Structure of the calcium pyro­phosphate monohydrate phase (Ca₂P₂O₇·H₂O): towards understanding the dehydration process in calcium pyrophosphate hydrates

Calcium pyrophosphate hydrate (CPP, Ca₂P₂O₇·nH₂O) and calcium orthophosphate compounds (including apatite, octa­calcium phosphate etc.) are among the most prevalent pathological calcifications in joints. Even though only two dihydrated forms of CPP (CPPD) have been detected in vivo (monoclinic and triclinic CPPD), investigations of other hydrated forms such as tetra­hydrated or amorphous CPP are relevant to a further understanding of the physicochemistry of those phases of biological inter­est. The synthesis of single crystals of calcium pyrophosphate monohydrate (CPPM; Ca₂P₂O₇·H₂O) by diffusion in silica gel at ambient temperature and the structural analysis of this phase are reported in this paper. Complementarily, data from synchrotron X-ray diffraction on a CPPM powder sample have been fitted to the crystal parameters. Finally, the relationship between the resolved structure for the CPPM phase and the structure of the tetra­hydrated calcium pyrophosphate β phase (CPPT-β) is discussed.

S1. Introduction

Calcium pyrophosphate hydrate (CPP) and calcium orthophosphate (including apatite, o­cta­calcium phosphate, tricalcium phosphate and whitlockite) compounds are among the most prevalent pathological calcifications in joints (MacMullan et al., 2011). CPP crystals are particularly involved in several kinds of arthritis, including osteoarthritis, a degenerative joint disorder affecting 80% of the population over 75 (Ea et al., 2011). Two different CPP phases have been detected in vivo in joints, viz. monoclinic and triclinic calcium pyrophosphate dihydrates (CPPD), referred to as m-CPPD and t-CPPD, respectively (Liu et al., 2009). In vivo studies have revealed that both are associated with a high inflammatory potential, probably due to their inter­action with cell membranes (Roch-Arveiller et al., 1990).

Several other forms of calcium pyrophosphate hydrates have also been synthesized in vitro, including monoclinic calcium pyrophosphate tetra­hydrates (CPPT), m-CPPT-a and m-CPPT-b, and an amorphous phase, a-CPP, which has been described as much more stable than the calcium phosphate and calcium carbonate amorphous phases (Brown et al., 1963; Slater et al., 2011).

Although the thermal decomposition of calcium orthophosphates has been studied extensively, few data are available on the behaviour of calcium pyrophosphate hydrates at high temperature. The dehydration process of m-CPPT-b is described as occurring in four different steps (Christoffersen et al., 2000; Gras et al., 2013). First, the loss of one water molecule occurs at low temperature (~323 K) to form a trihydrated calcium pyrophosphate (Gras et al., 2013). This phase was revealed to be highly metastable, rehydration occurring in less than 15 min under normal conditions of humidity at room temperature. The second step in the dehydration process corresponds to the loss of two water molecules at around 373 K to form calcium pyrophosphate monohydrate (CPPM). The next dehydration steps involve the formation of a calcium orthophosphate phase, monetite CaHPO₄, by hydrolysis of the pyrophosphate molecules and, finally, condensation into anhydrous b-Ca₂P₂O₇. The structures of the trihydrated and monohydrated phases have been described, but a full understanding of the different steps in the dehydration of CPP hydrates, particularly the involvement of hydrolysis reactions of the pyrophosphate molecules, is still lacking.

The present study focuses on the structure of calcium pyrophosphate monohydrate resolved by single-crystal X-ray diffraction (XRD) as a key step to further understanding of the m-CPPT-b dehydration process. The preparation of CPPM crystals by diffusion in gel is also described.

S2. Experimental

S2.1. Synthesis and crystallization

The technique used for the synthesis of hydrated CPP single crystals consists of the diffusion of separate calcium and pyrophosphate solutions into a silica gel, leading to a sufficiently high supersaturation in the gel to initiate homogeneous nucleation and slow growth of calcium pyrophosphate crystals. The gel method of crystal growth is often used for single-crystal synthesis because of its simplicity and the quality of the crystals produced, which are suitable for single-crystal characterization (Tamain et al., 2012).

Anhydrous tetra­sodium pyrophosphate (Na₄P₂O₇) was obtained by heating disodium phosphate (Na₂HPO₄, 100 g, Merck, >99% purity) in a muffle furnace at 673 K for 3 h. Calcium chloride (CaCl₂, Merck, >95% purity), [aqueous?] acetic acid solution (VWR, 100% purity, [What concentration?]) and sodium metasilicate penta­hydrate (Na₂SiO₃·5H₂O, Aldrich, >95% purity) were used as received without further purification. All solutions were prepared using deionized water.

