Crystal structure determination of In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O from X-ray powder diffraction

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1 Z. Kristallogr. 219 (2004) # by Oldenbourg Wissenschaftsverlag, München Crystal structure determination of In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O from X-ray powder diffraction Ludmila S. Ivashkevich*, Alexander S. Lyakhov, Anatoly F. Selevich and Anatoly I. Lesnikovich Physico-Chemical Research Institute of Belarusian State University, Leningradskaya 14, Minsk, , Belarus Received February 17, 2004; accepted May 13, 2004 Indiumhydrogenphosphates / Orthophosphates / Powder diffraction structure analysis / X-ray diffraction Abstract. The crystal structure of indium hydrogen phosphate hydrate, In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O, has been determined from X-ray powder diffraction data by using direct methods (EXPO program), and refined by FULLPROF package to R Bragg value of 5.6%. The structure is monoclinic, space group C2/c (No. 15), unit cell dimensions a ¼ (5), b ¼ (3), c ¼ (4) A, b ¼ (2), V ¼ (1) A 3, Z ¼ 4, D x ¼ 2.73 g/cm 3. Hydrogen atoms were placed on calculated positions. There are two unique indium atoms in the crystal structure. InO 6 octahedra are connected together via the corners of PO 4 tetrahedra to form layers parallel to the xy plane. The layers are linked together by water molecules, located in the interlayer space, through their hydrogen bonds. Within the layers there are hollows occupied by oxonium cations. The compound was found to be isostructural with salts of the composition M III 3M I (H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O, where M III trivalent metal, M I monovalent metal, oxonium or ammonium. 1. Introduction To date, a series of inorganic phosphates of the composition M III 3M I (H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O, where M III trivalent metal, M I monovalent metal, oxonium or ammonium, has been synthesized. Five representatives of the series have been structurally characterized: aluminium-oxonium (Brodalla, Kniep, 1986), iron(iii)-oxonium (Bosman, Beurskens, Smits, Behm, Mintjens, Meisel, Fuggle, 1986), iron(iii)- potassium (Anisimova, Chudinova, Serafin, 1997), iron(iii)- ammonium (Mgaidi, Boughzala, Driss, Clerac, Coulon, 1999), and mixed-valence thallium(iii)-[thallium(i), oxonium] (Chiadmi,Vicat,Tran Qui, Boudjada, 1985; Marsh, 1986) salts. All the compounds were found to be monoclinic, with space group C2/c, and are isostructural. This paper presents the crystal structure of the compound of the total composition In 3 H 15 (PO 4 ) 8 5H 2 O with rather heavy indium ions of large radius. Comparison of * Correspondence author ( iva@bsu.by) powder diffraction pattern of the compound with those for the above investigated salts did not allow to solve the question, whether it was isostructural to them. In view of this circumstance, we have undertaken a detailed investigation of the compound from unit cell determination to refinement of the solved structure. Because of small size of prepared crystals, the structure was determined from powder diffraction data. 2. Experimental 2.1 Sample preparation The sample was prepared by thin layer method using technique described earlier (Selevich, Lyakhov, Lesnikovich, 2000). A mixture of In(NO 3 ) 3 and H 3 PO 4 (molar ratio In 2 O 3 :P 2 O 5 ¼ 1 : 4) was held in thin layer of 2 3 mm in a wide quartz crucible at 20 C for several days until intensive crystallization began. The process was monitored by optical microscope and X-ray powder diffraction. The crystalline product was washed with acetone and air dried at room temperature. Composition of the compound was determined by chemical analysis, IR spectroscopy and thermal analysis. 2.2 Apparatus X-ray diffraction pattern was collected with a HZG 4A powder diffractometer (Carl Zeiss, Jena) equipped with a horizontal goniometer (a radius of 250 mm). The tube current and voltage were 30 ma and 40 kv, respectively. The CuK a radiation was selected by means of a Ni filter. Receiving slit of 0.22 mm and Soller slits positioned on the incident beam were used. The powder diffraction pattern was scanned in a step mode in the angular range 8 J 100, with t ¼ 20 s and DJ ¼ Results 3.1 Indexing procedure results The peak positions, necessary to perform the indexing procedure, were evaluated using the program POWDERX (Dong, 1999). The indexing of the diffraction pattern

