Effects of Cs/Sr intercalation on the structure and electrical conductivity of weathered biotite

Weathered biotite (WB) is a layered clay mineral composed of Si, Al, Fe, and Mg. After molten-salt treatment at temperatures above 700 °C, WB exhibits thermoelectric properties, making it a low-cost and thermally robust candidate for sustainable high-temperature applications. Using the molten-salt method, we previously showed that alkali ions can be intercalated into the interlayer space of WB. While environmental studies have described selective adsorption of Cs and co-uptake of Sr by WB, here we focus on intercalation into the solid lattice. In this study, we examine how Cs and Sr intercalation affect the local structure and electrical conductivity of WB. Samples were prepared by molten-salt treatment and analyzed by x-ray fluorescence (XRF), x-ray diffraction (XRD), x-ray absorption fine structure (XAFS), and four-probe conductivity measurements. XRF confirmed Cs and Sr incorporation, and XRD showed that the layered WB framework was preserved. XAFS revealed that, in the Cs/Sr-WB sample, the Sr–Si distance shortened by ≈0.2 Å at elevated temperature, consistent with Sr approaching the Si–O sheet. Electrical conductivity was measured from 630 to 860 °C, and in the 630–780 °C range, the Cs/Sr-WB sample showed higher conductivity than the Sr-WB sample. These observations are consistent with the rearrangement of interlayer ions that improves conduction pathways. The dominant carrier type is not identified from the present data, but a partial ionic contribution associated with interlayer ion motion at high temperature is suggested. The results highlight interlayer structural control as a lever for designing clay-based thermoelectric materials.