The diffusion cell implemented for the present study was composed of three compartments with the same volume (150 ml), each separated by a dialysis membrane. The gel was prepared by adding sodium metasilicate penta­hydrate (Na₂SiO₃·5H₂O, 25 g) to deionized water (300 ml); the gel obtained had a density of 1.04 Mg m^-3 (Deepa et al., 1994). After complete dissolution, the metasilicate solution was added continuously with stirring (400 r min^-1) to acetic acid (15 ml) at a constant volumetric flow rate (7.25 ml min^-1) using a peristaltic pump. The alkaline metasilicate was added until the pH of the final solution reached 5.8. The addition of the solution into the acid with stirring avoided the formation of inhomogeneities in the gel microenvironment. The solution was then poured into the central compartment of the diffusion cell, delimited on both sides by a dialysis membrane (Cellu Step T3, MWCO 12000-14000), and a homogeneous gel was obtained after 48 h of maturation.

The calcium and pyrophosphate reagent solutions were prepared separately by dissolving CaCl₂ (13.88 g, 12.50 × 10 ^-2 mol) and Na₄P₂O₇ (16.62 g, 6.25 × 10 ^-2 mol) in deionized water (250 ml). Each solution was poured into one side compartment of the cell. The system was then kept undisturbed for two weeks at room temperature.

m-CPPT-b crystal spherulites were formed with a diameter of around 1 mm, containing crystals large enough for X-ray characterization. The crystals showed a thin platelet morphology with an orientation perpendicular to (100). These large crystals of m-CPPT-b were then heated at 383 K for 30 min, resulting in their dehydration into the monoclinic calcium pyrophosphate monohydrate phase (m-CPPM).

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The selected crystal was mounted on a Microloop (MiTeGen) using perfluoro­polyether oil and cooled rapidly to 180 K in a stream of cold N₂. X-ray intensity data were collected on a Bruker Kappa APEXII Quasar diffractometer using a 30 W air-cooled microfocus source (ImS) with focusing multilayer optics at a temperature of 180 (2) K with Mo Kα radiation (λ = 0.71073 Å). The desired measurement temperature was achieved by injecting an airflow that was cooled using liquid nitro­gen.

The crystallographic cell was first determined by φ and ω scans. A strategy for data measurement was then calculated, and a complete data set was obtained using φ and ω scans with the APEX2 software (Bruker, 2007). The data were integrated using SAINT (Bruker, 2007) and an empirical absorption correction using SADABS was applied (Bruker, 2001).

The point-group determination was followed by determining the positions of all non-H atoms by direct methods using SHELXS97 (Sheldrick, 2008), and the structure was refined in the WinGX software package (Farrugia, 2012) using SHELXL97 (Sheldrick, 2008). The non-H atoms were easily localized after structural determination and subsequent Fourier analyses (Fourier difference maps) revealed the positions of the H atoms. The refinements were performed using anisotropic displacement parameters for all the non-H atoms. However, owing to the small size and rather poor quality of the m-CPPM crystals that were obtained by heating m-CPPT-b crystals at 383 K for 30 min, large and elongated reflections were registered, limiting the accuracy of the structural parameters.

Complementarily, we checked that the data (the structural parameters) obtained from single-crystal XRD fitted the X-ray powder diffraction data collected on the Cristal beamline at the SOLEIL synchrotron (Gif-sur-Yvette, France). A monochromatic beam was selected using an Si(111) double-crystal monochromator and its wavelength (0.72442 Å) determined using NIST standard LaB₆. The powder sample was placed in a 0.7 mm diameter glass capillary, mounted on a spinner to improve averaging. High angular resolution was obtained with the 21 perfect-crystal Si(111) rear analyser mounted on a two-circle diffractometer.

The powder diffraction pattern was indexed using the LSI method implemented in TOPAS (Coelho, 2003, 2009) in a monoclinic system of extension class P2₁/n or P2₁/c. The Rietveld refinement was performed using JANA2006 (Petříček et al., 2006).

S3. Results and discussion

The XRD data obtained from the single-crystal analysis led to the refined cell constants and additional crystal data reported in Table 1. The resolved molecular structure for the CPPM phase is presented in Fig. 1. Selected bond lengths are reported in Table 2.

The reciprocal lattice corresponds to a monoclinic system, with systematic extinctions consistent with the space group P2₁/n, similar to the space group of the m-CPPT-b structure determined by Balić-Žunić et al. (2000), and its asymmetric unit contains only one unit, Ca₂P₂O₇·H₂O.

