Ultrasonic investigation of the phase transitions in NH4Cl1–xBrx Mixed Crystals

Abstract

Studying the critical phenomena accompanying the order‐disorder phase transition (β‐δ, β‐γ) in NH4Cl1–xBrx mixed crystals especially at multicritical points, the velocity and the attenuation of the longitudinal sound wave at 10 MHz propagating along the [100] direction are measured as a function of mole fraction (x = 0 to 1) and of temperature (310 to 200 K). A large critical softening of velocity is observed in the disordered β‐phase of both β‐δ and β‐γ transitions. The results indicate that the fluctuations of two order parameters corresponding to the δ‐ and γ‐phases have a mutual interaction through elastic strain. The critical softening is expressed by a simple power low: [V(∞)‐V(0)]/V(∞) = A[(T – T0)/T] q. The critical index, q, varies with x. The fluctuations of the two order parameters in the disordered β‐phase are enhanced near the triple point, and are suppressed near the tricritical point.

λ‐transition of NH4Cl and ND4Cl at high pressures

“…(7)), or equivalently Eq. (17) was fitted to the experimental data for T-P phase diagrams of NH 4 Cl and ND 4 Cl [46]. As shown in Figs.…”

“…A master P-T phase diagram locating multicritical points in ammonium and deuteroammonium halides has been obtained experimentally [44]. Besides P-T phase diagrams in ammonium halides, the observed X-T phase diagrams for NH 4 Cl 1-x Br x system have been given in the literature [22,[45][46][47].…”

Calculation of the free energies for the solid phases of ammonium halides

“…Using the triple point temperature of T t ¼ 548.3358 C for NH 4 Cl (table 1), we plot C P (I) against temperature for the solid I-liquid transition in NH 4 CI according to equation (43) in figure 4. The same functional form of the specific heat, C P (II), according to equation (43), is also plotted against temperature for solid II-liquid phase transition of NH 4 Cl in figure 5. For the solid I-solid II transition we have the same functional form of the specific heat, C P (I) or C P (II), as plotted in figure 6 for NH 4 Cl according to equation (44).…”

Analysis of the Raman frequencies of a lattice mode in NH4Cl close to the tricritical and second order phase transitions

“…A general P-T phase diagram in ammonium and deutero-ammonium halides has been obtained experimentally [6]. There have been some experimental studies to obtain T-X phase diagrams for a mixed crystalline system of NH 4 Br x Cl 1 À x [7][8][9][10]. Apart from those experimental studies regarding the phase diagrams in ammonium halides, some theoretical phase diagrams in these crystalline systems have been obtained [11][12][13].…”