Structure of the β form of calcium pyrophosphate tetrahydrate

β-Ca₂P₂O₇·4H₂O is monoclinic, P2₁/c, a = 12.287 (6), b = 7.511 (3), c = 10.775 (5) Å, and β = 112.54 (1)°. Five of the terminal O atoms from a pyrophosphate group bind to Ca atoms, together with O atoms from three of the water molecules. The fourth H₂O forms only hydrogen bonds. Both Ca atoms have coordination number 7 and show characteristics between those of a capped octahedron and a pentagonal bipyramid. The analysis of coordination distortions suggests that regularity and volume efficiency of a Ca coordination polyhedron increases with the number of bound water O atoms. The structure is layered after {100}, reflected also in the morphology of crystals which are formed as extremely thin plates. The central parts of the layers are formed by chains of Ca coordination polyhedra which run along the b axis and are interconnected by pyrophosphate groups. Water molecules form the surfaces of the layers. A peculiar auto-inhibition of growth from supersaturated solutions is supposed to be caused by a direct attachment of CaP₂O₇²⁻ and P₂O₇⁴⁻ to the water molecules on the surfaces of layers. Ca₂P₂O₇·4H₂O is known in two polymorphs. The unit-cell volume of the β form compared with that of α suggests that the former is a low-temperature modification.

Computing details

Program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997).

(cppt)

Crystal data

Ca2O7P2·4(H2O)	Z = 4
Mr = 326.16	F(000) = 664
Monoclinic, P21/c	Dx = 2.359 Mg m^−3
Hall symbol: -P 2ybc	Mo Kα radiation, λ = 0.71073 Å
a = 12.287 (6) Å	Cell parameters from 173 reflections
b = 7.511 (3) Å	µ = 1.64 mm^−1
c = 10.775 (5) Å	T = 298 K
β = 112.542 (14)°	{100},{001},{011}, colourless
V = 918.4 (7) Å^3	0.24 × 0.06 × 0.01 mm

Data collection

Radiation source: fine-focus sealed tube	432 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.257
Absorption correction: gaussian face-indexed ?	θmax = 20.8°, θmin = 1.8°
Tmin = 0.891, Tmax = 0.987	h = −11→12
2928 measured reflections	k = −5→7
959 independent reflections	l = −10→9

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)] = 0.070	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.165	H atoms treated by a mixture of independent and constrained refinement
S = 0.87	w = 1/[σ^2(Fo^2) + (0.0459P)^2] where P = (Fo^2 + 2Fc^2)/3
959 reflections	(Δ/σ)max = 0.012
81 parameters	Δρmax = 0.70 e Å^−3
0 restraints	Δρmin = −0.68 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.

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 > σ(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 (Ų)

	x	y	z	Uiso*/Ueq
Ca1	0.4691 (3)	0.0022 (5)	0.7271 (4)	0.0174 (11)
Ca2	0.2613 (4)	0.1473 (5)	0.3157 (4)	0.0202 (12)
P1	0.7424 (5)	0.0268 (6)	0.9849 (5)	0.0179 (14)
P2	0.5607 (5)	0.2385 (7)	0.5199 (5)	0.0207 (15)
O1	0.5129 (10)	0.2410 (14)	0.1263 (10)	0.013 (3)*
O2	0.4707 (10)	0.2434 (14)	0.8748 (10)	0.010 (3)*
O3	0.6795 (10)	0.0141 (15)	0.8352 (10)	0.018 (3)*
O4	0.2187 (11)	0.1544 (16)	0.9504 (11)	0.027 (4)*
O5	0.6512 (10)	0.0938 (13)	0.0491 (11)	0.014 (3)*
O6	0.6311 (10)	0.0693 (14)	0.5232 (11)	0.019 (3)*
O7	0.8390 (11)	0.1595 (17)	0.0257 (12)	0.033 (4)*
OW1	0.2649 (11)	0.0895 (14)	0.6711 (12)	0.026 (4)*
OW2	0.8827 (12)	0.0078 (18)	0.6201 (13)	0.045 (4)*
OW3	0.0776 (11)	0.1621 (17)	0.1167 (11)	0.035 (4)*
OW4	0.8928 (12)	0.1621 (17)	0.3290 (12)	0.038 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca1	0.026 (3)	0.0016 (18)	0.022 (2)	0.000 (2)	0.007 (2)	0.001 (2)
Ca2	0.019 (3)	0.012 (2)	0.026 (3)	0.000 (2)	0.006 (2)	0.002 (2)
P1	0.018 (4)	0.012 (3)	0.025 (3)	−0.007 (3)	0.011 (3)	−0.002 (3)
P2	0.028 (4)	0.006 (3)	0.030 (4)	−0.002 (3)	0.013 (3)	−0.004 (3)

Geometric parameters (Å, º)

Ca1—O1^i	2.374 (11)	P1—O5^v	1.608 (12)
Ca1—O1^ii	2.369 (12)	P1—Ca2^ii	3.477 (6)
Ca1—O3	2.395 (12)	P2—O1^i	1.483 (11)
Ca1—O2	2.406 (11)	P2—O6	1.530 (12)
Ca1—OW1	2.436 (13)	P2—O2^iv	1.536 (11)
Ca1—O2^iii	2.481 (11)	P2—O5^i	1.629 (12)
Ca1—O6^ii	2.556 (11)	P2—Ca1^ii	3.121 (6)
Ca1—P2^ii	3.121 (6)	P2—Ca1^iv	3.506 (6)
Ca1—P2	3.365 (6)	P2—Ca1^vii	3.500 (6)
Ca1—P1	3.445 (7)	O1—P2^iv	1.483 (11)
Ca1—P2^i	3.506 (6)	O1—Ca1^iv	2.374 (11)
Ca1—P2^iii	3.500 (6)	O1—Ca1^ii	2.369 (12)
Ca2—O4^iv	2.276 (12)	O2—P2^i	1.536 (11)
Ca2—O3^ii	2.355 (11)	O2—Ca1^vii	2.481 (11)
Ca2—O6^ii	2.377 (11)	O2—Ca2^i	2.539 (12)
Ca2—OW2^ii	2.430 (14)	O3—Ca2^ii	2.355 (11)
Ca2—OW3	2.448 (12)	O4—P1^vi	1.520 (12)
Ca2—OW1^iv	2.527 (12)	O4—Ca2^i	2.276 (12)
Ca2—O2^iv	2.539 (12)	O5—P1^viii	1.608 (12)
Ca2—P1^ii	3.477 (6)	O5—P2^iv	1.629 (12)
Ca2—P2	3.555 (7)	O6—Ca2^ii	2.377 (11)
Ca2—Ca1^ii	3.690 (6)	O6—Ca1^ii	2.556 (11)
Ca2—Ca1^iv	4.029 (6)	O7—P1^viii	1.482 (13)
P1—O7^v	1.482 (13)	OW1—Ca2^i	2.527 (12)
P1—O3	1.501 (11)	OW2—Ca2^ii	2.430 (14)
P1—O4^vi	1.520 (12)
O1^i—Ca1—O1^ii	162.1 (2)	P1^ii—Ca2—P2	106.86 (16)
O1^i—Ca1—O3	78.5 (4)	O4^iv—Ca2—Ca1^ii	135.7 (4)
O1^ii—Ca1—O3	84.1 (4)	O3^ii—Ca2—Ca1^ii	39.4 (3)
O1^i—Ca1—O2	75.2 (4)	O6^ii—Ca2—Ca1^ii	66.9 (3)
O1^ii—Ca1—O2	99.5 (4)	OW2^ii—Ca2—Ca1^ii	132.5 (4)
O3—Ca1—O2	85.2 (4)	OW3—Ca2—Ca1^ii	118.3 (3)
O1^i—Ca1—OW1	93.4 (4)	OW1^iv—Ca2—Ca1^ii	86.1 (3)
O1^ii—Ca1—OW1	101.4 (4)	O2^iv—Ca2—Ca1^ii	42.1 (3)
O3—Ca1—OW1	158.1 (4)	P1^ii—Ca2—Ca1^ii	57.38 (12)
O2—Ca1—OW1	73.0 (4)	P2—Ca2—Ca1^ii	50.99 (11)
O1^i—Ca1—O2^iii	106.0 (4)	O4^iv—Ca2—Ca1^iv	92.6 (3)
O1^ii—Ca1—O2^iii	73.9 (4)	O3^ii—Ca2—Ca1^iv	77.4 (3)
O3—Ca1—O2^iii	78.2 (4)	O6^ii—Ca2—Ca1^iv	113.2 (3)
O2—Ca1—O2^iii	162.65 (19)	OW2^ii—Ca2—Ca1^iv	167.8 (4)
OW1—Ca1—O2^iii	123.7 (4)	OW3—Ca2—Ca1^iv	102.6 (3)
O1^i—Ca1—O6^ii	77.9 (4)	OW1^iv—Ca2—Ca1^iv	35.0 (3)
O1^ii—Ca1—O6^ii	115.2 (4)	O2^iv—Ca2—Ca1^iv	34.4 (2)
O3—Ca1—O6^ii	120.9 (4)	P1^ii—Ca2—Ca1^iv	78.94 (12)
O2—Ca1—O6^ii	137.3 (4)	P2—Ca2—Ca1^iv	54.64 (12)
OW1—Ca1—O6^ii	76.2 (4)	Ca1^ii—Ca2—Ca1^iv	59.22 (7)
O2^iii—Ca1—O6^ii	58.2 (3)	O4^iv—Ca2—Ca1	72.6 (3)
O1^i—Ca1—P2^ii	93.1 (3)	O3^ii—Ca2—Ca1	111.1 (3)
O1^ii—Ca1—P2^ii	94.2 (3)	O6^ii—Ca2—Ca1	30.0 (3)
O3—Ca1—P2^ii	100.8 (3)	OW2^ii—Ca2—Ca1	78.2 (3)
O2—Ca1—P2^ii	165.6 (3)	OW3—Ca2—Ca1	153.3 (3)
OW1—Ca1—P2^ii	99.9 (3)	OW1^iv—Ca2—Ca1	133.3 (3)
O2^iii—Ca1—P2^ii	29.1 (3)	O2^iv—Ca2—Ca1	71.4 (2)
O6^ii—Ca1—P2^ii	29.2 (3)	P1^ii—Ca2—Ca1	130.70 (15)
O1^i—Ca1—P2	22.5 (3)	P2—Ca2—Ca1	49.41 (11)
O1^ii—Ca1—P2	151.6 (3)	Ca1^ii—Ca2—Ca1	81.13 (12)
O3—Ca1—P2	74.3 (3)	Ca1^iv—Ca2—Ca1	103.22 (13)
O2—Ca1—P2	97.0 (3)	O7^v—P1—O3	112.6 (7)
OW1—Ca1—P2	105.6 (3)	O7^v—P1—O4^vi	113.0 (7)
O2^iii—Ca1—P2	83.5 (3)	O3—P1—O4^vi	112.2 (7)
O6^ii—Ca1—P2	63.8 (3)	O7^v—P1—O5^v	105.8 (7)
P2^ii—Ca1—P2	72.50 (17)	O3—P1—O5^v	109.0 (7)
O1^i—Ca1—P1	90.2 (3)	O4^vi—P1—O5^v	103.4 (6)
O1^ii—Ca1—P1	72.0 (3)	O7^v—P1—Ca1	134.2 (5)
O3—Ca1—P1	21.5 (3)	O3—P1—Ca1	35.8 (5)
O2—Ca1—P1	70.6 (3)	O4^vi—P1—Ca1	111.1 (5)
OW1—Ca1—P1	141.1 (3)	O5^v—P1—Ca1	74.9 (4)
O2^iii—Ca1—P1	92.0 (3)	O7^v—P1—Ca2^ii	104.6 (5)
O6^ii—Ca1—P1	142.0 (3)	O3—P1—Ca2^ii	32.6 (5)
P2^ii—Ca1—P1	118.55 (18)	O4^vi—P1—Ca2^ii	88.9 (5)
P2—Ca1—P1	92.08 (16)	O5^v—P1—Ca2^ii	139.2 (5)
O1^i—Ca1—P2^i	84.8 (3)	Ca1—P1—Ca2^ii	64.43 (13)
O1^ii—Ca1—P2^i	85.0 (3)	O1^i—P2—O6	117.1 (7)
O3—Ca1—P2^i	68.1 (3)	O1^i—P2—O2^iv	115.7 (7)
O2—Ca1—P2^i	21.3 (3)	O6—P2—O2^iv	106.1 (7)
OW1—Ca1—P2^i	91.0 (3)	O1^i—P2—O5^i	102.3 (6)
O2^iii—Ca1—P2^i	141.9 (3)	O6—P2—O5^i	107.4 (6)
O6^ii—Ca1—P2^i	157.7 (3)	O2^iv—P2—O5^i	107.4 (6)
P2^ii—Ca1—P2^i	168.99 (14)	O1^i—P2—Ca1^ii	140.2 (5)
P2—Ca1—P2^i	103.33 (12)	O6—P2—Ca1^ii	54.5 (5)
P1—Ca1—P2^i	50.78 (13)	O2^iv—P2—Ca1^ii	51.7 (4)
O1^i—Ca1—P2^iii	157.8 (3)	O5^i—P2—Ca1^ii	117.4 (5)
O1^ii—Ca1—P2^iii	19.2 (3)	O1^i—P2—Ca1	37.8 (4)
O3—Ca1—P2^iii	93.3 (3)	O6—P2—Ca1	82.9 (5)
O2—Ca1—P2^iii	83.6 (3)	O2^iv—P2—Ca1	115.4 (5)
OW1—Ca1—P2^iii	86.6 (3)	O5^i—P2—Ca1	131.0 (5)
O2^iii—Ca1—P2^iii	92.2 (3)	Ca1^ii—P2—Ca1	107.50 (17)
O6^ii—Ca1—P2^iii	123.4 (3)	O1^i—P2—Ca1^iv	123.5 (5)
