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Acta Crystallogr Sect E Struct Rep Online. 2008 September 1; 64(Pt 9): i58–i59.
Published online 2008 August 16. doi:  10.1107/S1600536808025865
PMCID: PMC2960660



Suitable single crystals of the title compound, barium dicobalt(II) bis­[orthoarsenate(V)], were prepared under hydro­thermal conditions. This phase belongs to a series of compounds with general formula AM 2(XO4)2, where A = alkaline earth metal, M = Mg or a divalent first-row transition element, and X = P, As or V. BaCo2(AsO4)2 is isotypic with BaNi2(XO4)2 (X = P, V or As) and is characterized by brucite-like sheets of edge-sharing CoO6 octa­hedra (3 symmetry) parallel to (001), with one-third of the octa­hedral positions being vacant. The sheets are capped above and below by AsO4 tetra­hedra (3 symmetry) and are inter­connected by distorted BaO12 cubocta­hedra (An external file that holds a picture, illustration, etc.
Object name is e-64-00i58-efi1.jpg symmetry).

Related literature

For isostructural compounds, see: Eymond et al. (1969a [triangle],b [triangle]); Bircsak & Harrison (1998 [triangle]); El-Bali et al. (1999 [triangle]); Faza et al. (2001 [triangle]); Rogado et al. (2002 [triangle]); Wichmann, & Müller-Buschbaum (1984 [triangle]). For magnetic properties of BaCo2(AsO4)2, see: Dojčilović et al. (1994 [triangle]); Regnault et al. (2006 [triangle]). For related compounds, see: Effenberger & Pertlik (1993 [triangle]); El-Bali et al. (1993a [triangle],b [triangle]); Hemon & Courbion (1990 [triangle]); Kreidler & Hummel (1961 [triangle]); Lucas et al. (1998 [triangle]); Mihajlović et al. (2004 [triangle]); Moquine et al. (1993 [triangle]); Osterloh & Müller-Buschbaum (1994 [triangle]). For general background, see: Brese & O’Keeffe (1991 [triangle]).


Crystal data

  • BaCo2(AsO4)2
  • M r = 533.04
  • Hexagonal, An external file that holds a picture, illustration, etc.
Object name is e-64-00i58-efi2.jpg
  • a = 5.007 (1) Å
  • c = 23.491 (5) Å
  • V = 510.02 (18) Å3
  • Z = 3
  • Mo Kα radiation
  • μ = 20.22 mm−1
  • T = 293 (2) K
  • 0.09 × 0.05 × 0.05 mm

Data collection

  • Nonius KappaCCD diffractometer
  • Absorption correction: multi-scan (Otwinowski & Minor, 1997 [triangle]; Otwinowski et al., 2003 [triangle]) T min = 0.221, T max = 0.362
  • 681 measured reflections
  • 341 independent reflections
  • 336 reflections with I > 2σ(I)
  • R int = 0.009


  • R[F 2 > 2σ(F 2)] = 0.014
  • wR(F 2) = 0.038
  • S = 1.30
  • 341 reflections
  • 22 parameters
  • Δρmax = 0.96 e Å−3
  • Δρmin = −1.09 e Å−3

Data collection: COLLECT (Nonius, 2002 [triangle]); cell refinement: SCALEPACK (Otwinowski & Minor, 1997 [triangle]); data reduction: DENZO-SMN (Otwinowski et al., 2003 [triangle]); method used to solve structure: starting parameters from an isostructural compound (Eymond et al., 1969a [triangle]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 [triangle]) and WinGX (Farrugia, 1999 [triangle]); molecular graphics: ATOMS (Dowty, 2000 [triangle]); software used to prepare material for publication: publCIF (Westrip, 2008 [triangle]).

Table 1
Selected bond lengths (Å)

Supplementary Material

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536808025865/wm2189sup1.cif

Structure factors: contains datablocks I. DOI: 10.1107/S1600536808025865/wm2189Isup2.hkl

Additional supplementary materials: crystallographic information; 3D view; checkCIF report


The author gratefully acknowledges financial support by the Austrian Science Foundation (FWF) (grant No. T300-N19).

