New arsenates (V) NaKAl 2 O[AsO 4 ] 2 and Na 2 KAl 3 [AsO 4 ] 4 Hamdi Ben Yahia, Tom Nilges, Ute Ch. Rodewald, Rainer Po ¨ ttgen * Institut fu ¨r Anorganische und Analytische Chemie, Universita ¨t Mu ¨nster, Corrensstrasse 30, D-48149 Mu ¨nster, Germany 1. Introduction Salt fluxes are frequently used as oxidation protective coatings in metallurgy or in preparative solid state chemistry to offer mild synthesis conditions at comparatively low reaction temperatures and to provide a solvent which favours the growth of single crystals for structure determination and property investigations. Such fluxes found also broad application in molecular chemistry for reactions in non-aqueous media [1,2], long before organic ionic liquids have been introduced [3]. NaCl/KCl fluxes have recently been used for the growth of pnictide oxide crystals REZnPO and REZnAsO [4–6] for investiga- tions of optical properties. Extension to other transition metals in the pnictide oxide series showed significant side reactions. Either the halide from the flux is included in the products as it was the case for Eu 5 [PO 4 ] 3 Cl [7] and Ce 3 OCl[AsO 3 ] 2 [8], or even alkali cations, e.g. in NaTiO[AsO 4 ] [9] and NaTi 2 [AsO 4 ] 3 [10]. During salt flux synthesis attempts for NdRuAsO in an alumina tube we observed an attack of the ceramic crucible, leading to few well shaped crystals of the new quinary oxide NaKAl 2 O[AsO 4 ] 2 . During the subsequent target-oriented synthesis of NaKAl 2 O[AsO 4 ] 2 we also obtained transparent plates of a second compound Na 2 K- Al 3 [AsO 4 ] 4 in the system Na 2 O–K 2 O–Al 2 O 3 –As 2 O 5 . Herein we report on an alternative synthesis strategy and the crystal structures of the title compounds. 2. Experimental 2.1. Synthesis NaKAl 2 O[AsO 4 ] 2 was first obtained as colorless needles as a by- product during the synthesis of the arsenide oxide NdRuAsO in an alumina tube, starting from Nd, Ru, RuO 2 , As and a NaCl/KCl flux. We then attempted to synthesize NaKAl 2 O[AsO 4 ] 2 starting from Al 2 O 3 , As and NaCl/KCl under a partial pressure of oxygen (KClO 4 ). The mixture was put in an alumina tube, which was sealed under vacuum in a silica tube. The tube was heated at 500 8C for 6 h and at 750 8C for 48 h. The sample was then cooled at a rate of 10 8C/h to room temperature. After washing the mixture with distilled water, we obtained a powder which consists mainly of AlAsO 4 and colorless plates of Na 2 KAl 3 [AsO 4 ] 4 . The latter was then synthesized successfully starting from Na 2 CO 3 ,K 2 CO 3 , Al 2 O 3 and NH 4 H 2 AsO 4 with a 2:1:3:8 molar ratio, respectively. The mixture was put into a gold tube which was fired at 400 8C for 24 h, at 500 8C for 24 h, at 600 8C for 24 h and at 700 8C for 48 h, always with intermediate grinding. 2.2. EDX data Semiquantitative EDX analyses of the single crystals investi- gated on the diffractometer (Fig. 1) were carried out with a Leica 420i scanning electron microscope using Albite (for Na), Al 2 O 3 , InAs, MAD-10 Feldspar (for K) as standards. The experimentally observed compositions Na 1.2 K 0.8 Al 2 As 2 and Na 1.9 K 0.9 Al 3 As 4 were close to the ideal ones, i.e. NaKAl 2 As 2 and Na 2 KAl 3 As 4 . The oxygen Materials Research Bulletin 45 (2010) 2017–2023 ARTICLE INFO Article history: Received 20 April 2010 Received in revised form 14 June 2010 Accepted 16 July 2010 Available online 23 July 2010 ABSTRACT The title compounds have been synthesized by a solid state reaction route using a salt flux. Their crystal structures were determined from single crystal X-ray data. NaKAl 2 O[AsO 4 ] 2 crystallizes with the orthorhombic K 2 Fe 2 O[AsO 4 ] 2 -type, Pnma, a = 8.2368(6) A ˚ , b = 5.5228(3) A ˚ , c = 17.0160(13) A ˚ and Z = 4, whereas Na 2 KAl 3 [AsO 4 ] 4 crystallizes with the orthorhombic K 3 Fe 3 [AsO 4 ] 4 -type, Cmce, a = 10.5049(9), b = 20.482(2), c = 6.3574(6) A ˚ and Z = 4. The NaKAl 2 O[AsO 4 ] 2 structure is built up of [Al 2 As 2 O 9 ] 2 layers perpendicular to the c-axis which are separated by A + alkali layers. The [Al 2 As 2 O 9 ] 2 layers consist of ribbons of edge-sharing AlO 6 octahedra, running along the a direction and which are connected through AsO 4 tetrahedra by sharing corners. The Na 2 KAl 3 [AsO 4 ] 4 structure contains [Al 3 As 4 O 16 ] 3 layers perpendicular to the b-axis separated by A + alkali layers. The [Al 3 As 4 O 16 ] 3 layer consists of a layer of corner-sharing AlO 6 octahedra which are also connected to the AsO 4 tetrahedra by sharing corners. ß 2010 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +49 251 83 36001; fax: +49 251 83 36002. E-mail address: pottgen@uni-muenster.de (R. Po ¨ ttgen). Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.07.009