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 softening 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.