supplementary crystallographic information


The crystal structures of phosphates, arsenates and vanadates with the general formula AM2(XO4)2 where A = alkaline earth metal, M = Mg or divalent first row transition elements and X = P, As or V, are relatively well known (Bircsak & Harrison, 1998; El-Bali et al., 1993a,b, 1999; Eymond et al., 1969a,b; Hemon & Courbion, 1990; Kreidler & Hummel, 1961; Lucas et al., 1998; Moquine et al., 1993; Osterloh & Müller-Buschbaum, 1994; Wichmann & Müller-Buschbaum, 1984, and references therein). These phases adopt different structure types and exhibit interesting physical properties (phase transitions, magnetism). Six compounds, viz. BaMg2(AsO4)2, BaCo2(AsO4)2 (Eymond et al., 1969b), BaNi2(AsO4)2 (Eymond et al., 1969a,b), BaCo2(PO4)2 (Bircsak & Harrison, 1998), BaNi2(PO4)2 (El-Bali et al., 1999; Faza et al., 2001) and BaNi2(VO4)2 (Wichmann & Müller-Buschbaum, 1984; Rogado et al., 2002) are isostructural with the title compound, BaCo2(AsO4)2. The underlying crystal structure was described for the first time for BaNi2(AsO4)2 by Eymond et al. (1969a). Although two-dimensional magnetic properties of these compounds have been widely studied (Dojčilović et al., 1994; Regnault et al., 2006, and references therein), a full determination of their crystal structures was not given for all members of this structural family. Hydrothermal synthesis and the crystal structure of BaCo2(AsO4)2 are presented in this communication.

Besides BaCoAs2O7 (Mihajlović et al., 2004), BaCo2(AsO4)2 represents the second compound structurally characterised in the system BaO–CoO–As2O5. Its crystal structure is made up of brucite-like sheets of edge-sharing CoO6 octahedra parallel to (001), with one-thirds of the octahedral positions being vacant. AsO4 tetrahedra are situated above and below the sheets. The resulting anionic [Co2(AsO4)2]2- layers are stacked with a sequence ABCABC along [001] and are laterally displaced by Δx = 2/3a, with Δx = 1/3b between the layers. Adjacent layers are interconnected by BaO12 polyhedra to form a three-dimensional framework (Fig. 1).

The Ba1 atom has site symmetry 3 and is coordinated by twelve oxygen atoms (Fig. 2) with an average Ba1—O bond length of 3.048 Å. This bond length compares well with the average bond length for Ba—O distances of 3.015 Å in the isostructural Ni compound BaNi2(AsO4)2 (Eymond et al., 1969a,b). The Ba1 atom is bonded to six O1 and six O2 atoms, resulting in a distorted cuboctahedral coordination polyhedron. The Co1 atom has site-symmetry 3 and is octahedrally coordinated to O1 atoms with a mean Co1—O1 distance of 2.091 Å. The As1O4 tetrahedron (3 symmetry) exhibits an average bond length of 1.692 Å, with two symmetrically independent As1—O bonds of 1.656 (3) Å (O1) and of 1.7050 (17) Å (O2, 3×) due to the different other coordination partners of the two oxygen atoms. O1, which bridges the barium ions, shows a shorter As1—O bond than O2, which bridges the cobalt ions (Fig. 2).

Bond-valence calculations for all atoms, using the parameters of Brese & O'Keeffe (1991), give 1.63 v.u. (valence units) for Ba1, 2.04 v.u. for Co1, 4.89 v.u. for As1, and 1.53 v.u and 1.96 v.u for O1 and O2, respectively. If one takes into account that the O2 atom is bonded to two Co1, one As1, and one Ba1 atom, and the O1 atom is bonded to one As1 and three Ba1 neighbours, the calculated values are close to the theoretical valences. However, it is worth mentioning that in BaCo2(AsO4)2 and all isostructural compounds the Ba1 atom is strongly undersaturated in terms of its bond valence.


Single crystals of BaCo2(AsO4)2 were obtained as reaction products from mixtures of Ba(OH)2.8H2O (Merck, >97%), Co(OH)2 (Alfa Products) and As2O5 (Alfa Products, >99.9%) under hydrothermal conditions. The mixture was transferred into a Teflon vessel and filled to approximately 70% of its inner volume with distilled water (pH of the resulting solution was ≈ 2.5). Finally, the vessel was enclosed into a stainless steel autoclave and was heated from room temperature to 493 K (2 h), held at 493 K for 24 h, then cooled to 393 K within 14 h, kept at this temperature for 24 h, and finally cooled to room temperature within 4 h. At the end of the reaction the pH was ≈ 6. The solid reaction products were filtered and washed thoroughly with distilled water. BaCo2(AsO4)2 (yield ca 50%) crystallized as transparent pink crystals and was accompanied with prismatic blue–green crystals of BaCoAs2O7 (Mihajlović et al., 2004) (yield ca 30%) and Co2As2O7(H2O)2 (Effenberger & Pertlik, 1993) (yield ca 15%), besides very few prismatic blue crystals of an yet unidentified compound (yield ca 5%). All crystals were up to 0.2 mm in length.


