| Summary: | <p>This work is focussed on the synthesis and characterisation of ferroelectric and multiferroic
materials which adopt n = 2 Dion-Jacobson or Ruddlesden-Popper structures.</p>
<p>A series of n = 2 Dion-Jacobson phases, RbNdM<sub>2</sub>O<sub>7</sub> and CsNdM<sub>2</sub>O<sub>7</sub> (M = Nb/Ta), were
investigated and it was observed that they adopt polar crystal structures, confirmed by both
second-harmonic generation and neutron diffraction experiments. Their polar structures are
driven by several tilting distortions and cation displacement modes. The different polar
structures adopted by the caesium and rubidium phases are attributed to the different sizes
of Rb<sup>+</sup> and Cs<sup>+</sup> cations.</p>
<p>To extend our understanding on the size effect of interlayer cations, the metastable
Ruddlesden-Popper phases NaNdM<sub>2</sub>O<sub>7</sub> and LiNdM<sub>2</sub>O<sub>7</sub> were prepared from RbNdM<sub>2</sub>O<sub>7</sub> via
cation exchange reactions. The lithium phases adopt a polar crystal structure, with lithium
cations ordered as stripes. In contrast, the sodium phases are non-polar and the sodium
cations adopt a checkerboard order pattern. Their different structures are again explained
by the size differences between lithium and sodium cations.</p>
<p>A temperature dependant phase transition study of polar RbNdM<sub>2</sub>O<sub>7</sub> and CsNdM<sub>2</sub>O<sub>7</sub>
reveals that upon heating, both rubidium and caesium phases undergo first-order phase
transitions to anti-polar structures before reaching the highest symmetry aristotype
structures. A complex competition between many distortion modes was observed during
their phase transitions.</p>
<p>An n = 2 Ruddlesden-Popper phase, Li2SrTa<sub>2</sub>O<sub>7</sub>, was prepared by a ceramic synthesis and
it adopts a polar crystal structure. A further cation exchange reaction using MnCl2 leads to
a polar and magnetically ordered phase MnSrTa<sub>2</sub>O<sub>7</sub>. This manganese phase adopts an
incommensurate crystal structure with an unambiguous modulation of the ordering pattern
of manganese cations. The superstructure is explained by a competition between
minimising Mn-Mn repulsion and optimising metal-oxide bonding. The unusual magnetolattice coupling behaviour observed from variable temperature neutron diffraction data
indicates a magnetoelectric effect in MnSrTa<sub>2</sub>O<sub>7</sub>.</p>
<p>Similar cation exchange reactions performed on Li<sub>2</sub>SrTa<sub>2</sub>O<sub>7</sub> using CoCl<sub>2</sub> or CuCl<sub>2</sub> produce
partially exchanged phases Co<sub>x</sub>Li<sub>2-2x</sub>SrTa<sub>2</sub>O<sub>7</sub> and Cu<sub>x</sub>Li<sub>2-2x</sub>SrTa<sub>2</sub>O<sub>7</sub> with different
structures with respect to their manganese analogue. Complex cation order is expected in
these two phases.</p>
<p>A polar oxynitride phase, Na<sub>1+x</sub>NdTa<sub>2</sub>O<sub>7-x</sub>N<sub>x</sub>, was prepared by a low temperature
ammonolysis from non-polar NaNdTa<sub>2</sub>O<sub>7</sub>, suggesting layered perovskite oxynitrides can
be a new family of polar materials. In addition, a series of transition metal oxynitrides,
M<sub>x</sub>Li<sub>y</sub>NdTa<sub>2</sub>O<sub>7-z</sub>N<sub>z</sub> (M = Mn/Fe/Co/Ni/Cu) were made from a four-step synthesis. Among
these materials, the nickel phase adopts a polar crystal structure and exhibit a magnetic
order below 20 K. </p>
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