Raman Scattering Studies on (NH₄)Cl_1−xBr_x Mixed Crystals in the Tetragonal Phase

Abstract

Raman measurements have been performed on the mixed system (NH,)C11 -.Br, in the tetragonal phase. For intermediate x-values, new peaks are found in the acoustical frequency region of the spectra, whose mode strengths are proportional to x and (1 -x ) , respectively. Adopting the assumption that the concept of a Brillouin zone is still useful in a mixed crystal, the Raman active modes of the tetragonal phase of the mixed crystals are identified with the M-point modes of the cubic p-phase. The experimental data are compared with a simple calculation based on the REI-model. Am Mischsystem (NH,)Cl1 -.Brz in der tetragonalen Phase wurden Ramanmessungen durchgefiihrt. Im akustischen Frequenzbereich der Spektren werden fur mittlere z-Werte neue Moden gefunden, deren Intensitaten proportional zu x beziehungsweise (1 -x ) sind.Unter der Annabme, daB das Konzept der Brillouin-Zone auch fur einen Mischkristall anwendbar ist, konnen die ramanaktiven Moden der tetragonalen Phase der Mischkristalle mit den M-Punkt-Moden der kubischen p-Phase identifiziert werden. Die experimentellen Ergebnisse werden mit einer einfachen, auf dem REI-Model1 basierenden Rechnung verglichen.

Far‐Infrared and Raman Studies on (NH4)Cl1−xBrx Mixed Crystals in the Cubic Phases β and δ

“…The Raman spectra in the P-phase, which reflect a weighted density of states, indicate that the high symmetry points of the Brillouin zone of the pure crystals remain points of a high density of states in the mixed crystals, too. The concentration dependence of the peaks of the polarized spectra leads to the conclusion that the system (NH,)Cll -%BIZ reveals an acoustical two-mode behavior, which is in good agreement with the results reported in the foregoing paper [14]. We found no evidence of (NH4)Cl1 -,Br, undergoing a room-temperature phase transition a t some value of x.…”

“…The same holds for the Tzg peaks a t 112 cm-l for NH4C1 and a t 76 em-l for NH,Br. These experimental results indicate that the optical one-mode system (NH,)CII -,BrZ reveals an acoustical two-mode behaviour as discussed in detail in the foregoing paper [14]. The solid lines in Fig.…”

“…The ir measurements and the low-temperature Raman measurements will be compared with a calculation based on a recent model calculation [13]. The Raman work on (NH4)C11-,Br, in the tetragonal y-phase is reported in a preceding paper [14].…”

Raman Spectra and Superconductivity of Various Phases of a High‐Tc Superconductor: NbN

“…For y-NblVo,vs, with regard to the narrow Ranian line a t 162 cni-l one may speculate as follows: The lattice of the tetragonal y-KbN has a superstructure of twice the lattice parameter c in one direction [ll, 121. Such a superstructure leads to a reduction of the Brillouin zone (BZ), thereby folding zone-edge phonons back to the zone centre [13]. If zone-edge TA phonons in [loo] of the normal BZ a t = 22 meV [14] are folded back to q = 0, a transition leading to a low-frequency Raman line a t z 170 cm-1 may occur.…”