We checked the structure determined from single-crystal XRD data by using a refinement of synchrotron X-ray powder diffraction data for the m-CPPM sample (Fig. 2). The unit-cell parameters obtained were a = 10.0058 (5) Å, b = 6.8629 (3) Å, c = 10.5596 (5) Å and β = 114.258 (2)°. The unit cell for the P2₁/c space group was obtained using the transformation matrix (001/010/???) [Please add third term to matrix] and refined with the following cell parameters: a = 10.5596 (4) Å, b = 6.8630 (2) Å, c = 11.1762 (5) Å and β = 125.257 (2)°, for comparison with the m-CPPT-b structure. The structure was fully resolved and refined. The results were in good agreement with the structure obtained by single-crystal XRD analysis.

We note that the resolved structure for m-CPPM is closely related to that of m-CPPT-b (Fig. 3). The structure of m-CPPT-b is formed by alternate layers of water molecules and calcium pyrophosphate oriented in the (100) direction (Balić-Žunić et al., 2000). In this structure, two different layers are linked together only by hydrogen bonds. In a second framework, the inner layer of calcium pyrophosphate can also be described as constituting layers of calcium and layers of pyrophosphate organized in the (001) direction, transverse to the water layers (Fig. 3a). This organization can be compared with that of brushite (CaHPO₄·2H₂O), described as a layered structure in which the layers are held together by water molecules via hydrogen bonding (Dosen & Giese, 2011). The dehydration of brushite leads to the formation of monetite, CaHPO₄; these two calcium orthophosphate phases have the same Ca/P atomic ratio as the m-CPPM and m-CPPT-b calcium pyrophosphate phases.

It has been shown by Balić-Žunić et al. (2000) that partial dehydration of m-CPPT-b involves the release of three water molecules, referred to as OW2, OW3 and OW4. These water molecules are held in the m-CPPT-b structure either by a weak inter­action with Ca2 (OW2 and OW3) or by hydrogen bonds only (OW4). They were described as being weakly bound compared with atom O8, which is coordinated to both Ca1 and Ca2. The dehydrated structure loses its initial layered organization based on hydrogen bonds with water molecules, with a closing of the gap resulting in the formation of bonds between the pyrophosphate molecules and calcium, as observed in the m-CPPM structure. The layered framework of calcium and pyrophosphate still remains in the (001) direction, but is slightly modified.

The transition between these inner calcium pyrophosphate structures could be described as a reorganization of the calcium (001) framework, related to the formation of the Ca2—O5 coordination between the different layers. The Ca layer is inclined at 2.2° relative to (001) in m-CPPM, compared with 11.5° for m-CPPT-b. The Ca2—O5 coordination is created in the former water layers and, as a consequence, the inner layered structure is slightly modified by this reorientation. Atoms Ca1 and P1, in particular, keep almost the same structural environment in m-CPPM as in m-CPPT-b, on the 2₁ axes for Ca1 and on the c-glide for P1.

This deformation, supposedly based on the orientation of the calcium framework, leads to greater deformations of the P2 tetra­hedron of the pyrophosphate molecules. It also involves a change in the Ca2 environment, which evolves from coordination polyhedra with a coordination number of 7 (CN7) to new ones with CN6. The coordination CN6 has not yet been observed for hydrated calcium pyrophosphate, but it is common for anhydrous calcium pyrophosphates b-Ca₂P₂O₇, other hydrated pyrophosphate compounds (X₂P₂O₇·2H₂O; X = Mg, Mn, Fe, Co) or calcium phosphate hydrates like o­cta­calcium phosphate, and could explain the dehydration mechanism without a major reorganization. The loss of water molecules OW2 and OW3 and the formation of the Ca2—O5 coordination deforms the Ca2 coordination polyhedra.

The new coordination provides the only link between the former calcium pyrophosphate layers, the remaining hydrogen bond of O8 being reoriented to the closest pyrophosphates on the same side of the initial gap between the calcium pyrophosphate layers. As a consequence, the Ca2 polyhedron is a highly distorted o­cta­hedron, resulting in a penta­gonal bipyramid system with a missing O atom due to dehydration. Indeed, atom O5 takes a position between the former OW2 and OW3 positions in the coordination sphere, closer to the OW3 position. Water molecule OW3 has already been reported by Balić-Žunić et al. (2000) to face atom O5 with matching surfaces. The Ca2 distorted o­cta­hedron has an O6—Ca2—O7 [171.85 (18)°] axis and an O1—O2—O5—O8 base included in an almost perpendicular plane [87.3(s.u.)°]. The different angles between the O atoms show the initial position of the water molecules by virtue of a larger distortion at that position: 78.5 (7) (O8—Ca2—O5), 78.6 (8) (O8—Ca2—O2), 80.1 (6) (O2—Ca2—O1) and 119.5 (6)° (O5—Ca2—O1). The next closest O atom in the m-CPPM structure is O3 at 2.97(?) Å [Please provide missing s.u.], but this cannot be considered part of the Ca2 coordination environment, due to its distance and position as a bridging atom in the pyrophosphate molecule.

