New Fluoride-based Magnetoelctrics

Many information storage technologies are based on either
magnetic or electrical ordering and switchability. If a material possesses both
these properties then, in principle, it should be possible to exploit this
advantageously, for example by storing data electrically, but reading it
magnetically. Most of the previous work on these systems had focussed on
mixed metal oxides. Our aim was to prepare and study specifically fluoride-based rather than oxide-based
multiferroics. Several exploratory synthetic approaches were taken, including solid-state,
solvothermal, and later, ionothermal methods. Several new compounds of
potential interest as multiferroics were prepared. Also, several previously
known, but poorly characterised, multiferroic fluorides were studied in order
to understand their behaviour in greater detail. Brief highlights of the
published work include:
1.The previously know multiferroic K0.6FeF3 was characterised by high-resolution
synchrotron powder diffraction. Contrary to previous work it was discovered
that these materials display a complex and intrinsic phase separation and
phase transition phenomenon as a function of temperature. This could only
have been discovered using this type of difffraction experiment, and may
have important consequences for the interpretation of the reported
multiferroic properties of this family of compounds.
2. The cryolite-type compound (NH4)3FeF6, previously reported to be
multiferroic, was shown to be magnetically inactive, and therefore no longer
a candidate for multiferroic behaviour.
3. Magnetic ordering in the kagome compound Cs2ZrCu3F12 was shown to
be associated with a structural phase transition at sub-ambient
temperatures, driven by a crystal-chemical coordination preference of the
Zr4+ cation. A comparison has been made with the related 'valence-bond solid'
phase Rb2SnCu3F12, which shows no magnetic ordering.
4. Various explorations in ionothermal fluoride chemistry led to the partially
serendipitous discovery of another kagome compound based on V4+. This is
the first of its type, and shows the exciting property of 'quantum spin liquid'
behaviour.