Phase composition and crystal structure of (La2-yNdy)1-x/2Sr1+xCu2O6 (0≤x≤1; 0≤y≤2)

In this paper the phase composition and crystal structure of (La2-yNdy)(1-x/2)Sr1+xCu2O6 were investigated and discussed based on the synthesis and powder X-ray diffraction studies of three series of Nd2-xSr1+xCu2O6 (0 less than or equal to x less than or equal to 1), La2-xSr1+xCu2O6 (0 less than or equal to x less than or equal to 1) and (La2-yNdy)SrCu2O6 (0 less than or equal to y less than or equal to 2). The solid solubility of Nd2+xSr1+xCu2O6 and La2-xSr1+xCu2O6 was determined as 0.08 less than or equal to x less than or equal to 0.3 and 0 less than or equal to x less than or equal to 0.25, respectively. The phase relations in (La2-yNdy)(1-x/2)Sr1+xCu2O6 were also given according to the above results. The existence of three sub-lattices as La2.1.3Nd0-0.7Sr1-1.25Cu2O6, La1.2-0.5Nd0.8-1.5Sr1-1.25Cu2O6 and Nd1.6-1.92La0.4 0Sr1+delta-1.2Cu2O6 (0 < delta less than or equal to 0.08) was supposed and this consideration was demonstrated by the structural refinements for the three ended compositions of La2SrCu2O6, LaNdSrCu2O6 and Nd1.8Sr1.2Cu2O6 using powder X-ray diffraction data. It can be concluded that the lattice of (La, Nd)(2-x)Sr1+xCu2O6 is very sensitive to the ionic size of rare-earth elements. Different rare-earth elements (such as La3+, Nd3+ and La3++Nd3+) may cause rather different distributions of RE3+ and Sr2+ in the lattices and thus give a series of sub-lattices. On the other hand, stabilizing this type of the lattice is indeed the combined effect of ionic size and ionic charge valence. The different solid solubility and ionic occupations of La2-xSr1+xCu2O6 and Nd2-xSr1+xCu2O6 can be explained using the concept of the combined effect. The chemical complexity of RE2MCu2O6 (RE=Rare-earth, M=Alkaine-earth) may help understanding the different conducting abilities of these compounds. The structural refinements on NdSr2Cu2O6 and LaSr2Cu2O6 give another example for the chemical complexity of RE-M-Cu-O system. (C) 1999 Elsevier Science S.A. All rights reserved.