Barium zinc diarsenate

The title compound, BaZnAs₂O₇, belongs to the family of isotypic series of compounds adopting the general formula M1²⁺M2²⁺X₂O₇ (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.

Related literature

For isostructural diarsenates, see: Pertlik (1986); Horng & Wang (1994); Wardojo & Hwu (1995); Chen & Wang (1996); Mihajlović et al. (2004). For the relationship to the known M1²⁺M2²⁺X₂O₇ compounds and the presence of non-merohedral twinning, see: Mihajlović et al. (2004). For related literature, see: Staack & Müller-Buschbaum (1998); Mihajlović & Effenberger (2006); Murashova et al. (1991); Nord et al. (1988).

Experimental

Crystal data

• BaZnAs₂O₇

• M_r = 464.55

• Monoclinic, P 2₁ /n

• a = 5.6260 (10) Å

• b = 8.557 (2) Å

• c = 13.317 (3) Å

• β = 90.01 (3)°

• V = 641.1 (2) ų

• Z = 4

• Mo Kα radiation

• μ = 20.08 mm⁻¹

• T = 293 (2) K

• 0.09 × 0.04 × 0.02 mm

Data collection

• Nonius KappaCCD diffractometer

• Absorption correction: multi-scan (Otwinowski & Minor, 1997) T_min = 0.265, T_max = 0.690

• 11068 measured reflections

• 2807 independent reflections

• 2747 reflections with I > 2σ(I)

• R_int = 0.040

Refinement

• R[F² > 2σ(F²)] = 0.022

• wR(F²) = 0.056

• S = 1.05

• 2807 reflections

• 102 parameters

• Δρ_max = 2.21 e Å⁻³

• Δρ_min = −1.77 e Å⁻³

Data collection: COLLECT (Nonius, 2002); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO-SMN (Otwinowski & Minor, 1997; Otwinowski et al., 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) and WinGX (Farrugia, 1999); molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: publCIF (Westrip, 2008).

Supporting information

S1. Comment

Diphosphates with the general formula M1M2P₂O₇ have been extensively studied during the last years. Among them are a couple of compounds with divalent M1 and M2 atoms (Mihajlović et al., 2004 and references therein). These diphosphates show a variety of crystal structure types that are controlled by the distinct stereochemical behaviour of the M1 and M2 cations. It affects the coordination numbers, the degree of distortion of the coordination polyhedra, and the conformation of the P₂O₇ groups (Murashova et al., 1991). By comparison much less is known about the structural chemistry of compounds with the formula M1²⁺M2²⁺X₂O₇ where X = V, Cr, Ge, As, and Si. Prior to this study, the crystal structures of only seven diarsenates M1²⁺M2²⁺As₂O₇ were known. Six of the first ones are isotypic with the title compound and crystallize in space group P2₁/n: PbCuAs₂O₇ (Pertlik, 1986), SrCoAs₂O₇ (Horng & Wang, 1994), BaCuAs₂O₇ (Wardojo & Hwu, 1995; Chen & Wang, 1996), BaMgAs₂O₇, BaCoAs₂O₇ (Mihajlović et al., 2004) and SrCuAs2O7 (Chen & Wang, 1996). Worthy to note is the five-coordinated M(2) position. CaCuAs2O7 has the same space-group symmetry; however, for the reduced cell the space group setting is P2₁/c and the structural features are different (Chen & Wang, 1996; Staack & Müller-Buschbaum, 1998).

The crystal structure of BaZnAs₂O₇ is characterized by a three-dimensional framework formed from corner-sharing square pyramids ZnO₅ and As₂O₇ groups. Within the framework bent chains running parallel to [010] are formed by the ZnO₅ and As1O₄ polyhedra. They are linked by the As2O₄ tetrahedra. Each of the five corners of the square pyramids around the Zn atoms are shared with a different As₂O₇ 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 BaMgAs₂O₇, BaCoAs₂O₇ and BaCuAs₂O₇ (2.852, 2.829 and 2.852 Å, respectively). The pyroarsenate groups involve two crystallographically non-equivalent AsO₄ 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) Å. The bond angles O_terminal—As—O_terminal are significantly larger than O_bridging—As—O_terminal. The As1—O_bridging—As2 angle is 124.67 (14)°.

