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MÖSSBAUER STUDY OF AN UNUSUAL IRON
ARRANGEMENT THE FERROUS PHOSPHATE
Fe2+3(PO4)2(H2O)
E. Mattievich, J. Danon
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, suppldment au no 12, Tome 37, Dtcembre 1976, page C6-483
MOSSBAUER
STUDY
OF
AN
UNUSUAL IRON ARRANGEMENT
THE FERROUS PHOSPHATE Fe3 +(P04)2(H2
0)
E. MATTIEVICH
Instituto de Fisica, Universidade Federal do Rio de Janeiro Rio de Janeiro, Brazil
and J. DANON
Centro Brasileiro de Pesquisas Fisicas Rio de Janeiro, Brazil
RBsumB.
-
Ayant comme point de depart la vivianite nous avons synthktisk le monohydrate F ~ ~ + ( P O ~ ) ~ ( H Z O ) , sous la forme de mor~osristaux. La structure aux rayons X montre trois sites differents pour l'ion Fez+. Deux des sites sont des octakdres distordus, le troisikme est une pyra- mide tbtragonale avec forte distorsion.La variation thermique des paramktres Mossbauer, la substitution isomorphe des ions Fez+ et I'oxydation du phosphate sont discutkes B la lumikre de cette structure.
Abstract.
-
By hydrothermal synthesis, starting from vivianite, the monohydratehas been prepared in the form of single crystals. X-ray structure analysis reveals the presence of three different sites for the Fez+ ion, two distorted octahedra and one very distorted tetragonal pyramid.
Thermal variation of the Mossbauer hyperfine parameters, isomorphous substitution of the iron ions and oxidation experiments are discussed in view of this unusual structure.
1. Introduction. -The homologous series
includes three well known minerals : vivianite, n = 8 ;
ludlamite, n = 4 ; and phosphoferrite, n = 3. In a recent study of this series [I], using a hydrothermal technique, several new compounds have, been synthe- sized, including the monohydrate, which is the subject of this study.
2. Experimental.
-
The hydrothermal synthesis was carried out in a pressure vessel of 30 ml, volume, made from austenitic steel. The starting material was chemically pure Fe,(P04)2(H,0), prepared according to the method of Evans [2], using a compound-to- water ratio of about 1:10 g/ml. In order to reduce contaminations of the phosphate with the metal from the pressure vessel the reaction was carried out in an open silica container inside the vessel. Each experiinent was performed for 40 hours, after which the product:was washed and finally stored in a vacuum dissecator. The ~ e g + (PO~),(H,O) species was synthetized a t a
temperature of 200- to 250 O C and a t a 'pressure of
400 bars. The best results we obtained when the pH of- water was slightly acidic. After 40 hours of synthesis-- we obtained pale green crystals of more than 0.1 mm
length, tabular parallel to (001) and striated parallel to (100).
The compounds were chemically analysed for ferrous iron content, phosphate and water ; the following table shows that the chemical composition closely agrees with the stoichiometric formula :
Exp. weight Ideal weight
%
-
-
%
FeO 57.39 57.39 PzO, 37.10 37.80 H20 5.36 4.81 Total 99.85 100.00The Mossbauer Spectra were obtained with poly- crystalline samples. Thin absorbers, containing 10 mg/ cm2 of iron were prepared in a dry box filled with inert gas and contact with air was avoided during the experiments.
An Elron Spectrometer with a 400-channel Nuclear Chicago multichannel analyser was used in recording the spectra. The Co57 in Pd source, which had a n
activity of 10 mCi, was kept a t room temperature in all the measurements. The isomer shifts (I. S.) are quoted relative t o metallic iron a t 295 K.
C6-484 E. MATTIEVICH AND J. DANON
The spectra were analyzed by graphical methods, except in the cases where the temperature variation of the Mossbauer parameters was studied, then computer fitting procedures were used to obtain the parameters.
3. Results and Discussion. - The spectra measured a t room temperature and at liquid nitrogen tempera- ture, shown in figure 1, differ notably from those of all
FIG. 1.
