Microstructural modelling of nanostructured fluoride powders prepared by mechanical milling

Nanostructured fluoride powders were prepared by the high-energy ball milli ng route. A combination of suitable techniques with complementary spatial s cales was used for the first time to investigate both structural and micros tructural properties: x-ray diffractometry, magnetic measurements and local probes such as Fe-57 Mossbauer spectrometry and F-19, Ga-69 and Ga-71 NMR. The set of data allows us to describe these nanostructured powders on the basis of nanocrystalline grains and grain boundaries. The relevant hyperfin e data support then modelling of the microstructure in terms of pseudo-cubi c and random packing of corner sharing octahedral units corresponding to na nocrystalline grains (similar to 15 nm diameter) and disordered grain bound aries (a few nanometres thick), respectively. Their different cationic topo logies are consistent with the antiferromagnetic and speromagnetic behaviou rs, respectively, as evidenced by in-field Mossbauer spectrometry. They als o support the temperature dependence of the magnetic properties which revea l a progressive magnetic decoupling of grains when the temperature increase s, originating a superparamagnetic behaviour above a blocking temperature w hich is dependent on the thickness of the grain boundaries, i.e. the millin g conditions.