In the 1000–800 cm^-1 range, Raman spectrum shows the As—O antisymmetric and symmetric stretching modes of the (As₂O₇)^4- groups [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 (As₂O₇)^4- group 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 (As₂O₇)^4- groups, and various lattice modes of the compound.

The presence of only two structure types among M1²⁺M2²⁺-diarsenates contrasts with the situation among the M1²⁺M2²⁺-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.

Related literature

For isostructural diarsenates, see: Pertlik (1986); Horng & Wang (1994); Wardojo & Hwu (1995); Chen & Wang (1996); Mihajlović et al. (2004). For the relationship to the known M1²⁺M2²⁺X₂O₇ compounds and the presence of non-merohedric twinning, see: Mihajlović et al. (2004). For related literature, see: Staack & Müller-Buschbaum (1998); Mihajlović & Effenberger (2006); Murashova et al. (1991); Nord et al. (1988).

S2. Experimental

Single crystals of BaZnAs₂O₇ were obtained as reaction products from mixtures of Ba(OH)₂^.8H₂O (Merck, > 97%), 2ZnO^.2CO₃^.4H₂O (Alfa Products), and As₂O₅ (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. BaZnAs₂O₇ crystallizes as colourless, prismatic crystals up to 0.11 mm in length (yield ca 25%). It was obtained together with colourless, prismatic crystals of Ba(AsO₃OH) (yield ca 60%) (Mihajlović & Effenberger, 2006) and uninvestigated amorphous mass yield ca 15%).

S3. Refinement

Several single crystals of the BaZnAs₂O₇ 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 P2₁/n based on the extinction rules. It crystal structure was refined starting from the atomic coordinates given for BaCuAs₂O₇ (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_obs² > F_calc²) 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°), non-merohedric 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.

Computing details

Data collection: COLLECT (Nonius, 2002); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO-SMN (Otwinowski & Minor, 1997; Otwinowski et al., 2003) and WinGX (Farrugia, 1999); program(s) used to solve structure: SIR97 (Altomare et al., 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: publCIF (Westrip, 2008).

Figures

	Fig. 1. The crystal structure of BaZnAs2O7 in a projection parallel to [010].
	Fig. 2. Atomic displacement ellipsoids at the 50% probability level.

Barium zinc diarsenate

Crystal data

BaZnAs2O7	F(000) = 832
Mr = 464.55	Dx = 4.813 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 5551 reflections
a = 5.626 (1) Å	θ = 0.4–35.0°
b = 8.557 (2) Å	µ = 20.08 mm^−1
c = 13.317 (3) Å	T = 293 K
β = 90.01 (3)°	Prismatic, colourless
V = 641.1 (2) Å^3	0.09 × 0.04 × 0.02 mm
Z = 4

Data collection

Nonius KappaCCD diffractometer	2807 independent reflections
Radiation source: fine-focus sealed tube	2747 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.040
ϕ and ω scans	θmax = 35.0°, θmin = 1.5°
Absorption correction: multi-scan (Otwinowski & Minor, 1997)	h = −9→9
Tmin = 0.265, Tmax = 0.690	k = −13→13
11068 measured reflections	l = −21→21