-
Spectrum of monohydrate F ~ ? ( P o ~ ) ~ ( H ~ o ) : a) at 295 K and b) at 80 K, showing the three non-equivalent sitesFe(I), Fe(I1) and Fe(II1).
the other ferrous phosphates and indicate the presence of Fe2+ occupying three non-equivalent sites in the crystal. The asymmetry of the spectra indicates that the isomer shifts (I. S.) for the three sites are different (see table of results for the Mossbauer parameters). The atomic structure determination was made by P. B. Moore and Takaharu Araki [3]. The unit cell parameters are : a = 9.431(1)
A,
b = 10.066(1)A,
c = 8.040(1)A,
/3 = 117.632(7)0 ; P 2,,,, Z = 4.A precise structure determination with refinements reveals an extraordinarily complex atomic arrange- ment. There are feeble resemblances to the other members of the homologous series, and it bears no obvious relationship with any known structure, the environments of Fe(1) and Fe(3) are distorted octahe- dra and the one of Fe(2) is a very distorted tetragonal pyramid. Figure 2 shows a polyhedral sketch of the
FIG. 2. - Polyhedral diagram of the Fe-0 arrangement between
114 d X'
<
112, where a' = a - c and c' = a+
c. The chain- like character is shown by stippling. Disks denote linkages ofcorners above and below this slab.
structure projected along the a' = a
-
c axis, showing the plane b x c' sinP',
where c' = a+
c. The stippled polyhedra reveal a chair that runs parallel to c' sinfi'
Table of the Mossbauer Parameters
I. S.
Q.
S. Chemical Formula Iron Sites- (mmls)
- L
(mmls)
MOSSBAUER STUDY OF UNUSUAL IRON ARRANGEMENT IN THE FERROUS PHOSPHATE C6-485 with a crankshaft motif. Equivalent chains are further
linked above and below to form a highly complex polyhedral framework.
In order to correlate the Mossbauer spectra with the X-ray structure, we have investigated the temperature variation of the Mossbauer parameters ; in addition we have performed isomorphous cationic substitution experiments and oxidation studies of the mono- hydrate.
4. Thermal variation .of the Mossbauer Parameters.
-
The variation of the parameters with temperature frequently permits an assignment of pairs of absorption lines of the spectra to different sites of the Fez+ ion in the crystal, using the following criteria :1) For the sites having the same coordination number, the I. S. should not differ too much.
2) The thermal variation of the I. S. in these sites should be identical (same law of thermal variation). 3) The thermal variation of the Q. S. in the three sites should be as regular as possible.
For the room temperature spectrum of figure la, the combination of lines that fulfill the first criterium is indicated by the scheme A of figure 3. If we adopt the
FIG. 3.
-
Temperature variation of the Mossbauer parameters in the monohydrate showing the schemes : a for T2
200 K andB for T 5 200 K.
scheme A as valid for all temperatures, we observe that below 200 K the variation of the I. S. is not identical. We should consider the scheme B (constructed from A through the crossing of the two extreme lines of higher velocity that cannot be resolved between the tempera- tures 80 K and 250 K).
The assignment of lines that fulfills all the criteria suggests the following solution : scheme A, for T
2
200 K and Scheme B, for T5
200oC.5. Isomorphous substitution experiments.
-
Although many experimental facts have been established on the interaction of isomorphous pairs, no quantitative theory of isomorphism exists at present. The experi- ments performed are based on the following rules :a) The cation which is more electronegative substi- tutes in these salts the less electronegative one ; b) In an isomorphous substitution of iron ions by more electronegative metal ions in high-spin compounds with multiple site occupancy, the determining factor in controlling the cationic order is not the structure of the oxygen-metal polyhedra framework, but the degree of their ionic character [4].
The cationic substitution experiments were done by hydrothermal techniques at the lowest temperatures compatible with the stability of the monohydrate (about 2200C, pH = 5).
In order to minimize a perturbation on the reticular parameter, we selected the cations Mg2+ and Zn2+ which have nearly the same ionic radius that, however, is smaller than the one of the Fez+ ion. The following isomorphous substitution is expected to occur :
The results obtained with Mg with an initial composi- tion of 117 molar relative to the total iron content are illustrated in figure 4a, b. At room temperature, the spectrum indicates a decrease of the intensity of the inner line of the group of three lines at higher velocity, the corresponding pair being unresolved at this tem- perature. At liquid nitrogen temperature, figure 4b, the resolution of the spectrum indicates that Mg2+ preferentially occupies the Fe(II1) sites. At a higher magnesium concentration the substitution is the same for all sites.
The spectrum in figure 4c, d illustrates the results of cationic substitution with an initial composition of 114 molar in Zn. The spectrum at room temperature indicates that z n 2 + occupies preferentially two diffe- rent sites. Since the lines are broadened it is difficult to resolve the spectrum completely ; the pair indicated by
I1 is the most probable assignment since the other
C6-486 E. MATTIEVICH AND J. DANON
I I I l I I l l I I l l 1
-2 0 2 4
velocity (mm/$)
FIG. 4.
-
Spectra of monohydrate showing the effects of iso- morphous substitution pairs : a) , b) (117 Mg, 617 Fe) andc), d) (114 Zn, 314 Fe) ; a), c) at 295 K and b), d) at 80 K. The less
intense absorption lines 111, 111 show the preferential site where
Mg2+ substitutes the Fez+ ion. One of the sites preferentially
occupied by Zn2+ is 11, 11. At d), the spectrum show that Zn2+ also substitutes the Fez+ in site 111, 111.
lower velocities that the preference of z n 2 + is for the sites Fe(I1) and Fe(II1).
Let us consider now the character of the chemical bonds. Due to differences in electronegativities the Fez+ ion forms bonds with oxygen in which the cova- lent character is more pronounced than with Mg2+ and z n 2 + . In high-spin compounds a correlation exist between the coordination number and the isomer shift [5]. By increasing the coordination number, the I. S is increased indicating an increase in the ionic character of the boud.
Figure 3 shows that the site corresponding to the larger I. S. is Fe(1) ; this suggests that the ionic cha- racter at this site predominates. Thus, Fez+ is less substituted from this site by Mg2+ and Zn2+ ions than in the other two Fe(I1) and Fe(II1) sites, which are preferentially occupied by Zn cations.
The smallest I. S. values arise from Fez+ at the Fe(I1) site, suggesting that this site corresponds to the pentacoordinated Fe(2) site. The Q. S. of Fe(I1) is larger than that of the two other sites. Apparently this is in contradiction with correlations reported between
the Q. S. and the coordination number. However, this discrepancy has also been observed with other iron phosphates such as synthetic graftonite [I] (A)
F~~+(Po,),. Moreover, the smaller values of the Fe-0 distances in the pentacoordinated Fe(2) site would cause a larger field gradient as well as a larger electronic density at the iron nucleus in this configura- tion.
6. Oxidation sequence of F~:+(Po,),(H,o).
-
The identification of the sites Fe(1) and Fe(II1) cannot be made from the previous experiments as in the case of the Fe(I1) site. However, since the oxidation process is associated with the H,O ligand, one of the sites can be identified as Fe(1) on the basis of discussion of
hydrogen bonds and the experiments on the oxidation process.
The two Fe(1)-0 octahedra share a common edge via two water ligands 0k-0,. The Ow molecule can donate two hydrogen bonds and the most likely acceptor is O(7). This coordinates to P(2) and Fe(2) only, accept- ing =
+
116 for the hydrogen bond strength. Moore adopts a model where O(7) accepts one strong and one weak hydrogen bond : Ow-H-...
O(7) = 2.72A
and OW-H-...
O(7)' = 3.43A.
