Effect of isovalent substitution on microstructure and phase transition of LaNb 1 À x M x O 4 (M ¼ Sb, V or Ta; x ¼ 0.05–0.3) S. Wachowski a,n , A. Mielewczyk-Gryn a,b , M. Gazda a a Faculty of Applied Physics and Mathematics, Department of Solid State Physics, Gdansk University of Technology, Narutowicza 11/12, 80-233 Gdansk, Poland b Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, CA 95616, USA article info Article history: Received 18 May 2014 Received in revised form 25 July 2014 Accepted 28 July 2014 Available online 1 August 2014 Keywords: Phase transition Thermal expansion coefficient Lanthanum niobate abstract LaNb 1Àx M x O 4 oxides with pentavalent elements of different ionic sizes (M¼Sb, Ta and V, x ¼0.05–0.3) were synthesized by the solid state reaction method. Special interest was devoted to the antimony substituted lanthanum niobate which is a new material in this group. Rietveld analysis of the X-ray diffraction patterns was used to determine the influence of the material composition on unit cell parameters. On the basis of dilatometric measurements phase transition temperatures and thermal expansion coefficients of the studied materials were determined. It was shown that with increasing concentration of Sb the phase transition temperature decreases. Thermal expansion coefficient of the antimony substituted samples above the transition temperature is in the range from 8.1 to 9.1 Â 10 À6 1/K, whereas below the transition temperature the TEC value is between 14 and 17.3 Â 10 À6 1/K. Influence of Ta, V and Sb substitutions on the microstructure and grain size was studied. & 2014 Elsevier Inc. All rights reserved. 1. Introduction Functional ceramics and electroceramics, with their interesting electrical, magnetic, optical, thermal and other properties, have become one of the most important subjects of modern materials science. The performance of electroceramic materials depends on their structure, microstructure and high temperature properties. Many ceramic materials, e.g. ABO 4 (fergusonites, scheelites and wolframites), ABO 3 (perovskites) and AB 2 O 4 (spinels) undergo structural phase transitions [1–4]. Since the structural phase transitions are important phenomena when it comes to analysis of the materials properties, the structural studies should be carried out. They include both high temperature and high pressure investigations. For instance, high pressure studies led to discovery of novel phases of strontium molybdate [5], characterization of the structural and electronic properties of various wolframites [6] and molibdates [7] and description of the mechanism of the phase transition in multiferroic CuWO 4 [8]. In this work the results of the structural and microstructural studies of lanthanum niobate, belonging to the ABO 4 group of oxides, substituted with the pentavalent elements on the niobium site, are reported and discussed. Special effort was devoted to the antimony substituted lanthanum niobate, LaNb 1 Àx Sb x O 4 (x ¼ 0.05– 0.3), which is a new material in this group. The aim of the studies was to determine the influence of antimony on the temperature of the phase transition. For comparison, also LaNb 1 Àx Ta x O 4 and LaNb 1 Àx V x O 4 (x ¼ 0.05–0.3) were studied. Lanthanum niobates belong to a group of electroceramics technologically interesting for their proton conductivity [9]. Acceptor doped lanthanum niobates have been studied for the last ten years because of their relatively high ionic conductivity and stability in carbon dioxide containing atmospheres. At approximately 500 1C lanthanum niobate undergoes a transforma- tion from the ferroeleastic phase with the monoclinic symmetry to the paraelastic tetragonal phase [1,2]. This leads to a thermal expansion coefficient (TEC) change which may worsen thermo- mechanical stability of material. The structural transformation also causes a change of the distances between the crystallographic planes between which hydrogen hopping occurs affecting the mobility of charge carriers. As a result the activation energy of conductivity is higher in the monoclinic phase than in the tetragonal one [10]. As a consequence, two strategies of the research concerning ABO 4 proton conductors have been carried out recently. One is to elaborate a material with the monoclinic fergusonite structure, stable up to high temperatures (e.g. 1000 1C) [11,12]. The other strategy is to stabilize the tetragonal scheelite structure in the whole temperature range above room tempera- ture [13]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jssc Journal of Solid State Chemistry http://dx.doi.org/10.1016/j.jssc.2014.07.041 0022-4596/& 2014 Elsevier Inc. All rights reserved. n Correspondence to: Centrum Nanotechnologii A, ul. Narutowicza 11/12, 80-233 Gdańsk, Poland. Tel.: þ48 58 348 66 12. E-mail address: swachowski@mif.pg.gda.pl (S. Wachowski). Journal of Solid State Chemistry 219 (2014) 201–209