AgNi₃(PO₄)(HPO₄)₂: an alluaudite-like structure

The new title phosphate, silver trinickel phosphate bis(hydrogenphosphate), has been synthesized by the hydro­thermal method. It has an alluaudite-like structure but shows some differences owing to the presence of the H atoms. The structure is isomorphous with the compounds of general formula AM₃(XO₄)(HXO₄)₂ (A is Na or Ag, M is Co, Zn or Mn, and X is As or P), with the Ag atom, one Ni atom and one P atom lying on twofold axes.

S1. Comment

Open-framework structures built up from MO₆ octahedra and XO₄ and/or X₂O₇ groups (M is a transition metal, X is P or As), with alkali or pseudo-alkali metals, are currently of great interest because of their potential applications in the fields of ion exchange, ionic conductivity etc. (Daidouh et al., 1997; Pintard-Scrépel et al., 1978; Winand et al., 1990; D'Yvoire et al., 1993; Couturier et al., 1991; Gueho et al., 1993; Haushalter, 1990; Piffard et al., 1985). During our recent investigation of the nickel system A₂O-NiO-X₂O₅ (A is an alkali or pseudo-alkali metal), four compounds with mixed open frameworks were found, namely K₄Ni₇(AsO₄)₆ (Ben Smail et al., 1999), KNi₃AsO₄As₂O₇ (Ben Smail & Jouini, 2000), AgNiPO₄ (Ben Smail & Jouini, 2002) and the title compound, AgNi₃(PO₄)(HPO₄)₂, which is a new open-framework phosphate. The present paper deals with the synthesis and structure determination of AgNi₃(PO₄)(HPO₄)₂.

The structure is built up from NiO₆ octahedra, and PO₄ and PO₃(OH) tetrahedra, sharing corners and edges to form a three-dimensional framework. Two types of six-sided tunnels running along the c axis are found. One tunnel, at (1/2, 0, z), is delimited by six octahedra, where Ag atoms reside, while the other tunnel, at (0, 0, z), is delimited by four octahedra and two tetrahedra. The OH groups point into this tunnel (Fig. 1).

The extended structure can be seen as parallel sheets, oriented perpendicular to the [010] direction, linked via O5 atoms and O6—H···O1 hydrogen bonds. Each sheet consists of [Ni₃O₁₂]_∞ infinite chains of edge-sharing Ni1O₆—Ni2O₆—Ni2O₆ octahedral units running along the [101] direction. Equivalent chains are linked together in the [101] direction by the phosphate tetrahedra (Fig. 2).

The title compound is isotypic with the following compounds: AgCo₃(PO₄)(HPO₄)₂ (Guesmi & Driss, 2001), AgCo₃(AsO₄)(HAsO₄)₂ and AgZn₃(AsO₄)(HAsO₄)₂ (Keller et al., 1981), NaCo₃(AsO₄)(HAsO₄)₂ and NaCo₃(PO₄)(HPO₄)₂ (Lii & Shih, 1994), and NaMn₃(PO₄)(HPO₄)₂ (Leroux et al., 1995). The structures of these compounds are related to the alluaudite structure type, X2X1M1M2(PO₄)₃ (Yakubovich et al., 1977; Moore, 1971), but there are important differences. The presence of H atoms and their need to form O—H bonds leads to a split of the (1/2, 1/2, 0) (X2) site into two H-atom positions. In addition, the X1 site is empty; it is replaced by the (0, ≈ 1/2, 1/4) position, which is occupied by monovalent Ag⁺ or Na⁺ cations.

The two Ni atoms are both octahedrally coordinated by O-atom neighbours, with average Ni—O bond distances of 2.073 (2) Å for Ni2 and 2.123 (2) Å for Ni1. The P—O bond lengths are in the range 1.533 (2)–1.578 (3) Å. The longest bond, at 1.578 (3) Å, occurs with the O6 atom, which is involved in the O6—H···O1 hydrogen bond. These values are in good agreement with those observed in other open-framework nickel phosphates (Abrahams & Eason, 1993; Jouini & Dabbabi, 1986; Erragh et al., 1998; Nord, 1983; Boudjada et al., 1978; Calvo & Faggiani, 1975; Hamanaka & Imoto, 1998).

The silver coordination in the title compound is very similar to that shown previously by arsenate (Keller et al., 1981) and phosphate (Guesmi & Driss, 2002) structures. The Ag⁺ cation forms a square plane, with Ag—O distances ranging from 2.364 (3) to 2.501 (3) Å.

