Electronic structure of barium-doped C₆₀

We have investigated the electronic structure of Ba-doped C60 films with Ba concentrations of up to x≈12 (BaxC60) by applying valence-band photoemission and x-ray-absorption spectroscopy. A crystal orbital (CO) formalism based on a semiempirical Hamiltonian of the intermediate-neglect-of-differential-overlap type has been employed to derive solid-state results for the Ba-doped C60 fullerides. Using x-ray diffraction, we show three distinct phases for the bulk BaxC60 system with Ba concentrations of up to x=6. In all cases, the experimental observations strongly indicate that fulleride formation leads to the occupation of hybrid bands on both sides of the Fermi level. The theoretical data indicate that the alkaline-earth atoms are essentially monovalent and hybridize strongly with the π-type functions of the C60 network. The Ba atoms in the BaxC60 fullerides deviate from the limit of complete charge transfer as a consequence of the competition between covalent Ba-C60 bonding and ionic contributions. Furthermore, it is shown that the calculated density-of-state profiles reproduce the photoemission data in the extreme outer valence-band region

Electronic Structure of the C₆₀ Fragment in Alkali- and Alkaline-earth-doped Fullerides

The electronic structure of the C60 fragment in alkali- and alkaline-earth-doped fullerides is studied theoretically. With increasing metal-to-C60 charge transfer (CT) the n electronic properties of the soccerball are changed. In the undoped solid and for not too high a concentration of doping atoms the hexagon-hexagon (6-6) bonds show sizeable double bond character while the hexagon-pentagon (6-5) bonds are essentially of single bond type. In systems with a high concentration of doping atoms this relative ordering is changed. Now the 6-5 bonds have partial double bond character and the 6-6 bonds are essentially single bonds. The high ability of the C60 unit to accomodate excess electrons prevents any sizeable weakening of the overall n bonding in systems with up to 12 excess electrons on the soccerball. A crystal orbital (CO) formalism on the basis of an INDO (intermediate neglect of differential overlap) Hamiltonian has been employed to derive solid state results for potassium- and barium-doped C60 fullerides. For both types of doping atoms an incomplete metal-to-C60 CT is predicted. In the potassium-doped fullerides the magnitude of the CT depends on the interstitial site of the dopant. The solid state data have been supplemented by INDO and ab initio calculations on molecular C60, C6-60 and C12-60. The calculated bondlength alternation in the neutral molecule is changed in C12-60 where the length of the 6-6 bonds exceeds the length of the 6-5 bonds. The geometries of the three molecular species have been optimized with a 3-21 G* basis. The theoretically derived modification of the C60 (π) electronic structure as a function of the electron count is explained microscopically in the framework of two quantum statistics accessible for π electronic ensembles. In the π ensemble of the C60 fragment so-called hard core bosonic properties are maximized where the Pauli antisymmetry principle has the character of a hidden variable only. Here the electronic degrees of freedom are attenuated only by the Pauli exclusion principle. This behaviour leads to the changes in the π electronic structure mentioned above

Crystal structure of polymeric carbon nitride and the determination of its process-temperature-induced modifications

Based on the arrangement of two-dimensional 'melon', we construct a unit cell for polymeric carbon nitride (PCN) synthesized via thermal polycondensation, whose theoretical diffraction powder pattern includes all major features measured in x-ray diffraction. With the help of this unit cell, we describe the process-temperature-induced crystallographic changes in PCN that occur within a temperature interval between 510 and 610 °C. We also discuss further potential modifications of the unit cell for PCN. It is found that both triazine- and heptazine-based g-C3N4 can only account for minor phases within the investigated synthesis products