| Summary: | <p>Q–dependent diffuse scattering was observed at low angles in a neutron diffraction experiment performed on the relaxor ferromagnet La<sub>3</sub>Ni<sub>2</sub>SbO<sub>9</sub> using a spin–polarised beam. The nuclear component could be modelled using direct Monte–Carlo methods if it was assumed that short (~5.5 Å) Sb–Sb contacts were energetically disfavoured. The magnetic component of the diffuse scattering at 110 K (T<sub>C</sub> = 108 K) was analysed using both direct and reverse Monte–Carlo techniques. When antiferromagnetic nearest–neighbour coupling was assumed to be dominant in this perovskite, second neighbour coupling was found to be weak but non–zero with third–neighbour coupling being negligible. The short–range structural and magnetic orderings were found to have correlation lengths of 15 and 9 Å, respectively.</p> <p>The consequences of replacing d<sup>10</sup> Sb<sup>5+</sup> by d<sup>0</sup> pentavalent cations were investigated; all the compounds reported are isostructural with La<sub>3</sub>Ni<sub>2</sub>SbO<sub>9</sub>. La<sub>3</sub>Ni<sub>2</sub>TaO<sub>9</sub> was found to be a relaxor ferromagnet but La3Ni2NbO9 behaved as a spin glass below 35 K. Solid solutions La<sub>3</sub>Ni<sub>2</sub>Sb<sub>x</sub>Ta<sub>y</sub>Nb<sub>1–x–y</sub>O<sub>9</sub> were synthesised and characterised. It was again shown that the relaxor behaviour is incompatible with the presence of Nb<sup>5+</sup> in the system. In contrast, doping La<sub>3</sub>Ni<sub>2</sub>B’O<sub>9</sub> (B’ = Sb, Ta or Nb) with Cu<sup>2+</sup> cations was found to increase TC and the bulk magnetisation. For example, La<sub>3</sub>Ni<sub>1.75</sub>Cu<sub>0.25</sub>NbO<sub>9</sub> exhibits ferrimagnetic behaviour below ~90 K with the development of long–range G–type magnetic ordering. It is proposed that relaxor regions and long–range ordered regions coexist in this system.</p> <p>The sensitivity of cation ordering, and hence magnetic behaviour, to the charge difference between Ni<sup>2+</sup> and the diamagnetic six–coordinate cation was studied by replacing Sb<sup>5+</sup> by Te<sup>6+</sup> or W<sup>6+</sup>. Electron microscopy revealed a significant level of cation disorder in a severely–twinned sample of SrLa<sub>2</sub>Ni<sub>2</sub>TeO<sub>9</sub>; spin–glass and antiferromagnetic phases coexist below 35 K. The Ni2+ cations in CaLn<sub>2</sub>Ni<sub>2</sub>WO<sub>9</sub> (Ln = La, Pr or Nd) form a spin glass, although the behaviour of each composition reflects the magnetic properties of Ln<sup>3+</sup>.</p>
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