Sr9La2(WO6)4 containing [WO6] octahedra

The crystal structure of Sr9La2(WO6)4 can be derived on the basis of the double-perovskite structure type. [WO6] octahedra and [(Sr/La)O x ] polyhedra are alternately placed in a distorted simple lattice with an additional vacancy (□).

Here, we report on synthesis and crystal structure analysis of Sr 9 La 2 (WO 6 ) 4 .
The crystal structures of alkaline-earth and rare-earth tungstates are often described in relation to the doubleperovskite structure type (Kemmler-Sack & Ehmann, 1981;Betz et al., 1982;Blasse & Kemmler-Sack, 1983;King et al., 2010;Ijdo et al., 2016). In the double-perovskite (A 2 BB 0 O 6 ) structure, B and B 0 atoms alternately occupy the B site of the perovskite (ABO 3 ) structure. The B site is at the center of an octahedron formed by O atoms, and the vertex-sharing [BO 6 ] and [B 0 O 6 ] octahedra regularly align in the A 8 simple cubic lattice frame in the double-perovskite structure. In case of the structure of Sr 9 La 2 (WO 6 ) 4 , a (Sr/La,&) 8 distorted simple lattice can be derived by connecting the Sr-rich sites of (Sr/La)2, (Sr/La)3, and (Sr/La)4 and a vacancy site at (1/2, 3/4, 1/8), as shown in Fig. 2. In the distorted lattice, the [WO 6 ] octahedra and the [(Sr/La)1O 8 ] polyhedra are alternately located by sharing four vertices and two edges of the [(Sr/ La)1O 8 ] polyhedra (Fig. 2).

Figure 1
The principal building units in the crystal structure of Sr 9 La 2 (WO 6 ) 4 with displacement ellipsoids drawn at the 99% probability level. Symmetry codes refer to Table 1.

Synthesis and crystallization
Raw powdered materials of SrCO 3 (Hakushin Chemical Laboratory, 98%), WO 3 (Furuuchi Chemical, 99.99%), and La 2 O 3 (FUJIFILM Wako Pure Chemical, 99.99%; calcined at 1273 K in advance) were weighed in a Sr:La:W molar ratio of 9:2:4, mixed in an agate mortar, and pressed into a cylindrical pellet with a diameter of 6 mm. The pellet was placed on a Pt plate in an alumina crucible with a lid (Nikkato, SSA-S) and heated to 1473 K at a rate of 300 K h À1 in a furnace. This temperature was maintained for 10 h, and the power to the heater of the furnace was then shut off. After the sample had cooled to room temperature, the sintered pellet was crushed, pressed into a pellet, and heated again under the same conditions. This procedure was performed three times. Part of the sintered pellet was then placed on a Pt plate in an alumina crucible, heated at 1673 K for 6 h, and cooled to room temperature at a rate of À400 K h À1 . The obtained crystalline sample was an aggregate consisting of $50 mm single-crystalline grains. A single crystal selected from the aggregate was placed on top of a glass fiber for X-ray structure analysis. Another single crystal was embedded in resin, mirror polished, and carbon coated in preparation for chemical analysis using an electron microprobe analyzer (EPMA; JEOL JXA-8200).
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )