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Abstract:

Polycrystalline samples of La₃Ni₂NbO₉ and La₃Ni₂TaO₉ have been characterised by X-ray and neutron diffraction, electron microscopy, magnetometry and muon spin relaxation (µSR); the latter technique was also applied to La₃Ni₂SbO₉. On the length scale of a neutron diffraction experiment, the six-coordinate sites of the monoclinic perovskite structure are occupied in a 1:1 ordered manner by Ni and a random â…“Ni/â…”B’ mixture. Electron microscopy demonstrated that this 1:1 ordering is maintained over microscopic distances, although diffuse scattering indicative of short-range ordering on the mixed site was observed. No magnetic Bragg scattering was observed in neutron diffraction patterns collected from La₃Ni₂µþ’O_9Ìý(B’ = Nb or Ta) at 5 K although in each case µSR identified the presence of static spins below 30 K. Magnetometry showed that La₃Ni₂NbO₉ behaves as a spin glass below 29 K but significant short-range interactions are present in La₃Ni₂TaO₉ below 85 K. The contrasting properties of these compounds are discussed in terms of their microstructure.