Full-multiple-scattering calculations on HgTe under high pressure at the mercury L 2,3 x-ray-absorption edges V. Briois LURE, Universite´ Paris-Sud, Baˆt 209D, 91405 Orsay Cedex, France Ch. Brouder and Ph. Sainctavit LURE, Universite´ Paris-Sud, Baˆt 209D, 91405 Orsay Cedex, France and LMCP, Universite´s Paris VI et VII, 4 place Jussieu, 75252 Paris Cedex 05, France A. San Miguel ESRF, BP 220, 38043 Grenoble, France J.-P. Itie ´ and A. Polian Physique des Milieux Condense´s, Universite´ Paris VI, 4 place Jussieu, 75252 Paris Cedex 05, France ~Received 17 March 1997! Full-multiple-scattering ~FMS! calculations have been performed to reproduce the modifications observed on the mercury L 3 edge in the x-ray-absorption near-edge structure ~XANES! spectra of the II-VI HgTe compound under increasing pressure. HgTe undergoes several structural phase transitions with pressure, i.e., zinc blende ~phase I!→cinnabar ~phase II!→rocksalt ~phase III!→unknown structure ~phase IV!. The best FMS simulations are obtained for the relaxed and screened final-state potential with a complex Dirac-Hara exchange potential with muffin-tin symmetry and nonoverlapping spheres. They reproduce satisfactorily the main edge changes occurring during the first two phase transitions. At P 512 GPa, HgTe undergoes a third transition ~phase IV! to an orthorhombic structure. The resolution of this high-pressure structure is controversial. Con- fident in the ability of FMS calculations to well reproduce the XANES modifications, we used this formalism to choose between structures in competition for phase IV. Calculations have been performed considering successively the three different structural models proposed for phase IV: a b-Sn-like structure and two ortho- rhombic structures with space group Cmcm, labeled Cmcm(1) and Cmcm(2), respectively. The trends observed on the XANES spectra during the rocksalt→phase IV transition are satisfactorily reproduced by two of the three models, Cmcm(2) being totally refuted. Although the differences between the calculations based on the two acceptable models are not great, the orthorhombic structure Cmcm(1) gives a slightly better agreement than the b-tin structure. @S0163-1829~97!02834-8# I. INTRODUCTION The effects of pressure on the phase transitions of the II-VI compounds have been well documented since the 1960s. 1–4 It has long been known that zinc and cadmium chalcogenides transform from the zinc-blende structure to the rocksalt structure under high pressure. For a long time, it has been also evidenced that for the telluride II-VI com- pounds or the mercury chalcogenides, the zinc-blende struc- ture transforms into the cinnabar structure first. 5–7 But it is only recently that the atomic positions of the cinnabar struc- ture were published for ZnTe, 8 CdTe, 9,10 and HgTe. 11,12 This systematic observation of the cinnabar structure for the tel- luride II-VI compounds and the relationships found among the zinc-blende, cinnabar, and rocksalt structures suggest that the cinnabar phase acts as a pivot during the zinc-blende to rocksalt transition. Studies at high pressure have also shown that for some of these chalcogenides a transition from the rocksalt structure to an orthorhombic crystal structure occurs. 13–21 Nevertheless, some controversies seem to exist about the determination of the structure of this high-pressure phase. Concerning mercury telluride, the zinc-blende phase ~phase I! at ambient pressure transforms to a cinnabar-type structure ~phase II! at 1.560.2 GPa, at 300 K, and then to a rocksalt-type structure ~phase III! at 8 GPa. Finally, an ortho- rhombic structure ~phase IV! appears above 12 GPa. Phase IV was first identified as belonging to the space group I 4 ¯ m 2, 15,16 and recently as belonging to the space group Cmcm . 20,21 Referring to this last study, 20,21 two orthorhom- bic structures with the same space group are a priori pos- sible. Simultaneously to the change of the crystal structure, important modifications in the resistivity of the material 22,23 occur under increasing pressure. In fact, HgTe is a semimetal in phase I, a semiconductor in phase II, and metallic in phase III. We have previously reported an x-ray-absorption spec- troscopy ~XAS! study of these first three pressure-induced phase transitions 24,25 at the mercury L 3 edge in the 0–20 GPa range. We have shown that important changes in the shape of the x-ray-absorption fine structures ~XAFS! occur under pressure. The goal of these full-multiple-scattering ~FMS! calculations is threefold: We first want to reproduce the XAFS modifications observed at the mercury L 3 edge under increasing pressure and consequently confirm the atomic po- sitions of phases I to III determined elsewhere by angular- dispersive x-ray diffraction ~ADX!~Refs. 11 and 12! and PHYSICAL REVIEW B 1 SEPTEMBER 1997-II VOLUME 56, NUMBER 10 56 0163-1829/97/56~10!/5866~10!/$10.00 5866 © 1997 The American Physical Society