Spin states and electronic conduction in Ni oxides

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Abstract
Magnetic and electronic properties of the mixed‐valence semiconductor Li x Ni2+ 1−2x Ni3+ x O are reinterpreted in terms of low‐spin states for both Ni ions. Anomalous decreases in hopping electron activation energies are discussed on the basis of (i) breakdown in antiferromagnetic ordering through spin canting of the Ni sublattices through exchange isolation caused by diamagnetic Li1+ ions that group with the low‐spin Ni3+ (S= (1)/(2) ) to form polarons, and (ii) enhanced disruption of magnetic superexchange that results from a combination of Li1+ dilutants and S=0 states of surrounding Ni2+ ions induced at low temperatures by static Jahn‐Teller tetragonal distortions of the oxygen octahedra around the Ni3+polarons. Reported magnetic ordering and conduction anomalies in La2−x Sr x NiO4 are then compared to the behavior of Cu in Li x Cu1−x O, and in the high‐T c superconducting La2−x Sr x CuO4 system. Spontaneous conduction through molecular‐orbital states involving zero‐spin Ni and Cu ions is discussed, together with the role of S=0 polarons in other oxide superconductors.

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