, vibrational and C-13 NMR spectroscopic features of C59HN are similar to those of the dimer(2), except for the signature of the sp(3) (C-H) carbon. C59HN should open the door to a new chemistry of heterofullerenes.

Prolonged exposure of colorless dry sodalite to alkali vapor causes the material to gradually turn blue, dark blue, and finally black. The blue color observed at low sodium uptake appears because the absorbed sodium atoms are spontaneously ionized. The electron produced by ionization is shared by the four sodium ions present in the sodalite cage (three initially there and the fourth originating from the absorbed atom). The color center created in this way is represented by the formula (Na+)4eF3−. Here, e stands for the electron and F3− for the negatively charged frame surrounding a zeolite cage. At the highest loading, when each cage contains an absorbed alkali atom, the color centers are arranged in a body‐centered cubic lattice, allowing the electrons associated with the centers to form bands. This may explain the black color observed at high concentration. In this paper we present measurements of the absorption coefficient of the black sodalite for photon energies between 0 and 3 eV, and interpret them by performing one‐electron band structure calculations for a fully loaded compound. These calculations deal only with the ‘‘solvated’’ electrons. The effect of the other electrons is taken into account through an empirical potential energy representing the interaction of a solvated electron with the zeolite frame. Because of this we study only the bands formed by the electrons of the color centers. Since the gap in the electron energy bands of the dry sodalite is over 6 eV, the color of the black sodalite is controlled by the solvated‐electron bands formed in this gap. The measured spectrum has a threshold of about 0.6 eV which seems to suggest that the system has a gap in the electronic structure and is therefore a semiconductor. The calculations indicate, however, that, if the one‐electron picture is valid, the fully doped black sodalite is a narrow‐band metal. The threshold in the spectrum appears because the transition matrix element is zero for transitions responsible for photon absorption, and not because of a gap in the density of states. The calculated spectrum is in reasonable agreement with the measured one. Conclusions based on one‐electron calculations can be altered by electron–electron interactions, which could turn a metal into an insulator. Two simple criteria, proposed by Mott and Hubbard, were used to test whether this transition might occur in our system. Unfortunately the results indicate that the system is close to the transition region which means that predictions made by these simple criteria are not reliable.

The influence of the magnetic ions dilution on the crystal and magnetic structures of CoxZn1-xO (x = 1, 0.90, 0.83, 0.75) solid solutions has been studied by the neutron diffraction and magnetic susceptibility measurements. It has been concluded that magnetic ordering of CoO is preserved in CoxZn1-xO regardless of the magnetic dilution for x greater than or equal to 0.75. This is confirmed independently by three different methods: Rietveld refinement of the magnetic structure, dependence of magnetic moments vs. temperature, and variation of the reduced Neel temperatures t = T-N(x)/T-N(1) with concentration. (C) 2004 Elsevier B.V. All rights reserved. International Conference on Magnetism (ICM 2003), Jul 27-Aug 01, 2003, Rome, Italy

This article summarizes our contribution to three distinct areas of research on thin films of fullerenes. In the first part we present results of an in-situ optical absorption study on K-doped thin films of C60 from which an unambiguous assignment of certain low energy electronic transitions in C60 and KxC60 was deduced. The second part of the article concerns transient and steady-state photoconductivity (PC) of pristine and oxygen-exposed thin films of C60 examined as a function of temperature and photon energy. We find that the presence of oxygen is responsible for the creation of deep traps which reduce the steady-state and transient PC by several orders of magnitude, particularly in the 1.6–2.3 eV excitation region. The thermally activated transport in an oxygen-free C60 sample is quenched upon exposure to oxygen so that both steady-state and transient PC become nearly temperature independent. In the last part we present two examples of utilizing C60 thin films for application in opto-electronics. The mechanism of light-induced charge transfer at the conducting polymer-C60 interface is described and the current versus voltage rectifying characteristics (> 104) of the MEH-PPV C 60 thin film heterojunction is presented. In addition, a non-linear transient photovoltaic response in a AlC 60 Au sandwich-type device featuring control of polarity with optical bias is described.

The intrinsic dynamics of photoexcited carriers in oxygen-free C 60 film and their remarkable evolution as the film is exposed to oxygen are revealed by transient photoconductivity (PC) and transient photoinduced absorption (PA) measurements at various temperatures, light intensities and photon energies. These studies reveal the carrier transport and carrier recombination mechanisms, and provide an estimate for the initial mobility in pristine C 60 film; these measurements also demonstrate that exposure C 60 film to oxygen creates deep traps that localize the photocarriers and effectively quenches the long-lived multiple trapping transport mechanism.

