Intergrowth and thermoelectric properties in the Bi-Ca-Co-O system

TL;DR

This study reports the growth and characterization of Bi-Ca-Co-O single crystals with misfit layered structures, revealing enhanced thermoelectric power and intergrowth phenomena that could improve thermoelectric materials.

Contribution

It introduces a method to grow Bi-Ca-Co-O single crystals with specific misfit structures and demonstrates how intergrowth affects thermoelectric properties.

Findings

Room-temperature thermoelectric power of 180 μV/K in quadruple RS layer crystals.

Intergrowth of different RS layer structures enhances thermoelectric power and resistivity.

Intergrowth does not change the room-temperature power factor.

Abstract

Single crystals of the Bi-Ca-Co-O system have been grown using the flux method with cooling from 900$\celsius$ and 950$\celsius$, respectively. The single crystals are characterized by transmission electron microscopy and X-ray diffraction. The misfit cobaltite [Ca$_2$Bi$_{1.4}$Co$_{0.6}$O$_4$]$^{RS}$[CoO$_2$]$_{1.69}$ single crystals with quadruple ($n$=4) rocksalt (RS) layer are achieved with cooling from 900$\celsius$. Such crystal exhibits room-temperature thermoelectric power (TEP) of 180$\mu$V/K, much larger than that in Sr-based misfit cobaltites with quadruple RS layer. However, intergrowth of single crystals of quadruple ($n$=4) and triple ($n$=3) RS-type layer-based misfit cobaltites is observed with cooling from 950$\celsius$. Both of TEP and resistivity were obviously enhanced by the intergrowth compared to [Ca$_2$Bi$_{1.4}$Co$_{0.6}$O$_4$]$^{RS}$[CoO$_2$]$_{1.69}$ single crystal, while the power factor at room temperature remains unchanged.