The Effect of Pressure on The Raman Spectra in Trigonal Se and Te
The crystal structure of trigonal Se and Te consists of infinite helical chains which spiral around the crystalline c axis with three atoms per turn. Each atom within the helical chains is tightly bonded to two neighbors with covalent character. The bonding between individual chains is much weaker. The lattice dynamics of trigonal Se and Te have been studied recently by infrared and Raman spectra [1,2], and neutron scattering , Richter et al.  have reported that the first-order Raman frequencies decrease linearly with increasing pressure up to 8 kbar. Trigonal Se and Te under pressure show a covalent-metallic transition accompanied by a discontinuous change in electrical resistivity at about 180 and 40 kbar, respectively [5,6]. The structure of the high-pressure phases has been studied by x-ray diffraction [7–9], but remains unsolved. Jamieson and McWhan  have reported another transition to a β-Po structure for Te at 115 kbar.
KeywordsRaman Active Mode Helical Chain Geometrical Coefficient Nonlinear Softening Raman Active Phonon
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