Data obtained from thin films of C60 using the techniques of absorption, photoluminescence, resonance Raman scattering, electroabsorption, and transient photomodulation are presented to develop an understanding of the energy levels and their dynamics in this fullerene solid.

The intrinsic dynamics of photoexcited carriers in oxygen-free C60 film and their remarkable evolution as the film is exposed to oxygen are revealed by transient photoconductivity (PC) and transient photoinduced absorption (PA) measurements at various temperatures, light intensities and photon energies. We demonstrate that exposure C60 film to oxygen creates deep traps which effectively localize the photocarriers, and in particular strongly effecting the long-lived transport mechanism.

We report the effect of oxygen on the action spectrum of the steady-state photoconductivity (PC) of C60 film and its temperature dependence. The steady-state PC decreases more than 4 orders of magnitude upon exposure to oxygen; the PC decreases faster below ħω≊2.3 eV. The low-temperature PC spectrum of the oxygen-free C60 is similar to the high-temperature PC spectrum of C60 exposed to oxygen. The steady-state PC exhibits a peak at T≊240 K in the oxygen-free C60, which disappears in C60 exposed to oxygen. The results indicate that the oxygen in C60 film acts as an efficient deep trap for photoexcited carriers and reduces the structural order.

The sodalite cage containing only an electron as a nonframework «anion» is well adapted for the study of solvated «electride» Wigner lattices and quantitative mapping of the intra- and intercage electronic potential surfaces. In this paper we report the direct synthesis of anion free cage structures of the form Na 3 [] (ABO 4 ) 3 .nH 2 O (A; B=Al, Ga; Si, Ge) as possible precursors for sodalite electride synthesis. A novel, low-temperature sodalite dehydroxylation method is presented for the preparation of Na 3 [] (AlSiO 4 ) 3 .4H 2 O from Na 4 [OH](AlSiO 4 ) 3 .1.72H 2 O

We study the properties of the electrons produced by absorbing sodium atoms into a zeolite which we call dry sodalite. Upon absorption, the sodium atom is ionized and the electron is shared with the other ions present in the sodalite cage. When the dry sodalite is saturated with sodium, each cage has one such electron and the system is periodic. We develop a simple model for the electron interaction with the zeolite framework and calculate the energy bands of the electrons in this periodic system. We find that in the one‐electron approximation, the system is a narrow band metal. It is very likely that the electron–electron interactions will transform it into a Mott insulator which is antiferromagnetic at low temperature. Particular attention is paid to understanding how the interactions between the electrons and the zeolite frame affect the electronic properties. Since the properties of the framework can be modified chemically, this understanding can help us determine how to use such modifications to our advantage.

The intrinsic dynamics of photoexcited carriers in an oxygen-free ${\mathrm{C}}_{60}$ film and their remarkable evolution as the film is exposed to oxygen are revealed by transient and steady-state photoconductivity (PC) measurements at various temperatures, light intensities, and photon energies. Exposure of ${\mathrm{C}}_{60}$ film to oxygen creates deep traps that reduce drastically the carrier lifetime and, consequently, the room-temperature steady-state photoconductivity by three to six orders of magnitude. Oxygen affects the steady-state photoconductivity excitation spectrum in a qualitatively similar way, as does decreasing the ambient temperature; in both cases, the photoconductivity decreases faster when carriers are photoexcited into the band edges. The transient and steady-state PC of a ${\mathrm{C}}_{60}$ film fully exposed to oxygen becomes temperature independent.

Permanganate ions were encapsulated in sodalite cages during the hydrothermal synthesis of basic sodalite in the presence of excess sodium permanganate. Both spectroscopic and structural studies of the solid solutions of basic sodium-permanganate-aluminosilicate sodalite focus on a particular stoichiometry: Na 4 -[AlSiO 4 ] 3 (MnO 4 ) x (OH) 1-x with x=0.73

The photo-luminescence emission of conjugated polymers exhibits a red-shift as the temperature is reduced. The gain curve red-shifts along with the emission and hence, the wavelength for amplified spontaneous emission (ASE) red-shifts on lowering the temperature. A similar, but not as pronounced, red-shift is also observed in optical absorption of these materials. The increased Stokes shift between the absorption and emission leads to lower self-absorption losses and hence to lower thresholds for ASE at lower temperatures.

