Temperature dependence, single-crystal

Fig. 31. Temperature-dependent single crystal conductivity of Ni(pc)I, Ni(tbp)I, and Ni(tatbp)I (Refs. 2, 132, 133)...

In conclusion, field dependent single-crystal magnetization, specific-heat and neutron diffraction results are presented. They are compared with theoretical calculations based on the use of symmetry analysis and a phenomenological thermodynamic potential. For the description of the incommensurate magnetic structure of copper metaborate we introduced the modified Lifshits invariant for the case of two two-component order parameters. This invariant is the antisymmetric product of the different order parameters and their spatial derivatives. Our theory describes satisfactorily the main features of the behavior of the copper metaborate spin system under applied external magnetic field for the temperature range 2+20 K. The definition of the nature of the low-temperature magnetic state anomalies observed at temperatures near 1.8 K and 1 K requires further consideration. 

Figure 10. Reciprocal dependence of magnetic susceptibility on temperature for single crystals of (a) KTmNbeClig and (b) TmNbeClig (c) Magnetic susceptibility of the cluster A/ 0f(TmNb6Cl s) -/(KTmNb6Cl 8)) below 100 K. The insert shows the reciprocal susceptibility of the cluster.

Results from magnetic susceptibiHty studies have been reported (50—53). Measurements (50) obtained by the Gouy method are shown in Figure 3. These are lower than those of other investigators. However, the temperature dependences of the magnetic susceptibiHties, for the various plutonium allotropes were similar. a-Plutonium single crystals show a slight anisotropy of (54). 

This kind of microstructure also influences other kinds of conductors, especially those with positive (PTC) or negative (NTC) temperature coefficients of resistivity. For instance, PTC materials (Kulwicki 1981) have to be impurity-doped polycrystalline ferroelectrics, usually barium titanate (single crystals do not work) and depend on a ferroelectric-to-paraelectric transition in the dopant-rich grain boundaries, which lead to enormous increases in resistivity. Such a ceramic can be used to prevent temperature excursions (surges) in electronic devices. 

Figure 4. Temperature dependence of CRSS for the six different deformation modes observed in Ti-56 at.%Al single crystals.

Figure 5. Temperature dependence of CRSS for <101] superlattice slip, <110] ordinary slip and twinning observed for Ti-54 at.%Al and Ti-56 at. SjAl single crystals.

In general, increasing the temperature within the stability range of a single crystal structure modification leads to a smooth change in all three parameters of vibration spectra frequency, half-width and intensity. The dependency of the frequency (wave number) on the temperature is usually related to variations in bond lengths and force constants [370] the half-width of the band represents parameters of the particles Brownian motion [371] and the intensity of the bands is related to characteristics of the chemical bonds [372]. 

Fig. 106. Temperature dependence of vs(NbO) and vs(NbF) wave numbers for a single crystal of RbsNbsOF/s- Reproduced from [442], A. I. Agulyansky, J. Ravez, R. Cavagnat, M. Couzi, Ferroelectrics 152 (1993) 373, Copyright 1993, with permission of Taylor Francis, Inc., http //www.routledge-ny.com.

In the case of single crystals of K5Nb3OF 8, a maximum in the dielectric permittivity 33 was observed at about 400K. Fig. 108 shows the temperature dependence of b at different frequencies. 

In retrospect, by inspecting the literature, we find a confirmation of this variance (see for instance Ref. [67]). Peak intensities of bands originally assigned to Franck-Condon components of the excilonic emission have random relative intensities. This would not be possible if the bands were intrinsically vibronic. Since we know that the excilonic emission, as it is observed in single crystals, is rather sharp at low temperatures, we were forced to reconsider the assignment of the fluorescence of thin films. From the temperature dependence of the fluorescence effi-... 

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