Thermochimica Acta 506 (2010) 20–27 Contents lists available at ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Thermodynamic modeling, structural and spectroscopic studies of the KNbWO 6 –KSbWO 6 –KTaWO 6 system Aleksandr V. Knyazev a,∗ , Miroslaw M ˛ aczka b , Nataliya Yu. Kuznetsova a a Nizhny Novgorod State University, Gagarin Prospekt 23/2, 603950 Nizhny Novgorod, Russia b Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw, Poland article info Article history: Received 11 March 2010 Received in revised form 6 April 2010 Accepted 7 April 2010 Available online 14 April 2010 Keywords: Thermodynamic modeling Defect pyrochlore IR and Raman spectroscopy X-ray diffraction Isomorphism Mixing diagram abstract In the present work some structural and thermodynamic features in KNb x Sb y Ta z WO 6 solid-state solutions were investigated. A mathematical subregular ternary solutions model is advanced. The compounds have been structurally studied using X-ray diffraction. In particular, structure of individual compounds (KNbWO 6 , KSbWO 6 and KTaWO 6 ) was refined by the Rietveld method (space group Fd3m, Z = 8). IR and Raman spectroscopies were used to assign vibrational bands and determine structural pecularities. The differential scanning calorimetry was applied to measure decomposition temperature of compounds under study and to detect any possible phase transitions. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Pyrochlore is important in nature as an ore mineral (Nb and Ta) and as a carrier of lanthanide and actinide (Th and U) elements in the Earth’s crust. The structure type exhibits a range of useful properties for materials science applications including ionic con- ductivity (both cations and anions) [1,2], electrical conductivity, magnetic [3–6] and ferroelectric [7] properties. Besides materials on basis of KA V WO 6 (A V – Nb, Sb, Ta) compounds can be used as catalysts due to content of transition metals. The object of our investigation is complex oxides containing potassium, tungsten and transition metals with oxidation num- ber +5 and solid-state solution on their base. Despite the extensive interest in defect pyrochlore oxides, thermodynamic and physic- ochemical data are presented in bare number of publications. Therefore detailed investigations of these compounds are espe- cially important. The goals of this work include detailed structural, spectro- scopic and thermal stability studies of individual phases (KA V WO 6 (A V – Nb, Sb, Ta)), as well as thermodynamic modeling of the KNb x Sb y Ta z WO 6 system. ∗ Corresponding author. Tel.: +7 831 465 62 06; fax: +7 831 434 50 56. E-mail address: knav@uic.nnov.ru (A.V. Knyazev). 2. Experimental 2.1. Samples KNb x Sb y Ta z WO 6 solid solution was prepared by the solid- state reaction between WO 3 , Nb 2 O 5 , Sb 2 O 3 , Ta 2 O 5 and KNO 3 . The synthesis was performed in a porcelain crucible, into which the reaction mixture with the corresponding atomic ratio 1K + 1W + xNb + ySb + zTa was loaded. In order to find out the mix- ing temperatures, the process was realized in several stages with temperature rise of 25 K and holding the reaction mixture at every temperature during 50 h. This process was started at 973 K. The choice of initial temperature is determined by the purity of stud- ied compounds with pyrochlore structure compared to additive products, which form at lower temperature. The phase individu- ality of the compounds was verified after every synthesis step by X-ray diffraction. The X-ray data and estimated impurity content (0.5–1 wt%) in the substances allowed us to conclude that every of the studied samples was composed of an individual crystalline compound. To prove the atomic ration 1K + 1W + xNb + ySb + zTa the obtained samples were analyzed on a Shimadzu energy-dispersive roentgen fluorescent spectrometer EDX-900HS (from 11 Na to 92 U) with sensitive detector without liquid nitrogen. In the case when at a given temperature a sample was detected to be two-phase, i.e. its X-ray diffraction pattern was a super- position of X-ray diffraction patterns of two solid solutions, the temperature was further increased by 25 K. Thus we took value 0040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2010.04.009