Three-Cation Scandium Borates Synthesis, Structure, Crystal Growth and Luminescent Properties

. Complex ortohoborates of rare earth metals with the general chemical formula R x La 1 − x Sc 3 (BO 3 ) 4 (R = Sm, Tb) have been obtained by solid state synthesis and spontaneous crystallization. These crystals belong to the huntite family with the space group R32 and for x = 0.5 have unit cell parameters a = 9.823(6), c = 7.975(3) (SLSB) and a = 9.803(3), c = 7.960(4) Å (TLSB).


Introduction
Orthoborates with the general formula RX 3 (BO 3 ) 4 , where R = Y, Ln; X = Al, Ga, Sc, Cr, Fe are practically important and interesting from the point of view of crystal chemistry objects for research. One of the important properties of these compounds is the ability to form a non-centrosymmetric structure, which is called huntite-like. Such a structure causes, for example, non-linear optical properties.
To understand the formation of the huntite-like structure of three-cation scandoborates, we consider the lanthanumscandium borate LaSc 3 (BO 3 ) 4 . The authors (He et al. 1999) distinguish three modifications of this crystal: high-temperature monoclinic with the C2/c space group, medium temperature trigonal with the R32 space group (huntitelike) and low-temperature monoclinic with the Cc space group. As a result of our research (Fedorova et al. 2013) identity of the X-ray patterns of polymorphic modifications high and low was shown.
The stabilization of the huntite-like structure can occur if an additional isomorphic cation is introduced into the LaSc 3 (BO 3 ) 4 structure, that was confirmed in (Li et al. 2001) who initiated the new three-cation scandoborate with the huntite-like structure Nd x La 1−x Sc 3 (BO 3 ) 4 . Further, in a number of works by adding a third cation R x La y Sc z (BO 3 ) 4 nonlinear optical crystals with a stable huntite-like structure were obtained with R = Gd (Xu et al., 2011);Y (Ye et al. 2005) and Lu (Li et al. 2007).
The existence of a huntite-like structure for the boundary members of the REE series suggests the stability of such a structure with the rest of the REE. This paper presents data on the huntite-like structures SLSB and TLSB for systems R x La 1−x Sc 3 (BO 3 ) 4 (R = Sm, Tb).

Methods and Approaches
Polycrystalline sample of R x La 1−x Sc 3 (BO 3 ) 4 (x = 0-0.5)were prepared by the method of two stage solid state synthesis in a Pt crucible. The stoichiometric mixtures of pure raw La 2 O 3 , Sc 2 O 3 , H 3 BO 3 and R 2 O 3 (R = Sm, Tb) reactants were heated at 800°C for 5 h to decompose H 3 BO 3 . At the second stage, the mixtures were grinded in an agate mortar and heated again at 1300°C for 12 h until the powder X-ray method showed no peaks of initial compounds (Fig. 1).
Spontaneous crystals of R x La 1−x Sc 3 (BO 3 ) 4 with dimensions 30 Â 30 Â 10 mm with a transparent area of 5 Â 5 Â 5 mm were grown from LiBO 2 -LiF flux Fig. 2. A Pt crucible containing R 0.5 La 0.5 Sc 3 (BO 3 ) 4 , Li 2 CO 3, H 3 BO 3 and LiF in the molar ratio of 1:1,5:1,5:3 was heated to 1000°C. The charge was held in a melted state for a day to achieve homogenization. After this stage a platinum wire with a loop was placed in the center of the melt surface and the temperature was decreased to 900°C. Then the melt was cooled with the 2°C/day to 850°C and following cooling at the rate of 15°C/day to room temperature. The crystal was chosen for x-ray analysis. Powder diffraction patterns were refined using the Rietveld method within the GSAS-II program.
The chemical composition of obtained crystals was measured by X-ray fluorescent analysis using XRF 1800 (Shimadzu, Japan). The results of the analysis are conformed with the formula obtained after crystal structure refinement: (Table 1) 3 Results and Discussion Structure. According to Rietveld refinement both SLSB and TLSB crystalize in the trigonal space group R32 with unit cell parameters: a = 9.823(6), c = 7.975(3) (SLSB) and a = 9.803(3), c = 7.960(4)Å (TLSB). The structure framework is composed of the R, La atoms, Sc atoms and B atoms occupy trigonal prisms, octahedra and planar triangle of oxygen, respectively. The isolated (R, La)O 6 trigonal prisms alternate along the c-axis with BO3 triangle that are perpendicular to the c-axis. ScO 6 octahedra link to each other along the edge and form twisted chain along c, which separate (R, La)O 6 prisms as well. The discrepancies between refined diffraction spectra with model calculations can be explained by crystal cleavage along {202} and {113}.  Luminescence. Typical excitation and luminescence spectra of SLSB are shown on Fig. 3(a). The strongest excitation peak of samarium crystal corresponds to 6 H 5=2 ! 4 F 7=2 transition located at 407 nm. Whereas luminescent spectrum of SLSB has some peaks corresponding to 4 G 5=2 ! 6 H J (J = 5=2, 7=2, 9=2 и 11=2) and located at 566, 602, 645 and 708 nm.

Conclusions
The formation of a huntite structure in systems R x La 1−x Sc 3 (BO 3 ) 4 , (R = Sm, Tb), as well as the dependence of the compositions stable in the required structure depending on the production method is shown. The spectral characteristics confirm the potential of using crystals as luminescent materials. Xu X, Ye N (2011)  Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made. The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.