Single crystal diffraction studies of phase transition of minerals across Fe high-low spin transition at high pressure
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Data di Pubblicazione:
2011
Citazione:
Single crystal diffraction studies of phase transition of minerals across Fe high-low spin transition at high pressure / M. Merlini, M. Hanfland. ((Intervento presentato al convegno AGU Fall Meeting tenutosi a San Francisco nel 2011.
Abstract:
The spin state of Fe in structure of minerals relevant for the lower mantle mineralogy, is known to undergo a high to low spin state change. This phenomena is often coupled to a remarkable volume contraction and from a structural point of view, often is associated to isosymmetrical phase transition. Recent improvements at X-Ray beamlines for diffraction at extreme conditions at synchrotron facilities allow the possibility to perform single crystal diffraction and determine crystal structure of minerals at extreme conditions, including also structural studies across first or second order phase transition. The accurate knowledge of crystal structure and of phase behaviour at high pressure is a very important step in order to: 1-understand the physical properties; 2- have an accurate experimental constraint on numerical simulation. We report here three examples of structure determination by single crystal X-Ray diffraction at extreme conditions concerning phase transition related to Fe spin state change, measured at ID09A beamline (ESRF, France). CaFe2O4 undergoes a spin transition at 50 GPa. XRD before and after indicate the symmetry and crystal structure is the same. The transition is marked by 10 % volume contraction. The use of He as pressure transmitting media strongly reduced strain induced by pressure and let the crystal survive this transition, allowing for the first time direct determination of Fe-O bond length changes related to variation of spin state. The main structural difference between high and low spin structure is simply a collapse of FeO6 polyhedra. FeCO3 has been also investigated, and the results are also compared with already present in literature. FeCO3 undergoes a transition around 45 GPa, with a remarked hysteresis. In the pressure range 20-45 however an anomalous behaviour is noticed, probably related to a different spin interaction due to reduced Fe-Fe distances. Fe1-xS pyrrhotite has been investigated in two different structure (a monoclinic and a incommensurately modulated hexagonal structure). Both samples present a continuous increase of compressibility in the pressure range 0-8 GPa. Above the volume data can be fitted with a conventional EoS, and, proved also by spectroscopic measurement, Fe is present in low spin state. Pyrrhotite have been also investigated at high pressure and high temperature and the effect of temperature is to shift the pressure of transition towards higher values. Crystal structure refinement below and above spin transition indicate that there is a local significative rearrangement of the structure evidenced also by a strong increase of modulation intensity in incommensurate pyrrhotite.
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
M. Merlini, M. Hanfland
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