2 544 L. S. Ivashkevich, A. S. Lyakhov, A. F. Selevich et al. was performed with the program TREOR90 (Werner, Eriksson, Westdahl, 1985). Using 41 experimental lines only one solution was found. The obtained orthorhombic unit cell dimensions were a ¼ (9), b ¼ 9.668(6), and c ¼ 17.92(1) A. All 41 lines were indexed according to this unit cell. Analysis of the Miller indices revealed the following reflections conditions: h k l, h þ k ¼ 2n; h 0 l, l ¼ 2n, h ¼ 2n (0 k 0 reflections were not detected). These conditions do not contradict a C-centered unit cell and presence of the glide plane c perpendicular to the b axis. Note, that the space group C2/c was found earlier for aluminium and iron(iii) salts of the composition M III 3M I (H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O, and their unit cell dimensions a, b, c are similar to those found for the present compound, but monoclinic b angles lie in the range of 90.64(6) 90.91(2). These circumstances allowed to assume that the indium salt might be also monoclinic, with C2/c space group and the monoclinic b angle very close to 90. This assumption was examined in further analysis. 3.2 Structure determination and refinement Structure determination of the investigated compound has been carried out by using EXPO package (Altomare, Burla, Camalli, Carrozzini, Cascarano, Giacovazzo, Guagliardi, Moliterni, Polidori, Rizzi, 1999). Attempts to solve the crystal structure were performed using the monoclinic space group C2/c (No. 15). Default run of the EXPO program, starting from the obtained unit cell dimensions and the b angle equal to 90, gave only indium atoms positions, combined figure of merit CFOM being No preferred orientation of crystallites in the sample was found during the program run. However, the run gave an important result, consisting in obtaining the b angle equal to This was achieved due to the use of a high angle part of the diffraction pattern. The second run of the EXPO program was performed with unit cell dimensions found in the previous calculation. The obtained results were: finding [001] preferred orientation with G ¼ 0.43; CFOM value was 0.49; all indium and phosphorus atoms and a part of oxygen ones were found on reasonable positions. The third run of the program was carried out using unit cell dimensions found in the second calculation. Good results were obtained in this case: preferred orientation effect for [001] direction was characterized by G ¼ 0.61; CFOM value was 0.54; reasonable positions of all indium, phosphorus and oxygen atoms were found. Fourier map was used for reliable identification of isolated oxygen atoms of water molecules (or oxonium cation). The discrepancy factor R F was 15.2% in this stage. The investigated compound presents an example when structure determination procedure is very sensitive to the values of initial cell dimensions. The reason probably is that, rather large unit cell parameters result in high density of overlapping reflections (especially in high-angle part of the pattern), what has an effect on finding the reflections intensities. Comparison of the found crystal structure with those of the aluminium and iron(iii) salts of the composition M III 3M I (H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O showed that all they were isostructural. In view of this fact, further the composition of the investigated compound can be written as In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O. Full profile fitting was performed with the FULLPROF program (Rodriguez-Carvajal, 1990) for the final refinement of the structure. The profile shape function applied was pseudo-voigt. Correction for profile asymmetry was performed for reflections below 40 (2J). Background values were first manually determined and