Article pubs.acs.org/JPCC
Lattice Dynamic Behavior of Orthoferrosilite (FeSiO3) toward Phase Transition under Compression Jennifer Kung*,† and Baosheng Li‡ †
Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan Mineral Physics Institute, Stony Brook University, Stony Brook, New York 11794, United States
‡
ABSTRACT: We performed elasticity and Raman scattering measurements of ortho-FeSiO3 up to ∼7.5 GPa. The velocities of the P and S waves were measured in conjunction with in situ X-radiation techniques. Above 5 GPa, the velocity of the S wave, but not the P wave, exhibited strong softening while the averaged crystal structure, as indicated by X-ray diffraction, maintained an orthorhombic symmetry. Within the same pressure range, the Raman shifts showed systematic increasing as a function of pressure. Based on the results of both Mg and Fe end-member orthopyroxenes, the observed velocity anomalies marked the onset of a phase transition to the high-pressure phase. The experimental results in this study provide insight into lattice dynamic behavior in terms of the composition effect at the onset of the phase transition in the orthopyroxene group under compression. Li+, Sc3+, Al3+, etc.). The size and charge of the cations chiefly determine the structure of a pyroxene in orthorhombic and monoclinic crystal structure groups. The (Mg,Fe,Ca)SiO3 pyroxene group is an important rockforming mineral of a shallow part of the earth’s mantle that undergoes a series of phase transitions with an increase in pressure and temperature. When the M1 and M2 sites are occupied by Mg2+ and Fe2+, the crystal structure of (Mg,Fe)SiO3 is orthorhombic under ambient conditions. Based on studies performed under high pressure and temperature, the composition of MgSiO3 with a pyroxene structure exhibits a complicated phase diagram at the P−T domains (Figure 1), including four polymorphs, space groups of P21/c, Pbca, HT phase (Pbcn or C2/c in different studies), and C2/c.12 The FeSiO3 compound shares a phase diagram topology similar to that of MgSiO3 pyroxene, but the phase boundaries for the Feend members are shifted to a lower pressure at a constant temperature.13,14 Studies of Mg−Fe orthopyroxene have mainly focused on the composition of the Mg side due to the implications for Earth sciences, which is the compositional and mineralogic models of the Earth silicate mantle.15−17 Little
I. INTRODUCTION The physical properties of a material are determined by its crystal structure and chemical composition. When the material is subjected to an external field (e.g., stress, electrical or magnetic field, etc.), its physical properties may change, leading in some cases to a structural phase transition. An understanding of structural phase transitions of materials allows for the control, enhancement, or optimization of the physical properties of devices in material sciences, (e.g., optics and multiferroics1) or for interpretation of the geophysical observations of the Earth’s interior (e.g., seismic discontinuities2). Some pyroxene group minerals were recently determined to comprise a new class of multiferroic materials.3 Investigation of the physical properties of pyroxene-structured groups is currently an important topic in materials science.4−10 Structurally, the silicate pyroxene group, ASiO3, can be described as a linkage of SiO4 tetrahedrons by the sharing of two of four corners to form continuous SiO3 chains. The SiO3 chains are bonded to form octahedral layers that contain two different cationic sites, M1 and M2. The M1 sites are the cavities located at the chain-forming, symmetrically equivalent, edge-sharing octahedral sites, and the M2 sites are the cavities tucked into the kinks of the M1 chains.11 Due to the flexibility of SiO3 chains, both cationic sites, M1 and M2, can accommodate a wide size range of cations with mono-, di-, and trivalent metals (A = Fe2+, Mg2+, Ca2+, Zn2+, Mn2+, Na+, © 2014 American Chemical Society
Received: November 17, 2013 Revised: May 19, 2014 Published: May 19, 2014 12410
dx.doi.org/10.1021/jp4112926 | J. Phys. Chem. C 2014, 118, 12410−12419
The Journal of Physical Chemistry C
Article
indicated that the transition pressure is Fe-content dependent. The differences in the transformation mechanism among those studies, however, require further clarification based on additional experiments. In the present study, we performed elasticity and Raman measurements of ortho-FeSiO3 at high pressure to investigate the composition effect on the lattice dynamic behaviors of both end-members, Mg- and FeSiO3.
II. EXPERIMENTAL METHODS A cylindrical polycrystalline specimen of FeSiO3 was synthesized and hot-pressed from a mixture of Fe2SiO4 and SiO2 at 3 GPa and 1000 °C, within the stability field of orthopyroxene (SG Pbca). The X-ray diffraction pattern indicated that the specimen was orthoferrosilite (Figure 2, “0 GPa_before high
Figure 1. Topology of a phase diagram for (Mg,Fe)SiO3 at high pressure and temperature. Black lines represent the boundaries for MgSiO3 (enstatite, En) and blue lines represent those for FeSiO3 (ferrosilite, FS).
