Thermal expansion and decomposition of jarosite: a high-temperature neutron diffraction study

The structure of deuterated jarosite, KFe(3)(SO(4))(2)(OD)(6), was investigated using time-of-flight neutron diffraction up to its dehydroxylation temperature. Rietveld analysis reveals that with increasing temperature, its c dimension expands at a rate similar to 10 times greater than that for a. This anisotropy of thermal expansion is due to rapid increase in the thickness of the (001) sheet of [Fe(O,OH)(6)] octahedra and [SO(4)] tetrahedra with increasing temperature. Fitting of the measured cell volumes yields a coefficient of thermal expansion, alpha = alpha(0) + alpha(1) T, where alpha(0) = 1.01 x 10(-4) K(-1) and alpha(1) = -1.15 x 10(-7) K(-2). On heating, the hydrogen bonds, O1 center dot center dot center dot D-O3, through which the (001) octahedral-tetrahedral sheets are held together, become weakened, as reflected by an increase in the D center dot center dot center dot O1 distance and a concomitant decrease in the O3-D distance with increasing temperature. On further heating to 575 K, jarosite starts to decompose into nanocrystalline yavapaiite and hematite (as well as water vapor), a direct result of the breaking of the hydrogen bonds that hold the jarosite structure together.