Phase equilibria in the system NaAl(WO 4 ) 2 –NaCr(WO 4 ) 2 I. Koseva *, A. Yordanova, D. Rabadjieva, P. Tzvetkov, V. Nikolov Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Building 11, 1113 Sofia, Bulgaria 1. Introduction There is increasing interest in tunable and very short-pulse solid-state lasers during the last years due to their promising applications in modern science and technology. This stimulated the studies on single crystal materials with broadband emission in the near-infrared region, as tungstates [1–4], molybdates [5,6], borates [7,8], garnets [9,10], forsterite [11], germanates [12], alexandrite [13], galates [14] and silicates [15,16]. Cr-doped NaAl(WO 4 ) 2 is a potential laser-active medium because of its high absorption, efficient pumping with visible-range semiconductor diode lasers and broad laser emission [2]. The absorption cross- section of NaAl(WO 4 ) 2 :Cr 3+ attains values of 32 Â 10 À20 cm 2 for the 4 A 2 ! 4 T 1 transition and 17 Â 10 À20 cm 2 for the 4 A 2 ! 4 T 2 transition, respectively. For the sake of comparison, the values of the absorption cross-section of the Cr 3+ -doped gadolinium scandium gallium garnet, GSGG:Cr 3+ , are 5.1 Â 10 À20 and 3.3 Â 10 À20 cm 2 [17], and for alexandrite, 10 Â 10 À20 and 20 Â 10 À20 cm 2 [18], respectively. In addition, according to the spectral data of NaAl(WO 4 ) 2 :Cr 3+ , this new material shows a strong optical absorption in the 660–680 nm range where efficient pumping with laser diodes is possible [19]. However, the production of single crystals from this tungstate as laser active media is related to a number of problems, mainly due to anisotropy and low growth velocity [3]. An effective approach to circumvent crystal growth problems is to produce transparent ceramics to replace single crystals. Moreover, trans- parent ceramics are low-cost products of high chemical homoge- neity and isotropy [20,21]. The technology of optical ceramics production includes three main stages: 1. synthesis of nanopowders. 2. preparation of high-density compacts. 3. sintering of the compacts to obtain non-porous ceramics. The first step of NaAl(WO 4 ) 2 ceramic production is to obtain nanosized powder. Nanosized powder of non-doped NaAl(WO 4 ) 2 was successfully synthesized by us by a co-precipitation method [22]. In this way powders with the required dimensions, particle size distribution and particle shape were obtained. These characteristics are important for the next step of ceramic preparation. The next step was the synthesis of Cr-doped NaAl(WO 4 ) 2 . To this purpose it was important to find out the maximum amount of chromium, which can be accommodated in NaAl(WO 4 ) 2 without altering the structure. NaCr(WO 4 ) 2 is an inorganic pigment with potential application in paints, ceramics, plastics and glasses [23,24]. The thermal and optical properties of this pigment may be modified by deep substitution of Cr by another element, e.g., Al. The aim of the present work was to study the phase equilibria in the system NaAl(WO 4 ) 2 –NaCr(WO 4 ) 2 , with a view to determining the concentration crystallization regions of the phases and the influence of the concentrations of chromium and aluminium, respectively, on their basic thermal and structural characteristics. On the basis of the obtained data, Cr-doped NaAl(WO 4 ) 2 nanosized powders were synthesized for the first time with chromium concentration up to the highest admissible level. Materials Research Bulletin 47 (2012) 3580–3585 A R T I C L E I N F O Article history: Received 9 March 2012 Received in revised form 22 May 2012 Accepted 14 June 2012 Available online 29 June 2012 Keywords: A. Optical materials B. Chemical synthesis C. Electron diffraction C. X-ray diffraction A B S T R A C T The phase equilibria in the system NaAl(WO 4 ) 2 –NaCr(WO 4 ) 2 are investigated. Nanopowders are synthesized in the whole concentration region from NaAl(WO 4 ) 2 to NaCr(WO 4 ) 2 by a co-precipitation method. Using X-ray, DTA, TEM and SEM analyses, three concentration regions in the system are established: single-phase region of NaAl 1Àx Cr x (WO 4 ) 2 solid solutions (x is between 0 and 0.08), with monoclinic structure, space group C2/c; single-phase region of NaAl 1Àx Cr x (WO 4 ) 2 solid solutions (x is between 0.40 and 1.0) with monoclinic structure, space group P2/c; and two-phase region, where the above mentioned phases crystallize simultaneously (x is between 0.08 and 0.40). The thermal behaviour of the two pure boundary phases, as well as the influence of chromium on the thermal stability and the structural characteristics of the solid solutions are discussed. ß 2012 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +359 2979 2786; fax: +359 2870 5024. E-mail address: ikosseva@svr.igic.bas.bg (I. Koseva). Contents lists available at SciVerse ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.06.061