Spectrochimica Acta, Vol. 28A, pp. 2306 to 2310. Pergamon Prem 1972. Printed in Northern Ireland

The vibrational spectrum of a-mercuric snlpbide

PETER DAWSON Department of Physics, King’s College, Strand, London. W.C.2.

(Received 4 March 1972)

Abstract-The Raman spectrum of mercuric sulphide in the a-phase has been studied down to 4 cm-l. The symmetry aaaignments have been made by examination of the polarization data and by reference to the infrared spectrum. A discussion ia given of the observed scattering intensity in a 180’ scattering geometry as a near resonance condition is approached. Such a near resonance condition was obtained in mercuric aulphide at room temperature when 0328 A radiation was used to excite the spectrum. Two phonona at frequenoies 96 and 363 cm-1 which have been shown to participate in interband transitions in this crystal have been identified aa longitudinal optic phonona of E-symmetry.

1. INTRODUCTION

MERCURIC sulphide can exist in two crystalline modifications, a low temperature a-phase with a trigonal unit cell end a high temperature p-phase with the zinc blende structure. The phase transition point lies in the temperature range 280-340 C [l] and its existence rules out the use of conventional methods for the preparation of synthetic crystals and natural mineral specimens must normally be used to study its physical properties.

The trigonal a-phase is of considerable interest because it exhibits large non- linear polarizability and large positive biretigence, simultaneous properties which have made it useful in recent second harmonic generation and parametric ampli- fication experiments [2, 31.

Understanding of its lattice vibrational spectrum is still, however incomplete. The Raman spectrum has been studied by SCHEUERMANN and RITTER [4], by ZALLEN et al. [5] and by COONEY and -L [6]. RICCIUS and SIEMSEN [7] have examined both Raman and i.r. spectra and have made some symmetry assignments from a comparison of the two. The results of these authors are, unfortunately, incomplete and, in some respects, mutually contradictory-a result no doubt of the severe difficulties presented by the combination of strong optical activity and poor optical quality found in these crystals. The usual technique for eliminating effectively the complications introduced into Raman studies by the former is to reduce the path lengths of both exciting and scattered radiations, but as the specific

[I] E. H. CANLSON, J. Cry&d Urowth 1,271 (1967). [2] W. L. BOND, G. D. BOYD and H. L. CAR-, J. AppZ. Phye. 88,409O (1967). [3] G. D. BOM, T. J. BRIDGES and E. G. BURKHARDT, J. Qualatum Electron& (I.E.E.E.)

4, 61.5 (1968). [4] W. SCHEUERMANN and G. J. RITTER, 2. Naturforsch. !& 408 (1969). [S] R. ZALLEN, G. LUCOVSICY, W. TAYLOR, A. PINCZUK and E. BURSTEIN, Bull. Am. P&s.

Sot. 12,422 (1967). 161 R. P. J. COONEY and J. R. HAL& Au&. J. Chem. 22,331 (1969). [7] H. D. RICCIIJS and K. J. SIEMSEN, J. Chem. Phys. 52,409O (1970).

2306

2306 PETER DAWSON

rotation of cc-mercuric sulphide is approximately 480’ per mm at 6000 L% [S] this procedure is not practical in the present case. It would be necessary to restrict the path of the laser beam to perhaps 0.01 mm thereby reducing the scattering volume to microsampling proportions, a problem which is taken up again in the next section.

The present paper describes a new Raman investigation of a-mercuric sulphide in which measurements down to 4 cm-r were achieved by exploitation of a resonance effect. The symmetry assignments made from polarization measurements have also been confirmed by the preliminary results of a polarized i.r. reflection study.

2. THEORETICAL CONSIDERATIONS In the a-phase the unit cell of mercuric sulphide is trimolecular and the trigonal

space group is _Z&4 [9]. The selection rules for the Brillouin zone-centre normal vibrational modes are summarised in Table 1.

