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Variants of the Bi6TiP2O16 structure: The preparation and crystal structure of the isomorph Bi6(Mn0.6Bi0.4)P2O15
Materials Research Bulletin 41 (2006) 1543–1549 www.elsevier.com/locate/matresbu
Variants of the Bi6TiP2O16 structure: The preparation and crystal structure of the isomorph Bi6(Mn0.6Bi0.4)P2O15 R.I. Dass a, V. Lynch b, R.L. Harlow c, H. Steinfink a,* a
Department of Chemical Engineering, Texas Materials Institute, University of Texas at Austin, 1 University Station C0400, Austin, TX 78712, United States b Department of Chemistry, University of Texas at Austin, Austin, TX 78712, United States c R. L. Harlow & Co. Inc. Silver Springs, MD 20906, United States Received 11 November 2005; accepted 18 January 2006 Available online 28 February 2006
Abstract Compounds with the composition Bi6(Bi1yMy)X2O16z, M = transition metal or Pb, X = P, V, As, display pseudo-tetragonal crystal systems. They are, however, monoclinic with space group I2 and the heavy atom positions mimic the d-Bi2O3 structure. The ˚ , b = 5.4259(11) A ˚ , c = 11.112(2) A ˚ , b = 96.25(3)8, I2, Z = 2. Least-squares title compound is monoclinic, a = 11.284(2) A refinement of single-crystal X-ray diffraction data on F 2 converged to R1 = 0.050, wR2 = 0.130. The crystal is twinned by two-fold rotation about [0 1 0] and each twin consists of its inverted component forming a racemate. The structure consists of chains of edge sharing (OBi4) tetrahedra parallel to [1 0 1]. The chains are bridged parallel to [1 0 1] by linked PO4 tetrahedra and (Mn/ Bi)O6 octahedra parallel to [1 0 1], into a three-dimensional structure. The lone-pair electrons of adjacent Bi atoms along the chain point in opposite directions along the b-axis. The Bi atoms are in distorted trigonal prismatic coordination that has one or two ˚ . The Mn/Bi atoms are disordered around the two-fold axis. faces capped. The Bi–O bond lengths vary from 2.08(5) to 3.05(2) A Three oxygen atom sites contain vacancies. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; Crystal growth; C. X-ray diffraction; D. Crystal structure
1. Introduction The ternary phase diagrams PbO–Bi2O3–X2O5, X = P, V, As, were studied in detail by Jie [1,2]. He reported compounds with the compositions PbBi6X2O15 although their crystal structures were unknown. During the synthesis of the recently reported compound Bi6TiP2O16 [3] we also synthesized Bi6MnP2Oy, grew single crystals and determined its crystal structure. It is isomorphous with the Ti phase. The Mn is in the Ti position. These structures might bear a relationship to the previously reported orthorhombic compounds Bi6PbP2O15 and the vanadate and arsenate analogues [1].
* Corresponding author. Tel.: +1 512 4715233; fax: +1 512 4717060. E-mail address: [email protected] (H. Steinfink). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.01.023
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2. Experimental Nominal MnBi6P2Oy was prepared by grinding analytical grade, stoichiometric quantities of Bi2O3, MnCO3, and (NH4)H2PO4 and reacting them in an alumina crucible. Prior to weighing, the Bi2O3 powder was dried in air at 150 8C for 24 h. The mixture was initially heated in air at 195 8C for 5 h to decompose (NH4)H2PO4 and then at 600 8C for 12 h. The product, a gray-brown powder, was ground, pelletized into 1/2 inch diameter pellets 3 mm thick, and annealed in air at 800 8C for 8 h. This sample was then ground, repelletized, and annealed in 8 h intervals at 800 8C in air for a total of 40 h with intermediate regrinding and repelletizing. The cooling rate was 2 8C/min. It was finally annealed in air at 850 8C for 8 h and cooled at a rate of 2 8C/min. The pellets were teal in color and were very dense. The thermal behavior of nominal Bi6MnP2Oy was determined by heating a small amount of the powder in air to 1000 8C with a heating and cooling rate of 10 8C/min in a Perkin Elmer Series 7 Differential Thermal Analyzer (DTA). Two endotherms at 873(4) and 917(6) 8C were observed on heating