Anisotropic magnetic susceptibility and phase transitions in intercalated graphite: KC24

We measured the magnetic susceptibility of KC₂₄ from 4.2 to 300 K and found no anomalies near the phase transitions at 95 and 123 K as observed in the resistivity. We conclude that the transitions must be due to order- disorder transitions of the K atoms and not charge density wave formation. The susceptibility is anisotropic; at room temperature $\text{χ}{}_{\mathrm{g}}\text{(}\text{H}\text{6}\text{c}\text{)= + 1.50 × 10}{}^{-6}\text{emu g}{}^{-1}\text{± 2\%}$ and $\text{χ}{}_{\mathrm{g}}\text{(}\text{H}\text{⊥}\text{c}\text{)= + 0.045 × 10}{}^{-6}\text{emu g}{}^{-1}\text{± 50\%}$. This anisotropy is not understood in terms of simple rigid band extensions of the band structure of graphite.     

Dielectric response and intraband plasmon dispersion in stage-1 FeCl3 intercalated graphite

A theory of the frequency and wavevector dependent longitudinal dielectric response functions in stage-1 FeCl₃-intercalated-graphite is developed from a two-band model for the graphite π electrons. The theory predicts the observed wavevector dependence of the energy and intensity of the intraband plasmon induced by intercalation, and comparison with experiment provides a measurement of the degree of charge transfer in C_6.6FeCl₃.     

Ultrasonic studies of structural transformations and phase transitions in liquid crystal emulsions

The propagation velocity and absorption coefficient of ultrasound in the range of 0.6–150 MHz in a series of liquid crystal emulsions prepared on the basis of thermotropic liquid crystals and isotropic liquids were studied. The dispersity parameters of emulsions were controlled by polarization microscopy and dynamic light scattering. Features of the temperature dependences of the acoustic parameters of emulsions due to the structural transformations and nematic-isotropic liquid transition in disperse and dispersion phases of liquid crystal emulsions were established.     

TDPAD studies of graphite intercalated by AsF5 vapour

The TDPAD technique has been used via the¹⁹F(p,p¹γ)¹⁹F reaction to study the graphite intercalation compound (GIC) formed in AsF₅ vapour. The spectra are dominated by the quadrupole interaction corresponding to the formation of C-F bonds, withv _q~58 MHz, η=0 and δ=0.04. However, peaks are also observed which can be ascribed tentatively to the presence of AsF₃. The strong textural features of the spectra can be related to the direction of the electric field gradient (efg) with respect to the incident beam direction and the detector plane. This suggests that potentially TDPAD can be useful for the characterization of GIC's.     

Intercalate lattice dynamics in stage-1 rubidium graphite

Collective intercalate models have been observed in C₈Rb by neutron scattering. The partial phonon density of states for in-basal-plane Rb vibrations agrees remarkably well with a calculation treating the Rb-Rb interactions as unscreened Coulomb forces.     

Elastic studies of phase transitions

We report some results on acoustic studies of phase transitions in which the order parameter is coupled to the elastic wave strain, when direct behaviour observation is not possible. First by Brillouin scattering it was possible to observe the softening of C₅₅ of ammonium oxalate hemihydrate and it was concluded that the order parameter is not the deformation. In the case of members of A₂MX₄ family a softening of C₆₆ related to a shear wave near the lock-in transition was observed for [N(CH₃)₄]₂ZnCl₄, as previously found for C ₅₅ of K₂SeO₄. Such a behaviour did not occur for (NH₄)₂BeF₄ in which a strong hysteresis effect appeared.     

NMR-NQR studies of structural phase transitions

Moving dislocations in II–VI semiconductors carry a large electric charge. This charge is not in thermal equilibrium, but is due to the sweeping up of electrons from point defects. Its movement produces a dislocation current during plastic deformation, and conversely, the application of an external field changes the flow stress. This paper reviews the structure and properties of these dislocations, the theory of their charge and the phenomena which are a consequence of the strong mutual interactions of the dislocation and electronic sub-systems in these crystals. The materials show a large photoplastic effect (a change in flow stress under illumination), and related effects due to the injection of electrons at an electrode. Deformation produces reversible changes in the conductivity, pulsed and continuous luminescence and the emission of electrons from the surface.     

Ultrasonic investigation of phase transitions in K2SnCl6

The velocities and the attenuation of ultrasonic waves have been investigated as a function of temperature (255KT350K) for the cubic phase of K₂SnCl₆ which undergoes a structural transition atT _c1263K. An anomalous decrease of the shear stiffness constantc=1/2(c ₁₁ –c ₁₂) of about 30% between room temperature andT _c1 is found in this cubic high temperature phase whenT _c1 is approached from above. Whereas the softening ofc extends over a substantial temperature range (about 50 K), the other shear constantc ₄₄ shows only a weak decrease, which is an order of magnitude smaller and restricted toT–T _c110 K. NearT _c1, a strong increase occurs in the attenuation of the shear acoustic wave propagated along the [111] direction, while such an anomalous attenuation is not observed for the transverse acoustic wave propagated along [100]. Furthermore, atT _c1 a hysteresis is detected for the longitudinal sound wave velocityv _L[100], which is restricted to the temperature region 262KT263.5 K. In addition, the room-temperature elastic constants of (NH₄)₂SiF₆ and the hydrostatic pressure derivatives of the elastic constants of K₂SnCl₆ and (NH₄)₂SiF₆ at room-temperature are presented and discussed in terms of mode softening behaviour.