In the m-CPPM structure, the Ca atoms of parallel Ca chains are surrounded by O polyedra sharing O—O edges. The Ca chains are connected together by the O1 vertices of the Ca polyhedra in an approximate [112] orientation. Pyrophosphate molecules are alternately oriented parallel to [110] and [110] in a complementary structure.

Pyrophosphate molecules are reported to have a very high flexibility with two characteristic configurations, staggered or eclipsed (Rulmont et al., 1991). Similar to pyrosilicates, pyroarsenates or pyrogermanates, crystals of pyrophosphate compounds of the composition X₂P₂O₇, with an ionic radius of X less than 0.97 Å (X = Mg, Mn, Fe, Co, Ni, Cu, Zn), are isostructural with thortveitite (Sc₂Si₂O₇), with the P–O–P bond angle varying from 140° to 180°, an O—P···P—O pseudo-torsion angle of 60° (staggered conformation) and the YO₄ [Define Y?] tetra­hedra showing a very low degree of distortion. For an ionic radius greater than 0.97 Å (X = Ca, Sr, Ba), or for hydrates, pyrophosphate molecules usually have the same configuration as the dichromate structure, with a P—O—P angle of approximately 120–135°, an O—P···P—O pseudo-torsion angle of 0–30° (eclipsed conformation) and distorted YO₄ [Define Y?] tetra­hedra (Davis et al., 1985). The angle of the P—O—P bridge, the torsion between the two phosphate tetra­hedra and the P—O3 distance correspond to an eclipsed configuration. The evolution of this structure leads to a highly deformed P2 tetra­hedron for m-CPPM, with the highest reported P—O3 distance of 1.68 Å. The two tetra­hedra have the same orientation, with a difference between the two bases of the phosphate group of 10.6° and a torsion angle of 2.1°. For comparison, the angle between the two terminal phosphate groups for m-CPPT-b is 19.6° and the torsion angle is 9.1°. Finally, the P—O—P angle was measured at 132.7°, compared with 134.1° for m-CPPT-b.

Dehydration leads to a higher density, from 2.36 Mg m^-3 for m-CPPT-b to 2.60 Mg m^-3 for m-CPPM. The volume of the cell also decreases by 25%, from 918.4 to 694.5 ų.

The distance between atoms O8 and O3 in the m-CPPM structure is 3.22 Å, at an angle of approximately 30° compared with the basal face of the pyrophosphate molecules. This configuration could favour the inter­nal hydrolysis of pyrophosphate ions into hydrogenphosphate ions. This would contribute to our understanding of the hydrolysis phenomenon occurring during the next step in the dehydration of the m-CPPM phase.

The next step that occurs upon heating m-CPPM corresponds, unexpectedly, to the transient hydrolysis of pyrophosphate ions and the formation of monetite (CaHPO₄). At higher temperatures, condensation of the hydrogenphosphate re-forms the pyrophosphate ions, i.e. b-Ca₂P₂O₇, and completes the dehydration process.

Related literature

For related literature, see: Balić-Žunić, Christoffersen & Christoffersen (2000); Brown et al. (1963); Bruker (2001, 2007); Christoffersen et al. (2000); Coelho (2003, 2009); Davis et al. (1985); Deepa et al. (1994); Dosen & Giese (2011); Ea et al. (2011); Farrugia (2012); Gras et al. (2013); Liu et al. (2009); MacMullan et al. (2011); Petříček et al. (2006); Roch-Arveiller, Legros, Chanaud, Muntaner, Strzalko, Thuret, Willoughby & Giroud (1990); Rulmont et al. (1991); Sheldrick (2008); Slater et al. (2011); Tamain et al. (2012).

Computing details

Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SORTAV (Blessing, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2014).

