Abstract
To correlate structural and compositional parameters in the bismuth 2212 system, single crystals with the composition (Bi1−yPby)2Sr2(Ca1−xRx)Cu2O8+δ (R = Y, Er, Ho, Tm, and Yb; y = 0, 0.1; 0 ≤ x ≤ 0.5) have been studied at room temperature by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The main results are as follows: (i) The actual content, x, of R (R = Y, Er) in samples is in significant excess over its content x′ in the melt for x′ < 0.5. The opposite effect (but several times smaller) takes place for Ca. At x′ = 0.5, the x value practically coincides with x′. (ii) For all R under examination and x′ = 0.1, the value of x is within the limits of 0.43 ≤ x ≤ 0.51; i.e., x exceeds x′ several times. (iii) The total content of Ca, R (R = Y, Er), and Sr is close to 3 through the whole range 0 ≤ x′ ≤ 0.5. At x′ < 0.5 Ca is partly substituted for Sr, while R occupies only Ca crystallographic positions. Thus the actual formula of the samples is (Bi1-yPby)2+∊Sr2-z(Ca1+z-xRx)Cu2O8+δ. (iv) The evidence was received that the nonlinear dependence c(x) at x < 0.5 is connected with the partial substitution of Sr with Ca. The dependence of c, namely on the R = Y content in the denoted range of x, is close to linear with the slope ∂c/∂x = −0.67(2) Å/at.