afterwards adjusted by a Fourier filtering technique. The full profile refinement involved the following variables: 71 non-hydrogen atomic coordinates, 10 isotropic displacement parameters, scale factor, zero-point, 4 unit cell dimensions, 1 preferred orientation parameter G 1 in Marsh Dollase function, 4 asymmetry and 3 half-width parameters. Parameters of pseudo- Voigt function were estimated at the first stage of the refinement and further were kept fixed to decrease the number of variables. Because of large number of variables, displacement parameters of all the oxygen atoms of the phosphate groups were refined as one parameter, and preferred orientation parameter G 2, which was found to be equal to 0, was excluded from the final refinement. Because there are 100 hydrogen atoms in the unit cell of In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O, it was advantageous to place them on calculated positions. The coordinates of the hydroxyl, water and oxonium hydrogen atoms were calculated on the basis of geometric and force-field considerations (Nardelli, 1999) using WINGX package (Farrugia, 1999). Oxygen atoms of POH groups were identified by comparing the crystal structure of the investigated compound with that of aluminium-oxonium isostructural analogue, for which hydrogen atoms were determined. Hydrogen atoms positions for oxonium cation were calculated by means of two sequential operation for water molecule H atoms location. Accepted model of H atoms disorder for H 3 O þ cation will be discussed below. Displacement parameters for H atoms were taken as 1.5B iso of carrier oxygen atom. The summary of the refinement details is given in Table 1. Figure 1 shows comparison of the observed and calculated diffraction patterns in the angular range 8 2J 70. The refined unit cell parameters are a ¼ (5), b ¼ (3), c ¼ (4) A, b ¼ (2), V ¼ (1) A 3, Z ¼ 4. Calculated substance density D x is 2.73 g/cm 3. Table 1. Summary of full profile refinement details for In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O. Number of points 4601 Number of reflections 1581 Number of fitted parameters 95 Zero-point (4) Preferred orientation parameter G (2) [plane (001)] R p (%) 9.2 R wp (%) 13.0 R expected (%) 15.4 R Bragg (%) 5.6 R F (%) 5.9

3 Crystal structure determination of In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O 545 Intensity (a.u.) Intensity (a.u.) a θ (º) H3 O17 O3 H18A H17B H17A P1 O18 O4 H8 O8 O2 H18B O5 H4 P2 In1 O7 O6 O16 H19B In2 O13 P4 O10 O15 H15 P3 O12 H12 O11 H11 O19 O14 O1 H19A H14 Fig. 2. Asymmetric unit of In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O with the atom labelling scheme. O9 b Discussion 2 θ (º) Fig. 1. The observed (dots) and calculated (line) patterns of In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O after the Rietveld refinement (two zoomed areas). The difference curve is shown in the lower part of the picture. Figure 2 shows the asymmetric unit of the investigated compound. Atomic positions and isotropic displacement parameters obtained after the final refinement are presented in Table 2. Selected interatomic distances are given in Table 3. There are two indium atoms in the asymmetric unit of the investigated compound. In1 atom is located on a general position, In2 on the (4e) Wyckoff site (International Tables for Crystallography, 1992). All the remainder atoms are on general positions, except for O19 atom lying on the (4e) Wyckoff site. Oxygen atoms O17, O18, and O19 are not included in phosphoric groups and belong to water molecules (O17, O18) or to oxonium cation (O19). This oxygen atoms assignment follows from the comparison of the obtained crystal structure with those for the investigated earlier isostructural aluminium-oxonium (Brodalla, Kniep, 1986) and iron(iii)-oxonium (Bosman, et al., 1986) salts. The comparison also indicated the oxygen atoms of POH groups to place their hydrogen atoms on calculated positions (Fig. 2). P O bond lengths in phosphoric groups are given in Table 3. O P O bond angles lie in the ranges 97(3) 119(4), 104(3) 119(4), 94(3) 117(4), and 107(4) 109(4) for P1, P2, P3, and