Crystal structures of Ca(ClO4)2·4H2O and Ca(ClO4)2·6H2O

The crystal structures of the tetra- and hexahydrate phases of Ca(ClO4)2 consist of Ca2+ ions in distorted square-antiprismatic environments and of perchlorate tetrahedra. O—H⋯O hydrogen bonds between water molecules and ClO4 units lead to the formation of a three-dimensional network in the structures.


Chemical context
Since the detection of perchlorates on Mars during the Phoenix Mission (Chevrier et al., 2009), interest in these salts, and especially their hydrates, has risen considerably (Kim et al., 2013;Quinn et al., 2013;Kerr, 2013;Davila et al., 2013;Schuttlefield et al., 2011;Navarro-Gonzá lez et al., 2010;Marion et al., 2010). To gain more knowledge about the behavior of salts and salt hydrates, it is essential to determine the corresponding phase diagrams. For calcium perchlorate, this was performed by several authors (Marion et al., 2010;Pestova et al., 2005;Dobrynina, 1984;Lilich & Djurinskii, 1956;Nicholson & Felsing, 1950;Willard & Smith, 1923) for different concentration areas with different results. The stable salt hydrate phase at room temperature in this system is calcium perchlorate tetrahydrate. At lower temperatures, a higher hydrated phase, i.e. the hexahydrate, occurs as the stable phase.

Structural commentary
The Ca 2+ cation in Ca(ClO 4 )Á4H 2 O is coordinated by four water molecules (O1, O2, O7, O8) and four O atoms from two pairs of symmetry-related perchlorate tetrahedra as shown in Fig. 1a. The resulting coordination polyhedron is a distorted square anti-prism (Fig. 1b) The two different Ca 2+ cations in Ca(ClO 4 )Á6H 2 O are each coordinated by six water molecules and two perchlorate tetrahedra (Fig. 2). Again, the bond lengths between the cations and water molecules [2.319 (6)-2.500 (6) Å ] are shorter than those to the perchlorate groups. For the latter, one of the two distances for each of the Ca 2+ cations is by 0.5 Å markedly longer than the other ($3.07 versus $2.53 Å ). Nevertheless, according to the bond-valence model (Brown, 2002), the longer bond contributes ca. 0.05 valence units to the overall bond-valence sum and hence should not be neglected. If this longer bond is considered to be relevant, again a square anti-prismatic coordination polyhedron is realised for both Ca 2+ cations, however with a much greater distortion. Two perchlorate tetrahedra in the hexahydrate are shared between two Ca 2+ ions, leading to the formation of [Ca(H 2 O) 6 (ClO 4 )] 2 dimers oriented in layers parallel to (001).

Supramolecular features
The perchlorate tetrahedra in the structure of Ca(ClO 4 )Á4H 2 O are shared between two adjacent Ca 2+ ions, forming chains extending parallel to [011] (Fig. 3) whereby each Ca 2+ ion is connected along the chain on one side with a pair of Cl1 perchlorate tetrahedra, and on the opposite side with a pair of Cl2 perchlorate tetrahedra. The chains are arranged in sheets parallel to (011) and are linked by O-HÁ Á ÁO hydrogen bonds into a three-dimensional network with similar OÁ Á ÁO distances between the water molecules and the perchlorate tetrahedra (Table 1).

Figure 5
Formation of layers parallel to (001) in Ca(ClO 4 ) 2 Á6H 2 O. Only the strongest hydrogen bonds are shown, represented by dashed lines.

Figure 4
Formation of perchlorate-bridged dimers in Ca(ClO 4 ) 2 Á6H 2 O and location of 'free' perchlorate tetrahedra in the gaps between the dimers (highlighted in dark green). Only the strongest hydrogen bonds are shown, represented by dashed lines.
The samples were stored in a freezer or a cryostat at low temperatures. The crystals were separated and embedded in perfluorinated ether for X-ray analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in   (9) Computer programs: X-AREA and X-RED (Stoe & Cie, 2009), SHELXS97 and SHELXL2012 (Sheldrick, 2008), DIAMOND (Brandenburg, 2006) and publCIF (Westrip, 2010 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.

Data collection
Stoe IPDS-2 diffractometer Radiation source: fine-focus sealed tube Graphite monochromator Detector resolution: 6.67 pixels mm -1 rotation method scans Absorption correction: integration (Coppens, 1970)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.41 e Å −3 Δρ min = −0.67 e Å −3 Absolute structure: Refined as an inversion twin Absolute structure parameter: 0.45 (9) 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. Refined as a 2-component inversion twin.