A Mössbauer study of synthetic leucophosphite, KFe2(PO4)2(OH)·2H2O is reported. The sample was prepared by the reaction of synthetic goethite (α-FeOOH) with a K-phosphate solution of pH 2 at 373 K for 20 days. The obtained cell parameters are a = 9.771(1), b = 9.675(2), c = 9.747(2) Å, β= 102.45(1)°. The Mössbauer spectra from 295 K down to 40 K show the existence of two Fe3+ doublets with ΔEQ1 ≈ 0.62 mm/s and ΔEQ2 ≈ 0.91 mm/s (at 80 K), respectively, whereas for lower temperatures the spectra are composed of two sextets with Hhf1 ≈ 48.3 T and Hhf2 ≈ 48.8 T at 4.2 K. The magnetic transition temperature was determined to be 36 K from a Mössbauer thermoscan experiment. The magnetic ordering is presumed to be antiferromagnetic. The temperature variations of the hyperfine fields cannot be explained by the simple molecular field approximation. Using a model that takes into account the exchange magnetostriction associated to a non-second-order transition, an excellent reproduction of the Hhf(T) curves was obtained.

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