Micro-Raman and X-ray diffraction study of Y 2BaNi 1− xM xO 5 (M=Mg, Zn) polymorphs

Abstract

The Y 2BaNiO 5 compound, prototypical Haldane system characterized by anti-ferromagnetic behaviour, and its analogs Y 2BaNi 1− x M x O 5 (M=Mg, Zn; 0.05≤ x≤0.65) are investigated by X-ray diffraction and Raman spectroscopy. Structural features and phase stability ranges of the doped samples are first determined. The first order Raman scattering for the different solid solutions is measured at room temperature in order to gain new insight on lattice dynamics of the Y 2BaNiO 5 compound when Zn 2+ or Mg 2+ ions progressively substitute into Ni site. In the case of Mg substitution, the Immm space group (sg) showing NiO 6 octahedra (C-6) is preserved; only a very slight lattice expansion is produced and the Raman spectrum remains practically unchanged. In Zn 2+ doped samples, both Immm and Pnma sgs coexist in samples with x≤0.13, while for higher x values only the Pnma structure, having NiO 5 pyramidal coordination (C-5), is present. Such a transformation is suitably observed through the changes in the Raman spectra. In particular, the lack of the inversion symmetry on the metal ion site causes the appearance of peculiar Raman modes in the frequency region between 260 and 340 cm −1. Energy and intensity of these modes are discussed taking into account the role of the doping level and of the phase abundances of the samples. The observed behaviour allows us to discuss a possible assignment of the modes in connection with the preferred insertion of Zn 2+ in the C-5 polyhedra.