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Acta Crystallogr C. 2010 March 15; 66(Pt 3): i33–i36.
Published online 2010 February 3. doi:  10.1107/S0108270110001344
PMCID: PMC2855575

The solid solution (Fe0.81Al0.19)(H2PO4)3 with a strong hydrogen bond


Single crystals of the solid solution iron aluminium tris(dihydrogenphosphate), (Fe0.81Al0.19)(H2PO4)3, have been pre­pared under hydro­thermal conditions. The compound is a new monoclinic variety (γ-form) of iron aluminium phosphate (Fe,Al)(H2PO4)3. The structure is based on a two-dimensional framework of distorted corner-sharing MO6 (M = Fe, Al) polyhedra sharing corners with PO4 tetra­hedra. Strong hydrogen bonds between the OH groups of the H2PO4 tetra­hedra and the O atoms help to consolidate the crystal structure.


Microporous materials find their origin in the discovery by Crönsted during the thirteenth century of the zeolitic property of the mineral stilbite. The zeolite family is made up of the aluminosilicate minerals with formula {M n+ x/n[(AlO2)x(SiO2)y]x·wH2O}, where x indicates the number of M n+ cations necessary to compensate the negative charge of the whole framework. All these phases exhibit three-dimensional structures built up exclusively from corner-sharing TO4 (T = Al, Si) tetra­hedra, defining tunnels in which the M n+ cations and water mol­ecules are located. Wilson et al. (1982 [triangle]) discovered a new family of compounds, the microporous alumino­phosphates. Since 1992, the research groups of Cavellec (Cavellec et al., 1995 [triangle]; Cavellec, Riou & Férey, 1997 [triangle]; Cavellec, Férey & Grenèche, 1997 [triangle]; Riou-Cavellec et al., 1998 [triangle]) have been inter­ested in the synthesis of these microporous materials. This work was followed by studies of microporous oxides by several groups (Debord et al., 1997 [triangle]; Lii & Huang, 1997a [triangle],b [triangle],c [triangle]; Huang et al., 1998 [triangle]; Zima et al., 1998 [triangle]; Zima & Lii, 1998 [triangle]). Microporous materials derived from octa­hedral and tetra­hedral frameworks currently boast an extensive chemistry and a number of them display useful properties as catalysts, sorbents and ionic exchangers (Davis & Lobo, 1992 [triangle]; Breck, 1974 [triangle]; Venuto, 1994 [triangle]).

Two polymorphs of Al(H2PO4)3 have been reported to date. The α-form is hexa­gonal with cell parameters a = 7.849 (1) Å and c = 24.87 (3) Å (Yoire, 1961 [triangle]), and the hexa­gonal β-form has parameters a = 13.69 (1) Å and c = 9.135 (1) Å (Yoire, 1961 [triangle]), also found by Brodalla et al. (1981 [triangle]). The α-form is isostructural with Fe(H2PO4)3 (Baies et al., 2006 [triangle]) and consists of a three-dimensional framework of corner-sharing FeO6 and PO2(OH)2 tetra­hedra. The synthesis of a new monoclinic variety of iron aluminium phosphate, (Fe0.81Al0.19)(H2PO4)3 (γ-form), is reported in this work.

(Fe0.81Al0.19)(H2PO4)3 is composed of a highly puckered sheet structure containing inter­connected M 2P2 units (M = Fe, Al) connected laterally by Fe–O–P mixed bridges to form two-dimensional layers perpendicular to the b axis (Fig. 1 [triangle]). The oligomeric M 2P2 units are built up from alternating corner sharing of octa­hedral MO6 and tetra­hedral PO4 units. The MO6 octa­hedra share six O atoms with adjacent P atoms, whereas the PO4 tetra­hedra share only two O atoms. The projection of the sheet is shown in Fig. 2 [triangle], viewed down the [010] axis. The M—O distances in (Fe0.81Al0.19)(H2PO4)3 have values inter­mediate between 1.944 (4) and 2.063 (4) Å (Table 1 [triangle]), consistent with the occupation of Fe and Al valencies in these sites. The inter­atomic angles reveal distortions of the octa­hedra, varying from O6—Fe1—O2(x, y, z − 1) = 86.66 (17)° to O1—Fe1—O6 = 178.84 (18)°. The dihydrogen phosphate ions, [H2PO4], can be described as slightly distorted tetra­hedra, with a mean value for the P—OH bond distances of 1.578 Å and with P=O bond distances ranging from 1.504 (4) to 1.525 (4) Å. The O—P—O angles are in the range 101.6 (3)–118.1 (2)°.

Figure 1
The structure of (Fe0.81 Al0.19)(H2PO4)3. Dashed lines show the intra- and inter-layer hydrogen bonds.
Figure 2
The sheet structure of (Fe0.81Al0.19)(H2PO4)3, viewed along the b axis, showing the oligomeric M 2P2 units (M = Fe, Al) connected laterally by Fe–O–P mixed bridges. The labels show the sites of the elements in the structure. ...
Table 1
Selected geometric parameters (Å, °)

The crystal structure of (Fe0.81Al0.19)(H2PO4)3 is characterized by an extended hydrogen-bonding network. The layers are held together through strong hydrogen bonds between the terminal O atoms attached to the two-connected phosphate groups in adjacent layers. Analysis of the hydrogen bonds in (Fe0.81Al0.19)(H2PO4)3 shows two different types of P—O—H(...)O—P bridges. Within the layer, adjacent [H2PO4] ions are connected into chains by short hydrogen bonds (Table 2 [triangle]) with an O(...)O distance of 2.614 (5) Å formed by one of the hydroxy groups, O3—H3(...)O6(x − An external file that holds a picture, illustration, etc.
Object name is c-66-00i33-efi1.jpg, −y + An external file that holds a picture, illustration, etc.
Object name is c-66-00i33-efi2.jpg, z + An external file that holds a picture, illustration, etc.
Object name is c-66-00i33-efi1.jpg). Adjacent layers are linked by longer hydrogen bonds, viz. O4—H4(...)O4(x, −y + 1, z + An external file that holds a picture, illustration, etc.
Object name is c-66-00i33-efi1.jpg) [2.775 (6) Å], O12—H12(...)O12(x, −y + 2, z − An external file that holds a picture, illustration, etc.
Object name is c-66-00i33-efi1.jpg) [3.055 (8) Å] and O11—H11(...)O12(x, −y + 2, z + An external file that holds a picture, illustration, etc.
Object name is c-66-00i33-efi1.jpg) [3.220 (7) Å], which allow the layers to connect as observed in Fig. 1 [triangle].

Table 2
Hydrogen-bond geometry (Å, °)

A comparison between (Fe0.81Al0.19)(H2PO4)3 and the series of compounds (NH4, H3O, K)(Fe,Al)3(HPO4)2(H2PO4)6·4H2O and (C6H8N)[Al2P3O10(OH)2] is shown in Fig. 3 [triangle]. Detailed descriptions of their topology are also reported here. The aim of this comparison is to provide a review of possible approaches that can be used to establish the topology of microporous structures. For obvious reasons, we do not consider related octa­hedral–tetra­hedral frameworks here. Most attention will be focused on network topology and the possibility of inter­calating alkaline cations or organic mol­ecules in the solid-state inorganic framework, which is important for both mineralogy and material sciences.

Figure 3
(a) The stacking sheets of (Fe0.81Al0.19)(H2PO4)3. (b) The structure of the series of compounds (NH4, H3O, K)(Fe,Al)3(HPO4)2(H2PO4)6·4H2O, with the water mol­ecules lying in the inter­layer space. (c) The two-dimensional layered ...

(Fe0.81Al0.19)(H2PO4) is considered a normal solid-state inorganic framework. A comparison between this compound and the series of compounds (NH4, H3O, K)(Fe,Al)3(HPO4)2(H2PO4)6·4H2O is shown in Figs. 3 [triangle](a) and (b). The common characteristic of these compounds is their bi­dimensionality. In (NH4, H3O, K)(Fe,Al)3(HPO4)2(H2PO4)6·4H2O, the NH4 +, H3O+ and K+ cations are located inside 12-sided polyhedra, which are generated by the corner-sharing MO6 (M = Fe, Al) and [H2PO4] units, while water mol­ecules are located in the inter­layer space (Mgaidi et al., 1999 [triangle]; Bosman et al., 1986 [triangle]; Anisimova et al., 1997 [triangle]). Figs. 3 [triangle](a) and (c) show the comparison between (Fe0.81Al0.19)(H2PO4)3 and the two-dimensional layered compound (C6H8N)[Al2P3O10(OH)2]. This structure contains macroanionic [Al2P3O10(OH)2] sheets that are charge-balanced by protonated 4-methyl­pyridine. The inorganic layers are constructed from alternating Al-centred units (AlO4 and AlO5) and P-centred units [PO4, PO3(OH) and PO2(=O)(OH)] with triply and doubly bridging phosphate groups (Yu et al., 2000 [triangle]). This comparison provides an example of the concept of scale chemistry (Férey, 2000 [triangle]). The cavities created by the framework, which are very small in typical solid-state inorganic frameworks and only able to accept alkaline cations or organic mol­ecules, become larger and larger.


The title compound was prepared from a reaction mixture of H3PO4 (4 mmol), FeO (5 mmol) and Al2O3 (5 mmol) in water (approximately 6 ml). The starting mixture was sealed inside a 23 ml Teflon-lined stainless steel Parr autoclave under autogenous pressure, filled to approximately 25% volume capacity, and the reactants were stirred briefly before heating. The reaction mixture was heated at 343 K for 3 d to obtain (Fe0.81Al0.19)(H2PO4)3, followed by slow cooling to room temperature. The product was filtered off, washed with de­ionized water and dried in air. A needle-shaped single crystal of (Fe0.81Al0.19)(H2PO4)3 was selected under a polarizing microscope.

Crystal data

  • (Fe0.81Al0.19)(H2PO4)3
  • M r = 341.32
  • Monoclinic, An external file that holds a picture, illustration, etc.
Object name is c-66-00i33-efi7.jpg
  • a = 11.700 (1) Å
  • b = 15.590 (1) Å
  • c = 5.030 (1) Å
  • β = 98.00 (1)°
  • V = 908.6 (2) Å3
  • Z = 4
  • Mo Kα radiation
  • μ = 1.98 mm−1
  • T = 293 K
  • 0.15 × 0.15 × 0.1 mm

Data collection

  • Enraf–Nonius CAD-4 diffractometer
  • Absorption correction: ψ scan (North et al., 1968 [triangle]) T min = 0.703, T max = 0.754
  • 1104 measured reflections
  • 1104 independent reflections
  • 1043 reflections with I > 2σ(I)
  • 2 standard reflections every 120 min intensity decay: 0.4%


  • R[F 2 > 2σ(F 2)] = 0.029
  • wR(F 2) = 0.074
  • S = 1.04
  • 1104 reflections
  • 166 parameters
  • 10 restraints
  • H atoms treated by a mixture of independent and constrained refinement
  • Δρmax = 0.55 e Å−3
  • Δρmin = −0.64 e Å−3
  • Absolute structure: Flack (1983 [triangle]), with 1104 Friedel pairs
  • Flack parameter: 0.00 (3)

During refinement, the occupancy of the Fe site exhibited a significant deviation from full occupancy, indicating a substitution with Al; the final occupancies were constrained to sum to 1.0 and refined to 0.807 (7) and 0.193 (7), respectively, for Fe1 and Al1. The positions of all H atoms were located from a difference electron-density map and were then refined with an O—H bond-length restraint of 0.95 (5) Å and with U iso(H) fixed at a value of 0.05 Å2.

Data collection: CAD-4 EXPRESS (Duisenberg, 1992 [triangle]; Macíček & Yordanov, 1992 [triangle]); cell refinement: CAD-4 EXPRESS; data reduction: MolEN (Fair, 1990 [triangle]); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 [triangle]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 [triangle]); molecular graphics: DIAMOND (Brandenburg, 1999 [triangle]); software used to prepare material for publication: WinGX (Farrugia, 1999 [triangle]).

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S0108270110001344/fn3041sup1.cif

Structure factors: contains datablocks I. DOI: 10.1107/S0108270110001344/fn3041Isup2.hkl


Supplementary data for this paper are available from the IUCr electronic archives (Reference: FN3041). Services for accessing these data are described at the back of the journal.


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