MODULATED STRUCTURE OF Bi,Y-2212 COMPOUND
A.V.Arakcheeva, V.F.Shamrai
Baikov
Institute of Metallurgy, Russian Academy of Sciences, Leninskii
pr. 49, Moscow, 117334 Russia
Single crystals of the incommensurably modulated Y-doped Bi-2212 phase of the composition Bi2Sr1.55CaY0.26Cu2O8.12(4) have been studied by X-ray methods (Syntex P1 diffractometer, MoKradiation, 277 basic reflections, and 290 and 62 first- and second-order satellite reflections, respectively). Experimental data were obtained for the unit cell with the parameters: a =5.409(3), b = 5b' = 5.408(3)x5, c = 30.56(1) Å. The incommensurability vector is q =0.213(3)b*. The refinement is performed in the commensurate (sp.gr. Pnnn, R = 0.0874) and incommensurate (group N: Bb2b / 1-11; R, R(0), R(1) and R(2) are equal to 0.755, 0.0548, 0.0823 and 0.1174, respectively) variants. The Y atoms are localized in the Sr positions [1].
The results obtained reflect the main features of the model of the incommensurately modulated Bi-2212 structure: the existence of relatively rigid two-dimensional (in xy-plane) perovskite-like blocks, in which the atomic displacements from their average positions along the x- and y-axis are rather small, and BiO two-layer blocks characterized by considerable displacements of atoms from their average positions along all three axes.
The data obtained in both refinement procedures confirm and complement each other. The nonmonotonic variation of the composition of the Sr positions was revealed only in the commensurate variant. The deficit in Bi position was recorded only in the incommensurate approximation. The modulations of atomic displacements are practically the same in both approximations. The presence of intercalated oxygen in Bi,O-layers follows from both variants. The most probable position of these extra atoms (Oextr) were obtained from the calculations in the commensurate approximation.
The crystallochemical conditionality of the presence of intercalated Oextr atoms in the structures of the Bi-2212 phases and their modulation is shown to be the consequence of the modular structure of the BiO layer. Bi and O atoms in this layer form two modules which are two links of the same hypothetical BiO chain along y-axis. The links differ in their length (2b' and 2.5b', where b' is parameter of basis subcell of the compound) and consequently in the structure of their ends. Every two neighboring links are related by a local mirror reflection plane, and two type of boundary between the links are formed. One of them passes through the Bi and O atoms shared by two links. The other boundary is the discontinuity between links ended by Bi atoms, i.e., the interlink space (~0.5b') into which the extra O atoms (Oextr) are incorporated. The presence of Oextr is necessary for the balance of local charges between ending Bi atoms. The more Oextr atoms, the shorter interlink space between ending Bi atoms and the lower modulation period T. Moreover, the more Oextr atoms between these Bi atoms, the more interaction between these Bi atoms and Oextr atoms from neighboring Bi,O layer, and consequently the shorter c period and the bigger amplitude of modulation along c axis. Content of Oextr is bound to increase with substitution of divalent cations (Ca and Sr) by trivalent Y. In fact, as is well known, Y-containing Bi-2212 phases with the oxygen content equal to 8.2 are characterized by even lower values of lattice parameter c (~30.6 Å) and the shorter values of T (~4.60 Å).
The oxygen content in Bi,O layers
is related to the critical temperature of supercondacting
transition (TC) via the hole-carrier concentration:
the lower the O content in BiO layer, the lower TC
value [2]. Y,Bi-2212 phases with oxygen content equal to 8.2 are
characterized by TC=95 K [3]. The Y-doped phase
studied in our work [1] has the c parameter ( 30.56 Å)
close to those usually observed but a somewhat overestimated
T=4.69 Å. The high value of last parameter is consistent with
the lower oxygen content in the BiO layers (8.12) and the lower
temperature of the superconducting transitions (TC=80
K).