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

The structure of deuterated jarosite, KFe3(SO4)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 approximately ten times greater than that for a. This anisotropy of thermal expansion is attributed to a rapid increase in the thickness of the (001) sheet of [Fe(O,OH)6] octahedra and [SO4] tetrahedra as temperature rises. Fitting the measured cell volumes yields a coefficient of thermal expansion, α = α0 + α1T, where α0 = 1.01 × 10^-4 K^-1 and α1 = -1.15 × 10^-7 K^-2. Upon heating, the hydrogen bonds, O1/C1/C1/C1 D–O3, that hold the (001) octahedral–tetrahedral sheets together, weaken, evidenced by an increase in the D/C1/C1/C1 O1 distance and a decrease in the O3–D distance with rising temperature. At further heating to 575 K, jarosite begins to decompose into nanocrystalline yavapaiite and hematite (along with water vapor), predominantly as a result of the breaking of the hydrogen bonds that stabilize the jarosite structure.