P4 phosphate groups, respectively. The indium atoms have octahedral oxygen coordination, with In O distances lying in the ranges 2.07(3) 2.19(4) A and 2.06(3) 2.22(4) A for In1 and In2 atoms, respectively (Table 3). The angles O In O are in the ranges 78(2) 102(2), 164(3) 179(3) for In1 atom, and 84(2) 101(2), 167(3) 171(3) for In2 one. Each indium atom is monodentately bonded with six phosphate groups. All the indium octahedrons are linked together via the phosphate group bridges to form layers parallel to the xy plane (Figs. 3, 4). Two such layers across the unit cell. Within the layers there are hollows, occupied by oxonium cations. Both water molecules, with O17 and O18 oxygen atoms, are positioned in the interlayer space. H 3 O þ cation, with pyramidal geometry, is located on twofold axis, so that there is no possibility to achieve a reasonable geometry of the cation without disorder of its H atoms. It should be noted that no hydrogen atoms were localized for isostructural iron(iii)-oxonium salt (Bosman, et al., 1986). H atom positions of H 3 O þ cation, found for B 0 Fig. 3. Atomic arrangement in a layer of In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O (view along the c axis). The largest circles show the indium atoms, remainder ones-phosphorus, oxygen and hydrogen atoms in size reduction sequence. Isolated circles present O19 atoms of oxonium cations located in hallows. A

4 546 L. S. Ivashkevich, A. S. Lyakhov, A. F. Selevich et al. C Table 2. Atomic coordinates and isotropic displacement parameters (A 2 ) for In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O obtained from the Rietveld refinement. Atom x y z B iso 0 Fig. 4. Layered structure of In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O (view along the b axis). Atoms representation corresponds to that in Fig. 3. aluminium-oxonium compound (Brodalla, Kniep, 1986), are erroneous, because unreasonable geometry of the cation is arrived from the given atomic coordinates, and also any disoder model was not discussed in the paper. So, no reliable information is available about the hydrogen atoms positions and H atom disoder model for H 3 O þ cation to evaluate adequacy of the calculated cation geometry for indium-oxonium salt. Figure 5 shows the calculated geometry of the cation and its environment in the structure of the investigated compound. Four H atom sites were found for oxonium cation: H19A, H19A i, H19B, H19B i (symmetry code: (i) x, y, 1 = 2 z). Two of them, H19B and H19B i, are separated by 0.607A. In view of this circumstance the site occupancy factor for H19B atom position was put to 0.5 to satisfy the composition of the cation. This model provides the cation with opportunity to formate three hydrogen A In (3) (6) (2) 1.9 (1) In (7) (2) P (1) 0.200(2) (8) 1.2(5) P (1) 0.239(2) (9) 2.0(5) P (1) 0.445(2) (9) 2.8(6) P (1) 0.015(2) 0.860(1) 2.8(6) O (2) 0.320(4) 0.700(2) 1.5(2) O (2) 0.124(4) 0.664(1) 1.5(2) O (2) 0.280(4) 0.562(2) 1.5(2) O (2) 0.114(4) 0.608(2) 1.5(2) O (2) 0.199(4) 0.672(2) 1.5(2) O (2) 0.387(4) 0.687(1) 1.5(2) O (2) 0.136(4) 0.682(2) 1.5(2) O (2) 0.241(4) 0.564(2) 1.5(2) O (2) 0.310(4) 0.833(2) 1.5(2) O (2) 0.531(5) 0.833(2) 1.5(2) O (2) 0.423(3) 0.945(2) 1.5(2) O (2) 0.519(4) 0.871(2) 1.5(2) O (2) 0.001(4) 0.815(2) 1.5(2) O (2) 0.164(5) 0.881(2) 1.5(2) O (2) 0.075(3) 0.934(2) 1.5(2) O (2) 0.027(4) 0.822(2) 1.5(2) O (3) 0.216(4) 0.495(1) 2.3(3) O (2) 0.451(4) 0.501(1) 3.0(4) O (6) (5) H H H H H H H H17A H17B H18A H18B H19A H19B a O1 i H19A i H19A O1 a: Site occupancy factor for H19B atom of oxonium cation is equal to 0.5. O13 i H19B i H19B O13 Fig. 5. Calculated geometry of the H 3 O þ cation and its environment in the In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O structure (view along the c axis). Dashed lines show hydrogen bonds of the cation. Site occupancy factor of H19B atom is of 0.5. Symmetry code: (i) x, y, 1 = 2 z. bonds: O19 H19A O1 and O19 H19A i O1 i, with D A distance of A and D H A angle of 173, and O19 H19B O13 or O19 H19B i O13 i with D A distance of A and D H A angle of 162. It is also of interest to compare calculated positions of H atoms, belonging to POH group and water molecules, with those, determined for aluminium-oxonium isostructural analogue. It was found from this comparison that except for H14, H17A, H17B, H18A and H18B, all the remainder hydrogen atoms were placed satisfactory. It should be noted that location of hydrogen atoms on the

