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Synthesis and characterization of a new iron phosphate KSrFe2(PO4)3 with a langbeinite type structure
Journal of Molecular Structure 1030 (2012) 145–148
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Synthesis and characterization of a new iron phosphate KSrFe2(PO4)3 with a langbeinite type structure Mourad Hidouri a,⇑, María Luisa López b, Carlos Pico b, Alain Wattiaux c, Mongi Ben Amara a a
UR Matériaux Inorganiques, 5019 Faculté des Sciences, Université de Monastir, Tunisia Departamento de Química Inorgánica I, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain c Institut de Chimie de la Matière Condensée de Bordeaux, CNRS, Université de Bordeaux I, 87 Avenue du Dr. A. Schweitzer, 33608 Pessac-Cedex, France b
a r t i c l e
i n f o
Article history: Received 16 January 2012 Received in revised form 31 March 2012 Accepted 2 April 2012 Available online 10 April 2012 Keywords: Inorganic materials Chemical synthesis Crystal structure Magnetic measurements Mössbauer spectroscopy
a b s t r a c t Synthesis, crystal structure, Mössbauer spectroscopy and magnetic susceptibility of a new phosphate KSrFe2(PO4)3 are described. This compound crystallizes with the unit cell parameter a = 9.809(2) Å of the cubic space group P213 and four formula units per cell. Its structure belongs to the well known langbeinite type. It consists of a 3D framework resulting from a corner-sharing between FeO6 octahedra and PO4 tetrahedra. This framework delimits two distinct cavities, statistically occupied by the K+ and Sr2+ ions. The Mössbauer spectroscopy gave clear evidence of the exclusive presence of octahedral Fe3+ ions. The magnetic susceptibility measurements revealed that the compound follows the Curie–Weiss behavior with an experimental effective moment per unit formula l = 8.16 lB, consistent with the ideal spin only value of 8.37 lB. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Iron phosphates are extensively studied for their interesting applications as prospective materials in corrosion inhibition [1] and heterogeneous catalysis [2]. These materials are also exhibited by the richness of their crystal chemistry owing to the possible +2/ +3 mixed valence of iron and its tendency to form with the phosphate groups various complex frameworks [3]. The interest devoted to these materials has been accentuated since the discovery, by Padhi et al. in 1997 of LiFePO4 as a promising electrode material for Li-ion batteries [4]. The langbeinite K2Mg2(SO4)3 [5] is the prototype of a large number of sulfates, vanadates and phosphates which have been the subject of intensive research for their potential applications in ferroelasticity, ferroelectricity and luminescence [6–9]. Most of them are cubic with space group P213 but some of them undergo P213 ? P212121 phase transitions [10,11]. The structure consists of a [M2(XO4)3]1 3D mixed framework, built up by corner-sharing MO6 octahedra and XO4 tetrahedra with each octahedron is linked to six tetrahedra and each tetrahedron to four octahedra. This framework delimits large interstitial cavities available to the alkaline metals and divalent cations. The present work is a part of our activity devoted to the synthesis and characterization of new iron phosphates belonging to the ⇑ Corresponding author. E-mail address: [email protected] (M. Hidouri). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.04.002
A2O–MO–Fe2O3–P2O5 systems where A and M are an alkali metal and a divalent cation, respectively. During this study, we recently described the langbeinite structures of the monophosphate NaBaFe2(PO4)3 [12] and the oxophosphate K11Fe15(PO4)18O [13]. In this paper, we report on the synthesis and characterization by single X-ray diffraction, Mössbauer spectroscopy and magnetic susceptibility of langbeinite-like monophosphate KSrFe2(PO4)3. 2. Experimental 2.1. Synthesis Single crystals of KSrFe2(PO4)3 were grown in a flux of potassium dimolybdate K2Mo2O7 with molar ratio of product: flux = 2:1 starting from 2.043 g of KNO3, 1.506 g of SrCO3, 8.203 g of Fe(NO3)39H2O, 4.002 g of (NH4)2HPO4 and 1.454 g of MoO3. These reactants were mixed in nitric acid and the resulting solution was evaporated to dryness by heating at 353 K. The obtained dry residue was ground in an agate mortar and then heated for 24 h at 673 K in order to remove the decomposition products: NH3, CO3, etc. The process was then followed by a further heating for 12 h at 873 K with intermediate grinding. After being reground, the product was melted for 1 h at 1373 K, and then slowly cooled at a rate of 10 K h1 to 673 K, then at a rate of 50 K h1 to room temperature. The crystals obtained by washing the final product with warm water, in order to dissolve the flux, are essentially composed by yellowish and irregularly shaped crystals. Their qualitative microprobe analysis indicated the exclusive presence of K, Sr, Fe
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M. Hidouri et al. / Journal of Molecular Structure 1030 (2012) 145–148
which significantly exceed those of the remaining atoms. This result was taken as an indication of the existence of some disorder around these atoms. However, a refinement based on this hypothesis shown to be unsuccessful since negative values of the thermal Ueq parameter were obtained. In order to take into account this apparent disorder of the O(2), O(3) and O(4) atoms, alternative refinements were performed using lower symmetries (ex: SG. P212121), associated to a possible distortion of langbeinite structure. Unfortunately, theses refinements did not led to better results. The proposed model is then the best one given the data obtained. At the end of refinement, anisotropic displacement were allowed to refine resulting in the final reliability factors R1 = 0.034 and wR2 = 0.088 and the atomic coordinates given in Table 2.