Table 1. The selection rules for the Brillouin zone-centre modes of vibration of a-HgS. f denotes forbidden and P denotes permitted

Ds4

H@ 3 molecules/unit cell

No of vibrations Selection rules Total Acoustic Optic Raman Infrared

AI -42

E

2 4 6

0 1 1

2 3 5

In an optically active uniaxial crystal the specific rotation, p, exhibits a depend- ence on propagation direction having a maximum value for radiation propagating along the principal axis and a (small) minimum value for radiation propagating in the plane perpendicular to it. Specific rotations are, in fact, very difbcult to measure in directions remote from that of the principal axis and very little work has been done in this field [lo]. Though it is not perfectly clear, the value of p quoted for mercuric sulphide is probably a maximum axial value. Consequently all the experi- ments described here were conducted in the plane perpendicular to the principal axis and crystal slices of maximum thickness 0.3 mm were used. It is known that residual optical activity in this plane can result in the conversion of plane polarized light to elliptically polarized light, though in practice this effect is small [ll]. Yet another reason for avoiding propagation of incident and scattered light along the principal axis of a uniaxial crystal has been given by PORTO [12]. Taking these precautions resulted in imperfect but clearly indicative polarization data and the symmetry assignments of the E-type phonons were in any event verified by reference to the i.r. spectrum.

[S] Ir&emationul Critical Tables, Vol VII, McGraw-Hill, New York (1930). [Q] R. W. G. WYCKOFF, cry8til Structures 2nd Edn., Interscience, New York (1964).

[lo] J. F. NYE, Phyeical Properties of Crystals, Clarendon Press, Oxford (1957). [ll] Encyclopedia of Physics (Edited by FL%QE S.) Springer, Berlin (1961). [12] S. P. S. PORTO, J. A. GIORDUINE and T. C. DAMEN, P&8. Rev. 147,608 (1966).

The vibrational spectrum of a-mercuric sulphide 2307

#--Is

Collection optics

10 -0 -------L

Fig. 1. Scattering from m single crystal in a, 180’ mode.

The forbidden energy gap of a-HgS is 2.03 eV at 300 K [l3] necessitating the use of a long wavelength exciting source. The helium-neon laser operating at 6328 A (1.96 eV) suggested itself as a possibility and was used in a 180’ scattering mode. It is interesting to consider the factors influencing the observed signal strength in this situation. Let cc1 and a2 be the absorption coefficients for the incident and scattered frequencies and let S be the Raman scattering efficiency (dependent on the incident frequency and on the phonon concerned), then referring to Fig. 1, if I,, is the incident laser intensity then the signal received at the detector I, is:

I, = K s

z [#(I, e-@l+s)z) &]e-asZ, 0

where K is a geometrical factor. This leads to:

I, = KI, - s

a1 + a2 + S - [ 1 - exp - (a, + a2 + iW1.

In reality a1 N a2 = a. One must choose an exciting frequency which gives e, large value of S and a

small value of a. This is generally difficult as both S and a increase with the incident photon energy RYE, as hvi approaches the forbidden energy gap from below i.e. approaches towards a resonance condition. As LOUDON h= shown the optimum exciting frequency to use is one at which the absorption coefficient just begins to rise significantly from its zero value below the absorption edge [14]. At higher frequencies, though a further gain may be achieved in the scattering efficiency, it is offset by the rapidly increasing absorption of the incident and scattered light. Aa only a limited number of discrete frequencies are available from current ow gas lasers it is not always possible to choose such an ideal frequency. However in the present case it was found that 6328 A excitation provided an almost ideal frequency and such it ‘near resonance’ condition was achieved permitting the use of very thin sample sections, a small cone of collection of the scattered light and high spectral resolution all with an exciting laser power of only 20 mW.

3. EXPERIMENTAL CONSIDERATIONS The crystals used in these measurements were natural specimens kindly donated

by the trustees of the British Museum. They were oriented by X-ray methods and cut into thin slices (co.3 mm) parallel to the crystal c-axis. The Raman spectra

[13] YA. 0. DOV~II and B. F. BILEN’KII, So&et Phys. SoZidState 8, 1280 (1966). [14] R. LOUDON, J. Phys. 26, 677 (1965).

2308 PETER DAWSON

were excited by the 6328 A output of a helium-neon laser operating at a power level of 20 mW in order to avoid thermal damage to the sensitive crystals. The scattering experiments were conducted in a 180’ mode in the plane perpendicular to the c-axis and the semi-angle of the cone of collection of the scattered light was reduced to less than 5“ in order to define thii plane more accurately. The back scattered light was analyzed by a Cary 81 Raman spectrometer and detection was via a EM1 9558 photomultip~er in a phase sensitive system. ~though such an a~angement is not usually compatible with measurements at very low frequencies, it was found that in the present case, due to the resonance effect discussed in the last section, it was possible to use very low amplifier gain and thereby approach within 4 cm-1 of the Rayleigh line when the strong~t band of the safe was full scale.

