Observation of soft-modes in a single crystal of Rb2ZnBr4 by Raman scattering

Francke, E.; Postollec, M. Le; Mathieu, Jean‐Paul; Poulet, H.

doi:10.1016/0038-1098(80)90240-9

Cited by 33 publications

(14 citation statements)

References 7 publications

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“…From the Raman study (Francke et al, 1980;Quilichini et al, 1982) and from group-theory analysis Dvorak & Kind, 1981) we think there is at least a doubling of the unit cell at the phase transition T o . Looking at these reflections we searched for systematically extinct reflections in the space group of phase V (Table 9a).…”

Section: Possible Space Group Of the Low-temperature Phasementioning

confidence: 97%

“…In Rb2ZnC14 the observed phases are (Table 1) 0108-7681/83/060657-07501.50 (Raman, optical) (Francke, Le Postollec, Mathieu & Poulet, 1980;Unruh & Str6mich, 1981;Giinter, Sanctuary & Rohner, 1982) and is also known to be ferroelectric. It has been shown (Unruh & Str6mich, 1981) that in phase V a spontaneous polarization P,, parallel to the pseudo-hexagonal a axis appears at T = T o and increases as T decreases while the spontaneous polarization Pb of the ferroelectric phase decreases with decreasing T. Furthermore, the behaviour of the relative permittivity K a at T o resembles that of a purely structural transition of second order and the high index of faintness n (= 4 or 6) suggests the formation of an improper ferroelectric.…”

Section: Introductionmentioning

confidence: 99%

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Neutron structural study of the successive phase transitions in Rb₂ZnCl₄

Quilichini¹,

Pannetier²

1983

Acta Crystallogr Sect B

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The crystal structures of Rb2ZnC14 at 300, 235, 100 and 60 K were investigated from neutron single-crystal diffraction data. Refinements in space groups Pnma (Z = 4, average structure of the incommensurate phase at 300 and 235 K) and Pna2~ (Z = 12, 100 and 60 K) converged to R values of 0.084, 0.085, 0.046 and 0.025, respectively. The strong anisotropy of the thermal parameters of chlorine atoms at 300 and 235 K suggests the presence of a static modulated distortion in the incommensurate phase. However, this displacive model is not sufficient to account for the successive phase transitions. Evidence is given that the ZnC14 group undergoes a large libration motion. The orientation of each group derives from its position in the paraelectric phase (T > 302 K) by a rotation around the pseudo-hexagonal axis a which is the direction of tripling of the ferroelectric-phase unit cell (T < 189 K). The structure of this ferroelectric phase is given. At 60 K a space group is proposed for the low-temperature phase (T < 74 K) based both on an analysis of the experimental data and on group-theory considerations. The present structural results are compared with those obtained by NMR and NQR spectroscopy.* Average symmetry of the incommensurate phase.

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Section: Possible Space Group Of the Low-temperature Phasementioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

Neutron structural study of the successive phase transitions in Rb₂ZnCl₄

Quilichini¹,

Pannetier²

1983

Acta Crystallogr Sect B

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“…On the other hand, neither the amplitude mode nor another soft mode was observed in our study. 2,[10][11][12][13][14] The temperature dependences of observed mode energies at 20 4 3 are shown in Fig. 2.…”

Section: Methodsmentioning

confidence: 99%

“…However, the Raman active modes were observed to soften below the N-INC transition point. [10][11][12][13][14] According to this evidence, it was assumed that a crossover between the displacive and the order-disorder regions occurred. 9,13,14) An empirical law between the pressure coefficient dT=dp of transition temperature and the mechanisms of the ferroelectric phase transition has been pointed out by Samara.…”

Section: Introductionmentioning

confidence: 99%

Neutron and X-ray Scattering Studies of Rb₂CoCl₄ and Successive Phase Transition in A₂BX₄-type Crystals

et al. 2014

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Soft phonon modes, which are related to the normal-incommensurate phase transition at T i = 294 K and the lowesttemperature phase transition at T 3 = 66 K in Rb 2 CoCl 4 , have been measured by inelastic neutron scattering. For the normal-incommensurate phase transition, phonon dispersion curves have been determined in the low-temperature commensurate phases, and the phase mode has been observed far below T i . The soft mode is fully overdamped above T i . Moreover, another soft phonon mode, which contributes to the phase transition at T 3 , has been observed. Furthermore, the space group of the lowest phase is directly confirmed to be C1c1 by verifying extinction rules of X-ray diffraction. It has been recognized that the ratio of the ionic radius of A + to the average bond length B-X in the tetrahedral BX 4 2¹ ion can control the phase transition sequence. Rb 2 CoCl 4 belongs to the group in which this ratio is in the range from 0.69 to 0.85, and the transition temperature decreases loosely with increasing the lattice parameter ratio c 0 /a 0 , where a 0 and c 0 are the lattice parameters of the normal phase.

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“…In some A,B&-type ferroelectrics below I;: the Raman soft mode was observed, with its frequency versus temperature dependence corresponding to the amplitudon. [14][15][16][17][18] In the C phase, a low-frequency mode corresponding to the phason was observed by Ramanism and neutron21 scattering. Besides, in the I phase an infrared-active phason was observed.,' At the same time very little is known of a real behaviour of the modulated structure and its evolution in the I For this reason, investigation of the temperature behaviour of the amplitudon and the phason is a current task when the dynamics of an incommensurably modulated structure is studied.…”

Section: Introductionmentioning

confidence: 99%