On the crystal structure and thermal decomposition of ammonium-iron(III) bis(hydrogenphosphate)

Article abstract of DOI:10.1039/b912427f

NH₄Fe(HPO₄)₂ and its deuterated form have been synthesized as monophasic polycrystalline materials. Their crystal structures, including hydrogen positions, were determined by Rietveld refinement and Fourier synthesis using

Full text of DOI:10.1039/b912427f

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PAPER
www.rsc.org/dalton | Dalton Transactions
On the crystal structure and thermal decomposition of ammonium-iron(III)
bis(hydrogenphosphate)†
Bele´n F. Alfonso,a Jesu´s A. Blanco,a Ma Teresa Ferna´ndez-D´ıaz,b Camino Trobajo,c Sergei A. Khainakovc and
Jose´ R. Garc´ıac
Received 24th June 2009, Accepted 18th November 2009
First published as an Advance Article on the web 11th January 2010
DOI: 10.1039/b912427f
NH4Fe(HPO4)2 and its deuterated form have been synthesized as monophasic polycrystalline materials.
Their crystal structures, including hydrogen positions, were determined by Rietveld refinement and
Fourier synthesis using constant-wavelength neutron powder diffraction data. In addition, the thermal
decomposition of NH4Fe(HPO4)2 was found to give mixtures of Fe4(P2O7)3 and Fe(PO3)3 via
NH4FeP2O7 formation, the crystal structure of which has also been refined from X-ray powder
diffraction data.
which such atoms can be located. This problem can be overcome
Introduction
by the use of deuterated samples.
Interest in phosphate-type materials has been recently renewed
The first part of this paper reports the neutron powder
by the success of synthesizing organic–inorganic hybrid nan-
diffraction data (a single crystal with adequate size has yet
otubes based on the g-titanium phosphate layered-type crystal
structure.1 Hydrogen is a key element whose functionalities
stretch to many inorganic compounds. Determining the role
of hydrogen in these materials is of paramount importance as
it often governs their physical-chemical properties and, there-
fore, their potential applications. Very recently, the synthesis
of several iron hydrogen phosphates have been reported by
Redrup and Weller.2 One of these compounds, ammonium-
iron(III) bis(hydrogenphosphate), NH4Fe(HPO4)2, formulated by
to be synthesized) of the fully deuterated ammonium-iron(III)
bis(hydrogenphosphate), while the second part is devoted to the
thermal decomposition of NH4Fe(HPO4)2.
Experimental
Synthesis
The synthesis of polycrystalline ND4Fe(DPO4)2 was carried out
via a hydrothermal route. The reagents used were: FeCl3 (Riedel-
of Nae¨n), D3PO4 (Aldrich, 85 wt% solution in D2O), (ND2)2CO
(Aldrich, 98% atom D) and D2O (Merck 99.8%). 3.246 g of FeCl3
were dissolved in 20 mL of D2O and mixed with D3PO4 and
(ND2)2CO in a molar ratio C/P = 0.25 and P/Fe = 15. The total
volume of the reaction mixture was 40 mL. Mixing was carried out
in a stainless steel (100 mL) Teflon-lined vessel under autogenous
pressure. The autoclave was sealed and heated to 180 ◦C for 7 days.
The solid product was filtered off, thoroughly washed with an
excess of D2O and dried in air at room temperature.
¯
these authors as (NH4)3Fe3(HPO4)6 (triclinic, P1 space group),
was previously hydrothermally synthesized,3 and its magnetic
properties4 were discussed at the beginning of this decade. The
body-centered unit cell (described with no conventional space
¯
group with a triclinic crystal structure I1 reported by Yakubovich
from single crystal X-ray diffraction data5) was transformed to a
¯
reduced primitive cell (P1 space group).
The structural data reported in ref. 2 and 5 coincide both in
terms of the framework description and the nitrogen positions,
but differ as regards the location of their hydrogen atoms. Whereas
direct visualization of hydrogen atoms using X-ray diffraction
is difficult due to the weak scattering, neutron diffraction is
a powerful tool for locating light atoms, especially hydrogen.
Unfortunately, the large incoherent scattering of hydrogen intro-
duces systematic errors (leading to high backgrounds, very long
counting times, and noisy data sets) that limit the precision with
NH4Fe(HPO4)2 was synthesized under hydrothermal conditions
as reported in the literature,3 while NH4FeP2O7 was obtained by
heating NH4Fe(HPO4)2 to 400 ◦C under an air atmosphere for
twelve hours.