Table 1 Crystal data and structure refinement parameters for KSrFe2(PO4)3. Crystal data Formula unit Crystal system Space group a (Å) V (Å3) Z qcal (g cm3)
KSrFe2(PO4)3 Cubic P213 9.809(2) 943.8(2) 4 3.68
Intensity measurements Crystal dimensions (mm3) Wavelength (Å) Monochromator l (mm1) h Range for data collection No. of unique reflections; Rin No. of retained reflections [Fo > 4r(Fo)]
(0.17 0.17 0.19) (Mo Ka) = 0.7103 Graphite 9.68 2.9–29° 821; 0.029 785
Structure solution and refinement Intensity correction Structure solution Agreement factors Number of refined parameters (Dq)max, min (e Å3)
Lorentz–Polarization Direct methods R1 = 0.036; wR2 = 0.089; S = 1.13 62 0.94; 0.83
2.3. Characterization
and P in an atomic ratio approximating 1:1:2:3, in accordance with the KSrFe2(PO4)3 composition. After its structure determination, the title compound was prepared in the powder form by the conventional solid state reaction. After an initial treatment similar to that undertaken for the synthesis of the single crystals until 873 K, the sample was subjected to final calcinations at 1273 K for 48 h with intermittent grinding. A yellowish powder was obtained by quenching in air.
The purity of the synthesized powder was checked from the examination of its X-ray powder diagram, registered in the range 5° 6 2h 6 80° on a PANalytical diffractometer using Cu Ka radiation (k = 1.5406 Å). The 57Fe Mössbauer spectrum was recorded at 293 K by a HALDER constant acceleration spectrometer using a room-temperature 57 Co/Rh source. The sample contained an average of 10 mg/cm2 of iron, a concentration for which the line broadening due to the thickness effects is not noticeable. Isomer shifts were calculated in reference to the natural a-Fe. Magnetic susceptibility measurements were performed in the 4.2–300 K temperature range by a Quantum Design SQUID MPMS-XL magnetometer at a constant magnetic field of 0.5 T. 3. Results and discussion
2.2. Structural determination
3.1. Description of the structure
The crystal structure of KSrFe2(PO4)3 was determined by singlecrystal X-ray diffraction, using a crystal of dimensions 0.17 0.17 0.19 mm3. Data were collected by a Bruker–Nonius diffractometer, equipped with a bidimensional CCD detector and using a graphite monochromated Mo Ka radiation (k = 0.71073 Å). Main crystallographic data and conditions for structure determination are reported in Table 1. Intensity data were corrected from absorption effects using SADABS (Bruker–Nonius area detector scaling and absorption correction version). All reflections could be indexed based on a primitive cubic unit cell with a = 9.809(2) Å. The systematic absences clearly indicated the space group P213. The starting structural model was extracted by direct methods [14] leading to two distinct positions for the Sr atoms. The atomic species were progressively located from Fourier difference maps [15] using the positions of the previously located atoms. Refinement of the occupancy factors suggested a statistical distribution between the Sr and K with identical partial occupancies. The refinement showed that O(2), O(3) and O(4) atoms exhibit large thermal parameters
KSrFe2(PO4)3 is isostructural with the langbeinite-like iron phosphates NaBaFe2(PO4)3 [12] and KBaFe2(PO4)3 [16]. A projection of the structure along the [1 1 1] direction (Fig. 1) shows that the [Fe2(PO4)3]1 framework is consisted by a three-dimensional assemblage of corner-sharing FeO6 octahedra and PO4 tetrahedra. Each octahedron connects six neighboring tetrahedra and reciprocally each tetrahedron links four adjacent octahedra. This association mode leads to the formation of identical Fe2P3O12 units (labeled U) formed by two octahedra, bridged by three tetrahedra (Fig. 2). The connection of adjacent through the common corners of their polyhedra leads to the formation of two kinds of large cavities A(1) and A(2), statistically occupied by the K+ and Sr2+ ions. In the other hand, the units and the cavities alternate along the ternary axes of the cubic cell generating infinite chains that are formed with a sequence of –U–U–A(1)–A(2)–. Table 3 lists main interatomic distances and angles in KSrFe2 (PO4)3. The octahedral Fe(1) and Fe(2) sites are located at the threefold axis. Fe(1)O6 is slightly more distorted than Fe(2)O6 as
Table 2 Atomic positions and thermal parameters Ueq for KSrFe2(PO4)3. Site
Wyckoff
Atom
x (r)
y (r)
z (r)
Ueq (r)
A(1) A(2) Fe(1) Fe(2) P O(1) O(2) O(3) O(4)
4a 4a 4a 4a 12b 12b 12b 12b 12b
0.17K + 0.83Sr 0.83K + 0.17Sr Fe Fe P O O O O
0.0650(1) 0.2986(2) 0.5865(1) 0.8541(1) 0.8731(2) 0.9221(5) 0.9724(7) 0.8431(9) 0.7377(9)
0.0650(1) 0.2986(2) 0.5865(1) 0.8541(1) 0.5385(2) 0.6851(5) 0.4490(6) 0.4903(7) 0.5225(8)
0.0650(1) 0.2986(2) 0.5865(1) 0.8541(1) 0.7706(2) 0.7671(7) 0.6940(8) 0.9107(6) 0.6993(11)
0.0204(3) 0.0275(5) 0.0133(3) 0.0121(3) 0.0092(3) 0.039(2) 0.053(2) 0.064(2) 0.094(4)
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b
Table 3 Selected interatomic distances (Å) and angles (°) in KSrFe2(PO4)3.
