Single crystal measurements

Typical current-voltage characteristics reflected the nonohmic nature of the Au/lead azide system. (Gold forms a nonohmic contact with lead). The voltage 

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Figure 41. PMC potential curves for a ZnO single crystal measured in contact with propylene carbonate (0.1 M TRAP) containing 10 mM ferrocene (curve 1), and with increasing concentrations (5,10, and 20%) of water (curves 2-4). Illumination with He-Cd U V laser. 5 mW.

The isotope is almost exclusively used for the investigation of chemical compounds. The resolving power of Mossbauer spectroscopy is demonstrated by the spectra from WS2 powder and single-crystal measurements as shown in Fig. 7.48 (from [222]). On the right-hand side of the picture, the angular dependence of E2 transitions for Aw = 2, 1,0 hyperfine components is indicated. 

One of the most promising techniques for studying transition metal ions involves the use of zeolite single crystals. Such crystals offer a unique opportunity to carry out single crystal measurements on a large surface area material. Suitable crystals of the natural large pore zeolites are available, and fairly small crystals of the synthetic zeolites can be obtained. The spectra in the faujasite-type crystals will not be simple because of the magnetically inequivalent sites however, the lines should be sharp and symmetric. Work on Mn2+ in hydrated chabazite has indicated that there is only one symmetry axis in that material 173), and a current study in the author s laboratory on Cu2+ in partially dehydrated chabazite tends to confirm this observation. 

Rotating single-crystal measurements also permitted the extraction of the orientation of the magnetic tensor in the molecular reference frame and the experimental easy axis was found to coincide with the idealized tetragonal axis of the coordination dodecahedron of Dy. Crystal field calculations assuming idealized tetragonal symmetry permitted the reproduction of magnetic susceptibility data for gz = 19.9 and gxy 0 [121]. More elaborated calculations such as ab initio post Hartree-Fock CASSCF confirmed this simple analysis [119]. 

Figure 5.8 Static magnetic data for the Er(trensal) complex in the case of a powdered sample and an oriented single crystal measured in parallel or perpendicular direction to the C3 rotation axis, (a) Temperature dependence of the magnetic susceptibility...

Ultrapure Th metal has been processed at the Ames Laboratory by solid-state electrotransport under very low pressures (on the order of 0.3 nPa), which has produced the purest Th metal known, that with a resistivity ratio of 4200 for doubly refined metal (99-101). This resistivity ratio of 4200 translates into probably <50 ppm total impurities in the metal (see footnote 1) (87-90, 104). Single crystals measuring 0.25 cm in diameter by 1.1 cm in length with resistivity ratios of 1700-1800 have also been grown (55). 

It would be best perhaps if this technique were illustrated by an example. In the case of single crystal measurements for a compound which crystallizes in the -K2S04 form (D ji-Pnma] factor group the Raman tensor for the Ag... 

IR and Raman single crystal measurements have been carried out for YVO4 61). 

Figure 3.7. TMAFM images of a (001) surface of an as-received EDT-TTF-(CONHMe)2 single crystal measured under ambient conditions (2.5 im x 2.5 j.m) (a) topography and (b) phase. The phase angle is defined as the phase shift observed between the cantilever oscillation and the signal sent to the piezo-scanner driving the cantilever.

No special precautions are required to prepare material of this quality. However, since the loss of conductivity in samples of (TTF)(TCNQ) has been attributed to inclusion of impurities18 it is appropriate to use purified starting materials and distilled solvents and to perform the reaction in the absence of oxygen. Although the product is easy to prepare, the crystals are small and therefore not suitable for single-crystal measurements. 

Measured on single crystals. Measured on compressed pellets. 

At very low oxygen pressures, Bevan and Anderson found that the graphs of conductivity against inverse temperature yield an activation energy of a few hundredths of an electron volt, and that the conductivity becomes independent of oxygen pressure. This activation energy is similar to that for ionization of the electrons from the interstitial zinc atoms, as obtained from low temperature and single-crystal measurements. This may indicate that the surface effects become unimportant at very low pressures. 

Fig. 19. Hc2(T) of a L11N12B2C single crystal measured parallel 10 the tetragonal c-axis (o). The solid curve was calculated using a two-band model (see text). Dashed lines isotropic single-band (ISB) models with two values...

Figure 3. Representative UV/Vis-spectra of (Na S), (K S), (Na Se) and (K Se) single crystals, measured in transmission.

The resulting principal values of the 13C chemical shift tensors of the C60 carbons are 8n = 228 ppm, 822 = 178 ppm, and 833 = -3 ppm. Tycko et al reportet the experimental values 8n = 213 ppm, S22 = 182 ppm, and 833 = 33 ppm obtained from low temperature measurements of a powder pattern spectrum (18). However, the spectra have a low signal to noise ratio and a wide slope so that a larger error for the experimental value can be assumed. The chemical shift anisotropy of 217 ppm corresponds quite well with the spectral range of about 200 ppm reported by Kerkoud et al for low temperature single crystal measurements (19). 

In principle, charge transfer spectra should be full of valuable information, because they depend on the properties of two atoms, bonded together. However, they have proved to be far less amenable to detailed analysis than d-d bands, mainly because the changes in interelectron repulsion which accompany charge transfer are not so easily parameterised. Single crystal measurements, using polarised light at low temperatures, are important and valuable in the study of d-d spectra, but... 

Figure 4.12 Polarized absorption spectra of a vivianite single crystal measured in three zones of increasing oxidation. (1) Nearly colourless (2) light blue and (3) dark blue. The spectra were measured with light polarized along the b axis corresponding to the Fe2+-Fe3+ vector of edge-shared [FeOg] octahedra. Note the intensification of the Fez+ crystal field bands at 1,200 nm and 800 nm by the Fe2+ — Fe3+ IVCT at 630 nm (from Amthauer and Rossman, 1984).

Fig. 2.5. Temperature-dependent mobility of ZnO single crystals measured with the current flowing parallel (a) or perpendicular (b) to the c-axis of the crystals. The theoretical mobilities for the different scattering processes (optical, acoustical, and piezoelectric as well as ionized impurity scattering) as calculated by Wagner and Helbig [34,35] are shown as differently dashed lines. The calculated combined mobility curves (solid lines) fit the experimental data quite well for temperatures above about 20-50 K. The carrier concentrations at 300 K were about Ad — Na = 2.25 x 1016 cm 3 and Na = 2.75 x 1016 cm 3 (compensation ratio Aa/Ad = 0.55)...

Figure 7. Temperature dependencies of kac for the Lu2Fei7 single crystal measured in magnetic fields of 1 - 1 kOe 2-2 kOe 3-3 kOe, 4-6 kOe, 5-9 kOe, 6-12 kOe and curve 7 - kac(T) for the Lu2Fe17E[15 single crystal measured in magnetic field of 12 kOe.

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