New structural model for GeO2/Ge interface: A first-principles study

First-principles modeling of a GeO2/Ge(001) interface reveals that sixfold GeO2, which is derived from cristobalite and is different from rutile, dramatically reduces the lattice mismatch at the interface and is much more stable than the conventional fourfold interface. Since the grain boundary between fourfold and sixfold GeO2 is unstable, the sixfold GeO2 forms a large grain at the interface. On the contrary, a comparative study with SiO2 demonstrates that SiO2 maintains a fourfold structure. The sixfold GeO2/Ge interface is shown to be a consequence of the ground-state phase of GeO2. In addition, the electronic structure calculation reveals that sixfold GeO2 at the interface shifts the valence band maximum far from the interface toward the conduction band.Comment: 18 pages, 5 figures, and 2 table

New structural model for GeO2/Ge interface: A first-principles study

First-principles modeling of a GeO2/Ge(001) interface reveals that sixfold GeO2, which is derived from cristobalite and is different from rutile, dramatically reduces the lattice mismatch at the interface and is much more stable than the conventional fourfold interface. Since the grain boundary between fourfold and sixfold GeO2 is unstable, the sixfold GeO2 forms a large grain at the interface. On the contrary, a comparative study with SiO2 demonstrates that SiO2 maintains a fourfold structure. The sixfold GeO2/Ge interface is shown to be a consequence of the ground-state phase of GeO2. In addition, the electronic structure calculation reveals that sixfold GeO2 at the interface shifts the valence band maximum far from the interface toward the conduction band.Comment: 18 pages, 5 figures, and 2 table

The Lunar Surface Gravimeter as a Lunar Seismograph

Introduction: The primary objective for the Lunar Surface Gravimeter (LSG) on Apollo 17 was to search for gravitational waves, but it failed in detecting them [1]. When the instrument was deployed on the Moon, the sensor beam could not be balanced in the proper equilibrium position. Consequently, the LSG was not able to function as originally designed. Lauderdale and Eichelman (1974) [1] concluded that “no provision has been made to supply data from the experiment to the National Space Science Data Center.” However, it was reported in Giganti et al. (1977) [2] that though they had not detected gravitational waves, after a series of reconfigurations the beam was recentered and the LSG gathered useful data. Besides the observation of gravitational waves, the LSG was also designed to observe seismic signals and tidal deformations [3]. According to Giganti et al. (1977) [2] LSG’s sensitivity covered the frequency range from 1~16Hz (Fig.1). There are several types of moonquakes reported, deep moonquakes, meteorite impacts, and high frequency teleseismic (HFT). Each of the moonquakes is known to have a resonant frequency around 1Hz and in addition, HFT has a predominant frequency around 10 Hz [4]. Therefore it is likely that the LSG was detecting the seismic events on the Moon. However, the LSG data have not been analyzed from a seismological point of view

5,5-Bis(4-methoxy­phen­yl)-2,8-bis­[3-(trifluoro­meth­yl)phen­yl]-5H-cyclo­penta­[2,1-b:3,4-b’]dipyridine

The title compound, C39H26F6N2O2, showed two melting transitions 477.4 and 506.5 K in a differential scanning calorimetry (DSC) study. The first of these can be attributed to a melting phase transition arising from the rotation of two trifluoro­methyl groups. In the crystal structure, both trifluoro­methyl groups are disordered over two sites with occupancy factors of 0.660 (17) and 0.340 (17) for the major and minor orientations, respectively. The introduction of trifluoro­methyl groups inhibits π-stacking between the diaza­fluorene (cyclo­penta­[2,1-b:3,4-b’]dipyridine) units. Three short F⋯O contacts between 2.80 (3) and 2.95 (1) Å are observed in the crystal structure

First-Principles Study on Structural Properties of GeO2_2 and SiO2_2 under Compression and Expansion Pressure

The detailed analysis of the structural variations of three GeO$_2$ and SiO$_2$ polymorphs ($\alpha$-quartz, $\alpha$-cristobalite, and rutile) under compression and expansion pressure is reported. First-principles total-energy calculations reveal that the rutile structure is the most stable phase among the phases of GeO$_2$, while SiO$_2$ preferentially forms quartz. GeO$_4$ tetrahedras of quartz and cristobalite GeO$_2$ phases at the equilibrium volume are more significantly distorted than those of SiO$_2$. Moreover, in the case of quartz GeO$_2$ and cristobalite GeO$_2$, all O-Ge-O bond angles vary when the volume of the GeO$_2$ bulk changes from the equilibrium point, which causes further deformation of tetrahedra. In contrast, the tilt angle formed by Si-O-Si in SiO$_2$ markedly changes. This flexibility of the O-Ge-O bonds reduces the stress at the Ge/GeO$_2$ interface due to the lattice-constant mismatch and results in the low defective interface observed in the experiments [Matsubara \textit{et al.}: Appl. Phys. Lett. \textbf{93} (2008) 032104; Hosoi \textit{et al.}: Appl. Phys. Lett. \textbf{94} (2009) 202112].Comment: 15 pages, 5 figures and 2 table