A RAMAN SCATTERING STUDY OF THE SOLID TO SOLID PHASE TRANSITION IN BENZIL

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A RAMAN SCATTERING STUDY OF THE SOLID TO SOLID PHASE TRANSITION IN BENZIL

D. Moore, V. Tekippe, A. Ramdas, J. Toledano

To cite this version:

D. Moore, V. Tekippe, A. Ramdas, J. Toledano. A RAMAN SCATTERING STUDY OF THE SOLID

TO SOLID PHASE TRANSITION IN BENZIL. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-

785-C6-787. �10.1051/jphyscol:19816232�. �jpa-00221317�

JOURNAL DE PHYSIQUE

CoZZoque C6, suppldment au n o 12, Tome 4 2 , dgcembre 1981 page c6-785

A RAMAN SCATTERING STUDY OF THE S O L I D TO S O L I D PHASE T R A N S I T I O N I N BENZ I L

** ***

D.R. Moore, V . J . ~ e k i p p e * , A.K. Ramdas and J.C. Toledano University of I l l i n o i s a t Chicago Circle, U.S.A.

* Gould Laboratories

** Purdue University

***c.B. E.T., fiance

Abstract. --tt 8 4 O ~ , benzil undergoes a first order phase transi- tion from Dj space group symmetry in the upper phase to C2 sym- metry in the lower temperature phase with a simultaneous four- fold expansion of the primitive unit cell. In addition to the effects of the phase transition on the zone center optical modes which are Raman active above Tc, our polarized Raman studies do indeed show the emergence of new modes, including a second soft mode, which are related to the enlargement of the unit cell.

I. Introductory Remarks

Room temperature benzil ( C H COCOC H ) has a trigonal structure

46 2

belonging to the space group D3. At Tg84 K, benzil undergoes a solid to solid phase transition in which the trigonal phase converts to a monoclinic one of space group symmetry C 2 . A previous Raman scatter-

ing studyL of this phase transition revealed a pronounced, nonlinear softening of the lowest frequency optic mode on both sides of Tc.

This soft mode is associated with the center of the Brillouin zone and typically induces a phase transition that preserves the number of at- oms per unit cell. However, x-ray diffraction studies show that the phase transition is accompanied by a four-fold expansion of the prim- itive unit cell 2 . This point was recently addressed by ~ o l e d a n o ~ in a phenomenological model of the phase transition in benzil. In this dual order parameter model, the primary order parameter is associated with the observed zone center soft mode and determines the trigonal to monoclinic symmetry change. The secondary order parameter corresponds

-+ *

to the star of 2 vectors (kM) at the M point of the Brillouin zone and is related to the observed four-fold expansion of the unit cell. The model allows a nonlinear coupling between the two order parameters and

the mechanism of the phase transition is assumed to proceed through a

"triggering" of the M point instability by the one at the r ^point.

-+ *

Although the phonon modes associated with kM are Raman inactive above Tc, they are expected to become Raman active below Tc. In addition, the model predicts that the soft optic mode at 1 zl=0 will trigger a second soft mode related to kM. Since neither this second soft mode +*

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816232

C6-786 JOURNAL DE PHYSIQUE

Fig. 1: Group theoretical -

analysis of the phase tran- - sition in benzil. The lines indicate the corre- lation of the modes in the trigonal phase above Tc

with those in the mono-

clinic phase below T , .

The solid lines indicate

those modes which are ex- '

0

.

^/'- -

pected to show strong

Raman activity.

( 3 ) A2

-

^I

nor the additional modes associated with the unit cell expansion had been observed in previous studies, a more comprehensive study of the phase transition by Raman scattering techniques was clearly indicated.

11. Experimental Results

Discussion

The decomposition of the lattice modes at the r ^and M points of the hexagonal Brillouin zone of the trigonal phase into the A and B irreducible representations at the r point of the monoclinic phase is summarized in Figure 1. The numbers in parentheses indicate the num- ber of modes of each symmetry type. At the transition temperature the decomposition proceeds as follows: A1+A, A2+B, E+A+B, r1+2A+B and r2+A+2B.

The temperature dependence of the polarized Raman spectra re- corded in the Y(XY+XZ)X scattering geometry is shown in Figure 2a.

As the temperature is lowered to Tc (dashed line) the intensity of the lowest E mode (le) increases dramatically while its frequency de- creases sharply. This anomalous behavior is characteristic of a soft optic mode inducing a phase transition and is similar to the results observed by Sapriel et a1.l The emergence of an additional mode (2b) positioned on the shoulder of the B component of the soft E mode (lb) is also observed at T;Tc. This new mode is interpreted as the strong- er B component of the soft r2 mode predicted by Toledano 3

The two soft mode components are clearly distinguishable since they exhibit a temperature dependence different from that of the other modes. As the temperature decreases, both modes become less intense and move towards higher frequency. This opposes the general increase in inten- sity exhibited by all of the other external modes. Since a maximum of eight B modes can be associated with the zone center modes of the trigonal phase, at least eleven of the modes observed in the spectrum

-f

*

at T = ~ ~ O K correspond to k M and, hence, to the enlargement of the unit

cell. Similar results were obtained for the modes of A1 symmetry

above Tc and A symmetry below Tc. The corresponding spectra were

( a ) RAMAN SHIFT

0 2 0 4 0 8 0 8 0 100

(b) RAMAN SHIFT _leni'l

Fig. 2: The polarized Raman spectra exhibited by benzil at selected temperatures aboye and below Tc. The spectra were excited with 5145 8 radiation and recorded in a 90° scattering geometry. The scattering geometries used allow a) E modes above Tc and B modes below T, and b) A1 modes above Tc and A modes below Tc.

recorded in a Y(ZZ)X scattering geometry and are shown in Figure 2b.

The A components of the two soft modes are denoted as la and 3a. Ad- ditional theoretical considerations predict that of the sixty-nine Raman active modes expected for T<Tc, seventeen A modes and eighteen B modes will be intense enough to observe 4 . This is in excellent agreement with the seventeen A modes and nineteen B modes observed in the present study.

In conclusion, our Raman scattering study of the phase transition in benzil has revealed effects due to both the symmetry change and the expansion of the unit cell, in excellent agreement with the predic- tions of a dual order parameter model.

References :

1. J. Sapriel, A. Boudou and A. Perigaud, Phys. Rev. =, 1484 (1979).

2. G. Odou, M. More and V. Warin, Acta. Cryst. A Z , 459 (1978).

3 . J. C. Toledano, Phys. Rev. B20, 1147 (1979).

4. D. R. Moore, Ph.D. Thesis (University of Illinois at Chicago

Circle, 1981) unpublished.