Electrical and Thermal Conduction Behaviors in La-Substituted GdBaCuFeO 5+d Ceramics Chengcheng Zeng, ‡ Bin Zhan, ‡ Sajid Butt, ‡,§ Yaochun Liu, ¶ Guangkun Ren, ‡ Yuan-Hua Lin, ‡,† Ming Li, ‡ and Ce-Wen Nan ‡ ‡ State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China § Department of Materials Science and Engineering, Institute of Space Technology, Islamabad 44000, Pakistan ¶ School of Materials Science and Engineering, University of Science and Technology, Beijing 100083, China Gd 1 – x La x BaCuFeO 5+d polycrystalline ceramics have been prepared by a sol–gel method combined with the traditional ceramic processing. Analysis of the microstructure and phase composition reveals that the pure GdBaCuFeO 5+d phase can be obtained even after the substitution of 50% La at Gd-site. The band gap can be tuned from 1.47 to 1.36 eV by La substi- tution, resulting in a significant increase in the electrical con- ductivity. In addition, the total thermal conductivity can also be suppressed by the substitution of La at Gd-sites. A ZT value of 0.02 at 1023 K is achieved in the 50% La-substituted samples, which is over 20 times higher than that of the pure GdBaCuFeO 5+d sample. I. Introduction P RESENTLY, more than 80% of energy we consume comes from the burning of fossil fuels which are decreasing day by day. 1 With the increasing clean energy demands, thermo- electric (TE) materials are receiving an immense interest due to environment friendliness and scalability. 2 TE material can directly transform the heat into electricity and vice versa, whose performance of conversion can be quantified by the thermoelectric figure of merit, ZT = rS 2 T/j tot , where r,S, j tot , and T represent the electrical conductivity, the Seebeck coefficient, the total thermal conductivity, and the absolute temperature, respectively. 3 Thus, the main goal of research on TE materials was to enhance the ZT value by improving the power factor (rS 2 ) and reducing j tot without changing r. Nowadays, TE materials are used in new applications such as thermopower wave energy sources because they are capa- ble of generating higher specific power in comparison to many of the conventional sources. 4–6 The extensive work on TE materials until now, has mainly focused on alloy compounds such as Bi 2 Te 3, 7 PbTe, 8,9 Half- Heusler, 10,11 etc. Although they owe high ZT value, most of them contain toxic elements and would be easily oxidized at higher temperature when exposed to air. Compared with alloy compounds, the oxide TE materials are regarded as potential competitor for high-temperature TE application due to the low cost and simple fabrication processing. 12,13 Furthermore, it has been investigated that the oxide TE materials comprised of transition metals are quite promising due to its tunable electronic and thermal properties. 14,15 Therefore, the discovery of new oxide TE materials with tun- able thermoelectric properties is indispensable. Due to their natural layered structure and excellent perfor- mance, 16 double perovskite-like oxides with general formula of A 0 A″B 2 O 5+d are gaining much attention, where, A 0 ,A″, and B represent rare-earth metal, alkaline-earth metal, and transition metal, respectively. 14 The B site atoms and oxygen atoms constitute pyramid structure to connect basic frame, with the A site atoms arrange into it. The structural diagram of GdBaCuFeO 5+d with [BO] x pyramid structure in white is shown in Fig. 1. Most of them have tetragonal structure with space groups of P4mm or P4/mmm, while LaBaCuFeO 5+d has a cubic structure with a space group Pm3m due to a little difference on the ionic radius between La 3+ and Ba 2+ . 17,18 The current research on these materials mainly focused on their structure, magnetic, and electrical performance. Roy et al. 19 have reported that the spin state of Co in RBaC- o 2 O 5+d is mainly controlled by oxygen vacancy d; Suescun et al. 20 have pointed out a relationship between structure and magnetic properties of LaBaCoCuO 5+d . What’s more, Kly- ndyuk et al. 17,21–24 have mainly investigated the electrical and thermoelectric properties of RBaCuFeO 5+d . Although, only a few systematical studies have been reported on the high-temperature thermoelectric properties of layered A 0 A″ B 2 O 5+d , its potential for TE applications cannot be neglected. As reported in the literature, GdBaCuFeO 5+d , a member of A 0 A″B 2 O 5+d family , exhibits a relatively higher Seebeck coefficient value of about 230 lV/K at high temperature and an appropriate low thermal conductivity of about 2.5 WÁ(mÁK) À1 , but the issue pertains to its lower electrical conductivity. As reported in our previous work, 25 La substi- tution can effectively improve the electrical conductivity in this type of material by decreasing the band gap. In this work, we report a systematic study on the thermoelectric properties of La-substituted GdBaCuFeO 5+d ceramics pre- pared by sol–gel method and traditional solid-state sintering. The effect of La substitution on the phase composition, mor- phology, and thermoelectric properties has been investigated systematically. The enhanced ZT values have been observed by optimizing the electrical properties and lowering the ther- mal conductivity through La substitution, revealing a candi- date for high-temperature thermoelectric applications. II. Experimental Procedure A series of Gd 1Àx La x BaCuFeO 5+d with nominal stoichiome- tric ratio (x = 0, 0.2, 0.3, 0.4, 0.5) were synthesized via sol– gel method followed by traditional solid-state sintering. The analytically pure Gd(NO 3 ) 3 Á6H 2 O, La(NO 3 ) 3 Á6H 2 O, Ba (NO 3 ) 2 , Cu(NO 3 ) 2 Á3H 2 O, Fe(NO 3 ) 3 Á9H 2 O were served as raw X.-D. Zhou—contributing editor Manuscript No. 36620. Received March 25, 2015; approved May 28, 2015. † Author to whom correspondence should be addressed. e-mail: ccc13@mails.tsing hua.edu.cn 3179 J. Am. Ceram. Soc., 98 [10] 3179–3184 (2015) DOI: 10.1111/jace.13728 © 2015 The American Ceramic Society J ournal