Solution of the structure of the high-coverage CO layer on the Ru(0001) surface-A combined study by density functional theory and scanning tunneling microscopy.

Structures formed by dense CO adsorption layers can provide information about the balance between molecule-surface and molecule-molecule interactions. However, in many cases, the structure models are not clear. Using density functional theory (DFT) and scanning tunneling microscopy (STM), we have investigated the high-coverage CO layer on the Ru(0001) surface. Previous investigations by low-energy electron diffraction (LEED) and vibrational spectroscopy led to conflicting results about the structure. In the present study, 88 models with coverages between 0.58 and 0.77 monolayers have been analyzed by DFT. The most stable structures consist of small, compact CO clusters with an internal pseudo 1×1 structure. The CO molecules in the cluster centers occupy on-top sites in an upright position, whereas the molecules farther outside are slightly shifted from these sites and tilted outward. STM data of the CO-saturated surface at low temperatures, corresponding to a coverage of 0.66 monolayers, show a quasi-hexagonal pattern of features with an internal hexagonal fine structure. Simulated images based on the cluster model agree with the experimental data. It is concluded that the high-coverage CO layer consists of the close-packed clusters predicted by DFT as the most stable structure elements. In the experiment, the sizes and shapes of the clusters vary. However, the arrangement is not random but follows defined tiling rules. The structure remains ordered, almost up to room temperature. The LEED data are re-interpreted on the basis of the Fourier transforms of the STM data, solving the long-standing conflict about the structure.

Reduction of Germanium Oxides-The Mixed-Valence Germanates A2Ge4O7 (A = Na, K).

We report on the synthesis of two-layered alkali germanates, Na2Ge4O7 and K2Ge4O7. Both compounds were synthesized by using the ammonothermal method at 823 K and 100 MPa. Under these conditions, germanium is partially reduced from the +IV state to +II, forming mixed-valence compounds with the rarely observed [Ge(II)O3]4- unit. The valence state was verified by X-ray photoelectron spectroscopy (XPS) and was accompanied by theoretical calculations alongside vibrational spectroscopy and single-crystal X-ray structure determination. The compounds crystallize in the trigonal space groups (Na2Ge4O7: P3̅c1 and K2Ge4O7: P3̅m1) and feature layers of corner sharing [Ge(II)O3]4- and [Ge(IV)2O7]6- units forming [Ge(II)2Ge(IV)2O7]2- polyanions. These layers are separated by alkali metal ions. The compounds are colorless insulators with band gaps of 4.0-4.2 eV. According to the Robin-Day classification, both compounds can be described as class I materials, where the valences are trapped on specific sites.