Figures

	Fig. 1. The molecular structure of m-CPPM, showing the atom-numbering scheme and symmetry-equivalent atoms. Displacement ellipsoids are drawn at the 50% probability level, except for H atoms which are at the 20% probability level. [Symmetry codes: (a) x + 1/2, -y + 1/2, z + 1/2; (b) -x + 3/2, y + 1/2, -z + 3/2; (c) -x + 1, -y + 1, -z + 1; (d) x, y + 1, z; (e) x + 1/2, -y + 3/2, z - 1/2; (f) x - 1/2, -y + 1/2, z - 1/2; (f) x + 1/2, -y + 1/2, z - 1/2; (g) -x + 1/2, y + 1/2, -z + 3/2.]
	Fig. 2. Rietveld plot for the m-CPPM phase based on the powder XRD pattern. Observed data points are indicated by dots, and the best-fit profile (upper trace) and the difference pattern (lower trace) are solid lines. The vertical bars indicate the positions of the Bragg peaks.
	Fig. 3. Comparison of projections along the b axis of (a) the structure of m-CPPM, showing the polyhedron frameworks in the structure with P21/c symmetry (Dashed lines idnicate hydrogen bonds?), and (b) the structure of m-CPPT-b.

Calcium pyrophosphate monohydrate

Crystal data

Ca2P2O7·H2O	F(000) = 544
Mr = 272.12	Dx = 2.602 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 1804 reflections
a = 10.16 (14) Å	θ = 3.7–27.8°
b = 6.97 (5) Å	µ = 2.11 mm^−1
c = 10.77 (10) Å	T = 180 K
β = 114.4 (4)°	Needle, colourless
V = 695 (12) Å^3	0.2 × 0.03 × 0.02 mm
Z = 4

Data collection

Bruker Kappa APEXII diffractometer with Quasar CCD area-detector	1402 independent reflections
Radiation source: micro-focus	996 reflections with I > 2σ(I)
Multilayer optics monochromator	Rint = 0.059
φ and ω scans	θmax = 26.4°, θmin = 5.3°
Absorption correction: multi-scan (SADABS; Bruker, 2001)	h = −12→12
Tmin = 0.665, Tmax = 0.957	k = −8→8
7104 measured reflections	l = −13→13

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)] = 0.051	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.133	All H-atom parameters refined
S = 1.06	w = 1/[σ^2(Fo^2) + (0.0566P)^2 + 4.017P] where P = (Fo^2 + 2Fc^2)/3
1402 reflections	(Δ/σ)max < 0.001
117 parameters	Δρmax = 0.89 e Å^−3
0 restraints	Δρmin = −0.69 e Å^−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
O1	0.4356 (5)	0.4417 (6)	0.6198 (4)	0.0238 (10)
O2	0.6208 (4)	0.2267 (6)	0.5986 (4)	0.0179 (9)
O3	0.4099 (4)	0.0743 (6)	0.6350 (4)	0.0205 (10)
O4	0.3620 (4)	0.2220 (6)	0.4061 (4)	0.0220 (10)
O5	0.4312 (5)	0.1701 (7)	0.8751 (4)	0.0304 (11)
O6	0.3854 (5)	−0.1793 (7)	0.7846 (5)	0.0282 (11)
O7	0.6306 (4)	−0.0269 (7)	0.8522 (4)	0.0235 (10)
O8	0.5937 (5)	0.5446 (9)	0.9158 (5)	0.0264 (11)
P1	0.45546 (16)	0.2487 (2)	0.56019 (15)	0.0184 (4)
P2	0.46790 (17)	0.0080 (2)	0.79898 (15)	0.0200 (4)
Ca1	0.70409 (13)	0.51047 (18)	0.74953 (12)	0.0186 (3)
Ca2	0.73672 (14)	0.3039 (2)	0.43544 (13)	0.0242 (4)
H1	0.572 (10)	0.421 (15)	0.905 (9)	0.05 (3)*
H2	0.526 (13)	0.631 (19)	0.885 (12)	0.10 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.023 (2)	0.022 (2)	0.018 (2)	0.0011 (19)	−0.0002 (19)	−0.0022 (19)
O2	0.018 (2)	0.019 (2)	0.0101 (19)	0.0007 (17)	−0.0011 (17)	0.0001 (17)
O3	0.021 (2)	0.021 (2)	0.011 (2)	−0.0021 (19)	−0.0020 (17)	−0.0002 (18)
O4	0.020 (2)	0.024 (3)	0.012 (2)	0.0000 (19)	−0.0039 (17)	0.0001 (18)
O5	0.033 (3)	0.035 (3)	0.017 (2)	0.005 (2)	0.005 (2)	−0.003 (2)
O6	0.024 (2)	0.026 (3)	0.024 (2)	−0.005 (2)	−0.0012 (19)	0.001 (2)
O7	0.016 (2)	0.033 (3)	0.013 (2)	0.0016 (19)	−0.0030 (17)	0.0048 (19)
O8	0.023 (3)	0.030 (3)	0.017 (2)	0.000 (2)	−0.0017 (19)	−0.004 (2)
P1	0.0146 (8)	0.0196 (8)	0.0115 (7)	−0.0007 (7)	−0.0041 (6)	−0.0011 (6)
P2	0.0166 (8)	0.0233 (9)	0.0135 (8)	−0.0003 (7)	−0.0003 (6)	−0.0004 (7)
Ca1	0.0163 (6)	0.0191 (7)	0.0126 (6)	−0.0009 (5)	−0.0020 (5)	−0.0005 (5)
Ca2	0.0192 (6)	0.0315 (8)	0.0141 (6)	0.0024 (6)	−0.0008 (5)	−0.0004 (6)

