STRUCTURAL TRANSITIONS IN THE NEODYMIUM NICKELATE AT ISO- AND HETEROVALENT SUBSTITUTIONS

S.A.Nedilko, O.F.Manchenko, T.P.Lishko

Department of Chemistry, Kiev Taras Shevchenko University, Kiev 252033, Ukraine

As the cuprate compound Nd_1.4Ce_0.2Sr_0.4Cu_O4- is known to be a superconductor [1] and Cu²⁺ and Ni²⁺ have rather close ionic radii, we have studied the formation, structure and electrical properties of Nd_1.4Ln_0.2Sr_0.4Ni_1-xCu_xO₄ (Ln = Ce, Pr, Eu, Tb; 0<=x<=1) solid solutions.

The samples were obtained from coprecipitated carbonates with postheat treatment in air at 900-950^oC for 10 hours. After calcination the specimens were regrounded, pelletized and then annealed in air and in O₂ at 900-1100^oC for 30 hours with an intermediate grinding.

It was found by powder X-ray diffraction analysis that the solid solutions of tetragonal structure (space group I4/mmm) known as T-phase (K₂NiF₄ type) and T'-phase (Nd₂CuO₄ type) were formed in the Nd_1.4Ln_0.2Sr_0.4Ni_1-xCu_xO₄ systems (Ln = Pr, Eu, Tb). The Nd_1.4Ce_0.2Sr_0.4Ni_1-xCu_xO₄ (0<=x<=1) systems contain CeO₂ as well as T and T'-phases.

A single phase regions of the phases obtained have been determined. They were found to be 0<=x<=0,4 (Pr, Tb) and 0<=x<=0.6 (Eu). The reducing of the annealing temperature below 1000^oC leads to the structural transition TT' and the formation of biphase systems.

The Ce, Pr, Eu - and Tb- containing neodimium and strontium cuprates crystallize with the Nd₂NiO₄ structure, whereas the nickelates of the same composition have the tetragonal K₂NiF₄ structure. Prolonged annealing of Nd_1.4Ce_0.2Sr_0.4CuO₄ in O₂ at 1050^oC results in the structural transition T'->T* and the formation of two phases. The lattice parameters are calculated for all single phase materials.

According to the data of iodometric analysis the concentration of the trivalent nickel decreases with the increase of the copper content in the Nd_1.4Ln_0.2Sr_0.4Ni_1-xCu_xO⁴ (Ln = Pr, Eu, Tb) solid solutions.

Resistance measurement carried out in the temperature range 77-300 K have revealed the semi-conducting behavior for the single phase materials.

1. M.Klauda, P.Lunz, J.Markl et al., Physica C. 191 (1992) 137.