Materials and methods
The phosphorus and iron contents of the solids were determined
with a SpectraSpectrometer DCP-AEC after dissolving a weighted
amount of sample in HF(aq). Microanalytical data (C, N and
H) were obtained with a Perkin-Elmer model 2400B. Elemental
analysis (w.t.%) for: ND4Fe(DPO4)2 [Fe 20.8, P 22.7, N 5.3 (calc.:
Fe 20.53, P 22.81, N 5.15)], NH4Fe(HPO4)2 [Fe 21.4, P 23.1,
N 5.4 (calc.: Fe 21.01, P 23.32, N 5.26)] and NH4FeP2O7 [Fe 22.7,
P 24.8, N 5.3 (calc.: Fe 22.53, P 25.01, N 5.65)].
aDepartamento de F´ısica, Universidad de Oviedo, 33007, Oviedo, Spain.
E-mail: mbafernandez@uniovi.es; Fax: +34 985182390; Tel: +34
985182080
bInstitute Laue-Langevin, B. P. 156X, F-38042, Grenoble, France. E-mail:
ferndiaz@ill.fr; Fax: +33(0) 4 7620 7648; Tel: +33 (0) 4 7620 7606
cDepartamento de Qu´ımica Orga´nica e Inorga´nica, Universidad de Oviedo,
33006, Oviedo, Spain. E-mail: jrgm@uniovi.es; Fax: +34 985103436;
Tel: +34 985103030
† CCDC reference numbers 737439, 737440, 751864 and 751865. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b912427f
Thermogravimetric analysis was carried out on a Mettler-
Toledo TGA/SDTA851e in a dynamic oxygen atmosphere
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Dalton Trans., 2010, 39, 1791–1796 | 1791
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(50 mL min-1) at a heating rate of 10 ◦C min-1. In the TG tests, a
Pfeiffer Vacuum ThermoStarTM GSD301T mass spectrometer was
used to determine the evacuated vapours. The masses 18 (H2O) and
15 (NH3) were tested using a C-SEM detector, operating at 1200 V,
with a time constant of 1 s. For NH4Fe(HPO4)2, after treatment up
to 900 ◦C, the residual solid is a mixture of two crystalline phases,
Fe4(P2O7)3 and Fe(PO3)3.
Table 1 Crystal data and structure refinements
ND4Fe(DPO4)2
NH4FeP2O7
Empirical formula
Crystal system
Space group
Z
FeP2O8ND6
Triclinic
P1 (No. 2)
1
Neutron
1.59
7.11830(3)
8.83828(4)
9.46407(4)
64.5802(4)
70.3127(4)
69.5733(5)
491.495(4)
5–160
FeP2O7NH4
Monoclinic
P21/c (No. 14)
4
¯
Radiation probe
X-ray
˚
l/A
1.5418
7.5219(2)
10.0018 (3)
8.2729(3)
90
105.887(3)
90
598.62(3)
10–140
0.0131
44
˚
a/A
Neutron and X-ray crystallographic analysis
˚
b/A
˚
For ND4Fe(DPO4)2, the powder neutron diffraction experiments
were performed at the Institute Laue Langevin (ILL), Greno-
ble (France), using the high-resolution two-axis diffractometer
(D2B) a description of which is given on the ILL web site
(http://www.ill.eu). The diffraction patterns were recorded for
a 2q range of 5–160◦, with a step size of 0.05◦. The measurements
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
2q range (◦)
Step
0.05
Parameters
Rp (%)
121
3.30
4.28
2.88
˚
were carried out at 50 K using a neutron wavelength l = 1.59 A.
2.81
4.07
5.45
4.73
Approximately 5 g of ND4Fe(DPO4)2 contained in a cylindrical
vanadium sample-holder was measured in an orange liquid-
helium cryostat. The FULLPROF program6 was employed to
the refinement by the Rietveld method to analyze the powder
patterns. A pseudo-Voigt function was chosen to generate the
line shape of the Bragg diffraction peaks. In the final run of the
analysis of patterns, the following parameters were refined: zero-
point, half-width, Pseudo-Voigt, isotropic thermal displacements,
and asymmetry parameters describing the line shape of the
Bragg diffraction peaks, scale factor, atom positions and unit
cell parameters. The final Rietveld plot is shown in Fig. 1.
Crystallographic parameters are shown in Table 1. Final atomic
positions are shown in Table 2. Selected bond distances and angles
are shown in Table 3.
