Polarization analysis of the Raman spectrum of As2S3 crystals

Solid State Communications, Vol. 29, pp. 361—364. Pergamon Press Ltd. 1979. Printed in Great Britain.

POLARIZATION ANALYSIS OF THE RAMAN SPECTRUM OF As

2S3 CRYSTALS

C.Razzetti and P.P.Lottici Istituto di Fisica de11’Universit~ and Grup~o Nazionale di Struttura della Materia del CNR, Parma,Italy Received on July 3rd, 1978 by R. Fieschi We present the first complete series of polarized Raman spectra of orpiment crystals and the corresponding possible symmetry assignments of the phonon peaks within the layer model structure. We observe new peaks and a low frequency mode which could be a rigid layer vibration. We discuss briefly on the possible implications of some inconsistencies between the expected and the observed results.

We have started a study of orpiment in the task of comparing the resonant behaviour of amorphous As2S3 with that

Table 1 — Correlation diagram relating the layer and crystal vibrations of As2S3. Raman and infrared activity is indicated.

of the corresponding crystalline form! While we are solving the particular problems connected with the full resonance conditions in this biaxial crystal, we present here our results obtained in the nonresonant case. Despite 2’3’4 the attention devoted many features ofto itsthis dynamic behaviour are still in discussion material and a complete series of polarized spectra has yet to be published. As 2S~ is a semiconductor which crystallizes as a layered molecular crystal with nearly perfect (010) micaceous cleavage planes. The structural molecular units (i.e. the layers) dimensions. The layers are formed are macroscopically extended in twoby AsS 3 pyramids linked together by

LAYER

CRYSTAL

2V

7~

(1W)

7Bu (~aIfc)

6A9 (aa bbccac) 682

(a~

As-S-As bonds. In the layers As-S spi ral chains run parallel to the c axis.3, presumably via Van der Waals forces: The interlayer interaction is weak due to this fact the system has been

7A2

(ab)

pictured in a twofold way 2,4 In the first picture (3D) the system is monoclinic (nearly orthorhombic, being

78i

(bc

= 90° 5) with 20 atoms per unit cell, and should show 57 optic modes obeying to the C~hspace group selection rules. In the second (2D), the system is again orthorhoinbic with 10 atoms per unit cell and 27 optic modes obeying to the ~ space group selection rules. The correlation shown in Table 1. the two pictures table are connected through This number correlation is obtained through Davydov splittings at the ex-

[~

(aabbcc)

(hu1~)

A1s.B1+B2 (acoustics

361

6B~(~aHc)

7Bg (ab) Th~(Mb)

[

7Au (fib) 7B 9 (~)

I1~

A9

I.

Ag 26.j

I

L

Au+ (acoustic)

362

RAMAN SPECTRUM OF As

2S3 CRYSTALS

Vol. 29, No. 4 356

312

307 360

293 289 26

I

158 I 154 137 1162

68

I 6270

II

I

I 106

145

I

I

52

204

I

384

180

~

188

263

326

i

3701

I

401 1

20

100

180

Raman

260

shift

340

420 cm

Fig. 1. — The non-polarized RT Raman spectrum of orpiment in the range 20—420 cm—1.

pense of the interlayer interactions, which last activate three more optic mode identified as rigid layer (RL) modes. No experimental evidence has yet supported either picture (mainly due to experimental problems - see for exam ple Ref. 6 - which have limited the available data), even if the 2D picture has been favoured in literature, We have obtained good samples from bulk Hungarian orpiment, kindly provided by Dr. Calzetti of the Mineralogy Dept. - Universit~ di Parma. These samples have been controlled with X ray diffraction and by direct inspection under the polarizing microscope. A series of selected crystals, oriented according to the cleavage plane (II b), the creep direction in the plane (1/ c) from the polarized light analysis and

2,3,5 peaks has increased with time together with the use of better samples and the refining of the experimental technique. We too have to add our contribution of previously unobserved peaks at 52, 162, 188, 263, 343, 401 cm~ in Table (2) we summarize and conpare the previous results and assignments with our data. The assi nment tasK is not a simple and straig~tforward cne: nonetheless our proposed assignments are the only ones based on a complete series of spectra. The biaxial character of the crystal could give a dependence of the polarization of the scattered light on the direction of propagation, so mixing in a spurious way the observed polarizations. A similar mixing contribution could be ascribed to unavoidable struc-

the known structure of orpiment,

tural defects.

