Experiments under extreme conditions are an important tool for synthesis of new materials and investigation of physical and chemical properties. Such experiments are of interest not only to physics, chemistry and materials science, but also to geoscience because they provide the opportunity to probe the properties of geophysically relevant materials at conditions of the deep Earth’s interior. The current work focuses on the design and testing of a portable system for simultaneous high pressure and high temperature generation and its application in resolving one of the fundamental problems of high-pressure mineral physics, namely the site occupancy and iron electronic configuration in Fe-, Al-bearing MgSiO3 perovskite at conditions of the Earth’s lower mantle. The work also demonstrates the application of the developed setup to the investigation of the high-pressure phases of a potential hydrogen storage material, ammonia borane complex.
I. Design of the portable laser-heating system for experiments with diamond anvil cells
The technique of laser heating in diamond anvil cells (DACs) is currently the only way to achieve temperatures of thousands of degrees statically at pressures in the multi-megabar range in order to simulate conditions of the deep Earth’s interior. We developed a portable double-sided laser-heating system that can be easily transferred between the laboratory and a synchrotron and between different synchrotron beamlines. The portable system allows for coupling of numerous analytical techniques with in situ experiments under extreme conditions. It gives the possibility to use the laser-heating DAC technique for experiments that require movement of the entire setup as a single unit, for example single crystal X-ray diffraction. Our technical developments reduced the time for installation of the laser-heating setup at beamlines to only 2-3 hours.
The system consists of two main parts – a source of laser light and the laser-heating heads. SPI fiber lasers are used as the laser sources, and fibers allow the power units of the laser sources (40 kg) to be separated from the optical parts of the system, hence decreasing the space required for the sample environment. The laser-heating heads are based on a fine cutting device produced by Precitec KG. The laser heads combine the functions of laser beam focusing, in situ sample imaging, illumination, and collection of the emitted thermal radiation for temperature measurements.
The first applications of the setup were demonstrated at the European Synchrotron Radiation Facility (ESRF). These comprise a single crystal X-ray diffraction investigation on (Mg0.87Fe0.13)(Si0.89Al0.11)O3 perovskite at the High Pressure Station of the White Beam beamline ID09a (Häuserma and Hanfland, 1996) and a Synchrotron Mössbauer Source study (Potapkin et al., 2012) on (Mg0.8Fe0.2)O ferropericlase at the Nuclear Resonance beamline ID18 (Rüffer and Chumakov, 1996).
II. Site occupancy and electronic configuration of iron in Fe-, Al-bearing magnesium silicate perovskite
Iron- and aluminum-bearing MgSiO3 in the perovskite structure (FeAlPv) is considered to comprise at least 75% of the Earth’s lower mantle (e.g., Irifune et al., 2010; Ringwood, 1982). Iron is expected to be incorporated into the structure in significant amounts as both ferric and ferrous iron. FeAlPv has two different cation sites – a large distorted dodecahedral site (A-site) and a small relatively undistorted octahedral site (B-site). While there is general consensus that Fe2+ occupies exclusively the A-site (i.e., Fe2+A) the site occupancy of Fe3+ is a matter of some debate. Ferric iron has been reported to occupy either exclusively the A-site (McCammon et al., 2008; Potapkin et al., 2013; Vanpeteghem et al., 2006) or to occupy both the A- and B- sites (McCammon, 1997; Catalli et al., 2011). Several studies have additionally proposed that a site exchange reaction takes place at high pressures and high temperatures: FeA3++Al3+B→Fe3+B+Al3+B (Catalli et al., 2011; Fujino et al., 2012). In order to test the site exchange hypothesis we conducted an in situ high-pressure high-temperature single crystal X-ray diffraction investigation on (Mg0.87Fe3+0.09Fe2+0.04)(Si0.89Al0.11)O3 perovskite at beamline ID09a at the ESRF. The chemical formula is based on the results of electron microprobe analysis and Mössbauer spectroscopy. We collected several datasets between 65 and 78 GPa: before, during, and after laser heating at 1750(50) K. In addition to the atomic coordinates and isotropic thermal parameters we refined the occupancy of the B-site considering Fe and Si (Si and Al are not distinguishable within our single crystal X-ray diffraction experiments) and of the A-site by Mg and Fe. We found that the refined amount of iron in the A-site coincided within uncertainty with the value determined by the electron microprobe. Extended laser heating at 1750 K did not cause any detectable exchange of Al and Fe between the sites; therefore, there is no evidence that Fe enters the B site. Our results correlate well with the recent results of Glazyrin et al. (2014) on Fe3+- and Al3+-rich perovskite. The spin state of iron in mantle phases may significantly influence the properties and dynamics of the Earth’s interior (e.g., Frost et al., 2004; Goncharov et al., 2008; Potapkin et al., 2013). In ferropericlase the high-spin (HS) to low-spin (LS) crossover in ferrous iron has been observed at pressures higher than 50 GPa (Badro et al., 2003). Reports of iron spin crossover(s) in FeAlPv in different valence states are controversial. Experimental investigations have shown that ferrous iron in FeAlPv exhibits HS-intermediate-spin (IS) crossover above ~ 35 GPa (McCammon et al., 2008; Potapkin et al., 2013). However, computational simulations find HS configuration of Fe2+ to be the most stable at lower mantle pressures (Hsu et al., 2012; Zhang and Oganov, 2006). Ferric iron in the A-site has been reported to remain in the HS state over all lower mantle pressures; whereas ferric iron in the B-site has been found to undergo HS-LS crossover above ~ 50 GPa (Catalli et al., 2011; Fujino et al., 2012). The latter process has been associated with the site-exchange reaction described above.
