Compression mechanisms in the anisotropically bonded elements Se and Te H. C. Hsueh, C. C. Lee, and C. W. Wang Department of Physics, Tamkang University, Taiwan 25137, Republic of China J. Crain Department of Physics and Astronomy, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, Scotland and IBM Research Division, 1623-14 Shimotsuruma, Yamato, 242-8502 Kanagawa, Japan Received 16 August 1999 The compression mechanisms of the elements selenium and tellurium which exhibit highly anisotropic bonding under ambient conditions are explored. A combination of experiments and ab initio simulation including generalized gradient corrections is used to examine the structural and dynamic properties of these elements in detail. The effect of pressure on both these systems is to enhance the weak interchain bonding at the expense of the stronger intrachain covalent interactions. This is manifested by a pronounced mode soften- ing of the intrachain vibrational modes under pressure as found from both Raman spectroscopy and simulation. A corresponding increase of the rigid-chain rotation mode is also revealed by the calculations. We also investigate pressure-induced polymorphism in these materials in order to resolve controversy concerning the high-pressure crystallographic structures. I. INTRODUCTION Observing the response of materials to hydrostatic com- pression is a powerful probe of complex bonding in solids. Of particular interest in this regard are anisotropic solids in which cohesive forces of very different strengths coexist. In recent years, major advances in experimental instrumentation for in situ high-pressure measurements and rapid increases in computer power have opened opportunities for detailed mea- surements of the structural and dynamical and electronic re- sponse to compression in complex materials. The combination of x-ray diffraction, high-resolution op- tical spectroscopy, and ab initio simulation has revealed complex compression mechanisms in layered solids where electronic and vibrational properties show a clear cross over from quasi-two-dimensional to three-dimensional character under pressure while the crystallographic structure remains layered. 1,2 In certain families of molecular crystals, compres- sion has been observed to induce intra- to inter-molecular electron transfer accompanied by a pronounced weakening of the intra-molecular bond strength and the formation of a two-dimensional layered solid. 3,4 Continued compression drives the layers closer together until a three-dimensional network solid of very low symmetry is formed. This process is accompanied by an unusual nonmonotonic variation of the intramolecular stretch frequency. In these cases ab initio simulation has been extremely effective in predicting and accounting for the complex compression mechanisms. The group-VI elements Se and Te are, in principle, sim- pler than the compounds discussed above but are, nonethe- less, known to be highly anisotropic under ambient condi- tions. These materials form chainlike structures with relatively strong covalent bonds in twofold coordination along the chain direction. Only weak cohesion exists be- tween the chains. Despite their status as prototype aniso- tropic materials their compression mechanisms have not been explored at the same level of detail as for more com- plex materials. Attention has tended to focus on high- pressure metalization transitions to structures where the an- isotropic chainlike bonding has been lost. Specifically, the equation of state has been measured using x-ray diffraction techniques 5–8 and detailed experimental structural studies have revealed a number of high-pressure phases of relatively low symmetry. 9,10 Theoretical calculations have been em- ployed to examine the electronic structure of selenium and tellurium with a view to exploring successive pressure- driven transitions 11,12 and the appearance of supercon- ductivity 13 . In the full-potential linearized-augmented-plane- wave method used by Geshi et al. 14 the calculated transition pressure is severely underestimated with respect to the ex- perimental value. Still there have been several candidate high-pressure structures of Se and x-ray-diffraction methods have not yielded a definitive structure solution. There has also been some attention given to the liquid state semicon- ductor to metal transition. 15–17 The compression mechanism of the ambient pressure phase of selenium and tellurium have not been explored in detail. Notable exceptions include recent Mo ¨ ssbauer studies 18 where the decrease of quadrupolar splitting with increasing pressure was attributed to the growing importance of interchain interlayer interactions in the trigonal phase. The purpose of this paper is to draw together experimen- tal results on Se and Te in order to understand the compres- sion mechanism and to consider the accuracy with which simulation can account for the observations and to relate the structural and vibrational changes to electronic structure. We will also briefly consider high-pressure polymorphism in or- der to resolve existing controversy. II. EXPERIMENTAL AND COMPUTATIONAL METHODS A. Sample preparation and Raman scattering Samples of Se and Te were obtained from Alfa Aesar products and used without further purification. The samples PHYSICAL REVIEW B 1 FEBRUARY 2000-II VOLUME 61, NUMBER 6 PRB 61 0163-1829/2000/616/38516/$15.00 3851 ©2000 The American Physical Society brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by Tamkang University Institutional Repository