Quantitative phase analysis of Bi2Sr2CaCu2O8+x and competing intergrowth and co-crystallizing phases via a Rietveld refinement study

Abstract

The Rietveld refinement technique has been used to determine the extent of intergrowth of the Bi2Sr2CuO6+x phase and co-crystallization of competing phases in the high-temperature superconductor Bi2Sr2CaCu2O8+x (Bi-2212). The refinement was done on powder diffractograms obtained on powders made by grinding single crystals of Bi2Sr2CaCu2O8+x grown using two different self-flux techniques, namely the pressure technique and the regrowth technique, and ground for either 2 min or 2 h. The Rietveld programs JANA and FULLPROF were used for the refinement and both gave consistent results. The Bi and Sr atom positions were refined in the average structure of centrosymmetric space group Bbmb. To incorporate Bi-atom modulation and extract information about the modulation vector, refinement was done in the centrosymmetric space group N^{Bbmb}_{1\overline 11}(Bbmb(0γ1)). The b* component of the modulation vector decreases with a decrease in the superconducting transition temperature in the pressure-technique sample compared with the regrowth sample, suggesting a better alignment of the CuO2 planes with respect to the Bi–O planes in the pressure-technique sample. All the samples exhibit a strong preferred orientation effect. Values of the March–Dollase parameters corresponding to the preferred orientation function were obtained. Brindley absorption contrast factors t ϕ were also calculated, together with the effect of microabsorption on the number of phases present in each sample. Rietveld refinements incorporating all the factors resulted in excellent values for the goodness-of-fit parameters for all the samples, with the lowest value of 2.08 for the pressure-technique sample ground for 2 min. Additionally, the powders corresponding to the pressure-technique crystals have no co-crystallizing phase and ∼94% of the Bi-2212 phase, suggesting that crystals grown by the pressure technique are of extremely good quality, much better than those grown by the regrowth flux technique.