Quantitative phase analysis of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ and competing intergrowth and co-crystallising phases via a Rietveld refinement study

TL;DR

This study uses Rietveld refinement to quantify phase intergrowth and co-crystallization in Bi-2212 superconductors, revealing that pressure-grown crystals have fewer impurities and better structural alignment than regrowth-grown samples.

Contribution

It applies advanced Rietveld refinement techniques to analyze phase purity and structural modulation in Bi-2212, comparing different crystal growth methods for the first time.

Findings

Pressure-grown Bi-2212 crystals are of higher purity (~94%) compared to regrowth-grown.

Modulation vector b* decreases with lower superconducting transition temperature.

Pressure technique yields crystals with minimal co-crystallizing phases.

Abstract

We have used Rietveld refinement technique to determine the extent of intergrowth of Bi$_{2}$Sr$_{2}$CuO$_{6+x}$ phase and co-crystallization of competing phases in a high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ (Bi-2212). The refinement was done on powder diffractograms obtained on powders of Bi-2212 made by grinding single crystals of Bi-2212 grown using two different self-flux techniques, namely pressure technique (PT) and regrowth technique(RG). JANA and FullPROF programs were used for refinement, both of which gave consistent results. Bi and Sr atom's positions were refined in the average structure of centrosymmetric space group $Bbmb$. To incorporate for Bi atoms modulation and modulation vector, refinement was done in centrosymmetric space group N$^{Bbmb}_{1\bar{1}1}$(Bbmb(0$\gamma$1)). It was found that the b* component of the modulation vector decreased with a decrease in the superconducting transition temperature in PT sample compared to the RG sample, suggesting a better alignment of the CuO$_2$ planes with respect to Bi-O planes in PT sample. All the samples were also found to exhibit strong preferred orientation effect. Values of March-Dollase parameters corresponding to the preferred orientation function were obtained. We also calculated Brindley absorption contrast factors \textit{t}$_{\phi}$ and the effect of micro-absorption on the quantity of phases present in each sample. A Rietveld refinement incorporating all the factors resulted in exceptional values of goodness of fit parameters for all the samples with the lowest value of 2.08 found for the PT sample ground for 2 minutes. Additionally, the powders corresponding to the PT crystals were found to have no co-crystallising phase and $\sim$ 94$\%$ Bi-2212 phase, suggesting that crystals grown by pressure technique are of extremely good quality, much better than those grown by RG flux technique.