5:15 PM - 7:15 PM
[SMP29-P06] High-Pressure Stability and Phase Transitions of Stishovite: Insights from First-Principles and Phonon Calculations
Keywords:Stishovite, High-pressure phase transition, Phonon, First-principles
Introduction. Stishovite, a high-pressure phase of SiO2 with a rutile structure, is an important mineral in Earth's lower mantle.[1-3] Stishovite is also found in meteorite craters and extraterrestrial materials, such as iron meteorites [4], eucrite [5], and lunar samples (e.g., from lunar meteorite [6], Apollo [7] and Chang'e-5 missions [8], etc.). Its high density, elastic modulus, and anisotropy make it significant for materials science and mineralogy.[9] Formed under high-pressure conditions (>10 GPa [10]), stishovite is crucial for understanding planetary impacts and geological history. This study investigates the high-pressure stability and phase transitions using phonon analysis of stishovite, to provide theoretical insights into its high-pressure behavior and applications in geophysics and planetary science.
Methods. First-principles calculations were performed using DFT [11, 12] in CASTEP [13, 14], with the GGA-PBE functional [15] and OTFG ultrasoft pseudopotentials [16]. A plane-wave cutoff of 630 eV and a 7×7×11 k-point grid (Monkhorst-Pack method [17]) were used. The finite displacement method [18, 19] was used for the phonon calculations. The crystal structure of stishovite is performed through structural optimizations and phonon calculations at pressures ranging from 0 to 100 GPa.
Results and Discussion. Under the space group P42/mnm, the lattice parameters of stishovite at 0 GPa are a = b = 4.235 Å and c = 2.692 Å, compressing to a = b = 3.896 Å and c = 2.543 Å at 100 GPa. Fitting P-V data with the Birch-Murnagham equation [20] yielded a bulk modulus B0 = 271 GPa and B0' = 4.5. Phonon dispersion spectra show no imaginary frequencies below 50 GPa, indicating dynamic stability within this pressure range. Most phonon branches exhibit a blue shift with pressure due to reduced interatomic distances and increased force constants. At 60 GPa, phonon softening near the Γ point along the M→Γ path suggests reduced stability and a potential phase transition. At 100 GPa, significant acoustic branch softening and an imaginary frequency mode (56i cm-1) indicate O atom shifts, distorting SiO6 octahedra. Analysis of atomic displacements predicts a transition to a stable Pnnm structure (i.e., the CaCl2-type structure). This study demonstrates the use of imaginary frequency modes to predict high-pressure stable structures and offers key insights into the phase transition mechanisms of stishovite under high pressure.
Acknowledgment. This study was supported by the Science and Technology Development Fund (FDCT) of Macau [Grants 0122/2022/A, 0117/2023/RIA2 and 002/2024/SKL].
References. [1] Earth Planet. Sci. Lett., 77(2), 245. [2] Earth Planet. Sci. Lett., 126(4),351. [3] Nature, 372(6508), 767. [4] Meteorit. Planet. Sci., 38(11), 1579. [5] PNAS, 111(30), 10939. [6] PNAS, 108(2), 463. [7] Am. Mineral., 100(5-6), 1308. [8] Geophys. Res. Lett., 49(12), e2022GL098722. [9] Phys. Earth Planet. Inter., 112(1999), 257. [10] Nature, 340, 217. [11] Phys. Rev. B, 136(3), B864. [12] PNAS, 76(12), 6062. [13] J. Phys.: Condens. Matter, 14(11), 2717. [14] Z. Kristallogr. − Cryst. Mater., 220, 567. [15] Phys. Rev. Lett., 77(18), 3865. [16] Phys. Rev. B, 41(11), 7892. [17] Phys. Rev. B, 13(12), 5188. [18] Phys. Rev. Lett., 74(10), 1791. [19] Phys. Rev. Lett., 78(21), 4063. [20] J. Geophys. Res., 83, 1257.
Methods. First-principles calculations were performed using DFT [11, 12] in CASTEP [13, 14], with the GGA-PBE functional [15] and OTFG ultrasoft pseudopotentials [16]. A plane-wave cutoff of 630 eV and a 7×7×11 k-point grid (Monkhorst-Pack method [17]) were used. The finite displacement method [18, 19] was used for the phonon calculations. The crystal structure of stishovite is performed through structural optimizations and phonon calculations at pressures ranging from 0 to 100 GPa.
Results and Discussion. Under the space group P42/mnm, the lattice parameters of stishovite at 0 GPa are a = b = 4.235 Å and c = 2.692 Å, compressing to a = b = 3.896 Å and c = 2.543 Å at 100 GPa. Fitting P-V data with the Birch-Murnagham equation [20] yielded a bulk modulus B0 = 271 GPa and B0' = 4.5. Phonon dispersion spectra show no imaginary frequencies below 50 GPa, indicating dynamic stability within this pressure range. Most phonon branches exhibit a blue shift with pressure due to reduced interatomic distances and increased force constants. At 60 GPa, phonon softening near the Γ point along the M→Γ path suggests reduced stability and a potential phase transition. At 100 GPa, significant acoustic branch softening and an imaginary frequency mode (56i cm-1) indicate O atom shifts, distorting SiO6 octahedra. Analysis of atomic displacements predicts a transition to a stable Pnnm structure (i.e., the CaCl2-type structure). This study demonstrates the use of imaginary frequency modes to predict high-pressure stable structures and offers key insights into the phase transition mechanisms of stishovite under high pressure.
Acknowledgment. This study was supported by the Science and Technology Development Fund (FDCT) of Macau [Grants 0122/2022/A, 0117/2023/RIA2 and 002/2024/SKL].
References. [1] Earth Planet. Sci. Lett., 77(2), 245. [2] Earth Planet. Sci. Lett., 126(4),351. [3] Nature, 372(6508), 767. [4] Meteorit. Planet. Sci., 38(11), 1579. [5] PNAS, 111(30), 10939. [6] PNAS, 108(2), 463. [7] Am. Mineral., 100(5-6), 1308. [8] Geophys. Res. Lett., 49(12), e2022GL098722. [9] Phys. Earth Planet. Inter., 112(1999), 257. [10] Nature, 340, 217. [11] Phys. Rev. B, 136(3), B864. [12] PNAS, 76(12), 6062. [13] J. Phys.: Condens. Matter, 14(11), 2717. [14] Z. Kristallogr. − Cryst. Mater., 220, 567. [15] Phys. Rev. Lett., 77(18), 3865. [16] Phys. Rev. B, 41(11), 7892. [17] Phys. Rev. B, 13(12), 5188. [18] Phys. Rev. Lett., 74(10), 1791. [19] Phys. Rev. Lett., 78(21), 4063. [20] J. Geophys. Res., 83, 1257.