the sample while two exotherms at 852(2) and 794(7) 8C were observed on cooling. The double thermal signatures may be indicative of a phase transition prior to melting, incongruent melting, or an impurity component. Single crystals were obtained by melting a small amount of the powder in a gold tube of inner diameter 2.7 mm that was crimped at both ends. The encapsulated powder was heated in air to 825 8C at a rate of 2 8C/min and finally at a rate of 0.2 8C/min to 925 8C where the melt was kept for 1.5 h. The melt was slowly cooled at a rate of 1 8C/h to 825 8C and then quenched in air to room temperature by removing the tube from the furnace. Examination of the product under a polarizing binocular microscope revealed dark green-blue crystals with platy habits. Single crystals were selected and analyzed quantitatively by inductively coupled plasma optical emission (ICP) spectroscopy yielding Bi 88.04 wt.%, Mn 3.05 wt.%, P 3.84 wt.% with a worst-case error limit of 10%. With the assumption of two P in the formula this yields Bi6.5Mn0.5P2 or Bi6(Bi0.5Mn0.5)P2. An X-ray diffraction powder pattern was obtained with a Philips X-pert diffractometer, Cu Ka radiation, equipped with a diffracted beam graphite monochromator, 45 KV, 40 mA, scan range 4–1608 2u, 0.0258 2u step size and 5 s/step. The sample holder was a zero-background quartz plate with a diameter of 32 mm. This powder pattern was eventually used in a Ritveld analysis. The powder pattern indicated that the compound was isostructural with Bi6TiP2O16 [3]. Several single crystals were selected using a polarizing optical microscope. The crystals displayed inclusions and multiple extinctions. Several crystals were mounted on a Weissenberg camera and photographs were obtained. The crystals displayed considerable mosaicity. The best crystal was transferred to a Nonius k automated CCD ˚ ). A total of 398 frames of data diffractometer using a graphite monochromator with Mo Ka radiation (l = 0.71073 A were collected using v-scans with a scan range of 18 and a counting time of 165 s per frame. The intensities were ˚ , b = 14.9068(11) A ˚ , c = 16.6672(13) A ˚ . Data reduction and scaling collected using lattice parameters a = 5.4219(4) A were performed using DENZO-SMN [4]. Details of crystal data, data collection and structure refinement are listed in Table 1. After processing of the data the linear overall R-merge was 0.133. The lattice parameters were transformed to ˚ , b = 5.422 A ˚, an I-centered monoclinic cell by the matrix (0 1/2 1/2, 1 0 0, 0 1/2 1/2) yielding a = 11.180 A ˚ c = 11.180 A, b = 96.388 similar to the parameters of Bi6TiP2O16 [3]. The most intense feature of the powder diffraction pattern of Bi6TiP2O16 is the superimposed (3 1 0) and (0 1 3), ˚ , line. A striking difference between this pattern and that of the Mn phase is that this line splits in the latter 3.07 A indicating that the a- and c-axes are no longer equal. Lattice parameters were derived from the Rietveld refinement of ˚ , b = 5.426(1) A ˚ , c = 11.114(2) A ˚, the above mentioned powder pattern. The lattice parameters are a = 11.282(2) A b = 96.257(1)8. 3. Structure solution The direct method using the SHELX suite of programs contained in WINGX [5] yielded Bi positional parameters. The Bi atoms were used for phasing the structure factors to calculate difference electron density maps that revealed the Mn, P and the oxygen atoms that are in tetrahedral interstices formed by four Bi atoms. It became evident that this compound was isostructural with Bi6TiP2O16 [3]. The Mn atom, similar to Ti, was situated in the 2a position of I2, (0, y, 0) [6], and a variable site occupancy factor (sof) in the least-squares calculation resulted in a larger value than required for this special position. Analogous to the Ti compound, Bi and Mn were placed into this position and the positional and atomic displacement parameters (ADP) were restrained in the refinement to be