a c
Fe(1)O6 Fe(1)–O(1) Fe(1)–O(3) hFe(1)–Oi
1.954(6) 1.999(7) 1.978(7)
(3) (3)
O(4)–Fe(4)–O(4) O(4)–Fe(1)–O(3) O(4)–Fe(1)–O(3)0 O(4)–Fe(1)–O(3)0 0 O(3)–Fe(1)–O(3)0 0 0
89.7(3) 167.5(4) 95.9(4) 79.3(4) 96.1(4)
(3) (3) (3) (3) (3)
Fe(2)O6 Fe(2)–O(1) Fe(2)–O(3) hFe(2)–Oi
1.980(5) 1.997(5) 1.987(5)
(3) (3)
O(1)–Fe(2)–O(1)0 O(1)–Fe(2)–O(2) O(1)0 –Fe(2)–O(2) O(1)0 0 –Fe(2)–O(1)0 O(2)–Fe(2)–O(2)0
93.8(2) 90.1(2) 173.8(3) 90.7(3) 85.1(3)
(3) (3) (3) (3) (3)
PO4 P–O(1) P–O(2) P–O(3) P–O(4) hP–Oi
1.518(5) 1.511(5) 1.487(5) 1.509(6) 1.506(6)
O(3)–P–O(2) O(3)–P–O(4) O(2)–P–O(4) O(3)–P–O(1) O(2)–P–O(1) O(4)–P–O(1) hO–P–Oi
113.7(4) 102.9(6) 106.1(5) 112.7(4) 109.6(3) 111.5(4) 109.41(5)
A(1)O12 A(1)–O(2) A(1)–O(3) A(1)–O(4) A(1)–O(4)0 hA(1)i
2.793(9) 2.99(2) 3.26(2) 2.962(9) 2.808(7)
A(2)O9 A(2)–O(1) A(2)–O(4) A(2)–O(3) hA(2)–Oi
2.761(5) 2.969(6) 3.206(7) 2.979(6)
FeO 6
PO 4 (K,Sr)
Fig. 1. A projection of the KSrFe2(PO4)3 structure along the [1 1 1] direction. The FeO6 octahedra and PO4 polyhedra are represented by solid and hacted polyhedra respectively and the K+ and Sr2+ ions by solid circles.
(3) (3) (3)
The oxidations of all the cations were evaluated by a valence sum calculation by the Brown and Altermatt method [19], using the cation-oxygen distances obtained from X-ray diffraction. The obtained valences of all these sites are in a good agreement with their formal charges. In particular, the BVS values of 3.3 and 3.2, obtained for Fe(1) and Fe(2) are consistent with the +3 oxidation state of iron.
c FeO6 PO4 Fig. 2. The Fe2(PO4)3 unit.
it can be indicated by the cis O–Fe(1)–O angles (from 79.3(4) to 96.1(4)°), more dispersed than the O–Fe(2)–O ones (from 85.1(3) to 93.8(2)°). Corresponding average values of 90.1° and 89.9°, respectively are close to the theoretical value of 90°. The Fe–O bond lengths vary between 1.954(6) and 1.999(7) Å for Fe(1) and between 1.980(5) and 1.997(5) Å for Fe(2). Their mean values of 1.978(6) Å and 1.987(6) Å, respectively are less than the value 2.03 Å, predicted by Shannon for octahedral Fe3+ ions [17]. The PO4 tetrahedron is fairly regular with classical P–O bond lengths ranging from 1.487(5) to 1.518(5) Å. Corresponding mean value of 1.506(6) Å is lower than 1.537 Å, calculated by Baur for the monophosphate groups [18]. The low mean values of the Fe–O and P–O bond lengths compared to the frequently observed ones reflects the high covalence of the structure. The A(1) and A(2) cavities are occupied by a random distribution of K+ and Sr2+. As often observed for alkali and alkaline earth ions, these cavities display a wide range of cation-oxygen distances, thus it is very difficult to distinguish between bonding and non-bonding contacts. A simple criterion is to consider all distances shorter than the shortest (K+, Sr2+) to next cation. Assuming this criterion, the coordination of the A(1) cavity is twelve with A(1)-O distances in the range 2.793(9)–3.26(2) Å while that of A(2) is nine with A(2)–O distances included between 2.761(5) and 3.206(7) Å.
