The crystal structure of the Li-exchanged form of natrolite, Li 1.6Na04AJ,Si301O . 2H20, was refined based on 568 F obs' R = 0.071. For comparison the structure of a natrolite, NazAJ,Si30lO· 2H10, was also refined (1278 F obs' R = 0.040). The diffraction data collected for K-natrolite were of insufficient quality, therefore a computer simulation of the K-form was based on the experimentally determined cell edges and space group. The mean rotation angles ljI of the chains (composed of 4 = 1 secondary building units) relative to the a and b cell axes are observed to be 250 (Li-compound), 240 (Na-compound) and 180 (K-compound). A detailed comparison shows that only the transformation to K-natrolite conforms approximately to the currently widely accepted model of rotating chains. Surprisingly the Li-form instead achieves the observed shortened cell constants a and b by a distortion of the chain itself, the rotation is only a secondary effect. Both cation exchanged forms display also shortened c-axes, which is an indication that the chains of 4 = I units are tilted and twisted within themselves. It is most likely that the angle of the hinge at the oxygen atom 02 is in Na-natrolite already at the lower attainable limit beyond wh ich it would become energetically unfavorable, and thus it cannot be reduced any furt her when the Na-natrolite is transformed by ion-exchange into a Li-natrolite.

We report on highly resolved core-level and valence-band photoemission spectroscopies of hydrogenated, unreconstructed 6H-SiC(0001) and (0001 ) using synchrotron radiation. In the C 1s core level spectra of 6H-SiC(0001 ) a chemically shifted surface component due to C-H bonds is observed at a binding energy (0.47Ϯ0.02) eV higher than that of the bulk line. The Si 2p core-level spectra of SiC (0001) suggest the presence of a surface component as well but a clear identification is hindered by a large Gaussian width, which is present in all spectra and which is consistent with values found in the literature. The effect of thermal hydrogen desorption was studied. On 6H-SiC(0001) the desorption of hydrogen at 700-750°C is accompanied by a simultaneous transformation to the Si-rich (ͱ3ϫͱ3)R30°reconstruction. On 6H-SiC(0001 ) first signs of hydrogen desorption, i.e., the formation of a dangling bond state in the fundamental band gap of SiC, are seen at temperatures around 670°C while the (1ϫ1) periodicity is conserved. At 950°C a (3ϫ3) reconstruction is formed. The formation of these reconstructions on thermally hydrogenated 6H-SiC (0001) and (0001 ) is discussed in the light of earlier studies of 6H-SiC͕0001͖ surfaces. It will be shown that by using the hydrogenated surfaces as a starting point it is possible to gain insight into how the (ͱ3ϫͱ3)R30°and (3ϫ3) reconstructions are formed on 6H-SiC(0001) and 6H-SiC(0001 ), respectively. This is due to the fact that only hydrogen-terminated 6H-SiC͕0001͖ surfaces possess a surface carbon to silicon ratio of 1:1.

Hydrogenation of 6H–SiC (0001) and (0001̄) is achieved by high-temperature hydrogen treatment. Both surfaces show a low-energy electron diffraction pattern representative of unreconstructed surfaces of extremely high crystallographic order. On SiC(0001), hydrogenation is confirmed by the observation of sharp Si–H stretching modes. The absence of surface band bending for n- and p-type samples is indicative of electronically passivated surfaces with densities of charged surface states in the gap of below 7×1010 cm−2 for p-type and 1.7×1012 cm−2 for n- type samples, respectively. Even after two days in air, the surfaces show no sign of surface oxide in x-ray photoelectron spectroscopy.

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Using Fourier-transform infrared absorption spectroscopy, we have studied the Si–H stretch mode on hydrogenated 6H-SiC(0001) and 3C-SiC(111). On 6H-SiC(0001) two signals at 2133.5 and 2128.0 cm−1 are observed. Their intensity ratio varies with sample preparation temperature. On 3C-SiC(111) essentially a single mode is observed at 2128.4 cm−1 at all temperatures. The origin of the two modes is explained in terms of different stacking arrangement below the surface. The possibility for a stacking rearrangement during the hydrogen treatment is discussed.