CRYSTAL-STRUCTURE AND MICROSTRUCTURE OF ND1.8SR0.2NIO3.72 - A K2NIF4-TYPE NICKELATE WITH MONOCLINIC SYMMETRY AND ORDERED OXYGEN VACANCIES

The crystallographic structure of the Ni1+/Ni2+ mixed valence Nd1.8Sr0.2NiO3.72 compound has been investigated over a wide temperature range (from 16 K to room temperature) by using neutron-powder diffraction. In contrast with stoichiometric Nd2NiO4.00, the symmetry of this new oxygen-defective phase is monoclinic, and it can be described by the B112/n(C2/c) space group in the whole range of temperatures. Between 200 and 16 K, a smooth decrease of the monoclinic strain s = 2(b singamma - a)/(a + b singamma) has been observed, which presents the same characteristics as the Bmab --> P4(2)/ncm first-order structural phase transition reported for Nd2NiO4.00 at T(c1) almost-equal-to 130 K. The oxygen vacancies are concentrated in the basal plane of the NiO6 octahedra along the [110] crystallographic direction and, in consequence, some of the Ni atoms are expected to be in square planar coordination. On the other hand, the strong additional broadening observed in the (hhl) reflections indicates the existence of microstrains, whose origin lies in the local fluctuations of the oxygen content along the ordering direction of the vacancies.