Moore has also pointed out that a change from 2 Fez+-(H,O) bonds to 2 Fe3+-(OH) is allowed without destruction of the structure for certain environments around the ligand, and some crystals may be capable of continuous oxidation to a ferric isotypjc end member, for example phosphoferrite- kryzhanovskite6. Since 2 F e ( 1 ) 2 + - ~ 2 ~ occurs in ~ e : + (PO,),(H,O), he proposed that a bounded mixed-
valence composition can occur as a stable crystal. The mid point of the
0,-0b
edge shared between two Fe(1) atoms is an inversion center. This suggests that an upper limit for a stable oxidized equivalent would be F~;+F~~+(Po,),(oH)-, where Fe(1) is completely oxidized.A sample of crushed monohydrate crystals were placed on a glass and heated in a muffle furnace to 230 OC. The oxidation process requires the presence of oxygen, since no oxidation occurs in vacuum as has been verified. A possible mechanism for this oxidation is the one in which the oxygen diffused into the crystal interchanges protons with the H,O ligand, forming hydroxil groups, ferric iron and free H,O :
Figure 5 indicates a sequence of spectra correspond- ing to different phases of the oxidation process. It is observed that the Fez+ ion at the sites Fe(I1) and Fe(1) are oxidized to the Fe3+ state, where the Fe(II1) sites remain unchanged. The oxidation of F~~+(Po,),(H,o) involve two Fe2+ ions, and this can occur in two ways :
M~SSBAUER STUDY OF UNUSUAL IRON ARRANGEMENT I N THE FERROUS PHOSPHATE C6-487
FIG. 5. - Spectrum at 295 K showing progressive oxidation of F e l ion in the monohydrate : a) after three days at 200 C, b) the previously oxidized compound submitted one more day at 230 "C and c) another three days at 230 "C. The last spectrum shows that FeZ+(I) and FeZ+(II) have been oxidized to Fe3+.
oxidized compound will satisfy the charge balance in the molecule. The extraction of 2 H + should influence the Fe(1) site since it is only in this site that the Fez+ ion is directly coordinated to the H,O molecule. The other oxidized ion necessarily comes from the Fe(I1) site which has already been identified as Fe(2) from its structure. In figure 5c, one can see that the Fe(II1) site remains unchanged. In this situation the strong undersaturation of the ligands 0--, resulting from the loss of two H + , cannot be compensated by the Fe3+ (I) ions and as a consequence the crystal struc- ture should break apart.
b) The second possibility consists of the isomor- phous transformation :
The presence of two Fe3+ in the Mossbauer spectrum (Fig. 5) can be interpreted as arising from an electron transfer process Fez+
s
Fe3+, analogous to the onereported for magnetite, F~,O;. This mechanism would also explain certain features of the Mossbauer spectra at 80 K, which seem to indicate the presence of some slow relaxation process such as the increase in the ratio Fe3+/Fe2+ of about 10
%,
an increase of about 20%
of the line widths of Fez+ in Fe(II1) sites and a marked asymmetry of the corresponding spectrum, as is illustrated in figure 6 .FIG. 6. - Spectrum at 295 K and 80 K of the oxidized mono- hydrate.
Acknowledgments.
-
It is a pleasure to acknow- ledge the contribution of Dr. Paul Brian Moore of the Department of the Geophysical Sciences of the University of Chicago in performing the atomic structure determinations. We also wish to thank Dr. Frangois Varret of the Centre dYEtudes Nucl6aires de Saclay (France) for the detailed investigation of the temperature dependence of the Mossbauer parameters.References
[I] MATTIEVICH, E. and DANON, J., J. Znorg. Nucl. Chem. [4] MALISHEVA, T. V. et aL, Proc. Conf. App. Moss. Effect,
(to be published). Tihany, 745 (1969).
[2] PASCAL, P., Nouveau Trait6 de Chimie Minerale XVIII, [5] CLARK, M. G. et al., J. Chem. Phys. 47 (1967) 10, 4250. Paris (1959). [6] MOORE, BRIAN, P., Nature 251 (1974) 305.
[3] MOORE, BRLAN, P. and ARAKI, TAKAHARU, Am. Mineral. 60 [7] GREENWOOD, N. N. and GIBB, T. C., Mossbauer Spectroscopy