Geometric parameters (Å, º)

O1—P1	1.540 (10)	P2—Ca2^iii	3.33 (2)
O1—Ca2^i	2.388 (18)	P2—Ca2^ii	3.48 (3)
O1—Ca1	2.54 (3)	P2—Ca2^v	3.50 (3)
O2—P1	1.56 (2)	Ca1—O7^vi	2.37 (2)
O2—Ca1^ii	2.389 (16)	Ca1—O2^vi	2.389 (16)
O2—Ca1	2.476 (14)	Ca1—O4^i	2.410 (14)
O2—Ca2	2.54 (2)	Ca1—O4^vii	2.410 (15)
O3—P1	1.627 (9)	Ca1—P1^i	3.48 (3)
O3—P2	1.680 (16)	Ca1—Ca1^ii	3.61 (2)
O3—Ca2^iii	2.968 (18)	Ca1—Ca1^vi	3.61 (2)
O4—P1	1.546 (15)	Ca1—Ca2^vi	3.79 (3)
O4—Ca1^i	2.410 (14)	Ca1—Ca2	3.81 (3)
O4—Ca1^iv	2.410 (15)	Ca2—O5^viii	2.33 (3)
O5—P2	1.530 (9)	Ca2—O6^iii	2.34 (2)
O5—Ca2^v	2.33 (3)	Ca2—O1^i	2.388 (18)
O6—P2	1.524 (11)	Ca2—O7^vi	2.427 (18)
O6—Ca2^iii	2.34 (2)	Ca2—O8^ii	2.554 (16)
O7—P2	1.53 (2)	Ca2—O3^iii	2.968 (18)
O7—Ca1^ii	2.37 (2)	Ca2—P2^iii	3.33 (2)
O7—Ca2^ii	2.427 (18)	Ca2—P2^vi	3.48 (3)
O8—Ca1	2.49 (2)	Ca2—P2^viii	3.50 (3)
O8—Ca2^vi	2.554 (16)	Ca2—Ca1^ii	3.79 (3)
P1—Ca1^i	3.48 (3)
P1—O1—Ca2^i	138.9 (4)	O2—Ca1—Ca1^vi	142.3 (4)
P1—O1—Ca1	95.6 (5)	O8—Ca1—Ca1^vi	95.7 (3)
Ca2^i—O1—Ca1	120.4 (6)	O1—Ca1—Ca1^vi	113.63 (15)
P1—O2—Ca1^ii	131.1 (5)	P1^i—Ca1—Ca1^vi	63.5 (3)
P1—O2—Ca1	97.7 (4)	Ca1^ii—Ca1—Ca1^vi	150.2 (4)
Ca1^ii—O2—Ca1	95.7 (6)	O7^vi—Ca1—Ca2^vi	126.4 (5)
P1—O2—Ca2	123.3 (6)	O2^vi—Ca1—Ca2^vi	41.3 (4)
Ca1^ii—O2—Ca2	100.4 (8)	O4^i—Ca1—Ca2^vi	95.8 (7)
Ca1—O2—Ca2	98.9 (6)	O4^vii—Ca1—Ca2^vi	85.6 (6)
P1—O3—P2	132.7 (4)	O2—Ca1—Ca2^vi	154.58 (16)
P1—O3—Ca2^iii	138.8 (5)	O8—Ca1—Ca2^vi	41.9 (4)
P2—O3—Ca2^iii	86.9 (4)	O1—Ca1—Ca2^vi	109.3 (5)
P1—O4—Ca1^i	121.7 (5)	P1^i—Ca1—Ca2^vi	115.9 (6)
P1—O4—Ca1^iv	140.2 (4)	Ca1^ii—Ca1—Ca2^vi	125.0 (3)
Ca1^i—O4—Ca1^iv	96.9 (6)	Ca1^vi—Ca1—Ca2^vi	62.0 (3)
P2—O5—Ca2^v	129.1 (4)	O7^vi—Ca1—Ca2	37.8 (3)
P2—O6—Ca2^iii	117.4 (4)	O2^vi—Ca1—Ca2	122.2 (6)
P2—O7—Ca1^ii	132.2 (7)	O4^i—Ca1—Ca2	77.1 (6)
P2—O7—Ca2^ii	121.6 (4)	O4^vii—Ca1—Ca2	95.2 (6)