We have used a variety of optical characterization techniques to study photoexcitations in C 60 and C 70 fullerenes as dispersion in polystyrene glass. These techniques include photoluminescence (PL), photomodulation (PM) and their respective versions of optically detected magnetic resonance (ODMR). In both fullerenes at 4 K we found long-lived (∼5 ms in C 60 and 0.5s in C 70 ) triplet excitons with two pronounced PA bands below the gap. The zero-field splitting parameters indicate that the triplet excitons are delocalized over the entire molecule: 6 A in C 60 and 7.8 A in C 70 .

Photoexcitations in C70 both as a dispersion in polystyrene (PS) glass and as films produced by evaporation, have been studied by photomodulation, photoluminescence and their respective versions of optically detected magnetic resonance. In C70: PS glass at 4 K we observed metastable triplet excitons with a distribution of lifetimes and a correlated distribution of magnetic powder patterns. In C70 films, on the other hand, in addition to triplet excitons we observed photogenerated charged excitations with spin 1 2 and a correlated photoinduced electroabsorption; we identify the charged excitations as C∓70.

A series of mixed sodalite samples, Na(8)[Al(6)Si(6)O(24)]Br(x).(H(3)O(2))(2-x), with the unit cell stoichiometries varying in the 0 < x

We have measured and calculated the absorption spectrum of the Na{sub 4}{sup 3+} clusters in Na{sub 3}[AlSiO{sub 4}]{sub 3} sodalite prepared by high-vacuum deposition of sodium atoms. The samples with a Na{sub 4}{sup 3+}:Na{sub 3}{sup 3+} cluster ratio up to 1:10 show a single-absorption feature with {lambda}{sub max} = 628 nm (1.99 eV). The absorption originates from the individual sodalite cages containing the Na{sub 4}{sup 3+} cluster. For the Na{sub 4}{sup 3+}:Na{sub 3}{sup 3+} cluster ratio larger than 1:10, when some of the Na{sub 4}{sup 3+} clusters are likely to interact, the changes in the absorption spectra indicate the onset of insulator-metal transition. Time-dependent quantum mechanical calculations of the photon absorption cross section of Na{sub 4}{sup 3+} clusters in sodalite at infinite dilution were carried out. The dependence of the calculated spectra on sodalite framework charges and cluster geometry was used to determine which of the proposed charge distribution models are consistent the the absorption spectra. The best agreement between measurements and calculations is obtained for Na = +1, Si = +1.9, Al =+0.9, and O = {minus}0.95. 24 refs., 7 figs., 1 tab.

We have used various optical characterization techniques to study the electronic excited states in icosahedral clusters of C 60 thin films produced by high-vacuum evaporation onto sapphire substrates. These techniques include c.w. photoluminescence (PL), photomodulation (PM) and spin-dependent PL and PM. We have found a modestly strong PL band at 1.72 eV, with two phonon replicas 185 meV apart. This phonon is the most strongly coupled C 60 pentagonal pinch mode and therefore shows that the PL band is intrinsic to C 60 . From its flat modulation frequency response up to 100 kHz, we conclude that the PL lifetime is shorter than 3 μs, implying that it is due to singlet exciton recombination. The PM spectrum contains a single band peaking at about 1 eV, which is correlated with a derivative-like feature around 2.3 eV; no photoinduced IR-active vibrations have been discovered. From the additional study of the frequency and excitation dependencies, we conjecture that the PM is due to charged carriers which recombine with bimolecular kinetics.

Raman spectra of the fullerenes C 60 and C 70 have been measured for photon energies from 2.1 to 3.6 eV. Resonant enhancement of the Raman cross section versus wavelength has been measured for the 270, 497, 1424, 1469, and 1568 cm −1 lines, with the diamond 1332 cm −1 line as a reference. A prominent enhancement occurs near 2.4 and 2.5 eV for the C 60 497 cm −1 and 1469 cm −1 lines, respectively. The C 70 1424 cm −1 and 1568 cm −1 lines increase monotonically in intensity between 2.3 and 2.7 eV. The 270 cm −1 line, which has contributions from both C 60 and C 70 , exhibits resonance enhancement which is qualitatively similar to that of the C 70 lines. All of the lines show resonance enhancement at 3.4 and 3.5 eV.