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Table 1 Crystal data and structure refinement for Bi6(Mn0.6Bi0.4)O15 Empirical formula Formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system, space group ˚) Unit cell dimensions (A a b c b ˚3 Volume A Z, calculated density (g cm1) Absorption coefficient (mm1) Crystal size (mm) Color Theta range for data collection Data completeness (%) Limiting indices Reflections collected/unique Absorption correction Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I>2sigma(I)] R indices (all data) Absolute structure parameter Extinction coefficient ˚ 3 Largest diff. peak and hole e A
Bi12.85Mn1.15O30.24P4 Bi6(Mn0.6Bi0.4)P2O15 1672.4 293(2) 0.71073 Monoclinic, I2 11.284(2) 5.4259(11) 11.112(2) 96.25(3)8 676.3(2) 2, 8.22 84.12 0.038 0.02 0.04 Dark green-blue 2.91–27.488 99.4 0 h 14, 6 k 6, 14 l 14 15325/1806 [R(int) = 0.133] Face indexed Full-matrix least-squares on F2 1294/21/84 1.108 R1 = 0.0503, wR2 = 0.1297 R1 = 0.0503, wR2 = 0.1297 0.00 0.00035(11) 5.13 and 3.67
equal; the sum of the sof for the two atoms was restrained to 1. The difference electron density map indicated that the Mn/Bi position was slightly displaced from the 2a equipoint and the x and z parameters were also permitted to vary in the least-squares refinements. The PO4 moiety was restrained to tetrahedral parameters. At this point of the refinement R1 was 0.092. An analytical absorption calculation using indexed crystal faces was carried out [7]. After sorting and merging of 3506 intensities R(merge) = 0.076 was obtained. The program TwinRotMat [8] indicated that pseudo-merohedral twinning was present by two-fold rotation about the b-axis and introduction of twin refinement lowered R to 0.057. Each twin fraction 0.22(6), 0.28(5) has also an inverted component 0.22(6), and 0.28(5). It was noted that Bi2 had a large value of U22 and this atom was split into two sites. The final cycle of least-squares refinement with anisotropic displacement parameters for the Bi atoms converged to the stoichiometry Bi6(Mn0.6Bi0.4)P2O15.2 and R = 0.050. This is in good agreement with the analytical determination of the cations. On the assumption that Mn is divalent the oxygen stoichiometry is 15 in agreement with the value from the structure determination. The single-crystal data collection and results of least-squares refinements are shown in Table 1, atomic parameters in Table 2 and selected bond lengths in Table 3. The program HYBRIDE [9,10] was used to calculate the lone-pair electron positions for Bi1, Bi2 and Bi3 and are listed in Table 4. 4. Discussion The crystal structure of this compound is isostructural with that of Bi6TiP2O16 [3] The heavy atom positions in these compounds mimic the d-Bi2O3 structure and impart features of pseudo-centrosymmetry. Small changes in the atomic coordinates occur because of the change in valence of Mn2+ for Ti4+. The oxygen coordination polyhedra around bismuth are similar to those in the Ti phase, consisting of trigonal prisms with an oxygen atom capping either one or two faces, Fig. 1(a)–(c). The lone-pair positions force distortions of the prisms as becomes evident from the O–O ˚ . In Fig. 1(b) O5–O9 is 4.64 A ˚ and O3–O9 is 4.03 A ˚ while all other O–O distances. In Fig. 1(a) O5–O5 is 5.426 A
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Table 2 ˚ 2 103) for Bi6(Mn0.6Bi0.4)P2O15 Atomic coordinates (104) and equivalent isotropic displacement parameters (A sof Bi(1) Bi(2) Bi(20 ) Bi(3) Mn Bi0 P O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) a
0.57(2) 0.43(1) 0.288(9) 0.212(6)
0.87(6) 0.86(6) 0.83(7)
x
y
z
U(eq)a
3309(2) 1710(10) 1720(6) 153(2) 4829(4) 4829(4) 1383(8) 0 0 1430(30) 1660(30) 2614(13) 3554(17) 660(30) 1460(20) 629(18)
279(11) 4461(10) 5701(1) 275(12) 4780(20) 4780(20) 4730(30) 2840(80) 2250(100) 2280(50) 2890(130) 4940(50) 620(50) 2520(50) 330(110) 2870(40)
58(2) 1591(9) 1654(6) 3364(1) 4874(5) 4874(5) 1483(7) 5000 5000 3290(20) 3290(30) 741(14) 2167(15) 990(30) 990(20) 1270(20)
27(1) 34(2) 17(1) 28(1) 23(1) 23(1) 20 15(3) 15(3) 4(6) 55(14) 16(6) 44(7) 66(17) 46(10) 21(6)
U(eq) is defined as one-third of the trace of the orthogonalized Uij tensor.
