Chemistry Reference
In-Depth Information
Fig. 4.20 Computer
modelled atomic structure of
grain boundaries in
nanostructured metallic
powders
means of a Monte Carlo scheme established from annealing/Metropolis algorithm
without using periodic boundaries conditions, with few free parameters [
160
].
This computer modelling approach allows to determine radial and angular
distribution function characteristic of both crystalline grains and grain boundaries:
the thickness of grain boundaries is estimated to 2-3 atomic layers, in perfect
agreement with experimental features while its atomic configuration which is
closer to the bcc structure than the fcc one in the case of Fe nanostructured
powders is rather new [
160
]. Despite no ab initio calculations were yet established,
the topology fairly supports the hyperfine structure observed by Mössbauer
spectrometry as afore described.
4.9 Ferric Nanostructured Fluoride Powders
The understanding of physical properties of ferric nanostructured fluoride powders
requires first a short review of the fundamental interest of ferric fluorides FeF
3
which
exhibit three polymorphic crystalline phases and several amorphous varieties. They
received great attention during the 1980s because of their simple chemical nature and
their non collinear magnetic structures which are governed by the cationic topology
and the antiferromagnetic superexchange interactions [
161
,
162
].
The crystalline and magnetic structures of the 3 polymorphic crystalline struc-
tures of FeF
3
are illustrated in Fig.
4.21
. The more stable crystalline rhombohedral r-
FeF
3
phase is a canted antiferromagnet below T
N
= 363 K, the canting is attributed
to the antisymmetric Dzialhozinky-Moriya exchange interaction; the hexagonal
tungsten bronze form orders below T
N
= 97 K with 3 ferromagnetic sublattices in