S2. Experimental

Crystals of AgNi₃(PO₄)(HPO₄)₂ were grown hydrothermally using a mixture of AgNO₃ (2 g; Fluka, 99%), Ni(NO₃)₂6H₂O (2.01 g; Fluka, 99%), H₃PO₄ (1 ml; Prolabo, 85%, density 1.70 Mg m ^-3) and distilled water (5 ml). The solution was heated in a sealed tube at 623 K for three weeks, followed by normal cooling to room temperature. Two types of crystals were formed, yellow-brown and green parallelepiped crystals. The green crystals were identified as AgNiPO₄ by X-ray studies, the yellow-brown crystals as AgNi₃(PO₄)(HPO₄)₂.

S3. Refinement

The unique H atom was located from a difference Fourier map and refined isotropically.

Computing details

Data collection: CAD-4 EXPRESS (Duisenberg, 1992; Macíček & Yordanov, 1992); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 1998); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A projection of the structure of AgNi3(PO4)(HPO4)2 along the c axis. O3* is the sixth vertex of Ni2O6 lying behind the equatorial plane.
	Fig. 2. A projection of a sheet of the anionic framework of AgNi3(PO4)(HPO4)2 along the b axis, with 75% probability displacement ellipsoids.

Silver trinickel phosphate bis(hydrogenphosphate)

Crystal data

AgH2Ni3O12P3	F(000) = 1096
Mr = 570.93	Dx = 4.458 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -C 2yc	Cell parameters from 25 reflections
a = 11.865 (4) Å	θ = 6.6–14.9°
b = 12.117 (3) Å	µ = 9.45 mm^−1
c = 6.467 (2) Å	T = 293 K
β = 113.82 (3)°	Parallelepiped, brown-yellow
V = 850.6 (4) Å^3	0.50 × 0.20 × 0.16 mm
Z = 4

Data collection

Enraf-Nonius CAD-4 diffractometer	916 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.024
Graphite monochromator	θmax = 27.0°, θmin = 2.5°
ω/2θ scans	h = −15→13
Absorption correction: ψ scan (North et al., 1968)	k = 0→15
Tmin = 0.119, Tmax = 0.221	l = 0→8
1012 measured reflections	2 standard reflections every 120 min
928 independent reflections	intensity decay: 1.0%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.020	All H-atom parameters refined
wR(F^2) = 0.051	w = 1/[σ^2(Fo^2) + (0.0174P)^2 + 8.9075P] where P = (Fo^2 + 2Fc^2)/3
S = 1.21	(Δ/σ)max = 0.004
928 reflections	Δρmax = 0.87 e Å^−3
93 parameters	Δρmin = −1.07 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0112 (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.

The Ni and P atoms were located by the direct method; the remaining atoms were located by successive difference Fourier maps.

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

	x	y	z	Uiso*/Ueq
Ag	0.0000	0.46944 (4)	0.2500	0.01576 (15)
Ni1	0.5000	0.27617 (5)	0.2500	0.00488 (16)
Ni2	0.28914 (4)	0.16290 (3)	0.37743 (7)	0.00391 (15)
P1	0.0000	0.18445 (10)	0.2500	0.0031 (2)
P2	0.22359 (8)	0.38701 (7)	0.11454 (14)	0.00311 (18)
O1	0.1079 (2)	0.10763 (19)	0.2648 (4)	0.0063 (5)
O2	0.0354 (2)	0.25737 (19)	0.4643 (4)	0.0050 (5)
O3	0.3430 (2)	0.17260 (19)	0.1127 (4)	0.0052 (5)
O4	0.2178 (2)	0.31833 (19)	0.3092 (4)	0.0052 (5)
O5	0.3362 (2)	0.00011 (19)	0.3945 (4)	0.0069 (5)
O6	0.3631 (2)	0.4049 (2)	0.1578 (4)	0.0057 (5)
H	0.371 (9)	0.480 (8)	0.176 (17)	0.10 (3)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ag	0.0063 (2)	0.0284 (3)	0.0109 (2)	0.000	0.00180 (16)	0.000
Ni1	0.0046 (3)	0.0053 (3)	0.0056 (3)	0.000	0.0029 (2)	0.000
Ni2	0.0041 (2)	0.0036 (2)	0.0048 (2)	0.00018 (15)	0.00269 (17)	0.00007 (15)
P1	0.0019 (5)	0.0036 (5)	0.0038 (5)	0.000	0.0009 (4)	0.000
P2	0.0036 (4)	0.0020 (4)	0.0042 (4)	−0.0003 (3)	0.0021 (3)	−0.0003 (3)
O1	0.0031 (11)	0.0039 (11)	0.0126 (12)	0.0006 (8)	0.0037 (9)	−0.0011 (9)
O2	0.0058 (11)	0.0062 (11)	0.0035 (10)	−0.0002 (9)	0.0023 (9)	−0.0006 (8)
O3	0.0061 (11)	0.0069 (11)	0.0042 (11)	−0.0015 (8)	0.0037 (9)	−0.0011 (8)
O4	0.0067 (11)	0.0055 (11)	0.0048 (11)	−0.0007 (9)	0.0037 (9)	0.0009 (8)
O5	0.0063 (11)	0.0040 (11)	0.0108 (12)	0.0004 (9)	0.0039 (10)	0.0003 (9)
O6	0.0037 (11)	0.0034 (11)	0.0112 (12)	0.0004 (9)	0.0041 (9)	−0.0010 (9)