The crystal structure was refined with the atomic coordinates of BaNi2(AsO4)2 (Eymond et al., 1969a) as starting parameters in the hexagonal setting of space group R3.


Fig. 1.
The crystal structure of BaCo2(AsO4)2 with sheets consisting of CoO6 octahedra and AsO4 tetrahedra (both in polyhedral representation) parallel to (001). Ba atoms are displayed as spheres.
Fig. 2.
Coordination of Ba and Co with atoms displayed as ellipsoids at the 50% probability level.

Crystal data

BaCo2(AsO4)2Z = 3
Mr = 533.04F000 = 720
Hexagonal, R3Dx = 5.206 Mg m3
Hall symbol: -R 3Mo Kα radiation λ = 0.71073 Å
a = 5.007 (1) ÅCell parameters from 675 reflections
b = 5.007 (1) Åθ = 1.0–30.0º
c = 23.491 (5) ŵ = 20.22 mm1
α = 90ºT = 293 (2) K
β = 90ºPseudo-hexagonal plate, pink
γ = 120º0.09 × 0.05 × 0.05 mm
V = 510.02 (18) Å3

Data collection

Nonius KappaCCD diffractometer341 independent reflections
Radiation source: fine-focus sealed tube336 reflections with I > 2σ(I)
Monochromator: graphiteRint = 0.009
T = 293(2) Kθmax = 30.0º
[var phi] and ω scansθmin = 2.6º
Absorption correction: multi-scan(Otwinowski & Minor, 1997; Otwinowski et al., 2003)h = −7→7
Tmin = 0.221, Tmax = 0.362k = −5→5
681 measured reflectionsl = −33→33


Refinement on F2Primary atom site location: isomorphous structure methods
Least-squares matrix: full  w = 1/[σ2(Fo2) + (0.0133P)2 + 3.104P] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.014(Δ/σ)max < 0.001
wR(F2) = 0.038Δρmax = 0.96 e Å3
S = 1.30Δρmin = −1.09 e Å3
341 reflectionsExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
22 parametersExtinction coefficient: 0.0118 (5)

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 F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2sigma(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)

Ba10.00000.00000.00000.01170 (15)
Co10.00000.00000.17014 (2)0.00614 (16)
As10.33330.66670.091941 (18)0.00473 (15)
O10.33330.66670.02145 (13)0.0118 (6)
O20.0163 (4)0.3375 (4)0.11476 (7)0.0077 (3)

Atomic displacement parameters (Å2)

Ba10.01138 (17)0.01138 (17)0.0124 (2)0.00569 (8)0.0000.000
Co10.00474 (19)0.00474 (19)0.0089 (3)0.00237 (9)0.0000.000
As10.00414 (16)0.00414 (16)0.0059 (2)0.00207 (8)0.0000.000
O10.0149 (9)0.0149 (9)0.0055 (13)0.0075 (5)0.0000.000
O20.0055 (7)0.0050 (7)0.0116 (8)0.0019 (6)0.0017 (6)0.0020 (6)

Geometric parameters (Å, °)

