Evolution of Thermal Hysteresis with Tt

In order to distinguish further between a W = Wb and a W — Wb exp (—/Iesc/Scoo), we compared (Fig. 17) two samples of different composition having the same Tt, one under pressure and the other at ambient. Under pressure, the locally cooperative oxygen-vibration frequency coq would increase with a reduction of the Ni—O bond length and of the bending angle 

We [85] also investigated the evolution at ambient pressure of the magnetic susceptibility of the NiOj array in these samples the data are shown in Fig. 18 together with p(T) to locate T(. The Ln -ion contribution was obtained from LnAlOj perovskites and subtracted out. Both Landau diamagnetism and Van Vleck paramagnetism are small in magnitude compared with the measured X(T), so no correction was made for these contributions. 

Interpretation of the x(T) data begins with a distinction between Stoner and mass enhancements. The electronic specific-heat parameter y increases with the mass enhancement [14]. By measuring both /(T) and y at low temperatures, Sreedhar et al. [80] determined a Stoner factor S = 0.58, well below the S = 1 for a ferromagnetic instability. These nickel oxides are clearly on the itinerant-electron side of the transition from localized to itinerant 

Qualitatively, the j (T) data for LaNiOs in Fig. 18a appears to fit the Mott prediction for a homogeneous electronic system. However, a weak temperature dependence persisting above the apparent Ta w 200 K does not conform to the Mott picture. Alternatively, this x(T) curve may be described by a model containing two electronic phases in which 

Significantly, the (T) curve for Ndo.5Smo.5Ni03, Fig. 18d, shows no anomaly at Tt as defined by p(T). This observation demonstrates that the CDW below a Tt Tn is not stabilized by either Fermi-surface nesting or the onset of a homogeneous Mott-Hubbard transition. However, it is compatible with an order-disorder transition for strong-correlation fluctuations. 

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