Author's personal copy LiZnNb 4 O 11.5 : A novel oxygen deficient compound in the Nb-rich part of the Li 2 O–ZnO–Nb 2 O 5 system Vladimir A. Morozov a,Ã , Alla V. Arakcheeva b,c , Vera V. Konovalova d , Philip Pattison b,e , Gervais Chapuis b , Oleg I. Lebedev f , Valery V. Fomichev d , Gustaaf Van Tendeloo f a Chemistry Department, Moscow State University, 119991 Moscow, Russia b Ecole Polytechnique Fe´de ´rale de Lausanne, Laboratoire de Cristallographie, BSP, CH-1015 Lausanne, Switzerland c Baykov Institute of Metallurgy and Material Sciences RAS, Leninsky pr., 49, 119991 Moscow, Russia d Material Science and New Materials Department, Moscow State Academy of Fine Chemical Technology, 119571 Moscow, Russia e Swiss–Norwegian Beamline, ESRF, BP-220, F-38043 Grenoble CEDEX, France f EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020, Antwerp, Belgium article info Article history: Received 29 October 2009 Received in revised form 3 December 2009 Accepted 8 December 2009 Available online 16 December 2009 Keywords: Complex niobium oxides a-PbO 2 related structure Superspace approach Supercell (3+1)D structure type X-ray diffraction Transmission electron microscopy abstract A novel lithium zinc niobium oxide LiZnNb 4 O 11.5 (LZNO) has been found in the Nb-rich part of Li 2 O–ZnO–Nb 2 O 5 system. LZNO, with an original a-PbO 2 related structure, has been synthesized by the routine ceramic technique and characterized by X-ray diffraction and transmission electron microscopy (TEM). Reflections belonging to the LZNO phase, observed in X-ray powder diffraction (XRPD) and electron diffraction, have been indexed as monoclinic with unit cell parameters a =17.8358(9) ˚ A, b =15.2924(7) ˚ A, c =5.0363(3) ˚ A and g =96.607(5)1 or as a-PbO 2 -like with lattice constants a =4.72420(3) ˚ A, b =5.72780(3) ˚ A, c =5.03320(3) ˚ A, g =90.048(16)1 and modulation vector q=0.3a*+1.1b* indicating a commensurately modulated a-PbO 2 related structure. The monoclinic cell is a supercell related to the latter. Using synchrotron powder diffraction data, the structure has been solved and refined as a commensurate modulation (superspace group P112 1 /n(ab0)00) as well as a supercell (space group P2 1 /b). The superspace description allows us to consider the LZNO structure as a member of the proposed a-PbO 2 -Z (3+1)D structure type, which unifies both incommensurately and commensurately modulated structures. HRTEM reveals several types of defects in LZNO and structural models for these defects are proposed. Two new phases in Li 2 O–ZnO–Nb 2 O 5 system are predicted on the basis of this detailed HRTEM analysis. & 2009 Elsevier Inc. All rights reserved. 1. Introduction Lithium niobate (LN: LiNbO 3 ) is an important material in the optical industry and it is of great interest for many applications, such as optical waveguides, surface acoustic wave devices, electro-optic modulators, second harmonic generators, holo- graphic storage and others [1]. Li-deficiency in LN crystals introduces intrinsic defects [2] which significantly increase the photorefractive effect and thus reduce the optical damage resistance. These defects can be reduced by increasing the stoichiometry or adding some dopants [3]. Usually, commercial LN crystals for laser applications are doped with MgO for increasing the damage threshold [4]. However, a different type of optical damage (the so-call dark trace) induced by a high- power laser beam still remained in the MgO-doped crystals. This limits their performance in high-power applications. Volk et al. [5] reported that the dark trace could be removed by using ZnO- doping. An optimization of the photorefractive and optical properties of ZnO-doped LN crystals requires a detailed knowl- edge of the phase formation in the Li 2 O–ZnO–Nb 2 O 5 system especially in the Nb-rich part. Two lithium zinc niobium oxides LiZnNbO 4 [6] and Li 2.98 Zn 0.51 Nb 2 O 7 [7] are known in the Li 2 O–ZnO–Nb 2 O 5 system. The first one crystallizes in the spinel-type structure while the structure of the second compound is unknown. The phase formation in the ZnO–Nb 2 O 5 system however is better studied in the Zn-rich part; the ZnO–Nb 2 O 5 phase diagram shows two congruent compounds having compositions Zn 3 Nb 2 O 8 (3ZnO:Nb 2 O 5 ) and ZnNb 2 O 6 (ZnO:Nb 2 O 5 ) [8]. Later Ballman and Brown showed that ZnNb 2 O 6 is not congruently melting [9]. ZnNb 2 O 6 forms in a columbite-type (or a-PbO 2 -type) structure [10] in direct equilibrium with b-Nb 2 O 5 and not as a low temperature transformation from the rutile structure [9,11]. Zn 7 Nb 2 O 12 (7ZnO:Nb 2 O 5 ) with a cubic spinel structure is formed in the presence of ZnO vapour [12]. The Zn 4 Nb 2 O 9 (4ZnO:Nb 2 O 5 ) phase with a corundum-related structure has been prepared at ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jssc Journal of Solid State Chemistry 0022-4596/$ - see front matter & 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2009.12.008 Ã Corresponding author. Fax: + 7 495 939 33 16. E-mail address: morozov@tech.chem.msu.ru (V.A. Morozov). Journal of Solid State Chemistry 183 (2010) 408–418