Mössbauer spectroscopic study of synthetic leucophosphite, KFe₂(PO₄)₂(OH)∙2H₂O

A Mossbauer study of synthetic leucophosphite, KFe2(PO4)(2)(OH) center dot 2H(2)O is reported. The sample was prepared by the reaction of synthetic goethite (alpha-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) angstrom, beta = 102.45(1)degrees. The Mossbauer spectra from 295 K down to 40 K show the existence of two Fe3+ doublets with Delta E-Q1 = 0.62 mm/s and Delta E-Q2 approximate to 0.91 mm/s (at 80 K), respectively, whereas for lower temperatures the spectra are composed of two sextets with H-hf1 approximate to 48.3 T and H-hf2 approximate to 48.8 T at 4.2 K. The magnetic transition temperature was determined to be 36 K from a Mossbauer 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 H-hf(T) curves was obtained