Sytle M. Antao', Jiuhua Chen', Wilson Crichton', Ishmael Hassan *, & John Parise',"
'MPI & Dept. of Geosciences, SUNY @ Stony Brook, NY 11794-2100.
"ESRF, Grenoble, France.
*Dept. of Chemistry, UWI, Kingston 7, Jamaica.
sytle.antao@stonybrook.edu
NSLS X7A, X7B, ESRF
The structure of magnesioferrite, MgFe2O4, was determined
at various fixed pressures (P) and variable temperature (T) conditions as a
function of time at ID30 beamline at the ESRF, France. Room P data at variable
T were obtained at beamlines X7A and X7B of the NSLS, Brookhaven, USA. This
study was carried out to determine the Mg2+ and Fe3+ cation
ordering in the octahedral and tetrahedral sites, the kinetics of order-disorder
in MgFe2O4, and also the equilibrium pathway at different
P and T conditions.
At room P, on heating, there is no change in cation ordering until the temperature
is high enough to cause exchange of Mg2+ and Fe3+ between
the two cation sites. At about Tb = 570°C, the sample achieves the maximum
ordered state on heating (x = 0.845). On further heating beyond Tb, the sample
continuously disorders along a smooth pathway to the maximum T studied (Tmax.
= 982°C, x = 0.751(3)). On cooling down from 982°C, the ordering reverses
along the equilibrium pathway up to Tb'. On cooling below Tb', the maximum equilibrium
order at Tb' is nearly constant to room temperature (x = 0.875). Data from X7A
confirm the equilibrium pathway. At low T, several days are required to reach
equilibrium compared to a few minutes at high T. P was increased to about 8
GPa and held constant, then the T was increased and held constant, and the structure
was determined as a function of time. This will determine the kinetics of the
approach to equilibrium. At a fixed P of 8 GPa, the T was increased in intervals
of about 100°C and data were collected as a function of time. At higher
T, the approach to equilibrium is rapid, therefore, the sample was kept for
a shorter amount of time at higher T. Two more cycles were done by decreasing
the P to about 5 and 3 GPa and diffraction traces were collected in a similar
way as a function of T and time. This in-situ study showed disordering of the
Mg2+ and Fe3+ cations as observed by the change in intensity
of the reflections affected by this process (e.g., 111). Structure
refinements are in progress but preliminary results confirm the ordering process.
At 3 GPa, the cations achieve a more ordered state (x = 0.86(1)) at 373°C.
The cations continuously disorder with increasing T to 926°C (x = 0.68(1)).