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.022	w = 1/[σ^2(Fo^2) + (0.0324P)^2 + 1.09P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.056	(Δ/σ)max = 0.001
S = 1.06	Δρmax = 2.21 e Å^−3
2807 reflections	Δρmin = −1.77 e Å^−3
102 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0037 (3)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ba1	0.21674 (4)	0.34610 (2)	0.715139 (16)	0.01062 (5)
Zn1	0.66892 (8)	0.14866 (4)	0.88747 (3)	0.01171 (8)
As1	0.75049 (6)	0.03337 (3)	0.65727 (2)	0.00781 (6)
As2	0.67177 (6)	0.31336 (4)	0.51369 (2)	0.00760 (6)
O1	0.7114 (5)	0.1140 (3)	0.53795 (18)	0.0143 (4)
O2	0.6667 (5)	−0.1533 (3)	0.6483 (2)	0.0130 (4)
O3	1.0297 (5)	0.0613 (3)	0.6914 (2)	0.0175 (5)
O4	0.5580 (4)	0.1208 (3)	0.73543 (19)	0.0111 (4)
O5	0.8103 (5)	0.3354 (3)	0.40399 (19)	0.0117 (4)
O6	0.3801 (5)	0.3486 (3)	0.5158 (2)	0.0114 (4)
O7	0.8042 (5)	0.4073 (3)	0.60893 (18)	0.0131 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ba1	0.01013 (8)	0.01003 (8)	0.01171 (8)	0.00076 (5)	−0.00048 (7)	−0.00098 (5)
Zn1	0.01165 (17)	0.01094 (16)	0.01254 (16)	−0.00053 (12)	−0.00077 (14)	0.00057 (11)
As1	0.00859 (13)	0.00678 (12)	0.00807 (12)	0.00043 (9)	−0.00037 (11)	0.00005 (9)
As2	0.00792 (12)	0.00819 (12)	0.00668 (11)	−0.00004 (10)	−0.00003 (10)	0.00028 (9)
O1	0.0249 (13)	0.0089 (9)	0.0092 (9)	0.0013 (9)	−0.0022 (9)	0.0006 (7)
O2	0.0166 (11)	0.0048 (9)	0.0176 (11)	−0.0001 (8)	0.0022 (9)	−0.0011 (7)
O3	0.0091 (10)	0.0163 (11)	0.0270 (13)	−0.0004 (8)	−0.0051 (9)	0.0005 (10)
O4	0.0105 (10)	0.0133 (10)	0.0097 (9)	0.0012 (8)	0.0021 (7)	−0.0029 (8)
O5	0.0094 (10)	0.0162 (10)	0.0096 (10)	0.0000 (8)	0.0035 (8)	0.0014 (7)
O6	0.0087 (10)	0.0145 (10)	0.0110 (10)	0.0005 (7)	0.0000 (8)	−0.0022 (8)
O7	0.0172 (10)	0.0110 (9)	0.0111 (9)	−0.0013 (9)	−0.0075 (8)	−0.0008 (7)

Geometric parameters (Å, º)