Electroabsorption spectra have been measured for undoped C 60 thin films at 80 and 300 K in the spectral range 1.76 to 5 eV. Absorption and thermal modulation spectra have also been obtained. The electronic energy levels of C 60 are reviewed, and the electroabsorption spectra are discussed in terms of the Stark effect on energy levels of A g , H g and T u symmetries.

Changes in optical absorption spectra of ≈ 500 A thick films of C 60 during doping with alkali metals (A = Li, Na, K, and Rb) were recorded. The stoichiometry of the potassium-doped C 60 films was monitored by simultaneous in situ measurements of electrical conductivity. The spectral changes indicate that the molecular solid behavior of C 60 bulk is preserved in the A 6 C 60 phase. Assignments for the C 60 and K 6 C 60 absorption spectra based on molecular orbitals of C 60 are discussed.

Low thresholds (∼500 W/cm2) for amplified spontaneous emission(ASE) are reported in films of soluble poly(paraphenylene vinylene)-based conjugated polymer blends. Efficient Forster energy transfer from the absorbing host polymer to the emitting guest polymer is observed. Emission in the blends originates predominantly from the guest polymer. The large spectral shift between the absorption and emission wavelengths lowers the self-absorption losses and results in low ASE thresholds. Initial results show an enhancement in photoluminescencequantum efficiency of the blends.

Described is the application of a combinatorial physical vapor deposition (CPVD) method for studying the growth dynamics of epitaxial films. The CPVD method takes advantage of the angle-dependent evaporation rate from a point source to produce thin film libraries whose deposition rate changes continuously for a factor of 50 across a 70-mm long-substrate. The link between the deposition rate and the resulting thin film morphology was made by spatially correlated absorption and atomic force microscopy measurements. It is shown that the growth of tryphenyldiamine derivate on a silica surface proceeds by three-dimensional growth of isolated islands which, at some critical coverage, coalesce to form uniform amorphous film. While the critical coverage of such films depends on the deposition rate in the 0.015-0.4 nm/s region, the particle size distribution function does not.

The two-photon absorption (TPA) properties of various terpyridines of the kind [4'-(C6H4-(P)-X)2,2':6',2"-terpyridine] and related homoleptic and heteroleptic bis(terpyridine) cationic zinc(11) complexes were investigated by the TPA induced photoluminescence (TPA-PL) method in a femtosecond regime. It appeared that terpyridines bearing an X donor group are characterized by TPA cross sections among the largest ever reported for a molecule with a dipole symmetry whereas coordination to a Zn(II) center leads to a decrease of the TPA response.

Hyperfine interactions between randomly distributed $\mathrm{F}$-center electrons and the surrounding magnetic nuclei in mixed $\mathrm{Cl}$/electrosodalite, ${\mathrm{Na}}_{8}{[\mathrm{Al}}_{6}{\mathrm{Si}}_{6}{\mathrm{O}}_{24}{]\mathrm{Cl}}_{x}\ifmmode\cdot\else\textperiodcentered\fi{}{({e}^{\ensuremath{-}})}_{2\ensuremath{-}x}$, give rise to multiple nuclear magnetic resonances due to quantized variations in the local electron density at the nuclear site. This enables one not only to account for the number of $\mathrm{F}$ centers surrounding a crystallographic site, but also to estimate the percentage of cages filled with $\mathrm{F}$ centers and their spatial distribution. Complete assignments of $^{23}\mathrm{Na}$, $^{27}\mathrm{Al}$, and $^{35}\mathrm{Cl}$ MAS NMR spectra are provided and the NMR evidence for spin correlation effects of interacting $\mathrm{F}$-center electrons is presented. It is proposed that the spin pairing is a general feature of paramagnetic cage-structures with $s$-like electrons, owing it to the Pauli exclusion principle.

MOLECULES of C60 (refs 1–4) are near-perfect spheres, and in the crystalline solid each molecule rotates rapidly while remaining at its lattice position. Speculation that this property may be conducive to good lubrication has not been borne out by experiments, at least for C60 on its own5–8. Here we report measurements of the intermolecular interactions of C60 adsorbed as monolayers on surfaces immersed in a liquid. We find that the adsorbed layers impart a surprising effect on the flow properties of the liquid at the surface. The measured short-range oscillatory force indicates weak adsorption of one to two layers of C60 on each surface. In the presence of adsorbed layers, fluid flow between two surfaces exhibits full-slip (zero drag) boundary conditions, giving rise to flow behaviour that is totally different from conventional fluid flow through narrow pores. This suggests that C60 might be effectively used as an additive to conventional lubricant fluids.