4. RESULTS AND DISCUSSION

Two Reman spectra of IIgS taken at 300 K are shown in Fig. 2. The first trace shows the tensor element asp which gives the two A, modes at frequencies 43 and 256 cm-l. The second trace shows the tensor element a,, and therefore E: modes only. Simple conservation of momentum considerations show that both longitu- dinal and transverse optic phonons should be observed in this scattering geometry. The spectrum of the E phonons is very weak and the A phonons are not completely extinguished, probably a result of crystal imperfections rather than of residual optical activity.

Figure 3 shows A and Z symmetry phonons together under greater amp~~tion. Much more of the E phonon spectrum is now discernible and there is also some multi- phonon structure in the frequency range 60-200 cm-l. This was clear from low temperature studies.

The phonon s~met~es and frequencies are collected in Table 2 together with the preliminary results of a polarised infrared refleotion study [15). The latter confirm the symmetry assignments and allow a positive distinction to be made between longitudinal and transverse optic modes of B symmetry.

The Edward ~~~a~ty in the Raman ~quen~ies is fl cm-l and in the i.r. frequencies, which were derived by a Kramers-Kronig inversion procedure from the reflection curves, somewhat larger and in any case these latter results are provisional. This could explain entirely the discrepancies in the frequencies as measured by the two cliques but it is possible there is another factor at work. In a uniaxial crystal the frequencies of polar phonons are dependent on their propagation direction and the poor optical quality of the crystal and the necessarily finite cone of collection of the scattered light may result in the observation of B phonons with a range of prop~ation directions resulting in a spreading of f!requencies. This would explain, for example, the failure to resolve the bands at 88 and 202 cm-l into their LO and TO components. The last of the five predicted fl phonons has not been observed in the i.r. spectrum.

One investigation of the fundamental absorption edge of HgS [13] has shown that it is associated with interband transitions in which at least two phonons participate. These have been assigned the frequencies 96 and 353 cm-l and Table 2 identiiies them as longitudinal optic phonons of .F symmetry.

[15] P. L. R. MORSE, Private communication.

The vibrational spectrum of a-mercuric snlphide

I I I I1 I I I I I ] !300 450 400 350 300 250 200 150 IO0 50 0

Frequency. cm-f

Fig. 2. Polarized Raman spectra of a-HgS at 300 R. The extinctions of the two Al spmhry modes me not complete & result primazily of poor cry&d optical quality ad they appear in the spectrum of the E-symmetry modes maked by

aate*.

Al 256

Al 44. -

-. . 500 450 400 350 300 250 200 150 loo so 0

Frequency, cm-l

Fig. 3. High g&n, high resolution scan of HgS showing both A, and E-modes together. Some of the structure in the frequency range W-200 cm is due to

multiphonon pr&sses.

2309

2310 PETER DAW~ON

Table 2. The frequencies and symmetry assignments of the fundamental vibrations of a-HgS.

Phonon symmetry Frequenoy (am-‘)

Reman Infra red

4 44 Forbidden

Al 256 Forbidden

TO

4 LO

TO

-4, LO

TO

A* LO

TO E

LO

TO E

LO

TO E

LO

TO E

LO

TO E

LO

Forbidden

Forbidden

Forbidden

88 1 105

147 1 285

292 1 345

1 353

202 1

36

41 1 102

139 1 336

361 1 83

91 1 100

145 1 280

289 1 343

1 349

Not observed

5. CONCLUSIONS

As discussed in the theoretical section, in the absence of a continuously tunable exciting source for Raman spectroscopy it is not always possible to study adequately or exploit resonance Raman effects in crystals. It should, however, be possible within limits to ‘tune’ the crystal to the exciting frequency. Thus if one of the now quite numerous discrete cw laser lines lies in approximately the right position for, say, maximum signal in a near resonance condition, then by careful variation of the temperature of the crystal its band could be moved to the optimum position. It would clearly be preferable to choose, where possible, an exciting frequency which is slightly too high (too much absorption) at room temperature so that the crystal must be cooled in order to shift the band gap down. This would have the simul- taneous effect of sharpening the spectral lines as an added bonus.

This simple technique seems not to have been much tried in crystal studies. Preliminary trials with mercuric sulphide (in which the energy gap moves by 6 x 1O-4 eV per degree Kelvin) showed that the procedure is certainly feasible and a more detailed study of this crystal along these lines will be the subject of a later paper.