Geometric parameters (Å, º)

Ag—O5^i	2.364 (3)	Ni2—O4^i	2.074 (2)
Ag—O5^ii	2.364 (3)	Ni2—O1	2.083 (2)
Ag—O5^iii	2.501 (3)	Ni2—O2^i	2.144 (3)
Ag—O5^iv	2.501 (3)	P1—O2	1.551 (2)
Ni1—O2^i	2.093 (2)	P1—O2^vii	1.551 (2)
Ni1—O2^v	2.093 (2)	P1—O1^vii	1.554 (2)
Ni1—O3	2.121 (2)	P1—O1	1.554 (2)
Ni1—O3^vi	2.121 (2)	P2—O5^iv	1.533 (2)
Ni1—O6^vi	2.155 (2)	P2—O4	1.534 (2)
Ni1—O6	2.155 (2)	P2—O3^viii	1.539 (2)
Ni2—O4	2.039 (2)	P2—O6	1.578 (3)
Ni2—O5	2.041 (2)	O6—H	0.9 (1)
Ni2—O3	2.058 (2)
O5^i—Ag—O5^ii	162.04 (12)	O4—Ni2—O4^i	86.18 (10)
O5^i—Ag—O5^iii	94.07 (9)	O5—Ni2—O4^i	99.85 (10)
O5^ii—Ag—O5^iii	83.26 (8)	O3—Ni2—O4^i	162.69 (10)
O5^i—Ag—O5^iv	83.26 (8)	O4—Ni2—O1	86.63 (10)
O5^ii—Ag—O5^iv	94.07 (9)	O5—Ni2—O1	85.96 (10)
O5^iii—Ag—O5^iv	162.91 (11)	O3—Ni2—O1	110.55 (10)
O2^i—Ni1—O2^v	157.61 (13)	O4^i—Ni2—O1	86.23 (10)
O2^i—Ni1—O3	78.43 (9)	O4—Ni2—O2^i	85.73 (10)
O2^v—Ni1—O3	88.33 (9)	O5—Ni2—O2^i	102.59 (10)
O2^i—Ni1—O3^vi	88.33 (9)	O3—Ni2—O2^i	78.67 (10)
O2^v—Ni1—O3^vi	78.43 (9)	O4^i—Ni2—O2^i	84.14 (9)
O3—Ni1—O3^vi	107.44 (13)	O1—Ni2—O2^i	168.08 (9)
O2^i—Ni1—O6^vi	107.08 (9)	O2—P1—O2^vii	110.57 (19)
O2^v—Ni1—O6^vi	89.27 (9)	O2—P1—O1^vii	108.37 (13)
O3—Ni1—O6^vi	168.70 (9)	O2^vii—P1—O1^vii	111.54 (13)
O3^vi—Ni1—O6^vi	82.86 (9)	O2—P1—O1	111.54 (13)
O2^i—Ni1—O6	89.27 (9)	O2^vii—P1—O1	108.37 (13)
O2^v—Ni1—O6	107.08 (9)	O1^vii—P1—O1	106.40 (19)
O3—Ni1—O6	82.86 (9)	O5^iv—P2—O4	110.04 (14)
O3^vi—Ni1—O6	168.70 (9)	O5^iv—P2—O3^viii	109.38 (14)
O6^vi—Ni1—O6	87.30 (13)	O4—P2—O3^viii	110.67 (14)
O4—Ni2—O5	170.12 (10)	O5^iv—P2—O6	108.69 (14)
O4—Ni2—O3	90.49 (9)	O4—P2—O6	108.70 (14)
O5—Ni2—O3	85.99 (10)	O3^viii—P2—O6	109.32 (14)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O6—H···O1^iv	0.9 (1)	1.5 (1)	2.503 (3)	174 (10)

Symmetry code: (iv) −x+1/2, y+1/2, −z+1/2.