COMPOSITION AND CRYSTAL-STRUCTURE PARAMETERS OF SINGLE-CRYSTALS (BI, PB)2SR2(CA1-XRX)CU2O8+DELTA (R = Y, ER, HO, TM, AND YB)

To correlate structural and compositional parameters in the bismuth 22 12 system, single crystals with the composition (Bi1-yPby)2Sr2(Ca1-xRx )Cu2O8+delta (R = Y, Er, Ho, Tm, and Yb; y = 0, 0.1; 0 less-than-or-eq ual-to x less-than-or-equal-to 0.5) have been studied at room temperat ure 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) ta kes 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 w ithin the limits of 0.43 less-than-or-equal-to x less-than-or-equal-to 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 less-t han-or-equal-to x' less-than-or-equal-to 0.5. At x' < 0.5 Ca is partly substituted for Sr, while R occupies only Ca crystallographic positio ns. Thus the actual formula of the samples is (Bi1-yPby)2+epsilonSr2-z (Ca1+z-xRx)Cu2O8+delta. (iv) The evidence was received that the nonlin ear dependence c(x) at x < 0.5 is connected with the partial substitut ion 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 partial-der ivative c/partial-derivative x = -0.67(2) angstrom/at.