effort has been made to investigate the properties of the Fe-end member compounds under compression. Raman spectroscopy and optical-acoustic measurements (Brillouin scattering and MHz-ultrasonic measurements) of ortho-MgSiO3 reveal anomalous behaviors in Raman-active modes and elasticity prior to phase transitions at high temperature,18−20 with Raman shifts and elastic moduli exhibiting strong nonlinear behavior at high temperature. High-temperature X-ray diffraction studies of ortho-MgSiO3 pyroxene show different phase transitions, identified as reconstructive phase transitions (Pbca →Pbcn or Pbca → C2/ c).21−25 High-pressure single-crystal studies reveal that orthoMgSiO3 remains the same structure up to ∼9.4 GPa.16,26 The elastic wave velocity shows strong softening behavior in both the P and S waves beyond 9 GPa and reaches minimum values between 12 and 14 GPa,27 at which point the Raman shifts exhibit discontinuities.28,29 Limited to the resolution of in situ energy dispersive X-ray data from Kung et al.,27 conclusions regarding phase transition have been drawn, but the possible high-pressure phase has not been identified. An atomistic simulation study30 suggested two possible high-pressure polymorphs for ortho-MgSiO3, P21ca. and Pbca. More recent in situ studies determined that the high-pressure phase is a P21/ c phase based on high-pressure single-crystal studies of the compositions of Mg-rich (Mg1.74Fe 0.16Al 0.05 Ca 0.04 Cr0.01)(Si1.94Al0.06)O6, En87, and Fe-rich (Mg0.32Fe1.64Al0.02Ca0.02)(Si1.98Al0.02)O6, Fs82, with transition pressures as low as ∼10 GPa to ∼14 GPa.31,32 Furthermore, a study by Dera et al.32 revealed another high-pressure phase beyond P21/c as a highpressure orthorhombic Pbca phase, demonstrating that this series of transitions is reversible and leaves no microstructural evidence, i.e., they transform back to the original Pbca phase upon recovery at ambient conditions. These findings, however, cannot explain the observations from an earlier study33 that the pure end-member ortho-FeSiO3 undergoes a phase transition at pressures above ∼4.2 GPa and that the quenched phase is predominantly a P21/c phase with a few small islands of metastable C2/c phase. Based on the morphology and microstructure of the recovered product,33 a reconstructive transition mechanism was suggested and the sequence of P → 4.2GPa
Figure 2. X-ray diffraction patterns collected at ambient conditions before and after the high pressure run, and three selected points at pressures of 2.5, 4.7, and 7.1 GPa. Red crosses and green lines represent the observed and calculated spectra, respectively. Black and red ticks represent the peak positions of orthorhombic symmetry (SG Pbca) and low pressure monoclinoferrosilite (SG P21/c), respectively.
pressure”). Based on Archimedes’ immersion technique, the bulk density of the specimen was 3.94(4) g cm−3 (or 98.5% of the X-ray value). The final size of the specimen was 0.805 mm in length with a diameter of ∼2.0 mm before the ultrasonic measurement. Ultrasonic measurements at high pressures were performed in a DIA-type, large-volume apparatus (SAM85) in conjunction with energy dispersive X-ray techniques at beamline X17B of the National Synchrotron Light Source at the Brookhaven National Laboratory. Details of the experimental setup were previously described.34−36 Briefly, the specimen was loaded in a cell assembly made of a mixture of amorphous boron and epoxy resin as the pressure medium, and embedded in a disk of NaCl, which provided a pseudohydrostatic environment and also served as a pressure marker. Two-way travel times of ultrasonic waves through the specimens were determined by using a transfer function method.37,38 A dual-mode LiNbO3 transducer was used to generate and receive ultrasonic signals (50 MHz
P → 1.5GPa
transition was postulated to be Pbca ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ C2/c ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ P21/c. Observations from the above studies suggest two different transformation mechanisms in orthopyroxene: displacive transition (Pbca → P21/c)31,32 and reconstructive transition (Pbca → C2/c).33 Findings from these studies 12411
dx.doi.org/10.1021/jp4112926 | J. Phys. Chem. C 2014, 118, 12410−12419
The Journal of Physical Chemistry C
Article
resonant frequency for P waves and 30 MHz for S waves). The mean standard deviation of travel times collected in the study was ∼0.3 ns for P waves and ∼0.5 ns for S waves. X-ray diffraction patterns were collected for both the specimen and NaCl. Determination of unit cell parameters of the specimen was refined by using the program package GSAS.39 The structural data for the FeSiO3 ferrosilite used in this study were for orthoferrosilite (SG Pbca) and low-pressure clinoferrosilite (SG P21/c).40,41 The experimental setup in this study allowed for direct measurement of the sample length at high pressure by using an X-radiographic imaging method.27,34 The image data of this run collected at high pressure were not adequate, however, and were therefore discarded. Alternatively, the specimen lengths at high pressures were calculated from the cell volume changes of the specimen and these were comparable to those directly measured by using an Xradiographic imaging method when the specimen was in the elastic deformation regime at high pressure.34 Symmetric diamond anvil cells were used to generate high pressure for the Raman experiments. The gasket material was initially 250 μm thick and it was compressed to ∼70−80 μm. A sample chamber (diameter, 150 μm) was drilled with use of a discharge machine in the center of the indented gasket. The cells were loaded with powdered sample and filled with 4:1 methanol−ethanol pressure medium. Both ruby fluorescence and Raman scattering measurements were performed with the micro-Raman system, Spectra-Physics Stabilite 2017.42 Raman spectra were excited with radiation of 514.5 nm from argon ion lasers and collected in the backscattered geometry with unpolarized light. Wavenumbers are accurate to ±1 cm−1 as determined from the plasma emission lines. Data were imported into Peakfit software and peak positions were determined by using the peak fitting utility. The uncertainty in peak position was
