ELSEVIER Physica B 234-236 (1997) 364-366

A cinnabar local order in liquid II-VI compounds

J.Y. R a t y a'*, J .P . G a s p a r d a, R. C601in b, R. Be l l i s s en t b

• University of Libge, Condensed Matter Physics B5, B4000 Sart-Tilman, Belgium b Laboratoire Ldon Brillouin (CEA-CNRS), Gif-sur-Yvette, France

Abstract

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A short wavelength (2 = 0.7 A) neutron scattering experiment has been performed on the II-VI compounds HgS, HgSe and Zn~Hg l_~Te. Liquid HgS can be viewed as a distortion of the blende structure; we show that this distortion decreases gradually with temperature. For ZnxHg 1 _xTe, we show that the transition between the six coordinated HgTe liquid and the four coordinated ZnTe liquid is not continuous, the intermediate phase being a deformed cinnabar one.

Keywords: Liquids; Semiconductors

1. Introduction

Most tetrahedral semiconductors get a six coor- dinated structure in the liquid. This change in local structure results in a semiconductor-to-metal transition and is accompanied by a change from the sp 3 resonance bonding towards a dominant p -p bonding mechanism. Recently, the II-VI tetrahed- ral compounds ZnTe, CdTe and HgTe have shown a different behaviour: ZnTe and CdTe keep their coordination four in the liquid and remain semiconductors while the heavier HgTe becomes six coordinated and metallic [1]. In this paper, we study the structure of liquid Zn~Hgl -~Te and HgS.

2. Results

We performed the neutron scattering experi- ments on the 7C2 spectrometer at the Orphee reactor of the L.L.B. We used hot neutrons

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(2 = 0.7035 A) in order to extend the q-range up to 15.9 A -1. After standard corrections of the scattered

*Corresponding author.

intensity, the pair correlation function g(r) is ob- tained by Fourier transformation.

The HgS cinnabar structure is the result of a Peierls-type distortion of the NaC1 structure which produces an assembling of helicoidal alter- nating chains. Each atom has 2 neighbours along the chain and 2 + 2 more distant neighbours be- tween the neighbouring chains. The pair correla- tion functions of liquid HgS are shown at 840°C and 930°C in Fig. 1. The area under the first peak of 4nprZg(r) gives a first neighbours number Z equal to 1.8. The second peak is merged in the third peak at 840°C and moves towards shorter distances at higher temperature. The number of second neigh- bours is constant and equal to 1.9. The deformed cinnabar order adopted at 840°C tends thus to become tetrahedral at 930°C.

ZnTe is a 3.7 coordinated liquid semiconductor and HgTe becomes 6.3 coordinated and metallic at the melting temperature. The addition of ZnTe to pure HgTe causes the first neighbour peak position to decrease below the first peak of pure ZnTe as shown in Fig. 2. For concentrations x -- 0.6 and 0.9 the 9(r) differ from other concentrations, with three pronounced peaks. The area under the first peak is equal to 1.7 for x = 0.6 and equal to 2.1 for x = 0.9

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J.Y. Raty et al. / Physica B 234-236 (1997) 364-366 365

g(r)

i

°i F 0

i i i i

930 *C

0

4 6 8 10 r (A °)

enium (crystal and liquid) local order. Indeed, the cinnabar structure can be considered as a distor- tion of the zinc-blende structure and is, from the geometrical point of view, an intermediate struc- ture between the zinc blende and the NaC1 struc- tures. Interestingly enough, the same behaviour is observed when applying pressure to Zn~Hg~_~Te. Indeed in the (P, x) phase diagram, the sequence of phases is NaCl-cinnabar-ZnS when increasing x at a constant 9 GPa pressure [2].

3. Conclusion

Fig. 1. Pair correlation functions of HgS at 840°C (full line) and 930°C (dotted line). The arrow shows the position of the second neighbour peak.

while the area under the second peak is close to 4 in both cases. These values are fully compatible with a "deformed cinnabar" structure or with the sel-

We have investigated the local structure of a series of liquid II-VI compounds (HgS, HgSe, ZnxHg~ _ ~Te). The HgS structure remains chainlike (cinnabar type)just above the melting point and tends to be tetrahedral at higher temperatures. A systematic trend is observed as function of the anion : HgS remains 2 coordinated in the liquid, HgSe is locally tetrahedral [3] and HgTe gets the

g(r)

4

3.5

3

2.5

2 -

1.5 -

1 ,-

0.5

0

I I I [

. . - •

e ' . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

' ZnTe " . . . . • o .

" \ . . . . . . .

• , J / "

, / J HgTe

2 4 6 8

HgTe

- - - - - Zn.2Hg.$Te

- - Zn.4Hg.6Te

. . . . . Zn.6Hg.4Te

. . . . . Zn.9Hg. 1Te

. . . . . . . . . ZnTe

10

r (A °)

Fig, 2. Pair correlation functions of Z nxHg l -xT e for x values of 0, 0.2, 0.4, 0.6, 0.9 and 1.0. Corresponding temperatures of measurements were (in units of °C): 670, 800, 950, 1000, 1150 and 1300, respectively.

366 J.Y. Rat)/et al. / Physica B 234-236 (1997) 364-366

NaC1 local order in the melt. So, the heavier the anion, the more compact the structure. The evolu- tion of ZnxHgl-xTe with concentration shows a deformed cinnabar structure for Zn concentra- tions between 0.6 and 0.9. Let us remark that the cinnabar structure is the intermediate phase ap- pearing between the zinc blende and NaC1 struc- tures when pressure is applied.

References

[1] J.P. Gaspard, J.Y. Raty, R. C6olin and R. Bellissent, J. Non-Cryst. Solids (1994) to appear.

I2] A. San Miguel, Ph.D. Thesis, University of Paris (1993). I-3] J.Y. Raty, J.P. Gaspard, R, C6olin and R. Bellissent, to be

published.

Acknowledgements

This work was supported by the Human Capital and Mobility contract no. ERB CHGECT 92001 and by the F.N.R.S. grant FRFC 2.4515.90.