Microstructural modelling of nanostructured fluoride powders prepared by mechanical milling

Nanostructured fluoride powders were prepared by the high-energy ball milling route. A combination of suitable techniques with complementary spatial scales was used for the first time to investigate both structural and microstructural properties: x-ray diffractometry, magnetic measurements and local probes such as 57 Fe Mössbauer spectrometry and 19 F, 69 Ga and 71 Ga NMR. The set of data allows us to describe these nanostructured powders on the basis of nanocrystalline grains and grain boundaries. The relevant hyperfine data support then modelling of the microstructure in terms of pseudo-cubic and random packing of corner sharing octahedral units corresponding to nanocrystalline grains (~15 nm diameter) and disordered grain boundaries (a few nanometres thick), respectively. Their different cationic topologies are consistent with the antiferromagnetic and speromagnetic behaviours, respectively, as evidenced by in-field Mössbauer spectrometry. They also support the temperature dependence of the magnetic properties which reveal a progressive magnetic decoupling of grains when the temperature increases, originating a superparamagnetic behaviour above a blocking temperature which is dependent on the thickness of the grain boundaries, i.e. the milling conditions.