An enhancement ZT and spin state transition of Ca 3 Co 4 O 9 with Pb doping S. Demirel a , E. Altin b , E. Oz a , S. Altin a,⇑ , A. Bayri a a Inonu University, Science and Art Faculty, Physics Department, 44280 Malatya, Turkey b Inonu University, Scientific and Technological Research Center, 44280 Malatya, Turkey article info Article history: Received 25 August 2014 Received in revised form 27 October 2014 Accepted 16 November 2014 Available online 27 December 2014 Keywords: Ca 3 Co 4 O 9 Pb-doping Thermopower abstract We reported the structural, electrical, thermal and magnetic properties of Pb-doped Ca 3 Co 4 O 9 in the range of 300–5 K. DTA analysis showed that the stability of the Ca 3 Co 4 O 9 was increased with Pb doping. According to XRD analysis, it is found that Pb ions were successfully doped in the Ca 3 Co 4 O 9 structure. The temperature of resistivity minima, T min , increased by increasing the Pb doping level and it is seen that incommensurate spin density wave state becomes more stable with Pb doping. The enhancement of ther- mopower was explained that Pb doping in Ca sites caused a decrease of Co 4+ fraction such that Co 4+ ions transformed into Co 3+ or Co 2+ . The room temperature ZT value of the polycrystalline sample reaches about 16 times larger value than that of the un-doped polycrystalline sample which is the promising candidate for high temperatures in the thin film applications. According to magnetic susceptibility measurement, the increase of effective magnetic moment by Pb concentration was explained by spin state transition of Co 3+ from low spin to intermediate spin and high spin state together with some orbital angular momentum contribution which comes from 5 T 2 term due to a decrease of the ligand field splitting energy. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction The thermoelectric (TE) material is one of the most important candidates for future technologies. TE materials convert the waste heat energy into electrical energy and may play important role for energy systems in the future. The thermoelectric materials characterized by the dimensionless figure of merit coefficient (ZT) including Seebeck coefficient, electrical conductivity, thermal conductivity and temperature. ZT value should be about one or above in order to apply technology such as TE module. So, thermopower and electrical conductivity should be large and ther- mal conductivity should be low for getting high ZT value [1–4]. Ca 3 Co 4 O 9 oxide compound is a promising material for thermoelectric research (especially at low temperatures) and high temperature power-generation applications in oxidative environ- ments. The layered Ca 3 Co 4 O 9 (Ca-349) has a reasonably high ther- mopower and electrical conductivity [5–7]. Moreover, Ca-349 has more characteristics which make it rather a compound than an alloy and the intrinsic properties of Ca-349 are also attractive fea- tures such as low temperature paramagnetic–ferrimagnetic phase transition, spin density wave ordering, metal–insulator transition, and high temperature spin state transitions [8–11]. Although the single crystalline form of Ca-349 has high ZT value (=0.87 at 700 °C), the polycrystalline form has lower ZT which is not useful for technological applications such as TE module [12]. There are many attempts to enhance of TE performance of Ca-349 polycrystalline samples by the modification of fabrication technique [13–17] and doping elements such as Sr, Y, Bi, Ag, Ti, Sb, Fe, Yb, Ga, Cr, La, Eu, Ir, Nd, Mg, Pr either Ca site or Co site in the system. Some of them caused the increase of ZT values (20%, 2%, 100% and 10% increase at room temperature for B, Cr, Ag and Ga doping, respectively) and the others decreased the thermoelec- tric performance [18–32]. To understand the unusual electrical and thermal behavior of Ca-349, substitutions at Co and Ca sites by transition metals with considering the valance state and lattice phonon modes are impor- tant. The studies showed that the substitutions at Ca and Co sites have different effects for TE properties. While the Co-sites in Ca 2 CoO 3 sub-layer effect the carrier concentration, the Co-sites in CoO 2 sub-layer have notable change in the electronic correlation of the system [33]. The Ca-sites doping studies show that doping affects the carrier concentration and phonon modes of the system [34], because the change of the weight and the size of atom in Ca-site causes the change of phonon frequency [35]. The spin state configurations of Ca-349 compounds have impor- tant concepts due to the relation of spin state and TE properties [36]. Three different spin states of Co ions in Ca-349 have been http://dx.doi.org/10.1016/j.jallcom.2014.11.200 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel./fax: +90 422 377 4978. E-mail address: serdar.altin@inonu.edu.tr (S. Altin). Journal of Alloys and Compounds 627 (2015) 430–437 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom