Thermoelectric properties of LaCo 1x Ni x O 3 polycrystalline samples and epitaxial thin films Rosa Robert a , Myriam H. Aguirre a , Laura Bocher a , Matthias Trottmann a , Sebastian Heiroth b , Thomas Lippert b , Max Do ¨beli c , Anke Weidenkaff a, * a EMPA, Solid State Chemistry and Catalysis, Ueberlandstr. 129, CH-8600 Duebendorf, Switzerland b Paul Scherrer Institut, CH-5232 Villigen, Switzerland c Ion Beam Physics, Paul Scherrer Institut and ETH Zurich, 8093 Zurich, Switzerland Received 15 October 2007; received in revised form 17 January 2008; accepted 21 January 2008 Available online 2 February 2008 Abstract Epitaxial thin films of LaCo 1x Ni x O 3 were deposited on MgO (100) substrates by pulsed reactive crossed-beam laser ablation (PRCLA). The electrical transport and thermopower of the epitaxial La(Co,Ni)O 3 thin films were measured in a broad temperature range and their thermoelec- tric activity was compared with bulk samples. The measured films show positive Seebeck coefficient values indicative of p-type conduction. The thin films display a better thermoelectric performance at high temperatures compared to the polycrystalline samples. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Thermopower; LaCoO 3 ; Thin films; PLD; Ni-substitution 1. Introduction The electronic properties of complex cobalt oxides are characterized by the contribution of the 3d orbitals. The interactions of the strongly correlated electrons in these mate- rials are very important and can lead to remarkable physical properties, i.e. magnetoresistance [1], superconductivity, ferro- electricity [2], and thermoelectric activity. The thermoelectric performance of a material is defined by the thermoelectric figure of merit ZT ¼ S 2 /(k  r), where S is the Seebeck coefficient or thermopower, r the electrical conductivity, and k the thermal conductivity. Enhanced ZT values are expected for materials with large Seebeck coefficient, low electrical resistivity and low thermal conductivity. Potential thermoelectric materials are found among the lanthanum cobaltates (e.g. LaCoO 3 ) due to their large Seebeck coefficient of S w 600 mV/K. It has been shown that the carrier concentration and spin entropy play an important role for the enhancement of the power factor (PF ¼ S 2 /r) in these phases [3,4]. The PF of these materials is tuned by suitable substitu- tions [5e7]. Previous studies revealed a lowering of the electrical resistivity for low level A- and B-site substituted LaCoO 3 , e.g. La 1x Sr x CoO 3 [8] and LaCo 1x Ni x O 3 [7,8], while, according to Heikes equation [9e11], the large thermo- power of LaCoO 3 is preserved. Recently, a number of structures with low dimensions, such as nano-structured materials, thin films, and superlatti- ces have been proposed to improve the thermoelectric prop- erties of materials. In particular, thin films should exhibit a decrease of the thermal conductivity due to increased phonon scattering, while an increased local electron density of states may improve the thermopower [12]. Intensive research is focused on the evaluation of the thermoelectric properties of complex oxide thin films [13e16]. Epitaxial thin films grown on chemically inert single crystalline substrate materials offer a suitable alternative to large single crystals as ‘‘two-dimensional’’ crystals and the possibility to study the influence of the dimensionality on the thermoelec- tric properties. * Corresponding author. E-mail address: anke.weidenkaff@empa.ch (A. Weidenkaff). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.01.017 Available online at www.sciencedirect.com Solid State Sciences 10 (2008) 502e507 www.elsevier.com/locate/ssscie