Ca1^ii—O7—Ca2^ii	105.4 (8)	O2—Ca1—Ca2	41.2 (3)
Ca1—O8—Ca2^vi	97.6 (7)	O8—Ca1—Ca2	154.8 (3)
O1—P1—O4	115.1 (4)	O1—Ca1—Ca2	84.7 (7)
O1—P1—O2	106.6 (3)	P1^i—Ca1—Ca2	60.6 (6)
O4—P1—O2	112.9 (5)	Ca1^ii—Ca1—Ca2	61.4 (2)
O1—P1—O3	109.5 (6)	Ca1^vi—Ca1—Ca2	104.5 (3)
O4—P1—O3	105.5 (6)	Ca2^vi—Ca1—Ca2	163.2 (2)
O2—P1—O3	106.9 (5)	O5^viii—Ca2—O6^iii	84.4 (7)
O1—P1—Ca1^i	83.6 (6)	O5^viii—Ca2—O1^i	119.5 (6)
O4—P1—Ca1^i	36.1 (4)	O6^iii—Ca2—O1^i	89.8 (5)
O2—P1—Ca1^i	109.0 (4)	O5^viii—Ca2—O7^vi	91.2 (7)
O3—P1—Ca1^i	136.0 (5)	O6^iii—Ca2—O7^vi	171.85 (18)
O6—P2—O7	111.4 (6)	O1^i—Ca2—O7^vi	86.5 (6)
O6—P2—O5	116.2 (6)	O5^viii—Ca2—O2	153.8 (2)
O7—P2—O5	112.5 (5)	O6^iii—Ca2—O2	114.7 (7)
O6—P2—O3	100.9 (4)	O1^i—Ca2—O2	80.1 (6)
O7—P2—O3	107.1 (4)	O7^vi—Ca2—O2	71.8 (8)
O5—P2—O3	107.7 (6)	O5^viii—Ca2—O8^ii	78.5 (7)
O6—P2—Ca2^iii	38.6 (4)	O6^iii—Ca2—O8^ii	108.1 (5)
O7—P2—Ca2^iii	114.8 (5)	O1^i—Ca2—O8^ii	156.5 (3)
O5—P2—Ca2^iii	132.4 (7)	O7^vi—Ca2—O8^ii	77.6 (6)
O3—P2—Ca2^iii	62.9 (5)	O2—Ca2—O8^ii	78.6 (8)
O6—P2—Ca2^ii	89.1 (7)	O5^viii—Ca2—O3^iii	113.1 (4)
O7—P2—Ca2^ii	36.4 (4)	O6^iii—Ca2—O3^iii	54.0 (3)
O5—P2—Ca2^ii	100.3 (7)	O1^i—Ca2—O3^iii	110.9 (8)
O3—P2—Ca2^ii	141.8 (4)	O7^vi—Ca2—O3^iii	134.2 (3)
Ca2^iii—P2—Ca2^ii	114.4 (6)	O2—Ca2—O3^iii	70.3 (4)
O6—P2—Ca2^v	85.8 (7)	O8^ii—Ca2—O3^iii	70.7 (7)
O7—P2—Ca2^v	135.3 (6)	O5^viii—Ca2—P2^iii	100.1 (7)
O5—P2—Ca2^v	31.1 (2)	O6^iii—Ca2—P2^iii	23.99 (16)
O3—P2—Ca2^v	109.5 (6)	O1^i—Ca2—P2^iii	97.6 (7)
Ca2^iii—P2—Ca2^v	104.2 (8)	O7^vi—Ca2—P2^iii	164.04 (14)
Ca2^ii—P2—Ca2^v	108.0 (8)	O2—Ca2—P2^iii	93.7 (7)
O7^vi—Ca1—O2^vi	85.2 (8)	O8^ii—Ca2—P2^iii	93.5 (7)
O7^vi—Ca1—O4^i	79.1 (5)	O3^iii—Ca2—P2^iii	30.3 (2)
O2^vi—Ca1—O4^i	84.4 (6)	O5^viii—Ca2—P2^vi	69.8 (9)
O7^vi—Ca1—O4^vii	81.7 (7)	O6^iii—Ca2—P2^vi	154.1 (3)
O2^vi—Ca1—O4^vii	81.6 (7)	O1^i—Ca2—P2^vi	101.3 (7)