The majority of previous reports extrapolate high-pressure iron spin state data at ambient temperature to lower mantle temperatures. In McCammon et al. (2008) the temperature was limited to 1000 K and results showed the stability of IS Fe2+ relative to HS Fe2+. In order to probe electronic configuration of iron at conditions of the deep Earth’s interior we conducted a first in situ investigation on Mg0.83Fe0.20Al0.06Si0.92O3 perovskite at pressures up to 81 GPa and temperatures up to 2000 K by means of Nuclear Forward Scattering (NFS) and Synchrotron Mössbauer Source (SMS) techniques complemented by room temperature conventional Mössbauer spectroscopic (MS) investigations.
The data measured by all three methods show the appearance of a new Fe2+ component assigned to the IS state at pressures above 35 GPa, similar to previous studies. The NFS data suggest the HS-IS crossover to be sharp (< 20 GPa); whereas the MS and SMS data show a more sluggish crossover that is not complete even at the highest pressure achieved in the study. The difference between previous observations appears to be due to the peculiarity of the NFS technique. NFS spectra at very early collection times are superimposed by electronic scattering and may provide not reliable results. These early times correspond to broad lines in absorption spectra, which appear in the case of HS Fe2+. No evidence of spin crossover of ferric iron in the A-site was found. After laser heating at pressures above 40 GPa, a minor component (~ 5% of the total iron content) thought to arise from LS Fe3+ in the B-site was observed. This low amount of iron in the B-site is below the detection limit of X-ray diffraction methods. The presence of LS FeB3+ is likely controlled by a redistribution of cation vacancies rather by the site exchange reaction between Fe3+ and Al3+. The low amount of LS Fe3+ detected in FeAlPv with composition relevant to the lower mantle shows that HS-LS crossover in ferric iron plays a negligible role in determining the properties of the deep Earth.
NFS and SMS high-temperature high-pressure spectra can be fitted as a superposition of subspectra corresponding to hot and relatively cold parts of the sample, possibly arising from temperature gradients within the sample. Our data show that the high ferric iron content observed previously in quenched samples (Frost et al., 2004) is also maintained at conditions relevant to the Earth’s lower mantle. Our data also show that the major fraction of ferric iron in FeAlPv remains in the HS state at conditions of the deep Earth’s interior; whereas all ferrous iron in FeAlPv converts to the IS state.
III. Pressure induced structural changes in the ammonia borane complex
The ammonia borane complex BH3NH3 (AB) is a potential candidate for hydrogen storage due its remarkably high hydrogen volume density (Stephens et al., 2007). At ambient conditions AB crystallizes in the tetragonal structure (space group I4mm) with disordered hydrogen (Bowden et al., 2007). Its high-pressure behavior has been intensively investigated during the last decade. Synthesis of dense AB polymorphs would imply the existence of materials with bulk hydrogen density even higher than that of the tetragonal AB phase. While there is general consensus regarding the phase transition from the tetragonal I4mm to the orthorhombic Cmc21 structure near 1 GPa (e.g., Chellappa et al., 2009; Filinchuk et al., 2009), reports on phase transitions at higher pressures remain controversial (e.g., Chen et al., 2010; Kumar et al., 2010; Lin et al., 2012). In order to investigate these phase transitions, we conducted an in situ high-pressure Raman spectroscopic investigation of AB up to 65 GPa. Our data show that AB behaves differently under compression at quasi-hydrostatic and non-hydrostatic conditions, which may explain the difference in observations of phase transitions below 12 GPa. Our study confirms the previously reported phase transition at ~ 12 GPa (Lin et al., 2008) and indicates a new transition at ~ 27 GPa under quasi-hydrostatic conditions.
We observed that AB has a nonlinear optical property, namely the capability of second harmonic generation (SHG) of laser light. The observation of the transmitted signal of SHG in the visible range was used for a development and testing of the portable laser-heating system described above, in particular for the improvement of the module for temperature measurements and for the investigation of laser beam focusing abilities.
The low scattering power of the elements constituting AB complicates X-ray diffraction studies of its structure, which makes any additional structural information valuable. SHG is possible only in anisotropic media without the inversion symmetry. We observed SHG during the passage of Nd:YAG (λ= 1072nm) laser light through AB up to 130 GPa, which suggests that the non-centrosymmetric point group symmetry is preserved in the material up to very high pressures.
|School:||Universitaet Bayreuth (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Keywords:||Diamond anvil cells, Magnesium silicate perovskite, Ammonia borane complex|
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