5 Crystal structure determination of In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O 547 Table 3. P O and In O bond lengths (A) in the crystal structure of In 3 (H 3 O)(H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O. Symmetry codes a: 1 = 2 x, y þ 1 = 2, 3 = 2 z; b: 1 = 2 x, y 1 = 2, 3 = 2 z; c: x, y, 3 = 2 z. P1 O1 1.64(4) P3 O9 1.43(4) P1 O2 1.59(4) P3 O (4) P1 O3 1.66(3) P3 O (4) P1 O4 1.56(4) P3 O (4) P2 O5 1.55(4) P4 O (4) P2 O6 1.58(4) P4 O (5) P2 O7 1.59(4) P4 O (3) P2 O8 1.66(3) P4 O (4) In1 O1 a 2.08(4) In1 O (3) In1 O2 2.13(3) In2 O7, O7 c 2.06(3) In1 O5 2.19(4) In2 O9, O9 c 2.22(4) In1 O6 b 2.17(3) In2 O13, O13 c 2.14(3) In1 O10 b 2.07(3) calculated positions have rather strong effect on the results, giving more reasonable values of P O bonds. In conclusion, we would like to remark the reasons responsible for rather low precision of the obtained structural parameters. The first reason consists in a large number of refined parameters (even if displacement parameters of all oxygen atoms of phosphate groups are refined as one parameter, this number is equal to 95). The second one is related to sharp decrease of reflections intensities with scattering angle rise, what reduces precision of the obtained experimental data. The third reason arises from essential preferred orientation of crystal grains in the sample, but all the used corrections for this effect are only a crude approximation. The forth complication is due to rather small crystal size resulting in broadening of reflections. Moreover, high density of reflections on powder diffraction pattern caused some problems at the stage of the structure solution. References Altomare, A.; Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Rizzi, R.: EXPO: a program for full powder pattern decomposition and crystal structure solution. J. Appl. Crystallogr. 32 (1999) Anisimova, N.; Chudinova, N.; Serafin, M.: Preparation and crystal structure of potassium iron hydrogen phosphate KFe 3 (HPO 4 ) 2 (H 2 PO 4 ) 6 4(H 2 O). Z. Anorg. Allg. Chem. 623 (1997) Bosman, W. P.; Beurskens, P. T.; Smits, J. M. M.; Behm, H.; Mintjens, J.; Meisel, W., Fuggle, J. C.: Structure of an oxonium iron(iii) orthophosphate hydrate. Acta Crystallogr. C42 (1986) Brodalla, D.; Kniep, R.: Phosphat mit Oxoniumionen enthaltenden Hohlräumen. Z. Naturforsch., Teil B. 35 (1980) Chiadmi, M.; Vicat, J.; Tran Qui, D.; Boudjada, A.: Structure de l orthophosphate acide de thallium a valence mixte, Tl 3 {Tl 0.5 (H 3 O) 0.5 }H 14 (PO 4 ) 8 4H 2 O. Acta Crystallogr. C41 (1985) Dong, C.: PowderX: Windows-95-based program for powder X-ray diffraction data processing. J. Appl. Crystallogr. 32 (1999) 838. Farrugia, L. J.: WinGX suite for small-molecule single-crystal crystallography. J. Appl. Crystallogr. 32 (1999) International Tables for Crystallography Vol. A (1992). Dodrecht: Kluwer Academic Publishers. Marsh, R. E.: Tl 3 {Tl 0.5 (H 3 O) 0.5 }H 14 (PO 4 ) 8 4H 2 O: corrigendum. Acta Crystallogr. C42 (1986) Mgaidi, A.; Boughzala, H.; Driss, A.; Clerac, R.; Coulon, C.: Structure et proprietes magnetiques du compose NH 4 Fe 3 (H 2 PO 4 ) 6 (HPO 4 ) 2 4H 2 O. J. Solid State Chem. 144 (1999) Nardelli, M.: Modeling hydroxyl and water H atoms. J. Appl. Crystallogr. 32 (1999) Rodriguez-Carvajal, J.: FULLPROF: A Program for Rietveld Refinement and Pattern Matching Analysis. Abstracts of the Satellite Meeting on Powder diffraction of the XV Congress of the IUCr., p Toulouse, France, Selevich, A. F.; Lyakhov, A. S.; Lesnikovich, A. I.: Phase Equilibria in the In 2 O 3 P 2 O 5 H 2 O System: The Synthesis and Properties of Indium Phosphates. Russ. J. Inorg. Chem. 45 (2000) Werner, P. E.; Eriksson, L.; Westdahl, M.: TREOR, a semi-exhaustive trial-and-error powder indexing program for all symmetries. J. Appl. Crystallogr. 18 (1985)