Rwp (%)
RF (%)
2
c
2.27
˚
Cu-Ka radiation (l = 1.5418 A). Data were collected at ambient
temperature over the angular 2q range 10–140◦, with a step size
of 0.0131◦. Indexing was performed with ITO12, TREOR90
˚
and DICVOL04. The monoclinic cell with a = 8.27 A, b =
◦
˚
˚
10.01 A, c = 7.52 A, and b = 105.9 (M20 = 67.4; 35.0; 51.0
in the corresponding program) was found and both chemical
composition and behaviour suggest that its structure is related
to KFeP2O7 (ICSD-202814). Rietveld refinement of the pattern
using the atomic coordinates from the isostructural KFeP2O7
compound with nitrogen replacing potassium was carried out
using the GSAS program (Fig. 2). Its monoclinic structure was
refined with the space group P21/c (No. 14) with the following
For NH4FeP2O7, powder X-ray diffraction patterns were col-
lected on X’Pert PRO MPD X-ray diffractometer with PIXcel
detector, operating in the Bragg-Brentano (q/2q) geometry,
Fig. 1 Powder neutron diffraction pattern of ND4Fe(DPO4)2. Observed (points), calculated (continuous line) and difference (below) profiles are plotted
on the same scale. Bragg peaks are indicated by tick marks. Background has been subtracted for more clarity. The lower Bragg position, around 2q =
151◦, is due to aluminium from the cryostat.
1792 | Dalton Trans., 2010, 39, 1791–1796
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Table 2 Final atomic positions and isotropic displacement (temperature)
parameters for the ND4Fe(DPO4)2 sample
˚
Table 3 Selected bond distances (A) and angles (deg) for ND4Fe(DPO4)2
sample
x
y
z
Biso
Fe1–O6
Fe1–O7
Fe1–O9
<Fe1–O>
Fe2–O1
Fe2–O2
Fe2–O3
Fe2–O4
Fe2–O5
Fe2–O8
<Fe2–O>
P1–O2
P1–O6
P1–O8
P1–O10
<P1–O>
O10–D1
P2–O3
2.029(8) ¥2
2.017(4) ¥2
1.947(5) ¥2
2.00
2.061(5)
2.020(8)
1.959(8)
1.930(6)
2.047(6)
1.943(6)
1.99
O6–Fe1–O6
O6–Fe1–O7
O6–Fe1–O9
180.0 (7)
89.4 (4)
87.1 (4)
Fe1
Fe2
P1
P2
P3
-0.5000
0.5000
0.5000
0.65(8)
0.33(6)
0.31(9)
0.28(9)
0.39(9)
0.54(9)
0.50(9)
0.56(8)
0.63(9)
0.54(8)
0.64(9)
0.58(9)
0.51(8)
0.52(9)
0.51(9)
0.64(9)
0.73(9)
1.14(8)
0.91(6)
1.8(1)
1.7(1)
1.90(9)
1.9(1)
2.1(1)
1.8(1)
1.9(2)
2.0(2)
2.2(2)
2.3(2)
-0.2975(5)
-0.5897(9)
0.0469(9)
-0.4152(9)
-0.2457(7)
-0.5127(8)
0.0951(8)
-0.4757(8)
-0.0657(8)
-0.4459(8)
-0.3424(8)
-0.6665(8)
-0.2383(8)
-0.2166(8)
-0.9023(8)
-0.3959(8)
0.0000
-0.2831(4)
0.6552(7)
0.5564(7)
0.1224(7)
-0.0446(6)
0.2425(7)
-0.6897(6)
0.1824(6)
-0.3759(7)
0.5887(7)
0.2616(6)
0.5164(7)
-0.4568(7)
0.2026(6)
0.6265(7)
-0.0874(7)
0.0000
-0.1654(5)
0.2582(8)
0.0388(7)
-0.0311(7)
-0.2043(7)
0.2170(7)
0.5489(7)
-0.2102(7)
-0.888(1)
0.966(2)
0.9499(3)
0.7802(7)
0.7780(7)
0.7677(7)
0.7957(6)
0.1872(6)
0.9377(6)
0.9100(6)
0.7874(6)
0.6444(6)
0.6131(6)
0.9281(6)
0.3582(6)
0.2857(6)
0.2601(6)
0.2507(6)
0.0000
0.4672(4)
0.2574(7)
0.2304(7)
0.4154(7)
0.5931(7)
0.5657(7)
0.2935(7)
0.4293(7)
0.972(1)
O1–Fe2–O2
O1–Fe2–O3
O1–Fe2–O4
O1–Fe2–O5
O1–Fe2–O8
85.8(5)
89.5(4)
91.8(3)
85.2(4)
172.3(9)
O1
O2
O3
O4
O5
O6
O7
O8
O9
O10
O11
O12
N1
N2
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D11
1.52(2)
O2–P1–O6
O2–P1–O8
O2–P1–O10
O6–P1–O10
109.8(8)
113.2(8)
103.1(7)
108.1(6)
1.545(8)
1.497(7)
1.602(7)
1.54
0.99(1)
P1–O10–D1
O3–P2–O5
O3–P2–O9
O3–P2–O11
O5–P2–O9
110(1)
1.526(7)
1.542(8)
1.526(7)
1.57(1)
112.3(6)
112.4(6)
104.6(6)
109.4(6)
P2–O5
P2–O9
P2–O11
<P2–O>
O11–D6
P3–O1
-0.1189(5)
-0.1032(9)
-0.4270(9)
-0.1603(9)
-0.1549(9)
-0.7991(9)
-0.7740(9)
0.0390(9)
0.027(2)
1.54
0.988(8)
1.530(7)
1.53(1)
1.520(7)
1.56(1)
P2–O11–D6
O1–P3–O4
O1–P3–O7
O1–P3–O12
O4–P3–O7
119.5(8)
111.0(9)
110.4(8)
110.0(8)
108.8(9)
P3–O4
P3–O7
P3–O12
<P3–O>
O12–D2
N1–D8
N1–D9
N1–D10
N1–D11
<N1–D>
N2–D3
N2–D4
N2–D5
N2–D7
<N2–D>
1.54
1.001(9)
0.98(2) ¥2
1.08(2) ¥2
1.19(1) ¥2
0.99(1) ¥2
1.06
1.044(6)
1.052(7)
1.04(1)
P3–O12–D2
D8–N1–D9
D8–N1–D10
D8–N1–D11
D9–N1–D11
108(1)
116(2)
110(2)
123(2)
104(2)
-0.992(2)
0.142(2)
-0.879(2)
0.900(1)
0.044(1)
0.919(1)
-0.114(1)
0.037(2)
2.1(2)
˚
˚
final unit cell parameters: a = 7.5219(2) A, b = 10.0018(3) A,
D3–N2–D4
D3–N2–D5
D3–N2–D7
D4–N2–D5
110.5(9)
109.1(9)
109.0(9)
109.3(9)
◦
3
˚
˚
c = 8.2729(3) A, b = 105.887(3) , V = 598.62(3) A (Z = 4).
Crystallographic parameters are shown in Table 1. Final atomic
positions are shown in Table 4. Selected bond distances and angles
are shown in Table 5.
1.043(6)
1.04
Powder X-ray diffraction patterns of the residual solid obtained
from NH4Fe(HPO4)2 thermogravimetric experiments were col-
lected on an Oxford Diffraction Gemini S diffractometer. Data
were collected at ambient temperature over the angular 2q range
10–70◦, with a step size of 0.01◦, by using CuKa1 radiation (l =
Table 4 Final atomic positions and isotropic displacement (temperature)
parameters for the NH4FeP2O7 sample
x
y
z
Biso
Fe1
P1
P2
0.2442(9)
0.454(1)
0.134(1)
0.312(2)
0.362(2)
0.123(2)
0.171(3)
0.014(2)
0.447(2)
0.335(3)
0.200(2)
0.5958(6)
0.632(1)
0.611(1)
0.565(2)
0.425(1)
0.740(1)
0.562(1)
0.493(2)
0.717(1)
0.619(2)
0.196(2)
0.7585(8)
0.187(1)
0.338(1)
0.279(3)
0.727(2)
0.860(2)
0.513(1)
0.750(2)
0.725(2)
1.004(1)
0.521(2)
0.69(9)
1.1(2)
1.1(2)
1.5(2)
1.5(2)
1.5(2)
1.5(2)
1.5(2)
1.5(2)
1.5(2)
3.0(4)
˚
1.5406 A). Rietveld refinement was used on the X-ray powder
patterns, employing GSAS software with a pseudo-Voigt function
and including a correction for asymmetry due to axial divergence.
The crystal structures used to interpret the powder patterns were
taken from the Inorganic Crystal Structure Database (ICSD).
The collection codes for each structure were: Fe4(P2O7)3 – 51768
and Fe(PO3)3 – 88848. Neither the positional nor the thermal
vibration parameters were refined. The parameters optimized
were: background coefficients, cell parameters, zero-shift error,
peak shape parameters, and phase fractions. Observed (vol.%):
Fe4(P2O7)3 50.2, Fe(PO3)3 49.8 (calc.: Fe4(P2O7)3 54.39, Fe(PO3)3
45.61).
