Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Antiferromagnetism and heat capacity of NaCo 2-x Cu x O 4 ceramics Sanja Pršić a, ⁎ , Slavica M. Savić a,b , Zorica Branković a , Zvonko Jagličić c,d , Stanislav Vrtnik e , Goran Branković a a Institute for Multidisciplinary Research, University of Belgrade, Kneza Višeslava 1a, 11030 Belgrade, Serbia b Biosense Institute-Institute for Research and Development of Information Technology in Biosystems, Dr Zorana Đinđića 1, 21000 Novi Sad, Serbia c Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova Cesta 2, 1000 Ljubljana, Slovenia d Institute of Mathematics, Physics and Mechanics, Jadranska 19, 1000 Ljubljana, Slovenia e Jožef Stefan Institute, Condensed Matter Physics, Jamova cesta 39, 1000 Ljubljana, Slovenia ARTICLE INFO Keywords: Powders: chemical preparation Solid state reaction Sintering Magnetic properties Heat capacity ABSTRACT Polycrystalline samples of NaCo 2-x Cu x O 4 (x=0, 0.01, 0.03 and 0.05) were synthesized in two different ways: 1) by a mechanochemically assisted solid-state reaction method (MASSR) and 2) by a citric acid complex method (CAC). In this work we examined the influence of these synthesis routes and small Cu concentrations on magnetic properties and the heat capacity of sintered samples. The magnetic susceptibility (χ) of all samples followed the Curie-Weiss law in the temperature range between 50 K and 300 K, while a negative Weiss constant (θ) implied an antiferromagnetic interaction. According to the magnetic susceptibility data, a peak around 30 K indicating the presence of Co 3 O 4 as a secondary phase appeared for all MASSR samples and CAC samples with Cu content above 1%. The effective magnetic moment (μ eff ) of CAC samples was lower than the theoretical, spin only value obtained for the Co 4+ ion in the low spin state indicating the presence of low spin Co 3+ (S =0). These values were also lower compared to the values obtained for MASSR samples. The highest μ eff of 1.75 μ B /atom Co was obtained for the undoped MASSR sample. The heat capacity of CAC samples at 2 K decreased with Cu concentration due to lowering of the electronic specific heat coefficient (γ). The highest γ of 63.9 mJ/molK 2 was obtained for the undoped CAC sample. This reduction in γ values was the result of the decrease of the density of state and/or mass enhancement factor. 1. Introduction In the past few decades, there has been a growing interest in alternative energy sources and new methods for energy conversion. Thermoelectric materials belong to the group of materials that directly convert waste heat into electric energy. Among them, layered oxides attract great attention because of their interesting structural, physical and chemical properties, such as NaCo 2 O 4 (NCO), which exhibits good thermoelectric properties [1–3]. Increase of the Seebeck coefficient (S), simultaneously with the decrease of thermal conductivity (κ) and electrical resistivity (ρ) are the main requests for high thermoelectric performance [4]. High S (also called thermopower) as a consequence of a strong electron correlation and low ρ make NaCo 2 O 4 a promising material for potential use in thermoelectric devices [5,6]. NaCo 2 O 4 belongs to compounds with a bronze-type crystal struc- ture of the A x BO 2 (0.5≤x≤1) general formula [7]. This layered oxide consists of Na and CoO 2 layers alternately stacked along the c- direction. The stoichiometry of Na in this compound is variable, and depending on the sodium content, there are three types of crystal structure: P3: β–Na x Co 2 O 4 (1.1≤x≤1.2), P2: γ–Na x Co 2 O 4 (1.0≤x≤1.4), O3: α–Na x Co 2 O 4 (1.8≤x≤2.0), whereby the P2 structure possesses the highest thermopower [8]. As arrangement of sodium ions in the crystal lattice depends on the temperature and Na content [9], diversity of Na x Co 2 O 4 properties comes from the different sodium content, accord- ingly [10]. In general, cobalt oxides are systems with a strong electron correlation, where 3d orbitals have a specific degeneration, due to the spin and orbital degrees of freedom. Two competitive processes are responsible for degeneration of electronic states of Co 3+ and Co 4+ ions: crystalline field and Hund's rule coupling [11]. Interactions between 3d electrons largely affect the transport properties of all cobaltites and they are expected to affect magnetic properties of these materials, as well [12]. In the octahedral crystal field, as is the case in the CoO 2 layer, the 3d orbitals split into two e g and three t 2g orbitals, which in the rhombohedral crystal field further split into e' g and a 1g orbitals [13– 15]. Hybridization between these orbitals causes the formation of two http://dx.doi.org/10.1016/j.ceramint.2016.10.170 Received 1 September 2016; Received in revised form 17 October 2016; Accepted 26 October 2016 ⁎ Corresponding author. E-mail addresses: sanjaprsic@imsi.rs, sanjaprsic@gmail.com (S. Pršić), slavicas@imsi.bg.ac.rs (S.M. Savić), zorica.brankovic@imsi.bg.ac.rs (Z. Branković), zvonko.jaglicic@imfm.si (Z. Jagličić), stane.vrtnik@ijs.si (S. Vrtnik), goran.brankovic@imsi.bg.ac.rs (G. Branković). Ceramics International 43 (2017) 2022–2026 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Available online 28 October 2016 crossmark