High-Pressure Phase Transformations in Hexagonal and Amorphous Selenium

McCann, D. R.; Cartz, L.

doi:10.1063/1.1677579

Cited by 61 publications

(16 citation statements)

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“…Under pressure, selenium exhibits a complex polymorphism, and the diversity of phases and transition sequences observed depend strongly on the starting material (12,(14)(15)(16)(17)(18)(19)(20)(21). First studied by x-ray diffraction (XRD) in 1972 (22), a-Se was found to crystallize in a trigonal structure (t-Se) at Ϸ10 GPa (23)(24)(25)(26)(27). However, its electrical resistance was clearly different from pure t-Se in the same pressure range.…”

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“…The discrepancy between the electrical resistance and structure of t-Se prompted further investigation into the nature of pressure-induced crystallization by using XRD techniques. With laboratory x-ray sources, exposure times per diffraction pattern for high-pressure samples varied from 100 h (22) to 24 h (25). The use of the second-generation synchrotron x-ray sources greatly reduced diffraction collection time to 10 minutes level (26,27).…”

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Anomalous high-pressure behavior of amorphous selenium from synchrotron x-ray diffraction and microtomography

Liu

Wang

Xiao

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

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The high-pressure behavior of amorphous selenium has been investigated with time-resolved diamond anvil cell synchrotron x-ray diffraction and computed microtomography techniques. A two-step dynamic crystallization process is observed in which the monoclinic phase crystallized from the amorphous selenium and gradually converted to the trigonal phase, thereby explaining previously observed anomalous changes in electrical conductivity of the material under pressure. The crystallization of this elemental system involves local topological fluctuations and results in an unusual pressure-induced volume expansion. The metastability of the phases involved in the transition accounts for this phenomenon. The results demonstrate the use of pressure to control and directly monitor the relative densities and energetics of phases to create new phases from highly metastable states. The microtomographic technique developed here represents a method for determination of the equations of state of amorphous materials at extreme pressures and temperatures.crystallization ͉ volume expansion ͉ equation of state ͉ phase transition ͉ metastability T he behavior of amorphous materials under pressure is a problem of great current interest. Unusual behavior, such as pressure-induced amorphization of crystalline forms, pressureinduced polyamorphism low-to high-density forms of insulating and metallic glasses, and the dynamics of pressure-induced crystallization are reported but not fully understood (1-10). The nature of the behavior of amorphous solids under pressure is complicated by their metastability and the possibility of irreversible relaxation of their properties and structure (11). Amorphous selenium (a-Se) is a model system for examining pressure effects in amorphous materials because of the wide range of structure and bonding properties expected based on the behavior of the crystalline polymorphs of the element. In experiments designed to understand the high-pressure behavior of a-Se, we have discovered an unexpected pressure-induced dynamic crystallization process associated with a volume expansion in the material. The origin of this unusual phenomenon is examined by using a microtomography technique that allows direct measurement of the equation of state (EOS) of the amorphous phase.Recent efforts to characterize the structural evolution of group VI elements under pressure, such as novel dense chain structures in sulfur and selenium (12), and the alternating phase transition sequence in sulfur at high pressure and temperature (13), have led to a new level of understanding of the phase diagrams and structures of these materials. Under pressure, selenium exhibits a complex polymorphism, and the diversity of phases and transition sequences observed depend strongly on the starting material (12,(14)(15)(16)(17)(18)(19)(20)(21). First studied by x-ray diffraction (XRD) in 1972 (22), a-Se was found to crystallize in a trigonal structure (t-Se) at Ϸ10 GPa (23-27). However, its electrical resistance was clearly different from pure t-Se i...

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confidence: 99%

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confidence: 99%

Anomalous high-pressure behavior of amorphous selenium from synchrotron x-ray diffraction and microtomography

Liu

Wang

Xiao

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

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“…1,2 At ambient conditions Se is a semiconductor with a trigonal structure comprising infinite helical chains. 3 While the radius of the helical chains decreases only very slightly with increasing pressure, the interchain distance decreases strongly, 4 and, at 14 GPa, Se-I transforms to Se-II [5][6][7][8] which is a semiconductor 9,10 with a C-centered monoclinic structure. 11 Se-II is stable to 23 GPa where it undergoes a semiconductor-metal transition to superconducting Se-III, 9,10 which in turn transforms to Se-IV at 28 GPa.…”

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Confirmation of the incommensurate nature ofSe−IVat pressures below70GPa

et al. 2004

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Se-IV, the high-pressure phase of selenium stable above 28 GPa, is shown to have an incommensurately modulated structure to a least 70 GPa, the highest pressure reached in this study. Although the previously reported phase transition at 40-60 GPa to primitive rhombohedral Se-V is not observed, extrapolation of the Se-IV structural parameters suggests a continuous transition to Se-V would occur at 82 GPa. The incommensurate wave vector of Se-IV is significantly pressure dependent, varying from 0.314(1) at 28.9 GPa to 0.277(1) at 56 GPa.

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“…We find that the large effects observed by KF are, in fact, precursors of the transition of Se to a metallic nonmolecular structure, which occurs at 130 kbar. 3 We further argue why one expects a band picture for the electronic states to be adequate and local-field corrections to be small in Se.…”

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confidence: 81%

“…12 Our high-pressure band structure for Se resembles the free-electron band structure of a hexagonal metal 20 : There is a large gap at K and a near degeneracy of valence and conduction states at H 0 These changes in the band structure are a precursor to a matallic phase which occurs at 130 kbar. 3 At 80-kbar pressures Se is nearly isotropic. Its band structure resembles that of Te 12 at zero pressure.…”

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confidence: 99%