X-ray investigation of the incommensurate modulated structure of Bi_2.08Sr_1.84CuO_6−δ

Abstract

At room temperature the bismuth strontium cuprate Bi2.osSrl.8aCuO6 [a = 5.3791 (6), b = 5.3811 (9), c = 24.589 (3) A, fl = 89.93 (1) °, basic space group A2/a, superspace group pAZ/~, M, = 755.44, Dx = 7.05 g cm -3, V = 711.7 ,/k 3, Z = 4, Cu Ka radiation, = 1.541838/~, /z = 1203 cm-i, F(000) = 1278.24, T = 294 K] exhibits a one-dimensional incommensurate modulated structure with a modulation wavevector q* having components 0.2030 (2), 0, 0.467 (2). Both a displacive modul~ition wave, acting on all atoms and a density modulation wave, governing the occupancy probabilities of the Bi, Sr and apical 0(2) sites are involved in the crystal. 1117 unique hklm reflections with I_> 3o-(1) were used to refine the modulated structure. The final R factors (based on F) of main reflections (292), and first- and secondorder satellite reflections (563 and 262) are 0.061, 0.061 and 0.086 respectively. The modulation is anharmonic for all atoms. Evolution of the Bi, Sr and apical 0(2) site occupancies throughout the crystal is correlated in a spectacular way and shows the existence of Bi and 0(2) vacant sites in some unit cells; moreover, some Sr sites are occupied by Bi atoms. These features give rise to various coordinations i.e. square planar, pyramidal, octahedral for