Barium zinc diarsenate

The title compound, BaZnAs2O7, belongs to the family of isotypic series of compounds adopting the general formula M12+ M22+ X 2O7 (M12+ = Ca, Sr, Ba or Pb; M22+ = Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn or Cd; X = P or As). Suitable single crystals were prepared under hydrothermal conditions. The framework structure is characterized by corner-sharing ZnO5 square pyramids and As2O7 groups where the Zn atoms occupy channels. X-ray diffraction analysis of single crystals twinned by non-merohedry [twin plane is (100)] yielded formula BaZnAs2O7. Raman spectra confirmed the presence of a non-linear As—O—As linkage.

The title compound, BaZnAs 2 O 7 , belongs to the family of isotypic series of compounds adopting the general formula M1 2+ M2 2+ X 2 O 7 (M1 2+ = Ca, Sr, Ba or Pb; M2 2+ = Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn or Cd; X = P or As). Suitable single crystals were prepared under hydrothermal conditions. The framework structure is characterized by corner-sharing ZnO 5 square pyramids and As 2 O 7 groups where the Zn atoms occupy channels. X-ray diffraction analysis of single crystals twinned by non-merohedry [twin plane is (100)] yielded formula BaZnAs 2 O 7 . Raman spectra confirmed the presence of a nonlinear As-O-As linkage.
The crystal structure of BaZnAs 2 O 7 is characterized by a three-dimensional framework formed from corner-sharing square pyramids ZnO 5 and As 2 O 7 groups. Within the framework bent chains running parallel to [010] are formed by the ZnO 5 and As1O 4 polyhedra. They are linked by the As2O 4 tetrahedra. Each of the five corners of the square pyramids around the Zn atoms are shared with a different As 2 O 7 group. The Ba position is located within channels parallel to [010] ( Fig. 1). The Ba atoms are coordinated by nine oxygen atoms, and the coordination polyhedron can be described as a distorted tricapped trigonal prism. The average <Ba-O> bond length of 2.829 Å compare well to that of BaMgAs 2 O 7 , BaCoAs 2 O 7 and BaCuAs 2 O 7 (2.852, 2.829 and 2.852 Å, respectively). The pyroarsenate groups involve two crystallographically non-equivalent AsO 4 tetrahedra. As expected, the longest As-O bonds are to the bridging oxygen atoms: <As1-O1> = 1.746 (2) Å and <As2-O1> = 1.751 (3)  In the 1000-800 cm -1 range, Raman spectrum shows the As-O antisymmetric and symmetric stretching modes of the (As 2 O 7 ) 4groups [904 (sh), 892 (vs), 847 (m), 825 (m)]. The bands at 784 (w) and 585 (s) cm -1 can be assigned to the asymmetric [ν as (As-O-As)] and symmetric bridge stretching vibration [ν s (As-O-As)], respectively and they are characteristic of the (As 2 O 7 ) 4group with a non-linear As-O-As bond (Nord et al., 1988). In the region below 450 cm -1 appear the bending modes of the (As 2 O 7 ) 4groups, and various lattice modes of the compound.
supplementary materials sup-2 The presence of only two structure types among M1 2+ M2 2+ -diarsenates contrasts with the situation among the M1 2+ M2 2+ -diphosphates where a greater variety of structure types is known; however, it probably reflects to some extent the different number of diphosphates and diarsenates studied so far.

Experimental
Single crystals of BaZnAs 2 O 7 were obtained as reaction products from mixtures of Ba(OH) 2 . 8H 2 O (Merck, > 97%), 2ZnO . 2CO 3 . 4H 2 O (Alfa Products), and As 2 O 5 (Alfa Products, > 99.9%). The mixture was transferred into Teflon vessel and filled to approximately 70% of their inner volume with distilled water (pH of the mixture was 2.5). Finally it was enclosed into stainless steel autoclave. The mixture was heated under autogeneous pressure from 293 to 493 K (4 h), held at that temperature (72 h) and rapidly cooled to room temperature. At the end of the reaction the pH of the solvent was 6.

Refinement
Several single crystals of the BaZnAs 2 O 7 were studied with an automatic four-circle X-ray diffractometer equipped with a CCD area detector. Preliminary measurements showed sharp reflection spots and a pseudo-orthorhombic unit cell. However, closer inspections of the recorded CCD frames revealed that at higher diffraction angles some very slight splitting or slight broadening of the reflection spots was evident; this was later found to be due to twinning. The space-group symmetry was confirmed as P2 1 /n based on the extinction rules. It crystal structure was refined starting from the atomic coordinates given for BaCuAs 2 O 7 (Chen & Wang, 1996). Although the structure models appeared to be crystal-chemically correct, the refinements initially converged unsatisfactorily. Distinct discrepancies between measured and calculated structure factors were observed (F obs 2 > F calc 2 ) for the most disagreeable reflections). The weighting scheme used by the programme SHELXL97 (Sheldrick, 2008) suggested unexpectedly large values for the second weighting parameters. Furthermore, in the residual electron densities remained unusually high peaks which indicated the presence of 'phantom' atoms apparently mirroring atom positions across the (100) plane. Because of the pseudo-orthorhombic metrics of the unit cells (β close to 90°), nonmerohedric twinning with a twin plane parallel to (100) was assumed. The application of the twin matrix (-1 0 0, 0 1 0, 0 0 1) during the refinement procedures reduced the R-values significantly. During the last stages anisotropic displacement parameters were allowed to vary for all atoms.