O4^i—Ca1—O4^vii	157.0 (3)	O7^vi—Ca2—P2^vi	22.0 (2)
O7^vi—Ca1—O2	74.0 (6)	O2—Ca2—P2^vi	90.4 (9)
O2^vi—Ca1—O2	155.4 (3)	O8^ii—Ca2—P2^vi	69.2 (6)
O4^i—Ca1—O2	103.9 (7)	O3^iii—Ca2—P2^vi	138.1 (2)
O4^vii—Ca1—O2	82.6 (6)	P2^iii—Ca2—P2^vi	161.11 (12)
O7^vi—Ca1—O8	163.91 (19)	O5^viii—Ca2—P2^viii	19.8 (2)
O2^vi—Ca1—O8	82.9 (8)	O6^iii—Ca2—P2^viii	83.1 (7)
O4^i—Ca1—O8	110.4 (5)	O1^i—Ca2—P2^viii	99.6 (7)
O4^vii—Ca1—O8	85.9 (7)	O7^vi—Ca2—P2^viii	90.3 (7)
O2—Ca1—O8	114.6 (5)	O2—Ca2—P2^viii	162.09 (16)
O7^vi—Ca1—O1	121.9 (8)	O8^ii—Ca2—P2^viii	97.6 (8)
O2^vi—Ca1—O1	145.2 (3)	O3^iii—Ca2—P2^viii	125.3 (4)
O4^i—Ca1—O1	80.6 (4)	P2^iii—Ca2—P2^viii	104.1 (7)
O4^vii—Ca1—O1	120.7 (5)	P2^vi—Ca2—P2^viii	72.0 (8)
O2—Ca1—O1	59.4 (3)	O5^viii—Ca2—Ca1^ii	116.9 (5)
O8—Ca1—O1	73.4 (8)	O6^iii—Ca2—Ca1^ii	121.9 (6)
O7^vi—Ca1—P1^i	75.4 (7)	O1^i—Ca2—Ca1^ii	116.9 (4)
O2^vi—Ca1—P1^i	105.8 (6)	O7^vi—Ca2—Ca1^ii	66.3 (6)
O4^i—Ca1—P1^i	22.21 (12)	O2—Ca2—Ca1^ii	38.3 (4)
O4^vii—Ca1—P1^i	155.12 (17)	O8^ii—Ca2—Ca1^ii	40.5 (4)
O2—Ca1—P1^i	81.9 (6)	O3^iii—Ca2—Ca1^ii	68.1 (5)
O8—Ca1—P1^i	118.3 (7)	P2^iii—Ca2—Ca1^ii	98.3 (6)
O1—Ca1—P1^i	66.1 (4)	P2^vi—Ca2—Ca1^ii	73.8 (6)
O7^vi—Ca1—Ca1^ii	70.3 (2)	P2^viii—Ca2—Ca1^ii	133.8 (5)
O2^vi—Ca1—Ca1^ii	119.3 (6)	O5^viii—Ca2—Ca1	127.8 (3)
O4^i—Ca1—Ca1^ii	138.3 (4)	O6^iii—Ca2—Ca1	146.5 (6)
O4^vii—Ca1—Ca1^ii	41.6 (3)	O1^i—Ca2—Ca1	67.5 (4)
O2—Ca1—Ca1^ii	41.2 (4)	O7^vi—Ca2—Ca1	36.8 (6)
O8—Ca1—Ca1^ii	106.5 (2)	O2—Ca2—Ca1	39.9 (4)
O1—Ca1—Ca1^ii	92.1 (4)	O8^ii—Ca2—Ca1	89.7 (6)
P1^i—Ca1—Ca1^ii	119.1 (3)	O3^iii—Ca2—Ca1	110.1 (4)
O7^vi—Ca1—Ca1^vi	82.9 (3)	P2^iii—Ca2—Ca1	131.6 (6)
O2^vi—Ca1—Ca1^vi	43.1 (3)	P2^vi—Ca2—Ca1	58.5 (7)
O4^i—Ca1—Ca1^vi	41.6 (4)	P2^viii—Ca2—Ca1	123.4 (4)
O4^vii—Ca1—Ca1^vi	123.5 (5)	Ca1^ii—Ca2—Ca1	56.6 (5)

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