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

Abstract

In this paper the phase composition and crystal structure of (La 2− y Nd y ) 1− x /2 Sr 1+ x Cu 2 O 6 were investigated and discussed based on the synthesis and powder X-ray diffraction studies of three series of Nd 2− x Sr 1+ x Cu 2 O 6 (0≤ x ≤1), La 2− x Sr 1+ x Cu 2 O 6 (0≤ x ≤1) and (La 2− y Nd y )SrCu 2 O 6 (0≤ y ≤2). The solid solubility of Nd 2− x Sr 1+ x Cu 2 O 6 and La 2− x Sr 1+ x Cu 2 O 6 was determined as 0.08≤ x ≤0.2 and 0≤ x ≤0.25, respectively. The phase relations in (La 2− y Nd y ) 1− x /2 Sr 1+ x Cu 2 O 6 were also given according to the above results. The existence of three sub-lattices as La 2∼1.3 Nd 0∼0.7 Sr 1∼1.25 Cu 2 O 6 , La 1.2∼0.5 Nd 0.8∼1.5 Sr 1∼1.25 Cu 2 O 6 and Nd 1.6∼1.92 La 0.4∼0 Sr 1+ δ ∼1.2 Cu 2 O 6 (0< δ ≤0.08) was supposed and this consideration was demonstrated by the structural refinements for the three ended compositions of La 2 SrCu 2 O 6 , LaNdSrCu 2 O 6 and Nd 1.8 Sr 1.2 Cu 2 O 6 using powder X-ray diffraction data. It can be concluded that the lattice of (La, Nd) 2− x Sr 1+ x Cu 2 O 6 is very sensitive to the ionic size of rare-earth elements. Different rare-earth elements (such as La 3+ , Nd 3+ and La 3+ +Nd 3+ ) may cause rather different distributions of RE 3+ and Sr 2+ 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 La 2− x Sr 1+ x Cu 2 O 6 and Nd 2− x Sr 1+ x Cu 2 O 6 can be explained using the concept of the combined effect. The chemical complexity of RE 2 MCu 2 O 6 (RE=Rare-earth, M=Alkaline-earth) may help understanding the different conducting abilities of these compounds. The structural refinements on NdSr 2 Cu 2 O 6 and LaSr 2 Cu 2 O 6 give another example for the chemical complexity of RE–M–Cu–O system.