Creation of ionic defects in transition-metal-free oxides with K2NiF4 structure

A number of oxides with the K2NiF4 crystal structure are known, with La2NiO4 being a prototypical example. Common to most of the known compositions is the presence of one or more transition metals on the B-site. Though the materials typically have very large concentrations of mobile oxygen ion defects, the transition metals impart electron (or hole) conduction that prevents their use as solid electrolytes. Here, a series of new oxide materials with the K2NiF4 structure has been synthesized by solid-state reaction in order to reduce and, ultimately, remove the B-site transition metal. X-ray diffraction patterns of La1.6Sr0.4Al0.4Ni1−xMgx−0.4O4 show no obvious impurity peaks for x values up to 1, even when the La/Sr ratio was adjusted to create oxygen vacancy or interstitial defects. The stability of the crystal structure, despite the presence of high concentrations of ionic defects, is believed to be related to the high charge separation between the perovskite and rocksalt layers in the crystal structure. The total electrical conductivity decreased by orders of magnitude as Ni was removed, consistent with a polaron hopping conduction mechanism across the B-site. Low apparent mobility of the oxygen defects in La1.6+xSr0.4−xAl0.4Mg0.6O4+x/2 caused little improvement in ionic conductivity compared to stoichiometric samples.