O1
O2
O3
O4
O5
O6
O7
N1
of the three crystallographically distinct PO4 groups is linked
to one deuterium atom. The [Fe(DPO4)2]- anionic framework
contains infinite tunnels along the [100] and [010] directions,
where the [ND4]+ groups are located (Fig. 3). The tetrahedral
[DPO4]2- anions form groups linked by hydrogen bonds, whose
Results and discussion
˚
˚
The ND4Fe(DPO4)2 structure, related to NH4(Al0.64Ga0.36)-
(HPO4)2,7 consists of isolated FeO6 octahedra linked to one
another by PO4 tetrahedra through common corners. Each Fe
octahedron is surrounded by six PO4 tetrahedra, while each
P tetrahedron is linked to three FeO6 polyhedra. One oxygen
distances vary from 2.587(8) A to 2.706(7) A. Each P1 is involved
in three hydrogen bonds, O2 and O6 acting as acceptors and
O10 as donor; each P2 is involved in two hydrogen bonds,
O5 acting as acceptor and O11 as donor; and each P3 is
involved in one hydrogen bond, O12 acting as donor (see Fig. 4).
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Fig. 2 Powder X-ray diffraction pattern of NH4FeP2O7 (symbols as in Fig. 1). The second line of Bragg peaks is associated with a small amount of iron
˚
oxide (cubic, a = 8.41 A) originated on the NH4Fe(HPO4)2 thermal decomposition.
˚
˚
Table 5 Selected bond distances (A) and angles (deg) for NH4FeP2O7
Table 6 B–C and A–C (less than 3.20) distances (A) and A–B–C
sample
angles (deg) obtained from neutron powder diffraction data for the
ND4Fe(DPO4)2 sample
Fe1–O2
Fe1–O3
Fe1–O4
Fe1–O5
Fe1–O6
Fe1–O7
<Fe1–O>
P1–O1
P1–O2
P1–O6
P1–O7
<P1–O>
P2–O1
1.973(7)
2.01(1)
1.978(7)
1.99(1)
2.025(4)
1.972(7)
1.99
1.616(3)
1.486(4)
1.548(7)
1.541(7)
1.55
1.620(2)
1.499(7)
1.484(4)
1.556(7)
1.54
O2–Fe1–O3
O2–Fe1–O4
O2–Fe1–O5
O2–Fe1–O6
O2–Fe2–O7
161.6(8)
74.0(7)
88.2(7)
96.5(7)
101.5(5)
A
B
C
B–C
A–C
A–B–C
O12
O10
O11
N1
N1
N1
N1
N1
N1
N1
N1
N2
N2
N2
N2
N2
N2
N2
D2
D1
D6
D8
D11
D9
D10
D11
D8
D10
D9
D4
D7
D5
D3
D7
D5
D7
O2
O5
O6
O3
O3
O3
O3
O12
O12
O12
O12
O5
1.597(8)
1.69(1)
1.718(8)
1.85(2)
2.44(2)
2.48(2)
2.73()
2.12(2)
2.77(2)
2.96(2)
2.98(2)
1.951(7)
2.998(7)
2.250(9)
1.874(6)
2.44(2)
2.587(8)
1.980(8)
2.587(8)
2.677(9)
2.706(7)
2.822(6)
2.822(6)
2.822(6)
2.822(6)
3.100(5)
3.100(5)
3.100(5)
3.100(5)
2.869(6)
2.869(6)
3.020(8)
2.906(6)
2.946(8)
2.946(8)
3.001(6)
169.3(9)
172(1)
178.7(8)
173(1)
O1–P1–O2
O1–P1–O6
O1–P1–O7
O2–P1–O6
105(1)
104(2)
98(2)
102.7(8)
96.9(9)
81.9(7)
173(2)
100.5(9)
85.1(6)
86.0(9)
143.9(6)
72.9(5)
129.7(8)
169.2(6)
109.0(7)
99.8(7)
165.5(7)
111.0(8)
O1–P2–O3
O1–P2–O4
O1–P2–O5
O3–P2–O4
113(2)
105(2)
102(2)
103(1)
P2–O3
P2–O4
P2–O5
<P2–O>
O5
O12
O10
O11
O11
O7
Fig. 5 shows the local structure around N1 with the nearest
deuterium atoms presenting an occupation of 0.5. They are
positioned forming two tetrahedra in two different orientations,
with the N1 located at the inversion centre of symmetry. The
strongest interactions between [ND4]+ and [DPO4]2- groups take
place through D8(N1) ◊ ◊ ◊ O3(P2) and D11(N1) ◊ ◊ ◊ O12(P3), in the
two different tetrahedral orientations; and D3(N2) ◊ ◊ ◊ O10(P1),
D4(N2) ◊ ◊ ◊ O5(P2) and D7(N2) ◊ ◊ ◊ O7(P3) (see Table 6).
