Synthesis and physical properties of layered Ba_xCoO₂

Abstract

A layered cobaltite Ba(x)CoO2 (x = 0.19, 0.28, 0.30, 0.33) was synthesized by an ion exchange technique from the layered Na(x)CoO2 precursors. The phase composition and physical properties were investigated. Ba(x)CoO2 is isomorphic to the precursor Na(x)CoO2. The magnetic susceptibility of Ba(x)CoO2 decreases with increasing barium content and shows a Curie-Weiss-like behavior at temperatures above 50 K. The resistivity is sensitive to the barium content. As the barium content increases from 0.19 to 0.33, a crossover from a semiconducting behavior to a metallic behavior was observed. The Seebeck coefficient of Ba(x)CoO2 is insensitive to the barium content due to the tradeoff effect between the carrier concentration and the Co(4+) content, while the thermal conductivity increases with the increasing barium content from 0.19 to 0.33 owing to the ordered state of Ba ions between the CoO2 layers.

High-temperature thermoelectric properties of layered Ba CoO2

“…For the Ba 0.30 CoO 2 , Ba 0.28 CoO 2 and Ba 0.19 CoO 2 samples, the r increases with increasing temperature and shows semiconducting behavior. This semiconducting behavior of the Ba-poor samples might be related to the carrier localization [14,22]. On the other hand, the r decreases with decreasing barium content from 0.33 to 0.19, and the r of Ba 0.33 CoO 2 is the largest of the four samples, which is attributed to the result of a welldefined Ba order in Ba 0.33 CoO 2 [14,20].…”

“…The behavior of r for Ba x CoO 2 with different barium content at high temperatures is the continuation of that for Ba x CoO 2 at temperatures below 300 K [14]. For the Ba 0.33 CoO 2 sample, the r decreases with increasing temperature and is typical of simple metallic behavior.…”

“…It was reported that they are isostructural to their precursor Na x CoO 2 , and also exhibit a large Seebeck coefficient or low resistivity, which is necessary for a good thermoelectric material. In a previous study [14] we successfully prepared the Ba xCoO 2 material and found that both the conductivity and the Seebeck coefficient are quite large at room temperature. In addition, the thermal stability of Ba x CoO 2 is very good.…”

“…[14]. The actual Ba contents x of the ion exchange products Ba x CoO 2 were 0.19, 0.28, 0.30 and 0.33 for the four samples.…”

“…In recent years, a number of novel layered cobaltites with the same CoO 2 block have been prepared by an ionexchange technique from Na x CoO 2 precursor, such as Ca xCoO 2 [10,11], Sr x CoO 2 [12,13], Ba x CoO 2 [14,15] and Ln 0.30 CoO 2 (Ln = La, Pr and Nd) [16,17]. It was reported that they are isostructural to their precursor Na x CoO 2 , and also exhibit a large Seebeck coefficient or low resistivity, which is necessary for a good thermoelectric material.…”