3.2. Mössbauer spectroscopy The room temperature Mössbauer spectrum for KSrFe2(PO4)3 (Fig. 3) consists of two large absorption lines, characteristic of a paramagnetic compound. It was refined in two steps. In the first treatment, Lorentzien peaks were assumed and the position (isomer shift, d), amplitude and width of each peak were refined. This preliminary calculation allowed the determination of experimental hyperfine parameters for the various iron sites present in the compound. The second computation allowed the analysis of spectra in terms of quadrupole splitting distribution P(D) using the method of Hesse and Rubartsch [20]. This method is often used for disordered compounds in which can be found a significant distribution of environments, which gives rise to strong line broadening and to line shapes differing from a Lorentzian profile. For this calculation the half height width C was fixed at 0.20 mm/s and the isomer
VELOCITY (mms-1)
ABSORPTION (%)
b a
(3) (3) (3) (3)
0
-4
-3
-2
-1
0
1
2
3
4
6
12
Fig. 3. The room temperature Mössbauer spectrum for KSrFe2(PO4)3.
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M. Hidouri et al. / Journal of Molecular Structure 1030 (2012) 145–148
Distribution
d (mm s1)
C (mm s1)
Dmax (mm s1)
% Fe
Site
value taking only into account the spin contribution for Fe3+ is 8.37 lB. On the other hand the sign of the Weiss constant points to AFM interactions predominating at very low temperatures.
DS1 DS2
0.451 0.443
0.20 0.20
0.80 0.40
49 51
Fe3+ [Oh] Fe3+ [Oh]
4. Conclusions
Table 4 Room temperature Mössbauer parameters for KSrFe2(PO4)3.
50
0.12
45
-1
-1
40 35
0.08
30 25
0.06
20 15
-1
0.04
1 / (mol.Oe.emu )
(emu.mol .Oe )
0.10
10
0.02
KSrFe2(PO4)3 has been prepared and its structure shown to exhibit a [Fe2(PO4)3]1 3D framework of corner-sharing FeO6 octahedra and PO4 tetrahedra, analogous to those of the langbeinite type. The K+ and Sr2+ ions are statistically occupying two interstitial cavities. The Mössbauer spectroscopy results were consistent with the structure, predicting the exclusive presence of Fe3+ ions occupying two distinct octahedral sites. The Magnetic susceptibility measurements indicated a Curie–Weiss behavior within the temperature range 4.2–300 K. The structural similarity of this compound with the langbeinitelike iron phosphates NaBaFe2(PO4)3 and KBaFe2(PO4)3 emphasizes the flexibility of the [Fe2(PO4)3]1 framework suggesting that it should be possible to prepare new isostructural iron phosphates including other alkali and alkaline earth elements.
5 0
50
100
150
200
250
300
T (K) Fig. 4. Molar magnetic susceptibility and its reciprocal versus temperature for KSrFe2(PO4)3.
shifts fixed at values determined in the first treatment. This refinement showed that the spectrum is well described by considering that it consists of two distributions of quadrupolar splittings, labeled DS1 and DS2. The derived hyperfine parameters are listed in Table 4. The isomer shifts, d1 = 0.451 mm s1 and d2 = 0.443 mm s1 are typical of high spin Fe3+ ions in an octahedral oxygen environment [21]. The difference in maximum quadrupolar splittings (D1max = 0.80 mm s1; D2max = 0.40 mm s1) is in accordance with the difference in the octahedral geometry between Fe(1)O6 and Fe(2)O6, revealed by the crystallographic study. The relative populations (49% for DS1 and 51% for DS2) are consistent with those 50%, 50%, calculated by considering that the Fe(1) and Fe(2) sites are occupying two positions of similar multiplicities. 3.3. Magnetic properties Variable temperature susceptibility measurements for KSrFe2 (PO4)3 have been carried out on a powdered sample in the 4.2– 300 K temperature range. Fig. 4 shows the temperature dependence of the molar magnetic susceptibility vm and its reciprocal v1 m . In all temperature range the thermal evolution of vm follows a Curie– Weiss law, v = C/(T–h), with Cm = 8.33 emu K/mol and h = 72 K. The experimental magnetic moment is 8.16 lB, and the theoretical
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2012. 04.002. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
W. Meisel, H.J. Guttmann, P. Gütlich, Corros. Sci. 23 (1983) 1373. J.B. Moffat, Catal. Rev. – Sci. Eng. 18 (1978) 199. C. Gleitzer, Eur. J. Solid State Inorg. Chem. 28 (1991) 77. A. Padhi, K. Nanjundaswamy, J. Goodenough, J. Electrochem. Soc. 144 (1997) 1168. A. Zeeman, J. Zeeman, Acta Crystallogr. 10 (1957) 409. F. Jona, R. Pepinsky, Phys. Rev. 103 (1956) 1126. T. Hikita, S. Sito, H. Sekiguchi, T. Ikeda, J. Phys. Soc. Jpn. 42 (1977) 1656. D. Speer, E. Salite, Phys. Chem. Miner. 13 (1986) 17. Z.-J. Zhang, J.-L. Yuan, Solid State Sci. 11 (2009) 549. B. Bezina, M. Glogarova, Phys. Status Solidi A 22 (1979) 69. T. Hikita, M. Kitabatake, T. Ikeda, J. Phys. Soc. Jpn. 49 (1980) 1421. M. Hidouri, H. Jerbi, M.B. Amara, Acta Crystallogr. E64 (2008) i51. B. Lajmi, M. Hidouri, A. Wattiaux, L. Fournes, J. Darriet, M.B. Amara, J. Alloys Compd. 361 (2003) 77. A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, J. Appl. Crystallogr. 26 (1993) 343. G.M. Sheldrick, SHELXL97, A Program for the Solution of the Crystal Structures, University of Göttingen, Göttingen, Germany (1997). P.D. Battle, A.K. Cheetham, W.T.A. Harrison, G.J. Long, J. Solid State Chem. 62 (1986) 16. R.D. Shannon, Acta Crystallogr. A32 (1976) 751. W.H. Baur, Acta Crystallogr. B 30 (1974) 1195. I.D. Brown, D. Altermatt, Acta Crystallogr. B41 (1985) 244. J. Hesse, A. Rubartsch, J. Phys. E: Sci. Instrum. 7 (1974) 526–532. F. Menil, J. Phys. Chem. Solids 46 (7) (1985) 763.
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Synthesis and characterization of a new iron phosphate KSrFe2(PO4)3 with a langbeinite type structure Mourad Hidouri a,⇑, María Luisa López b, Carlos Pico b, Alain Wattiaux c, Mongi Ben Amara a a
UR Matériaux Inorganiques, 5019 Faculté des Sciences, Université de Monastir, Tunisia Departamento de Química Inorgánica I, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain c Institut de Chimie de la Matière Condensée de Bordeaux, CNRS, Université de Bordeaux I, 87 Avenue du Dr. A. Schweitzer, 33608 Pessac-Cedex, France b
a r t i c l e
i n f o
Article history: Received 16 January 2012 Received in revised form 31 March 2012 Accepted 2 April 2012 Available online 10 April 2012 Keywords: Inorganic materials Chemical synthesis Crystal structure Magnetic measurements Mössbauer spectroscopy
a b s t r a c t Synthesis, crystal structure, Mössbauer spectroscopy and magnetic susceptibility of a new phosphate KSrFe2(PO4)3 are described. This compound crystallizes with the unit cell parameter a = 9.809(2) Å of the cubic space group P213 and four formula units per cell. Its structure belongs to the well known langbeinite type. It consists of a 3D framework resulting from a corner-sharing between FeO6 octahedra and PO4 tetrahedra. This framework delimits two distinct cavities, statistically occupied by the K+ and Sr2+ ions. The Mössbauer spectroscopy gave clear evidence of the exclusive presence of octahedral Fe3+ ions. The magnetic susceptibility measurements revealed that the compound follows the Curie–Weiss behavior with an experimental effective moment per unit formula l = 8.16 lB, consistent with the ideal spin only value of 8.37 lB. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Iron phosphates are extensively studied for their interesting applications as prospective materials in corrosion inhibition [1] and heterogeneous catalysis [2]. These materials are also exhibited by the richness of their crystal chemistry owing to the possible +2/ +3 mixed valence of iron and its tendency to form with the phosphate groups various complex frameworks [3]. The interest devoted to these materials has been accentuated since the discovery, by Padhi et al. in 1997 of LiFePO4 as a promising electrode material for Li-ion batteries [4]. The langbeinite K2Mg2(SO4)3 [5] is the prototype of a large number of sulfates, vanadates and phosphates which have been the subject of intensive research for their potential applications in ferroelasticity, ferroelectricity and luminescence [6–9]. Most of them are cubic with space group P213 but some of them undergo P213 ? P212121 phase transitions [10,11]. The structure consists of a [M2(XO4)3]1 3D mixed framework, built up by corner-sharing MO6 octahedra and XO4 tetrahedra with each octahedron is linked to six tetrahedra and each tetrahedron to four octahedra. This framework delimits large interstitial cavities available to the alkaline metals and divalent cations. The present work is a part of our activity devoted to the synthesis and characterization of new iron phosphates belonging to the ⇑ Corresponding author. E-mail address: [email protected] (M. Hidouri). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.04.002
A2O–MO–Fe2O3–P2O5 systems where A and M are an alkali metal and a divalent cation, respectively. During this study, we recently described the langbeinite structures of the monophosphate NaBaFe2(PO4)3 [12] and the oxophosphate K11Fe15(PO4)18O [13]. In this paper, we report on the synthesis and characterization by single X-ray diffraction, Mössbauer spectroscopy and magnetic susceptibility of langbeinite-like monophosphate KSrFe2(PO4)3. 2. Experimental 2.1. Synthesis Single crystals of KSrFe2(PO4)3 were grown in a flux of potassium dimolybdate K2Mo2O7 with molar ratio of product: flux = 2:1 starting from 2.043 g of KNO3, 1.506 g of SrCO3, 8.203 g of Fe(NO3)39H2O, 4.002 g of (NH4)2HPO4 and 1.454 g of MoO3. These reactants were mixed in nitric acid and the resulting solution was evaporated to dryness by heating at 353 K. The obtained dry residue was ground in an agate mortar and then heated for 24 h at 673 K in order to remove the decomposition products: NH3, CO3, etc. The process was then followed by a further heating for 12 h at 873 K with intermediate grinding. After being reground, the product was melted for 1 h at 1373 K, and then slowly cooled at a rate of 10 K h1 to 673 K, then at a rate of 50 K h1 to room temperature. The crystals obtained by washing the final product with warm water, in order to dissolve the flux, are essentially composed by yellowish and irregularly shaped crystals. Their qualitative microprobe analysis indicated the exclusive presence of K, Sr, Fe
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M. Hidouri et al. / Journal of Molecular Structure 1030 (2012) 145–148
which significantly exceed those of the remaining atoms. This result was taken as an indication of the existence of some disorder around these atoms. However, a refinement based on this hypothesis shown to be unsuccessful since negative values of the thermal Ueq parameter were obtained. In order to take into account this apparent disorder of the O(2), O(3) and O(4) atoms, alternative refinements were performed using lower symmetries (ex: SG. P212121), associated to a possible distortion of langbeinite structure. Unfortunately, theses refinements did not led to better results. The proposed model is then the best one given the data obtained. At the end of refinement, anisotropic displacement were allowed to refine resulting in the final reliability factors R1 = 0.034 and wR2 = 0.088 and the atomic coordinates given in Table 2.
Table 1 Crystal data and structure refinement parameters for KSrFe2(PO4)3. Crystal data Formula unit Crystal system Space group a (Å) V (Å3) Z qcal (g cm3)
KSrFe2(PO4)3 Cubic P213 9.809(2) 943.8(2) 4 3.68
Intensity measurements Crystal dimensions (mm3) Wavelength (Å) Monochromator l (mm1) h Range for data collection No. of unique reflections; Rin No. of retained reflections [Fo > 4r(Fo)]
(0.17 0.17 0.19) (Mo Ka) = 0.7103 Graphite 9.68 2.9–29° 821; 0.029 785
Structure solution and refinement Intensity correction Structure solution Agreement factors Number of refined parameters (Dq)max, min (e Å3)
Lorentz–Polarization Direct methods R1 = 0.036; wR2 = 0.089; S = 1.13 62 0.94; 0.83
2.3. Characterization
and P in an atomic ratio approximating 1:1:2:3, in accordance with the KSrFe2(PO4)3 composition. After its structure determination, the title compound was prepared in the powder form by the conventional solid state reaction. After an initial treatment similar to that undertaken for the synthesis of the single crystals until 873 K, the sample was subjected to final calcinations at 1273 K for 48 h with intermittent grinding. A yellowish powder was obtained by quenching in air.
The purity of the synthesized powder was checked from the examination of its X-ray powder diagram, registered in the range 5° 6 2h 6 80° on a PANalytical diffractometer using Cu Ka radiation (k = 1.5406 Å). The 57Fe Mössbauer spectrum was recorded at 293 K by a HALDER constant acceleration spectrometer using a room-temperature 57 Co/Rh source. The sample contained an average of 10 mg/cm2 of iron, a concentration for which the line broadening due to the thickness effects is not noticeable. Isomer shifts were calculated in reference to the natural a-Fe. Magnetic susceptibility measurements were performed in the 4.2–300 K temperature range by a Quantum Design SQUID MPMS-XL magnetometer at a constant magnetic field of 0.5 T. 3. Results and discussion
2.2. Structural determination
3.1. Description of the structure
The crystal structure of KSrFe2(PO4)3 was determined by singlecrystal X-ray diffraction, using a crystal of dimensions 0.17 0.17 0.19 mm3. Data were collected by a Bruker–Nonius diffractometer, equipped with a bidimensional CCD detector and using a graphite monochromated Mo Ka radiation (k = 0.71073 Å). Main crystallographic data and conditions for structure determination are reported in Table 1. Intensity data were corrected from absorption effects using SADABS (Bruker–Nonius area detector scaling and absorption correction version). All reflections could be indexed based on a primitive cubic unit cell with a = 9.809(2) Å. The systematic absences clearly indicated the space group P213. The starting structural model was extracted by direct methods [14] leading to two distinct positions for the Sr atoms. The atomic species were progressively located from Fourier difference maps [15] using the positions of the previously located atoms. Refinement of the occupancy factors suggested a statistical distribution between the Sr and K with identical partial occupancies. The refinement showed that O(2), O(3) and O(4) atoms exhibit large thermal parameters
KSrFe2(PO4)3 is isostructural with the langbeinite-like iron phosphates NaBaFe2(PO4)3 [12] and KBaFe2(PO4)3 [16]. A projection of the structure along the [1 1 1] direction (Fig. 1) shows that the [Fe2(PO4)3]1 framework is consisted by a three-dimensional assemblage of corner-sharing FeO6 octahedra and PO4 tetrahedra. Each octahedron connects six neighboring tetrahedra and reciprocally each tetrahedron links four adjacent octahedra. This association mode leads to the formation of identical Fe2P3O12 units (labeled U) formed by two octahedra, bridged by three tetrahedra (Fig. 2). The connection of adjacent through the common corners of their polyhedra leads to the formation of two kinds of large cavities A(1) and A(2), statistically occupied by the K+ and Sr2+ ions. In the other hand, the units and the cavities alternate along the ternary axes of the cubic cell generating infinite chains that are formed with a sequence of –U–U–A(1)–A(2)–. Table 3 lists main interatomic distances and angles in KSrFe2 (PO4)3. The octahedral Fe(1) and Fe(2) sites are located at the threefold axis. Fe(1)O6 is slightly more distorted than Fe(2)O6 as
Table 2 Atomic positions and thermal parameters Ueq for KSrFe2(PO4)3. Site
Wyckoff
Atom
x (r)
y (r)
z (r)
Ueq (r)
A(1) A(2) Fe(1) Fe(2) P O(1) O(2) O(3) O(4)
4a 4a 4a 4a 12b 12b 12b 12b 12b
0.17K + 0.83Sr 0.83K + 0.17Sr Fe Fe P O O O O
0.0650(1) 0.2986(2) 0.5865(1) 0.8541(1) 0.8731(2) 0.9221(5) 0.9724(7) 0.8431(9) 0.7377(9)
0.0650(1) 0.2986(2) 0.5865(1) 0.8541(1) 0.5385(2) 0.6851(5) 0.4490(6) 0.4903(7) 0.5225(8)
0.0650(1) 0.2986(2) 0.5865(1) 0.8541(1) 0.7706(2) 0.7671(7) 0.6940(8) 0.9107(6) 0.6993(11)
0.0204(3) 0.0275(5) 0.0133(3) 0.0121(3) 0.0092(3) 0.039(2) 0.053(2) 0.064(2) 0.094(4)
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b
Table 3 Selected interatomic distances (Å) and angles (°) in KSrFe2(PO4)3.
a c
Fe(1)O6 Fe(1)–O(1) Fe(1)–O(3) hFe(1)–Oi
1.954(6) 1.999(7) 1.978(7)
(3) (3)
O(4)–Fe(4)–O(4) O(4)–Fe(1)–O(3) O(4)–Fe(1)–O(3)0 O(4)–Fe(1)–O(3)0 0 O(3)–Fe(1)–O(3)0 0 0
89.7(3) 167.5(4) 95.9(4) 79.3(4) 96.1(4)
(3) (3) (3) (3) (3)
Fe(2)O6 Fe(2)–O(1) Fe(2)–O(3) hFe(2)–Oi
1.980(5) 1.997(5) 1.987(5)
(3) (3)
O(1)–Fe(2)–O(1)0 O(1)–Fe(2)–O(2) O(1)0 –Fe(2)–O(2) O(1)0 0 –Fe(2)–O(1)0 O(2)–Fe(2)–O(2)0
93.8(2) 90.1(2) 173.8(3) 90.7(3) 85.1(3)
(3) (3) (3) (3) (3)
PO4 P–O(1) P–O(2) P–O(3) P–O(4) hP–Oi
1.518(5) 1.511(5) 1.487(5) 1.509(6) 1.506(6)
O(3)–P–O(2) O(3)–P–O(4) O(2)–P–O(4) O(3)–P–O(1) O(2)–P–O(1) O(4)–P–O(1) hO–P–Oi
113.7(4) 102.9(6) 106.1(5) 112.7(4) 109.6(3) 111.5(4) 109.41(5)
A(1)O12 A(1)–O(2) A(1)–O(3) A(1)–O(4) A(1)–O(4)0 hA(1)i
2.793(9) 2.99(2) 3.26(2) 2.962(9) 2.808(7)
A(2)O9 A(2)–O(1) A(2)–O(4) A(2)–O(3) hA(2)–Oi
2.761(5) 2.969(6) 3.206(7) 2.979(6)
FeO 6
PO 4 (K,Sr)
Fig. 1. A projection of the KSrFe2(PO4)3 structure along the [1 1 1] direction. The FeO6 octahedra and PO4 polyhedra are represented by solid and hacted polyhedra respectively and the K+ and Sr2+ ions by solid circles.
(3) (3) (3)
The oxidations of all the cations were evaluated by a valence sum calculation by the Brown and Altermatt method [19], using the cation-oxygen distances obtained from X-ray diffraction. The obtained valences of all these sites are in a good agreement with their formal charges. In particular, the BVS values of 3.3 and 3.2, obtained for Fe(1) and Fe(2) are consistent with the +3 oxidation state of iron.
c FeO6 PO4 Fig. 2. The Fe2(PO4)3 unit.
