Journal of Alloys and Compounds 380 (2004) 248–254 Structure and properties of the KNbW 2 O 9 hexagonal bronze doped with Eu 3+ ions as an optically active probe L. Macalik a,∗ , M. M˛ aczka a , J. Hanuza a,b , A. Bednarkiewicz a , D. Hreniak a , W. Str˛ ek a , A. Majchrowski c a Institute of Low Temperatures and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50950 Wroclaw, Poland b Department of Bioorganic Chemistry, Faculty of Engineering and Economics, Wroclaw University of Economics, Wroclaw, Poland c Institute of Applied Physics, Military University of Technology, 00-908 Warsaw, Poland Abstract Luminescence properties of KNbW 2 O 9 crystal doped with Eu 3+ , investigated at room and liquid nitrogen temperatures, showed that the active ions occupy two crystallographically different sites. The structural and IR spectra investigations were also carried for better understanding of Eu 3+ ion behavior. The luminescence decay profiles were slightly non-exponential. The luminescence lifetimes of Eu 3+ were approximated by biexponential decays. Both were ascribed to two different Eu(III) sites. The longer component was determined to be ca. 1 ms, whereas the shorter one was about 0.5 ms. It was found that the Eu sites characterized by longer decay times were not affected by temperature, whereas the sites characterized by shorter decay times revealed strong decrease of decay times with temperature. Infrared investigation demonstrated the presence of water molecules occupying hexagonal tunnels in the KNbW 2 O 9 crystal. The UV irradiated crystal showed the presence of blue coloration. The observed color was attributed to small W 5+ polarons or intervalence charge transfer between W 5+ and W 6+ states. The electrons, necessary for the formation of the W 5+ states, originate most likely from UV photodissociation of water molecules. © 2004 Elsevier B.V. All rights reserved. Keywords: Inorganic compounds; Infrared spectroscopy; Luminescence; Crystal structure 1. Introduction Tungsten bronzes of the A x B y W 1-y O 3 family, where A is the alkali metal and B the Nb, Ta, Zr, Sc etc., are valence-balanced dielectrics, whereas A x WO 3 bronzes exhibit metallic type conductivity [1]. These compounds undergo a number of phase transitions and may display ferroelectric or anti-ferroelectric properties [2–7]. It is the reason why these materials are extensively studied and their physical properties characterized. This paper concerns the luminescence properties of KNbW 2 O 9 bronze doped with Eu 3+ ions. These RE ions in the crystal studied play the role as optically active probes to the structural studies of the host. This compound was studied by means of dielectric, electron microscopy, X-ray diffraction methods as well as IR and Raman spectro- scopic methods [2–8]. These studies suggested that the high-temperature phase could be described in the P6 3 22 ∗ Corresponding author. Fax: +48-71-441-029. E-mail address: lmacalik@int.pan.wroc.pl (L. Macalik). space group, which was also found for K 0.26 WO 3 [9]. Our previous work showed that the structure of KNbW 2 O 9 is lower than hexagonal with a small distortion of the lattice [8]. Because the Raman bands were found to be asymmetric and broad it was not possible to establish the structure of the sites occupied by the W, Nb, and K atoms [8]. 2. Experimental Pure KNbW 2 O 9 single crystals were obtained by means of spontaneous crystallization from 20 mol% solution in K 2 W 2 O 7 (High Temperature Solution Growth, HTSG) according to the procedure described by us previously [8]. This method of crystallization has several drawbacks, namely non-controllable crystallization, contact of a grow- ing crystal with the crucible, and the necessity of separation of the as-grown crystals from the rest of the solidified flux by dissolution in hot water. To overcome the mentioned above disadvantages of spontaneous HTSG method, we used top seeded solution growth (TSSG) technique to obtain 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.03.052