Redrup and Weller stated that the ammonia is lost from
the framework in two stages, probably corresponding to the
decomposition of two ammonium cations. Further decomposition
of the residual iron hydrogen phosphate occurs with the loss of
the hydroxyl groups as water, prior to simple iron phosphate
structure [FePO4 or Fe4(P4O12)3] formation.2 From the study of
our sample, however, the on-line mass spectrometric analysis of
the evacuated vapours in the TG experiment (Fig. 6) indicates
a different reaction route. The mass spectrometric curve related
to water has two maxima, while that associated with ammonia
has only one (coinciding with the second stage of water loss).
This feature indicates that the dehydroxylation process takes place
before ammonium cation decomposition. Finally, after treatment
at 900 ◦C, a mixture of two crystalline phases [Fe4(P2O7)3 and
Fe(PO3)3] is found.
When NH4Fe(HPO4)2 is stabilized at 400 ◦C, it irreversibly
transforms to NH4FeP2O7. In the last three decades, subsequent
to the first characterization of KFeP2O7,8 several studies have been
devoted to AIMIIIP2O7-type diphosphates, with three different
structural forms [denoted as NaFeP2O7–I (low-temperature form),
NaFeP2O7-II (high-temperature form) and LiFeP2O7 (III-type
structure)] being reported.9 As far as we know, the NH4FeP2O7
crystal structure has not yet been resolved. All I to III structures
may be described as an alternating succession of MIIIO6 octahedral
1794 | Dalton Trans., 2010, 39, 1791–1796
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Fig. 3 Projection of the structure of ND4Fe(DPO4)2 viewed down the
b-axis (PO4, tetrahedra; FeO6, octahedra; N, green circles; D, green-pale
circles).
Fig. 6 NH4Fe(HPO4)2 thermal behaviour: (a) TG (solid line) and DTG
(dashed line) curves, and (b) m/z 18 (H2O) and m/z 15 (NH3) MS signals
of evacuated vapours.
[FeP2O7]- anionic framework can also be described by the associ-
ation of FeP2O11 units formed by corner-sharing FeO6 octahedra
and P2O7 di-tetrahedra (Fig. 7). Host lattice ammonium iron
diphosphate form cages resulting from intersecting tunnels where
[NH4]+ ions are located. Two sorts of tunnels are in fact observed:
wide tunnels running along the [001] direction and distorted
hexagonal ones running along the [110] direction formed by two
types of alternating windows.
Fig. 4 Hydrogen bond scheme between [DPO4]2- groups.
Fig. 5 Coordination environment of nitrogen atom N1. Solid and dashed
lines represent the two tetrahedral polyhedra (see text).
and P2O7 di-tetrahedral layers (where tetrahedra share corners
with octahedra). The difference between them lies mainly in
the conformation of the diphosphate groups. The structure of
the NH4FeP2O7 crystals obtained in this study is I-type. Their
Fig. 7 Projection of the structure of NH4FeP2O7 viewed down the c-axis
(symbols as in Fig. 3).
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B. F. Alfonso acknowledges the Department of Physics at the
University of Oviedo for the reduction in teaching workload.
Conclusion
Combining neutron and X-ray powder diffraction data, we have
been able to determine the crystal structure of NH4Fe(HPO4)2
including the determination of the position of the hydrogen atoms.
Furthermore the thermal decomposition of this material yields a
diphosphate NH4FeP2O7, which appears at temperatures around
350 ◦C, and their crystal structure has also been refined using X-
ray powder diffraction. Finally, at 900 ◦C, two crystalline phases
[Fe4(P2O7)3 and Fe(PO3)3] are formed.
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This work has been carried out with the financial support of
FEDER-MEC (MAT2006-01997, MAT2008-06542-C04-03, and
Factor´ıa Espan˜ola de Cristalizacio´n-Consolider Ingenio 2010).
1796 | Dalton Trans., 2010, 39, 1791–1796
This journal is
The Royal Society of Chemistry 2010
©