The energetics of la_2-xa_xnio_4-y (A = ba, sr)

The enthalpy of formation of the solid solution series La_2-xA_xNiO_4-y (A = Ba, Sr; 0 ≤ x ≤ 0.7 and -0.18 ≤ y ≤ 0.04) has been measured using high-temperature reaction calorimetry. As barium or strontium is substituted for lanthanum, the enthalpy of formation from the oxides becomes more exothermic. The enthalpy of formation from the component oxides and the enthalpy of oxidation for the barium- and the strontium-doped material for a given value of x is the same within the experimental uncertainty. From the measurements of the enthalpy of oxidation for the oxygen interstitial regime (y < 0) and the oxygen vacancy regime (y > 0), the oxygen Frenkel energy for the lanthanum nickelates has been found to be 43 ± 20 kJ/(mole O). Based on the oxidation enthalpy measurements, we argue that the excess holes in lanthanum nickelate are predominantly in Ni 3d-type bands.