Disorder-induced Raman scattering in ammonium halides

The mechanism responsible for the lambda-phase transitions in ammonium halides is investigated within the framework of an extended theory of the disorder-induced Raman scattering. The Raman spectra of ammonium halides have been predicted for both the disordered beta phase (NH4Cl and NH4Br) and the ordered gamma phase (NH4Br). It is postulated that those terms which dominate the dynamic distortions of the lattices and thereby cause the bulk of the disorder-allowed Raman scattering, will also dominate the quasi-static distortion which is the lambda-phase transition in these crystals. From this we find that the four-spin correlation function for the M-point boundary phonons in the disordered phase (beta), and the two-spin correlation function for the zone centre phonons in the ordered phases (gamma and delta) dominate the mechanism of the lambda-phase transitions in ammonium halides. Predictions obtained for the Raman scattering agree in most cases with our Raman observations on NH4Cl and NH4Br.


Landau theory of the nematic-smectic a tricritical phase transition in a binary mixture of liquid crystals
Yurtseven, Hasan Hamit (Informa UK Limited, 1995-01-01)
We have developed in this study a thermodynamic model for the nematic-smectic A (NA) tricritical phase transition of a binary mixture of liquid crystals in analogy with the model for quasi fluid mixtures due to Griffiths. Using the Landau theory with a symmetrical TCP, we have predicted the critical behaviours of the nematic susceptibility, order parameter and the specific heat discontinuity in the vicinity of the TCP.
Yurtseven, Hasan Hamit (Informa UK Limited, 1994-01-01)
This work is mainly concerned with the application to the ammonium chloride system of the model that was developed by Yurtseven (1993).
A phase diagram near the NAC* point in liquid crystals
Salihoglu, S; Tublek, A; Yurtseven, Hasan Hamit (Informa UK Limited, 2000-01-01)
Wt study here a mean field model to obtain the phase diagram (concentration versus temperature) near the NAC* point in a binary mixture of liquid crystal. Wr have Fitted our phase line equations to the experimental data for the mixture of SCE9 + SCE10 liquid crystals. We deduce fi om our analysis that there should exist a tricritical point close to the NAC* point on the AC* phase line.
The lambda-phase transition in NH4Br
Yurtseven, Hasan Hamit; Sherman, WF (Informa UK Limited, 1996-01-01)
This study gives the predicted frequencies for the Raman modes of v(7)TA(56 cm(-1)), v(5)LO(177 cm(-1)) and v(2)(1684 cm(-1)) of NH4Br at various temperatures for zero pressure across the lambda-phase transition in this crystal system. We calculate the Raman frequencies by means of a constant mode Gruneisen parameter for each mode across the lambda-phase change region of NH4Br.
Ultrasonic frequencies correlated with the volume changes of the q[1(1)over-bar-0] mode for the first-order phase transition in NH4Cl
Yurtseven, Hasan Hamit (Informa UK Limited, 2000-01-01)
We correlate here by means of gamma-Gruneisen relations the volume changes with the ultrasonic frequencies of the q[1 (1) over bar 0] mode of NH4Cl for the first-order phase transition in this crystal. The ultrasonic frequencies were calculated by a method, which we have developed, using the length-change data from the literature at pressures of 0, 0.2, 0.3 and 0.6 kbar in the first-order phase region of NH4Cl.
Citation Formats
H. H. Yurtseven, “Disorder-induced Raman scattering in ammonium halides,” PHASE TRANSITIONS, pp. 165–179, 1995, Accessed: 00, 2020. [Online]. Available: https://hdl.handle.net/11511/56975.