it can be indicated by the cis O–Fe(1)–O angles (from 79.3(4) to 96.1(4)°), more dispersed than the O–Fe(2)–O ones (from 85.1(3) to 93.8(2)°). Corresponding average values of 90.1° and 89.9°, respectively are close to the theoretical value of 90°. The Fe–O bond lengths vary between 1.954(6) and 1.999(7) Å for Fe(1) and between 1.980(5) and 1.997(5) Å for Fe(2). Their mean values of 1.978(6) Å and 1.987(6) Å, respectively are less than the value 2.03 Å, predicted by Shannon for octahedral Fe3+ ions [17]. The PO4 tetrahedron is fairly regular with classical P–O bond lengths ranging from 1.487(5) to 1.518(5) Å. Corresponding mean value of 1.506(6) Å is lower than 1.537 Å, calculated by Baur for the monophosphate groups [18]. The low mean values of the Fe–O and P–O bond lengths compared to the frequently observed ones reflects the high covalence of the structure. The A(1) and A(2) cavities are occupied by a random distribution of K+ and Sr2+. As often observed for alkali and alkaline earth ions, these cavities display a wide range of cation-oxygen distances, thus it is very difficult to distinguish between bonding and non-bonding contacts. A simple criterion is to consider all distances shorter than the shortest (K+, Sr2+) to next cation. Assuming this criterion, the coordination of the A(1) cavity is twelve with A(1)-O distances in the range 2.793(9)–3.26(2) Å while that of A(2) is nine with A(2)–O distances included between 2.761(5) and 3.206(7) Å.
3.2. Mössbauer spectroscopy The room temperature Mössbauer spectrum for KSrFe2(PO4)3 (Fig. 3) consists of two large absorption lines, characteristic of a paramagnetic compound. It was refined in two steps. In the first treatment, Lorentzien peaks were assumed and the position (isomer shift, d), amplitude and width of each peak were refined. This preliminary calculation allowed the determination of experimental hyperfine parameters for the various iron sites present in the compound. The second computation allowed the analysis of spectra in terms of quadrupole splitting distribution P(D) using the method of Hesse and Rubartsch [20]. This method is often used for disordered compounds in which can be found a significant distribution of environments, which gives rise to strong line broadening and to line shapes differing from a Lorentzian profile. For this calculation the half height width C was fixed at 0.20 mm/s and the isomer
VELOCITY (mms-1)
ABSORPTION (%)
b a
(3) (3) (3) (3)
0
-4
-3
-2
-1
0
1
2
3
4
6
12
Fig. 3. The room temperature Mössbauer spectrum for KSrFe2(PO4)3.
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M. Hidouri et al. / Journal of Molecular Structure 1030 (2012) 145–148
Distribution
d (mm s1)
C (mm s1)
Dmax (mm s1)
% Fe
Site
value taking only into account the spin contribution for Fe3+ is 8.37 lB. On the other hand the sign of the Weiss constant points to AFM interactions predominating at very low temperatures.
DS1 DS2
0.451 0.443
0.20 0.20
0.80 0.40
49 51
Fe3+ [Oh] Fe3+ [Oh]
4. Conclusions
Table 4 Room temperature Mössbauer parameters for KSrFe2(PO4)3.
50
0.12
45
-1
-1
40 35
0.08
30 25
0.06
20 15
-1
0.04
1 / (mol.Oe.emu )
(emu.mol .Oe )
0.10
10
0.02
KSrFe2(PO4)3 has been prepared and its structure shown to exhibit a [Fe2(PO4)3]1 3D framework of corner-sharing FeO6 octahedra and PO4 tetrahedra, analogous to those of the langbeinite type. The K+ and Sr2+ ions are statistically occupying two interstitial cavities. The Mössbauer spectroscopy results were consistent with the structure, predicting the exclusive presence of Fe3+ ions occupying two distinct octahedral sites. The Magnetic susceptibility measurements indicated a Curie–Weiss behavior within the temperature range 4.2–300 K. The structural similarity of this compound with the langbeinitelike iron phosphates NaBaFe2(PO4)3 and KBaFe2(PO4)3 emphasizes the flexibility of the [Fe2(PO4)3]1 framework suggesting that it should be possible to prepare new isostructural iron phosphates including other alkali and alkaline earth elements.
5 0
50
100
150
200
250
300
T (K) Fig. 4. Molar magnetic susceptibility and its reciprocal versus temperature for KSrFe2(PO4)3.
shifts fixed at values determined in the first treatment. This refinement showed that the spectrum is well described by considering that it consists of two distributions of quadrupolar splittings, labeled DS1 and DS2. The derived hyperfine parameters are listed in Table 4. The isomer shifts, d1 = 0.451 mm s1 and d2 = 0.443 mm s1 are typical of high spin Fe3+ ions in an octahedral oxygen environment [21]. The difference in maximum quadrupolar splittings (D1max = 0.80 mm s1; D2max = 0.40 mm s1) is in accordance with the difference in the octahedral geometry between Fe(1)O6 and Fe(2)O6, revealed by the crystallographic study. The relative populations (49% for DS1 and 51% for DS2) are consistent with those 50%, 50%, calculated by considering that the Fe(1) and Fe(2) sites are occupying two positions of similar multiplicities. 3.3. Magnetic properties Variable temperature susceptibility measurements for KSrFe2 (PO4)3 have been carried out on a powdered sample in the 4.2– 300 K temperature range. Fig. 4 shows the temperature dependence of the molar magnetic susceptibility vm and its reciprocal v1 m . In all temperature range the thermal evolution of vm follows a Curie– Weiss law, v = C/(T–h), with Cm = 8.33 emu K/mol and h = 72 K. The experimental magnetic moment is 8.16 lB, and the theoretical
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2012. 04.002. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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