Ba1—O3^i	2.641 (3)	Zn1—O7^vi	2.071 (3)
Ba1—O3^ii	2.674 (3)	Zn1—O6^vii	2.082 (3)
Ba1—O4	2.734 (3)	Zn1—O4	2.132 (3)
Ba1—O7^ii	2.768 (3)	As1—O3	1.652 (3)
Ba1—O6	2.809 (3)	As1—O2	1.669 (2)
Ba1—O2^iii	2.821 (3)	As1—O4	1.678 (2)
Ba1—O4^iii	2.889 (3)	As1—O1	1.746 (2)
Ba1—O5^iv	3.002 (3)	As2—O5	1.667 (3)
Ba1—O5^v	3.157 (2)	As2—O6	1.669 (3)
Zn1—O2^i	1.989 (2)	As2—O7	1.676 (2)
Zn1—O5^iv	2.034 (3)	As2—O1	1.751 (2)
O3^i—Ba1—O3^ii	154.69 (6)	O5^iv—Ba1—O5^v	151.15 (6)
O3^i—Ba1—O4	93.77 (8)	O2^i—Zn1—O5^iv	115.47 (11)
O3^ii—Ba1—O4	69.23 (8)	O2^i—Zn1—O7^vi	145.21 (11)
O3^i—Ba1—O7^ii	124.18 (8)	O5^iv—Zn1—O7^vi	97.87 (11)
O3^ii—Ba1—O7^ii	77.40 (8)	O2^i—Zn1—O6^vii	85.52 (11)
O4—Ba1—O7^ii	140.60 (7)	O5^iv—Zn1—O6^vii	118.44 (11)
O3^i—Ba1—O6	105.27 (9)	O7^vi—Zn1—O6^vii	87.17 (10)
O3^ii—Ba1—O6	91.37 (8)	O2^i—Zn1—O4	90.21 (11)
O4—Ba1—O6	82.47 (7)	O5^iv—Zn1—O4	79.64 (10)
O7^ii—Ba1—O6	77.89 (8)	O7^vi—Zn1—O4	86.10 (10)
O3^i—Ba1—O2^iii	96.19 (9)	O6^vii—Zn1—O4	161.45 (10)
O3^ii—Ba1—O2^iii	77.12 (8)	O2^i—Zn1—Ba1^viii	142.43 (9)
O4—Ba1—O2^iii	118.34 (7)	O5^iv—Zn1—Ba1^viii	59.85 (7)
O7^ii—Ba1—O2^iii	71.81 (8)	O7^vi—Zn1—Ba1^viii	48.98 (7)
O6—Ba1—O2^iii	149.22 (8)	O6^vii—Zn1—Ba1^viii	130.97 (7)
O3^i—Ba1—O4^iii	67.32 (8)	O4—Zn1—Ba1^viii	52.43 (7)
O3^ii—Ba1—O4^iii	123.92 (8)	O2^i—Zn1—Ba1	77.81 (9)
O4—Ba1—O4^iii	157.94 (3)	O5^iv—Zn1—Ba1	51.31 (7)
O7^ii—Ba1—O4^iii	60.91 (7)	O7^vi—Zn1—Ba1	120.23 (7)
O6—Ba1—O4^iii	112.59 (7)	O6^vii—Zn1—Ba1	150.26 (7)
O2^iii—Ba1—O4^iii	56.12 (7)	O4—Zn1—Ba1	44.28 (7)
O3^i—Ba1—O5^iv	82.60 (8)	O3—As1—O2	115.24 (14)
O3^ii—Ba1—O5^iv	72.34 (8)	O3—As1—O4	112.25 (14)
O4—Ba1—O5^iv	55.22 (7)	O2—As1—O4	106.77 (13)
O7^ii—Ba1—O5^iv	132.28 (8)	O3—As1—O1	108.22 (15)
O6—Ba1—O5^iv	137.57 (7)	O2—As1—O1	106.10 (13)
O2^iii—Ba1—O5^iv	66.11 (8)	O4—As1—O1	107.88 (13)
O4^iii—Ba1—O5^iv	108.88 (7)	O5—As2—O6	117.03 (14)
O3^i—Ba1—O5^v	70.18 (8)	O5—As2—O7	113.66 (13)
O3^ii—Ba1—O5^v	135.12 (8)	O6—As2—O7	109.72 (13)
O4—Ba1—O5^v	133.79 (7)	O5—As2—O1	102.27 (12)
O7^ii—Ba1—O5^v	62.59 (7)	O6—As2—O1	107.34 (13)
O6—Ba1—O5^v	62.25 (7)	O7—As2—O1	105.74 (12)
O2^iii—Ba1—O5^v	106.58 (7)	As1—O1—As2	124.67 (14)
O4^iii—Ba1—O5^v	52.15 (7)

Symmetry codes: (i) −x+3/2, y+1/2, −z+3/2; (ii) x−1, y, z; (iii) −x+1/2, y+1/2, −z+3/2; (iv) x−1/2, −y+1/2, z+1/2; (v) −x+1, −y+1, −z+1; (vi) −x+3/2, y−1/2, −z+3/2; (vii) x+1/2, −y+1/2, z+1/2; (viii) −x+1/2, y−1/2, −z+3/2.