L Journal of Alloys and Compounds 341 (2002) 333–337 www.elsevier.com / locate / jallcom 2 1 Site symmetries of Cu ions in LiNbO crystals 3 a b c, * T. Tsuboi , M. Grinberg , S.M. Kaczmarek a Faculty of Engineering, Kyoto Sangyo University, Kamigamo, Kita-ku, Kyoto 603-8555, Japan b ´ ´ Institute of Experimental Physics, University of Gdansk, Wita Stwosza 57, 80-952 Gdansk, Poland c Institute of Optoelectronics, Military University of Technology,2 Kaliski Str, 00-908 Warsaw, Poland Abstract 9 9 2+ Absorption spectra of (3d) →(3d) electronic transition have been investigated for Cu ions in LiNbO crystals at 16-340 K. A triplet 3 2 2 2 1 structure is observed for the absorption band due to the E → T transition in a crystal with 0.05 at.% Cu ions, while a doublet g 2g 2 1 2 1 structure is observed in a crystal with 0.5 at.% Cu ions. It is suggested that such a difference is due to a different site symmetry of Cu 2 1 ion because the location of Cu ions in LiNbO single crystals depends on the crystal growth conditions.  2002 Published by 3 Elsevier Science B.V. 2 1 Keywords: Absorption bands; Parity-forbidden transition; Jahn–Teller effect; Cu ion; LiNbO 3 1. Introduction Although a considerable amount of optical studies have been done on the impurity ions in LiNbO crystals there 3 2 1 Lithium niobate, LiNbO , is of 3 m point group symme- are only a few reports on the Cu ions. Optical properties 3 2 1 try, rhombohedral, with the oxygen atoms arranged in of the Cu ions in LiNbO crystals has been mainly 3 planar sheets, forming a network of distorted octahedra studied by Kobayashi et al. [1] and Petrosyan et al. [5]. 2 1 9 [1,2]. In the LiNbO single crystal there are distinguished The Cu ion has 3d outer electron configuration that in 3 2 crystallophysical axes: X , X and X , perpendicular each the octahedral crystal field splits into the E ground and 1 2 3 g 2 to the other, crystallographical axes: X [2, 21, 21, 0], T excited states. The absorption band related to the cryst 2g 2 2 Y [21, 2, 21, 0], U [21, 21, 2, 0], which lie in E → T transition has been observed at 800–1200 nm. cryst cryst g 2g the same plane and are rotated from each other by an angle Actually Kobayashi et al. [1] and Petrosyan et al. [5] of 1208, and, Z [0, 0, 0, 1] perpendicular to the plane observed slightly different absorption spectra. Kobayashi cryst formed by X , Y , U axes, and technological et al. reported a triplet structure with components peaked at cryst cryst cryst growth directions, X ;X ;X , Y ;X and is ro- about 850, 960 and 1100 nm at temperature 23 K, while tech cryst 1 tech 2 tated by 308 with respect to Y , and Z ;X ;Z ; Petrosyan et al. observed doublet structure where the two cryst tech 3 cryst c-optical axis. For further description we use in this paper components were peaked at about 885 and 1040 nm at 77 the following notations: X 5X , Y 5Y and Z 5Z . K. The present work was undertaken to clarify the relations tech tech tech Lithium niobate crystal is of great interest because of between the growing conditions of the LiNbO crystals 3 2 1 good electrooptic, acoustooptic, ferroelectric and and the symmetry of Cu ion manifested by different piezoelectric properties which allow its use in various absorption. applications, e.g. as basic material for waveguides and amplifiers. Since LiNbO doped with TM ions (Fe, Cu and 3 Mn) exhibits significant photorefractive properties it can be 2. Experimental procedure used also for holographic memory [3,4]. Both, photorefrac- tivity and quality of holographic memory depend on the Single crystals of LiNbO were grown by the Czoch- 3 presence of defects generated by growth conditions as well ralski method starting from the congruent melt (51.4 as on controlled impurities, usually TM ions [3]. mol.% Li O and 48.6 mol.% Nb O ) mixed with various 2 2 5 CuO concentration [3]. Crystals up to 40 mm in diameter and up to 70 mm in length were pulled. The intentional * Corresponding author. Tel.: 148-22-683-9019; fax: 148-22-666- dopand concentrations were: 0.05 (crystal [1) and 0.5 8950. 2 1 E-mail address: smkaczmar@wp.pl (S.M. Kaczmarek). (crystal [2) at.% of Cu , nevertheless, electron para- 0925-8388 / 02 / $ – see front matter  2002 Published by Elsevier Science B.V. All right reserved. PII: S0925-8388(02)00032-4