has

been nolarization analyzed at RT under 5145 X excitation. The spectra were taken with 1 cm~ band-pass and stored with averaging technique in a multichannel analyzer. In Fig. (1) we show the non-polarized spectrum (i.e. not analyzed in scattered light polarization), and in Figs. (2) and (3) the series of polarized spectra, which are complete according to the 2D space

group selec-

tion rules. The spectrum is very rich in features: the number of observed

The layered crystals such As 2S3 should show three zone center RL ortical phonons, namely two nondegenerate shear modes (of Ag symmetry in the 3D scheme), and a compressional one, expected to lie at higher frequency than the former two (and of Bg symmetry). Zallen et al. interpret the pair of low frequency vibrations (25 and 36 cm~ which they assign to the same symmetry) as the two shear RL modes. They have not observed a third

Vol. 29, No. 4

RAMAN SPECTRUM OF As

2S3 CRYSTALS

363

cc

ac 1 20

100

180Raman shift260

340

420 cm

Fig. 2. — The occ (upper) and nac (lower) components of the RT Raman spectrum of orpiment, obtained in b(cc)a and b(ac)a scattering geometries, respectively.

cb

ab 20

100

180 260 Raman shift

340

Fig. 3. — The acb (upper) and nab (lower) components of the RT Raman spectrum of orpiment, obtained in b(cb)a and b(ab)a scattering geometries, respectively.

420

cm1

364

RANAN SPECTRUM OF As

2S3 CRYSTALS

Table 1) observed 2 — One phonon in As Rainan frequencies (in cm 2S3 and assignments ac— cording to C2~symmetry. This work reports a number of peaks which cannot be expected from C2v. The assignment is for comparison with previous results. This worI~~

Ref.3

Ref.2

26 A1? 37 A2 52? *

25 36

25 35

—

—

62 B1

63

60

—

—

70 68 AA2 1 137 106?A 1 145 B1 154 A2 158 Bi 162 ~ * 180 B1 188 ? * 204 ~ 263 ? * 289 7 293 B~ 307 A1 312 A2? 326 ? 343

?

69 107 136 A1

370 ? 384 A1 401 A1

69 105 A2 134 B2

—

—

154 A1

152 A1 156 A1

— —

—

179 Bi —

179

62

lying modes belong to different symmetries, and that there is a third low lying (52 cm~) previously unobserved peak which could be the missing RL node; however its symmetry assigneinent is ambiguous (see Figs. (2) and (3) ). But, given that this mode is the third RL and that it may have the “right’ symmetry (i.e. the same as the 26 or 37 cm~ peaks), then we remain with the following point: the RL compressional mode has not either the higher fre1 is quency as suggested, but a frequency of 26 or 37 cn ; or the 52 cm not RL; not RL, but thea1could on thebe same basisor also 26 and 37 cm the factor space group predictions are incorrect. Apart from these considerations on the RL modes, we note an apparent “rigidity” of the spectrum, with very few phonons sensitive to polarization.

—

204 A1 —

201

A1

—

—

293 —

311 A1 326

*

354 A2 356 ~ 360 B1?

B2 A2

Vol. 29, No. 4

—

290

B2+A2

—

300

B1

309 A1 — — —

354 A1 —

355 A1 —

359

359 364

—

A1 A2

—

382 —

381

~i

—

This point could be ascribed (beyond the quoted above) either rules to sone problems relaxation of the selection or to some dynamic fact which could mask the predictions of the space factor group analysis. We are presently working at the series of all the possible different polarized spectra in order to analyze the complete Raman tensor. We feel that the ambiguity of the results heretofore obtained may be connected to the fact that the model picture does not contain some fundamental feature, due, for example, to the chain character of As-S bonds which could lower the dimensionality of the system. In particular we think that the backscattering geometries could provide use

E*previ008iy unreported)

line in proximity of the former two (the B compressional mode), and have sugges~ed that it could be a ijiember of a doublet in the 60-70 cni region. Our results indicate that the two low

ful hints to separate spurious geometrical effects from substantial dynamic ones. Studies in this sense are in progress.

REFERENCES 1)

C.RAZZETTI and P.P.LOTTICI, phys.stat.sol.

(b) 87,

479 (1978).

2) J.M.MATI-IIEU and H.POULET, Bull.Soc.Fr.Mineral.Cristall. 93, 532 (1970). 3) R.ZALLJ2N, M.L.SLADE and A.T.WARD, 4) R.ZALLEN and M.L.SLADE,

Phys.Rev. B 3, 4257 (1971).

Phys.Rev. B 9, 1627 (1974).

5) N.MORIMOTO, Mineral.J. Tokio 1,160 (1954). 6)

E.J.FLYNN,

S.A.SOLIN and J.N.PAPATHEODOROU,

Phys.Rev.

7)

R.J.KOBLISKA and A.SOLIN, Phys.Rev. B 8, 756 (1973).

B 13,

1752

(1976).