Ba1—O1i2.9344 (8)Ba1—O2ix3.1611 (17)
Ba1—O1ii2.9344 (8)Co1—O2x2.0791 (17)
Ba1—O1iii2.9344 (8)Co1—O2xi2.0791 (17)
Ba1—O12.9344 (8)Co1—O2xii2.0792 (17)
Ba1—O1iv2.9344 (8)Co1—O2vii2.1017 (17)
Ba1—O1v2.9344 (8)Co1—O22.1017 (17)
Ba1—O2vi3.1611 (17)Co1—O2vi2.1017 (17)
Ba1—O23.1611 (17)As1—O11.656 (3)
Ba1—O2vii3.1611 (17)As1—O2xiii1.7050 (17)
Ba1—O2iii3.1611 (17)As1—O21.7050 (17)
Ba1—O2viii3.1611 (17)As1—O2xiv1.7050 (17)
O1i—Ba1—O1ii180.00 (12)O2vii—Ba1—O2iii126.22 (5)
O1i—Ba1—O1iii117.11 (3)O1i—Ba1—O2viii52.94 (6)
O1ii—Ba1—O1iii62.89 (3)O1ii—Ba1—O2viii127.06 (6)
O1i—Ba1—O162.89 (3)O1iii—Ba1—O2viii82.85 (6)
O1ii—Ba1—O1117.11 (3)O1—Ba1—O2viii97.15 (6)
O1iii—Ba1—O1180.00 (12)O1iv—Ba1—O2viii106.72 (6)
O1i—Ba1—O1iv117.11 (3)O1v—Ba1—O2viii73.28 (6)
O1ii—Ba1—O1iv62.89 (3)O2vi—Ba1—O2viii180.00 (7)
O1iii—Ba1—O1iv117.11 (3)O2—Ba1—O2viii126.22 (5)
O1—Ba1—O1iv62.89 (3)O2vii—Ba1—O2viii126.22 (5)
O1i—Ba1—O1v62.89 (3)O2iii—Ba1—O2viii53.78 (5)
O1ii—Ba1—O1v117.11 (3)O1i—Ba1—O2ix82.85 (6)
O1iii—Ba1—O1v62.89 (3)O1ii—Ba1—O2ix97.15 (6)
O1—Ba1—O1v117.11 (3)O1iii—Ba1—O2ix106.72 (6)
O1iv—Ba1—O1v180.00 (12)O1—Ba1—O2ix73.28 (6)
O1i—Ba1—O2vi127.06 (6)O1iv—Ba1—O2ix52.94 (6)
O1ii—Ba1—O2vi52.94 (6)O1v—Ba1—O2ix127.06 (6)
O1iii—Ba1—O2vi97.15 (6)O2vi—Ba1—O2ix126.22 (5)
O1—Ba1—O2vi82.85 (6)O2—Ba1—O2ix126.22 (5)
O1iv—Ba1—O2vi73.28 (6)O2vii—Ba1—O2ix180.00 (3)
O1v—Ba1—O2vi106.72 (6)O2iii—Ba1—O2ix53.78 (5)
O1i—Ba1—O273.28 (6)O2viii—Ba1—O2ix53.78 (5)
O1ii—Ba1—O2106.72 (6)O2x—Co1—O2xi92.91 (7)
O1iii—Ba1—O2127.06 (6)O2x—Co1—O2xii92.91 (7)
O1—Ba1—O252.94 (6)O2xi—Co1—O2xii92.91 (7)
O1iv—Ba1—O297.15 (6)O2x—Co1—O2vii92.34 (6)
O1v—Ba1—O282.85 (6)O2xi—Co1—O2vii174.36 (6)
O2vi—Ba1—O253.78 (5)O2xii—Co1—O2vii88.86 (9)
O1i—Ba1—O2vii97.15 (6)O2x—Co1—O2174.36 (6)
O1ii—Ba1—O2vii82.85 (6)O2xi—Co1—O288.86 (9)
O1iii—Ba1—O2vii73.28 (6)O2xii—Co1—O292.34 (6)
O1—Ba1—O2vii106.72 (6)O2vii—Co1—O285.72 (7)
O1iv—Ba1—O2vii127.06 (6)O2x—Co1—O2vi88.86 (9)
O1v—Ba1—O2vii52.94 (6)O2xi—Co1—O2vi92.34 (6)
O2vi—Ba1—O2vii53.78 (5)O2xii—Co1—O2vi174.36 (6)
O2—Ba1—O2vii53.78 (5)O2vii—Co1—O2vi85.72 (7)
O1i—Ba1—O2iii106.72 (6)O2—Co1—O2vi85.72 (7)
O1ii—Ba1—O2iii73.28 (6)O1—As1—O2xiii108.33 (6)
O1iii—Ba1—O2iii52.94 (6)O1—As1—O2108.33 (6)
O1—Ba1—O2iii127.06 (6)O2xiii—As1—O2110.59 (5)
O1iv—Ba1—O2iii82.85 (6)O1—As1—O2xiv108.33 (6)
O1v—Ba1—O2iii97.15 (6)O2xiii—As1—O2xiv110.59 (5)
O2vi—Ba1—O2iii126.22 (5)O2—As1—O2xiv110.59 (5)
O2—Ba1—O2iii180.00 (7)

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


Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2189).


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