An intercalation compound of azafullerene, K6C59N, was prepared and structurally characterized. It is isostructural with the fullerene compound K6C60, adopts a body-centered-cubic structure (lattice constant a = 11.31 angstroms), and consists of quasi-spherical monomeric (C59N)(6-) ions. Density functional calculations of the structural and electronic properties confirm the similarity to K6C60 but also suggest a sizable deformation, principally confined in the vicinity of the nitrogen atom, of both the molecular structure and the electron states. These results show that study of the intercalation chemistry of azafullerene promises to reveal a rich family of both n- and p-doped systems with novel conducting and magnetic properties, like their fullerene antecedents.

The solid solution behavior of mixed halide (Cl/Br, Cl/I, and Br/I) sodalite systems as prepared by hydrothermal synthesis has been studied in detail by X-ray diffraction and solid-state nuclear magnetic resonance (NMR). At a synthesis temperature of 450 K the rate of halide incorporation into the cages of mixed Cl/Br, Br/I, and Cl/I sodalites depends on the halide stoichiometry in the precursor gel and on the particular halogen anion. 2 3 Na and 2 7 Al NMR spectra acquired with magic angle spinning (MAS) are sensitive to both halide composition and distribution and can be used to monitor the onset of phase separation. The Cl/Br and Br/I sodalites show close to ideal miscibility behavior, whereas in the Cl/I sodalites domain segregation effects prevail. The NMR spectra of the quadrupolar 2 3 Na and the halogen nuclei also serve as sensitive indicators of the local symmetry perturbations caused by the disorder inherent to solid solution systems. Furthermore, the continuous variation of lattice parameters in sodalite solid solutions provides an opportunity to probe the fundamental dependence of chemical shift on Na-halogen distances. In this respect the experimental 2 3 Na NMR data reveal an exponential relationship, in agreement with theoretical calculations based on the Hartree-Fock method. In contrast, for the halogen chemical shifts the correlation is found to be approximately linear, with slopes of 17, 35, and 75 ppm/A for 3 5 Cl, 8 1 Br, and 1 2 7 I, respectively. All of these results reflect the high sensitivity of chemical shifts to the extent of anion-cation overlap for nuclei in ionic environments.

The magnetic ordering in the ${\mathrm{Eu}}_{4}{\mathrm{Ga}}_{8}{\mathrm{Ge}}_{16}$ clathrate compound was studied using magnetic susceptibility, M\"ossbauer spectroscopy, and heat-capacity measurements. Antiferromagnetic ordering is observed at 8 K. M\"ossbauer spectroscopy shows the Eu atoms to exist as ${\mathrm{Eu}}^{2+}$ with strong room-temperature absorption at -11.8 mm/s vs ${\mathrm{EuF}}_{3}.$ The temperature dependence of the Lamb-M\"ossbauer factor can be fitted to an Einstein model with an effective Einstein temperature of 80 K. In combination with heat-capacity measurements, this result suggests that the Eu atoms behave like isolated oscillators that are effectively decoupled from the lattice. The magnetic susceptibility data are interpreted using the Curie-Weiss law for ordering with ${\ensuremath{\mu}}_{\ensuremath{\parallel},\mathrm{eff}}=8.00\ifmmode\pm\else\textpm\fi{}0.03{\ensuremath{\mu}}_{B}/{\mathrm{Eu}}^{2+}$ ion and $\ensuremath{\theta}=4.43\ifmmode\pm\else\textpm\fi{}0.08\mathrm{K}.$ The positive Weiss constant is rationalized by a nearest-neighbor ferromagnetic exchange interaction within an overall antiferromagnetically ordered system. The spin-flop phase diagram is established and used to extract the next-nearest-neighbor interaction energy. The location of the tri-critical point in H,T space is determined. The magnetic heat capacity ${C}_{m}$ is extracted by modeling the lattice contribution using a weighted Debye-Einstein model with fitted Debye temperature ${\ensuremath{\Theta}}_{D}=314\ifmmode\pm\else\textpm\fi{}4\mathrm{K}$ and using a fixed Einstein temperature ${\ensuremath{\Theta}}_{E}=80\mathrm{K}.$ Integration of ${C}_{m}/T$ gave 15.1 J/K mol ${\mathrm{Eu}}^{2+}$ in fair accordance with expectations based on configurational considerations.