PERSISTENCE OF THE LA2NIO4 CRYSTAL-STRUCTURE IN LA2-XBAXNIO4 SAMPLES WITH HIGH BA CONTENTS (X-LESS-THAN-OR-EQUAL-TO-1)

Black prismatic crystals of tetragonl La2-XBaXNiO4, K2NiF4 type, were grown. Their crystal structure was determined by standard methods of single-crystal X-ray-diffraction in space group 14/mmm (No. 139). Ba contents of x = 0.48(6) and 0.94(6) were established in two isomorphic polycrystalline samples by Rietveld profile refinements from neutron diffraction data. The persistence of the tetragonal K2NiF4 structure-type through progressively substituting Ba for La in La2CuO4 is shown. Along this substitution, which takes place without noticeable changes in the oxygen content and goes as far as a composition near LaBaNiO4, the Ni oxidation-state increases from 2 (x = O) to 3 (x = 1), and the elongated NiO6 octahedra become more and more regular. The shortest Ni-O-Ni distances and, hence, the strongest Ni-Ni interactions are found for x = 0.5 in samples which contain equal amounts of Ni(II) and low-spin Ni(III). Some signs of La and Ba ordering have been detected in the sample for which x = 1.