Angelika D. Rosa

In situ monitoring of phase transformation microstructures at Earth's mantle pressure and temperature using multi-grain XRD

Microstructures govern the mechanical properties of materials and change dramatically during phase transformations. A detailed understanding of microstructures at different stages of a transformation is important for the design of new materials and for constraining geophysical processes. However, experimental studies of transformation microstructures at the grain scale have been mostly based on ex situ observations of quenched products, which are difficult to correlate with bulk sample properties and transformation kinetics. Here, it is shown how multi-grain crystallography on polycrystalline samples, combined with a resistively heated diamond anvil cell, can be applied to investigate the microstructural properties of a material undergoing a phase transition in situ at high pressure and high temperature. This approach allows the extraction of the crystallographic parameters and orientations of several hundreds of grains inside a transforming sample. Important bulk information on grain size distributions and orientation relations between the parent and the newly formed phase at the different stages of the transformation can be monitored. These data can be used to elucidate transformation mechanisms (e.g. coherent versus incoherent growth), growth rates and orientation-dependent growth of individual grains. The methodology is demonstrated on the α–γ phase transitions in hydrous Mg2SiO4·H2O up to 22 GPa and 940 K. This transformation most likely occurs in the most abundant mineral of the Earths upper mantle (Mg0.8Fe0.2SiO4) in deep cold subducted slabs and plays an important role in their subduction behaviour.

Evolution of grain sizes and orientations during phase transitions in hydrous Mg2SiO4.

Transformation microstructures in mantle minerals, such as (Mg,Fe)2SiO4, are critical for predicting the rheological properties of rocks and the interpretation of seismic observations. We present in-situ multi-grain X-ray diffraction experiments on hydrous Mg2SiO4 at the P/T conditions relevant for deep cold subducting slabs (up to 40 GPa and 850 °C) at a low experimental strain rate (~4*10-6s-1). We monitor the orientations of hundreds of grains and grain size variations during the series of α-β-γ (forsterite-wadsleyite-ringwoodite) phase transformations. Microtextural results indicate that the β and an intermediate γ* phase grow incoherently relatively to the host α-phase consistent with a nucleation and growth model. The β and γ-phase exhibit orientation relationships which are in agreement with previous ex-situ observations. The β and intermediate γ* show texturing due to moderate differential stress in the sample. Both the α-β and α-γ transformation induce significant reductions of the mean sample grain size of up to 90% that starts prior to the appearance of the daughter phase. Apart from the γ*, in the newly formed β and γ-phases, the nucleation rate is faster than the growth rate, inhibiting the formation of large grains. These results on grain orientations and grain size reductions in relation to transformation kinetics should allow refining existing slab strength models.

Exploring liquid–liquid transitions in ZnSe at extreme conditions

Here, we report on the fingerprints of 4-fold to 6-fold transition in liquid ZnSe at extreme pressure and temperature conditions up to 42 GPa and more than 3000 K using X-ray absorption spectroscopy (XAS) combined with laser-heated diamond anvil cell techniques and complementary ab initio molecular dynamics simulations. Among pressure induced structural modifications, liquid–liquid’ (L-L’) transitions have attracted much interest both experimentally and theoretically due to their peculiarity. Unlike most L-L’ transitions that only show slight modifications of the bond distances between atoms (i.e., P, Na, supercooled Si) [1, 2, 3], strong L-L’ transitions have been theoretically predicted for a few II-VI semiconductor compounds such as ZnSe, CdSe and CdTe, given the following two conditions are satisfied: Firstly, the melting of the sp3 bonded phase (4-fold coordination) at ambient pressure results in a liquid structure that remains approximately 4-fold coordinated; and secondly, the presence of a semiconductor to metal phase transition under pressure [4, 5, 6, 7]. Our results show that solid and liquid ZnSe undergoes a series of structural modifications at various P, T values that satisfy the conditions above. The red shift in Zn K edge energy observed at around 7 GPa upon increasing temperature suggests a metallization event before melting to a possible 6-fold coordinated liquid structure. These findings are supported by our simulation results showing a pronounced difference in the first diffraction peak of the calculated structure factor at high pressures indicating a 4-fold to 6-fold coordination change in liquid ZnSe. Our results may provide additional insight for such transitions that may be observed for similar tetrahedrally coordinated II-VI systems.