Synthesis and crystal structure of ABW-type SrFe1.40V0.60O4

A member of the novel solid-solution series SrFexV2–xO4 (x = 1.40) has been structurally characterized. Topologically, the compound belongs to the zeolite-type ABW.


Chemical context
Solid oxide fuel cell (SOFC) technology is considered as particularly promising for energy storage applications (Larminie et al., 2003). SOFCs are electrochemical devices that consist of three main parts: (i) a redox-capable porous cathode that reduces O 2 to O 2anions, (ii) an electrolyte transporting these anions to the anode, and (iii) the anode, where the fuel (hydrogen or carbon-containing fuels) is electro-oxidized by the O 2anions to CO 2 and H 2 O (Huang & Goodenough, 2009). Double perovskites with the general composition A 2 (BB 0 )O 6 have been studied intensively as potential anode materials in SOFCs (Xu et al., 2019). In the course of an explorative study on double perovskites combining mixed ionic-electronic conductivity with catalytic activity for fuel oxidation, we tried to synthesize Sr 2 FeVO 6 using a ceramic synthesis route in the range between 1473 and 1573 K. For the highest reaction temperature, where partial melting occurred, a member of the previously unknown SrFe x V 2-x O 4 solid-solution series was observed as a sideproduct, and the crystal structure of the member with x = 1.40 is reported here.
All atoms occupy general positions. Fe <-> V substitutions occur on each of the four symmetrically non-equivalent T-sites occupying the centers of distorted tetrahedra formed by oxygen atoms. Site-population refinements indicate no clear trend when comparing the individual Fe:V distributions. The Fe:V population at the T-sites is more or less balanced ranging from 64 (3) to 75 (3)% of iron. Individual T--O distances adopt values between 1.820 (6) and 1.901 (5) Å . The distortion of the tetrahedra is also reflected in the variation of the O-T-O bond angles scattering between 98.2 (2) and 129.9 (2) . According to Robinson et al. (1971), the distortions can be expressed numerically by means of the quadratic elongation and the angle variance 2 . These two parameters exhibit values between 1.009 and 1.016 for and 34.72 and 59.96 for 2 .
Each of the two symmetrically independent Sr II cations is coordinated by seven oxygen atoms within the channels of the framework. They are located off-center and have irregular coordination spheres formed by the oxygen atoms of two adjacent six-membered tetrahedral rings (Figs. 2, 3). Bondvalence-sum calculations using the parameter sets for the Sr-O bonds given by Brown & Altermatt (1985) resulted in the following values (in v.u.) considering cation-anion interactions up to 3.2 Å : Sr1: 1.911 and Sr2: 1.692. The considerable underbonding of the Sr2 position indicates that the bonds are stretched and that this Sr site resides in a cavity that is too large. A similar situation has been observed in isostructural SrFe 2 O 4 and -SrGa 2 O 4 .

Topological features
SrFe 1.40 V 0.60 O 4 belongs to the ABW zeolite structure type (Baerlocher et al., 2007). This class of materials comprises a large number of representatives and has been investigated in great detail because of the complex phase transitions and interesting ferroic effects (Bu et al., 1997). The polyhedral connectivity results in a three-dimensional network built from six-, four-and eight-membered rings. Perpendicular to [100], for example, the structure can be decomposed into layers consisting of six-membered rings (S6R) of [TO 4 ]-tetrahedra forming honeycomb nets (Fig. 4). Within a single S6R, three tetrahedra with vertices up (U) alternate with three tetrahedra having their vertices down (D) (sequence of directedness: UUUDDD). Using the terminology of Flö rke (1967), the relative orientation of paired tetrahedra belonging to different adjacent layers can be approximately classified as a transconfiguration ( Fig. 1). Alternatively, the layers can be regarded as being constructed from the condensation of unbranched vierer single-chains via common corners. Representation of the coordination polyhedron around Sr1. Displacement ellipsoids are drawn at the 70% probability level. [Symmetry codes:

Figure 1
Projection of the framework structure along [100]. [TO 4 ] tetrahedra are shown in blue. Oxygen and strontium atoms are given in red and orange, respectively. Displacement ellipsoids are drawn at the 70% probability level.

Synthesis and initial characterization
Single-crystals of SrFe 1.40 V 0.60 O 4 were obtained in the course of a series of synthesis experiments aimed at the preparation of a possible double perovskite phase with composition Sr 2 FeVO 6 . Therefore, mixtures of the dried starting materials SrCO 3 , Fe 2 O 3 and V 2 O 5 were homogenized in the molar ratio 4:1:1 using a ball mill operated at 600 r.p.m. for 45 min under ethanol. The resulting slurry was dried for 24 h at 323 K and subsequently re-ground by hand. An amount of about 0.5 g was pressed into a pellet having a diameter of 12 mm. Thermal treatment was performed in a resistance-heated horizontal tube furnace in air. Therefore, the tablet was placed on a platinum foil contained in an alumina-ceramic combustion boat. The sample was heated from 298 K to 1473 K with a ramp of 100 K h À1 , followed by 25 K h À1 to 1423 K and finally at 10 h K À1 to 1573 K. After annealing for 48 h at the maximum temperature, the container was quenched to room temperature. The partially melted pellet was removed from the foil, crushed in an agate mortar and transferred to a glass slide under a reflected-light microscope. A first optical inspection revealed the presence of at least two different crystalline phases: (a) larger, transparent-colorless crystals up to 150 mm in size and (b) (Westrip, 2010) and WinGX (Farrugia, 2012).

Figure 5
Strongly folded tetrahedral layer with four-and eight-membered rings in a projection along [010]. Displacement ellipsoids are drawn at the 70% probability level.

Figure 4
Single tetrahedral layer with six-membered rings in a projection along [100]. T-sites in the centres of the tetrahedra are shown in blue. Displacement ellipsoids are drawn at the 70% probability level.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Initial coordinates for the refinement calculations were taken from the crystal structure refinement of SrFe 2 O 4 (Kahlenberg & Fischer, 2001) after transformation to monoclinic second setting. Site-population refinements of the Fe:V ratios on the T-sites indicated the presence of a member of the solid-solution series SrFe x V 2- x O 4 .  (Rigaku OD, 2018); cell refinement: CrysAlis PRO (Rigaku OD, 2018); data reduction: CrysAlis PRO (Rigaku OD, 2018); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: publCIF (Westrip, 2010) and WinGX (Farrugia, 2012).

Strontium tetraoxidodi[ferrate(III)/vanadate(III)]
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. 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 > 2σ(F 2 ) is used only for calculating R-factors(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.