Crystal growth and optical characterizations of Yb 3+ -doped LiGd 6 O 5 (BO 3 ) 3 single crystal: a new promising laser material† V eronique Jubera, * a Philippe Veber, a Marie Chavoutier, a Alain Garcia, a Fr ed eric Adamietz, b Vincent Rodriguez, b Jean-Pierre Chaminade a and Matias Vel azquez a Received 29th June 2009, Accepted 22nd September 2009 First published as an Advance Article on the web 2nd October 2009 DOI: 10.1039/b912819k Single crystals of a new oxyborate, Yb-LGOB, were obtained by an original flux method and interesting spectroscopic properties of this new laser crystal are reported. As borates are generally chemically stable over broad temperature ranges, many of them can be synthetically made, 1 in both single crystalline or powder forms for various purposes such as lighting, displays, 2 high energy particles detection (scintillators), 3 and all solid- state laser sources. 4 In the latter two realms of applications, centi- meter-sized high quality single crystals hold a strategic position, since they constitute the essential link in the high-tech manufacturing chain that goes from raw materials to devices and systems. Such a capability of introducing major technological breakthroughs arises from several reasons. Firstly, the single crystals allow for the investigations of their intrinsic optical properties (microstructural defects are avoided). Secondly, they permit the characterization of the anisotropy of these optical properties (absorption and emission cross-sections, for instance, can depend on the electric field polarization). Broadly speaking, borate crystal structures contain boron atoms that are either three- or four-fold coordinated in triangles, rings, chains or a three-dimensional (3D) framework. In many rare earth (Re) borate materials, the isolated triangle form is predominant. 1 When (BO 3 ) 3 oxoanions are packed along one single direction, the crystals may exhibit non linear optical properties. 1f,5 In several crys- talline matrices, additional oxygen atoms located in a tetrahedral position surrounded by four rare earth cations, 6 lead to the existence of chemically different Re–O bonds which can provide interesting luminescent properties. 7 The crystallographic structure of LiGd 6 O 5 (BO 3 ) 3 (LGOB) has been published in 1999. 6a The unit cell is monoclinic (space group P2 1 / c, Z ¼ 4) with a ¼ 8.489(4) A ˚ , b ¼ 15.706(3) A ˚ , c ¼ 12.117(6) A ˚ , b ¼ 132.27(2)  (r ¼ 6.706 g/cm 3 ). GdO 8 and GdO 7 polyhedra are connected by common edges and corners, giving rise to a 3D array. In this compound, the different triangular borate groups connected to the Gd 3+ polyhedra are not parallel to one single plane, but the strong chemical bonds between boron and oxygen atoms induce a distortion of the Gd 3+ coordination polyhedra. Doping this crystal host with optically active ions resulted in complicated fluorescence spectra because of the possibility for the rare earth ions to occupy six different crystallographic positions. 7 Ytterbium ions-doped borate crystals constitute prime materials for diode pumped high power continuous wave (CW) lasers as well as femtosecond, broadly tunable and compact near infrared (NIR) laser sources. 8 The symmetry and strength of the crystal field exerted around the ytterbium ions depend on their coordination polyhedron and the Re–O distances. A strong crystal field is required to achieve a large enough splitting of the two 2 F 5/2 and 2 F 7/2 energy levels, likely to broaden the emission bands and allow for tunable and short pulse laser operation to be demonstrated. Our group recently reported the ytterbium doped LiY 6 O 5 (BO 3 ) 3 powder spectroscopic properties in the ultraviolet (UV) and NIR spectral ranges. 7b,c The rather broad absorption and emission bands encouraged us to elaborate this compound in a single crystalline form. LGOB decomposes with a peritectic transformation at 1080  C entailing that it has to be grown from a high-temperature solution (HTS). Hence, a suitable flux has to be identified and we propose to investigate the pseudo-ternary diagram Li 2 O–B 2 O 3 –Gd 2 O 3 which exhibits an interesting pseudo-binary system between LGOB and Li 6 Gd(BO 3 ) 3 (LGB). 9,10 A careful XRD characterization of the remaining load contained in the crucible at the end of a Czochralski growth attempt of the latter compound has firmly established the easiness of formation of millimetre-sized LGOB crystallites. There- fore, because of the selective volatilization of Li 2 O and B 2 O 3 observed during the LGB growth trial, such crystallization of LGOB in the molten LGB bath leads us to assume the absence of any eutectic point in the LGOB-LGB phase diagram. In addition, the LGB phase exhibits a low and congruent melting point (860  C) which makes it a convenient solvent for using in the LGOB HTS growth. Two runs of single crystal growth are undertaken. Low molar fractions of 8% (solution A) and 10% (solution B) of LGOB as solute are chosen in order to both avoid the decomposition of LGOB above 1080  C during the growth process and minimize flux volatilization. A stable melt could thus be produced between 860 and 1080  C. LGB and LGOB are prepared from gadolinium and ytterbium oxides in order to obtain a molten bath containing 4.5% molar ytterbium doped compounds. This concentration was determined according to previous results obtained with an yttrium isostructural phase on a powder sample. 7c The starting reactants (99.99% H 3 BO 3 , 99.999% Li 2 CO 3 , 99.99% Gd 2 O 3 and Yb 2 O 3 ) are mixed according to stoi- chiometric proportions and thermally treated in successive stages of 4 h at 400  C and 12 h at 700  C. a CNRS, Universit e de Bordeaux, ICMCB, 87 av. Dr. A. Schweitzer, Pessac, F-33608, France b Institut des Sciences Mol eculaires, UMR 5255 CNRS, Universit e de Bordeaux, 351 cours de la lib eration, Talence Cedex, F-33405, France † Electronic supplementary information (ESI) available: Setup experiment, thermal expansion and gain cross section. See DOI: 10.1039/b912819k This journal is ª The Royal Society of Chemistry 2010 CrystEngComm, 2010, 12, 355–357 | 355 COMMUNICATION www.rsc.org/crystengcomm | CrystEngComm