Defect-chemical analysis of the nonstoichiometry, conductivity and thermopower of La(2)NiO(4+delta).

La(2)NiO(4+delta) is an oxygen excess compound (delta > 0) with oxygen interstitials (O(i) and holes (h*) in majority, which deviates positively from the ideal-dilute-solution behavior of defects. It was earlier attempted to interpret this positive deviation by taking into account the activity coefficients of both O(i) and h*. In this work, we examined the nonstoichiometry, electrical conductivity, and thermopower against oxygen activity in the entire stability range of the oxide at 800 degrees , 900 degrees and 1000 degrees C, respectively. It has been found that the positive deviation is ascribed essentially to the hole degeneracy and quantitatively described by using the Joyce-Dixon approximation of the Fermi-Dirac integral. Consequently evaluated are the effective density of states of the valence band (20.3 <or= log(N(v)/cm(-3)) <or= 20.5), the Fermi energy relative to the valence band edge (-5 <or= (E(F)-E(V))/kT<or= 1.5), and the effective mass (1.2<or=m(h)*/m(0)<or=1.3)), mobility (0.14-0.17 cm(2) V(-1) s(-1)), and reduced heat-of-transport (ca. 0.02 eV) of holes. The stability limit of the oxide is also evaluated and compiled together with the reported values.