University:
Widener University
Course:
PHYS-313 | Classical Mechanics
Academic year:

2023
Views:

423

Pages:

16
Author:

r1haroc

Tetragonal structure of CaSiO3 perovskite above 20 GPa Crystallography Review Images removed due to copyright considerations. Miller Index Octahedra = CN: 6 Xray Diffraction 6 Oxygens 1 Silicon Perovskite Structure Cubic a=b=c Images removed due to copyright considerations. Orthorhombic a>b>c Previous Studies • Determined EOS to CMB with Energy dispersive X-ray diffraction • CaSi Perovskite has a cubic structure (Pm3m) • Theoretical study proposed a tetragonal to cubic phase transformation in the lower mantle. Raw Xray Pattern Integrated Diffraction Pattern Images removed due to copyright considerations. Expected Peak Positions Integrated Diffraction Pattern Results • Pressure measurements from 19.7 – 45.8 GPa • Peak splitting observed for 200 and 211 • 0.4% shorter c-axis than a-axis • This amount of distortion is smaller than resolution for energy dispersive diffraction • Intensity of splitting is a function of geometry 200>211>110 • More intense peak of the doublet occurs at lower angle • Intense line should originate from atomic planes normal to the two identical a-axes of the tetragonal unit cell (200) • Less intense line should originate from planes normal to the c-axis (002) Images removed due to copyright considerations. Unit Cell Parameters Image removed due to copyright considerations. • Isothermal bulk modulus = 255 +/- 5 GPa Tetragonal Perovskite Distortion Octahedral Rotation Image removed due to copyright considerations. I4/mmm • Octahedra around a-axis • Octahedra rotate by 4.9 +/- 1.7o @ 45.8 GPa • Si-O bond length along c-axis is shorter by 0.6 +/- 0.3% • Three crystallographically distinct Ca sites Tetragonal Perovskite Distortion Octahedral Rotation Image removed due to copyright considerations. P4/mbm & I4/mcm • Octahedra rotation about the c-axis • Contraction of the a-axis • Contrary to diffraction pattern Tetragonal Perovskite Distortion Octahedral Rotation Image removed due to copyright considerations. MgPv & CaPv Si-O Bond Length vs. Pressure Image removed due to copyright considerations. “There appears to be an intrinsic anisotropy in these two materials” Discussion • More pronounced peak splitting with more pressure medium indicates distortion is not due to differential stress • Recent 1st principle calculations suggest an orthorhombic structure • Resolution of current x-ray diffraction study can not resolve orthorhombic distortion (0.13%) • Is tetragonal distortion stable at High T and High P? Conclusions • If tetragonal distortion is real at ambient conditions in the mantle – elastic properties and their directional dependence can be very different from cubic perovskite • Since the dodecahedral site of perovskites is the main repository of large cations in the mantle (U, Th, Sr, etc.) – distortion of this site may have important consequences for the partitioning of these elements in the lower mantle