Cs10Ta29.27O78

Single crystals of caesium tantalate(V), Cs10Ta29.27O78, were obtained as a serendipitous product in a welded tantalum ampoule by a blank reaction of CsBr and bismuth subnitrate [Bi5O(OH)9(NO3)4] with the container material. The crystal structure of the title compound is made up of a three-dimensional framework constituted by two types of layers, viz. (Ta6O15)n and (Ta3O9)n, parallel to (001), which are linked together by TaO6 octahedra (3m. symmetry) along [001]. This framework has cavities where three independent Cs+ ions (3m. and m2 symmetry, respectively) are located. The compound reveals a Ta deficiency at one trigonal prismatic coordinated site ( m2 symmetry). The composition of the title compound was verified by energy-dispersive X-ray analysis of single crystals.

Single crystals of caesium tantalate(V), Cs 10 Ta 29.27 O 78 , were obtained as a serendipitous product in a welded tantalum ampoule by a blank reaction of CsBr and bismuth subnitrate [Bi 5 O(OH) 9 (NO 3 ) 4 ] with the container material. The crystal structure of the title compound is made up of a threedimensional framework constituted by two types of layers, viz. (Ta 6 O 15 ) n and (Ta 3 O 9 ) n , parallel to (001), which are linked together by TaO 6 octahedra (3m. symmetry) along [001]. This framework has cavities where three independent Cs + ions (3m. and 6m2 symmetry, respectively) are located. The compound reveals a Ta deficiency at one trigonal prismatic coordinated site (6m2 symmetry). The composition of the title compound was verified by energy-dispersive X-ray analysis of single crystals.
Based on a previous powder diffraction study of Cs 10 Ta 29.27 O 78 reported by Michel et al. (1978), an additional Ta site was found, but was not observed in the current re-investigation. The closely related structure of Tl 10 Nb 29.2 O 78 was also reported (Marini et al., 1979) with an additional Nb site with too short interatomic distances between the Nb atoms.
The title compound crystallizes isotypically with Rb 5 VONb 14 O 38 (Haddad & Jouini, 1997), in which the vanadium atom occupies the trigonal-prismatic Ta deficiency site of the title compound. The related compound Cs 3 Ta 5 O 14 was first investigated by powder diffraction studies by Serafin & Hoppe (1982) and was later re-investigated on the basis of single-crystal X-ray diffraction by du Boulay et al. (2003). An overview on the related hexagonal tungsten bronzes of potassium, rubidium and caesium was presented by Magnéli (1953).

Experimental
The title compound was obtained as a serendipitious product by the reaction of CsBr and bismuth subnitrate (Bi 5 O(OH) 9 (NO 3 ) 4 , 98%, Sigma-Aldrich) with the tantalum container material. It was synthesized under argon at temperatures of 1523 K in a welded tantalum ampoule. The air stable title compound crystallizes in brown rods. The chemical composition of the single-crystal was confirmed by energy-dispersive X-ray (EDX) analysis which revealed no impurity elements.

Refinement
The site occupation factor (s.o.f.) of the Ta-deficient Ta4 site was refined freely. The maximum residual electron density lies 0.95 Å from O1 and the minimum residual electron density lies at the position of the Cs1 atom. Fig. 1 Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.