P2^ii—Ca1—P2^iii	108.87 (11)	O6—P2—Ca1^iv	117.8 (5)
P2—Ca1—P2^iii	167.53 (13)	O2^iv—P2—Ca1^iv	34.6 (4)
P1—Ca1—P2^iii	76.30 (15)	O5^i—P2—Ca1^iv	72.8 (4)
P2^i—Ca1—P2^iii	72.95 (16)	Ca1^ii—P2—Ca1^iv	70.24 (11)
O4^iv—Ca2—O3^ii	169.8 (5)	Ca1—P2—Ca1^iv	144.7 (2)
O4^iv—Ca2—O6^ii	100.5 (4)	O1^i—P2—Ca1^vii	31.6 (4)
O3^ii—Ca2—O6^ii	85.4 (4)	O6—P2—Ca1^vii	132.8 (5)
O4^iv—Ca2—OW2^ii	76.2 (4)	O2^iv—P2—Ca1^vii	119.5 (5)
O3^ii—Ca2—OW2^ii	113.6 (4)	O5^i—P2—Ca1^vii	71.3 (5)
O6^ii—Ca2—OW2^ii	74.1 (4)	Ca1^ii—P2—Ca1^vii	168.2 (2)
O4^iv—Ca2—OW3	99.7 (4)	Ca1—P2—Ca1^vii	67.69 (10)
O3^ii—Ca2—OW3	81.0 (4)	Ca1^iv—P2—Ca1^vii	107.05 (16)
O6^ii—Ca2—OW3	137.6 (5)	O1^i—P2—Ca2	82.9 (5)
OW2^ii—Ca2—OW3	75.2 (4)	O6—P2—Ca2	107.2 (5)
O4^iv—Ca2—OW1^iv	86.8 (4)	O2^iv—P2—Ca2	38.5 (4)
O3^ii—Ca2—OW1^iv	83.9 (4)	O5^i—P2—Ca2	137.4 (5)
O6^ii—Ca2—OW1^iv	148.1 (4)	Ca1^ii—P2—Ca2	66.75 (14)
OW2^ii—Ca2—OW1^iv	137.5 (5)	Ca1—P2—Ca2	77.26 (14)
OW3—Ca2—OW1^iv	69.7 (4)	Ca1^iv—P2—Ca2	69.58 (14)
O4^iv—Ca2—O2^iv	95.0 (4)	Ca1^vii—P2—Ca2	101.46 (16)
O3^ii—Ca2—O2^iv	77.8 (4)	P2^iv—O1—Ca1^iv	119.7 (6)
O6^ii—Ca2—O2^iv	79.1 (4)	P2^iv—O1—Ca1^ii	129.2 (6)
OW2^ii—Ca2—O2^iv	149.6 (4)	Ca1^iv—O1—Ca1^ii	107.5 (4)
OW3—Ca2—O2^iv	135.3 (4)	P2^i—O2—Ca1	124.1 (6)
OW1^iv—Ca2—O2^iv	69.3 (4)	P2^i—O2—Ca1^vii	99.3 (6)
O4^iv—Ca2—P1^ii	156.4 (3)	Ca1—O2—Ca1^vii	103.0 (4)
O3^ii—Ca2—P1^ii	20.1 (3)	P2^i—O2—Ca2^i	119.4 (6)
O6^ii—Ca2—P1^ii	103.2 (3)	Ca1—O2—Ca2^i	109.1 (4)
OW2^ii—Ca2—P1^ii	109.5 (3)	Ca1^vii—O2—Ca2^i	94.6 (4)
OW3—Ca2—P1^ii	61.5 (3)	P1—O3—Ca2^ii	127.3 (7)
OW1^iv—Ca2—P1^ii	73.6 (3)	P1—O3—Ca1	122.7 (7)
O2^iv—Ca2—P1^ii	90.3 (3)	Ca2^ii—O3—Ca1	101.9 (4)
O4^iv—Ca2—P2	85.1 (3)	P1^vi—O4—Ca2^i	140.3 (7)
O3^ii—Ca2—P2	90.4 (3)	P1^viii—O5—P2^iv	134.1 (7)
O6^ii—Ca2—P2	61.7 (3)	P2—O6—Ca2^ii	137.5 (7)
OW2^ii—Ca2—P2	127.6 (3)	P2—O6—Ca1^ii	96.4 (6)
OW3—Ca2—P2	157.1 (3)	Ca2^ii—O6—Ca1^ii	122.3 (4)
OW1^iv—Ca2—P2	88.4 (3)	Ca1—OW1—Ca2^i	108.5 (5)
O2^iv—Ca2—P2	22.1 (2)

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