Table 3 ˚ ) and angles (deg) for Bi6(Mn0.6Bi0.4)O15 Bond lengths (A Bi(1)–O(4)#1 Bi(1)–O(3)#2 Bi(1)–O(1)#1 Bi(1)–O(2)#3 Bi(1)–O(8)#4 Bi(1)–O(6)#4 Bi(1)–O(5)#5 Bi(1)–O(5)
2.24(5) 2.26(2) 2.33(2) 2.34(3) 2.43(3) 2.524(16) 2.83(3) 3.04(3)
Bi(2)–O(4) Bi(2)–O(3)#7 Bi(2)–O(8) Bi(2)–O(4)#8 Bi(2)–O(3)#9 Bi(2)–O(9)#4 Bi(2)–O(5)#9
2.08(5) 2.29(3) 2.35(6) 2.61(6) 2.63(3) 2.76(3) 2.91(2)
Bi(20 )–O(3)#9 Bi(20 )–O(4)#8 Bi(20 )–O(4) Bi(20 )–O(8)#5 Bi(20 )–O(3)#7 Bi(20 )–O(7)#5 Bi(20 )–O(5)#9 Bi(20 )–O(8) Bi(20 )–O(9)#4
Mn–O(8)#1 Mn–O(7)#15 Mn–O(7)#1 Mn–O(8)#15 Mn–O(9)#15 Mn–O(9)#1
1.65(3) 2.05(2) 2.05(2) 2.11(3) 2.183(19) 2.22(2)
P–O(6)#12 P–O(5) P–O(7) P–O(9)#6
1.521(16) 1.540(15) 1.580(18) 1.587(17)
O(6)#12–P–O(5) O(6)#12–P–O(7) O(5)–P–O(7) O(6)#12–P–O(9)#6 O(5)–P–O(9)#6 O(7)–P–O(9)#6
2.18(3) 2.18(5) 2.38(5) 2.63(6) 2.64(3) 2.88(3) 2.968(19) 3.01(6) 3.05(2)
Bi(3)–O(3)#4 Bi(3)–O(2) Bi(3)–O(1) Bi(3)–O(4) Bi(3)–O(9)#4 Bi(3)–O(5)#11 Bi(3)–O(6)#12 Bi(3)–O(7)
2.27(3) 2.27(3) 2.28(3) 2.50(5) 2.72(3) 2.810(16) 2.95(3) 3.04(3)
113.6(10) 109.3(11) 109.7(10) 109.2(10) 109.2(10) 105.5(9)
Symmetry transformations used to generate equivalent atoms: #1 x + 1/2, y + 1/2, z + 1/2; #2 x + 1/2, y 1/2, z + 1/2; #3 x + 1/2, y 1/2, z + 1/ 2; #4 x, y, z; #5 x, y 1, z; #6 x, y + 1, z; #7 x 1/2, y 1/2, z 1/2; #8 x 1/2, y 1/2, z 1/2; #9 x, y 1,z; #10 x 1/2, y + 1/ 2, z 1/2; #11 x + 1/2, y 1/2, z 1/2; #12 x + 1/2, y + 1/2, z 1/2; #13 x + 1/2, y 1/2, z 1/2; #14 x, y, z 1; #15 x + 1/2, y + 1/2, z + 1/2; #16 x + 1/2, y + 3/2, z + 1/2; #17 x 1/2, y + 1/2, z 1/2; #18 x + 1/2, y + 1/2, z 1/2; #19 x, y, z 1; #20 x, y + 1, z; #21 x, y, z + 1; #22 x 1/2, y 1/2, z + 1/2; #23 x 1/2, y + 1/2, z + 1/2; #24 x 1/2, y + 1/2, z + 1/2; #25 x 1/2, y 1/2, z + 1/2.
Table 4 Electron lone-pair localization in Bi6(Mn0.6Bi0.4)P2O15
Bi1 Bi2 Bi3
x
y
z
˚ d, A
0.317 0.837 0.0313
0.959 0.482 0.940
0.0151 0.869 0.685
0.275 0.506 0.336
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Fig. 1. The oxygen environments around: (a) Bi1, (b) Bi2, (c) Bi3 in Bi6(Mn0.6Bi0.4)P2O15. The large spheres represent the lone-pair electrons. The interatomic oxygen–oxygen distances are shown in (c) to illustrate distortions of the coordination polyhedron to accommodate the lone-pair electrons.
˚ or less. All O–O distances are labeled in Fig. 1(c) to illustrate the severe distortion occurring there distances are 3 A (Fig. 2). Mn/Bi0 is slightly displaced from the 2a position (0 y 0), of space group I2 and is disordered. This site is nearly equally occupied by Mn, sof = 0.56(2) and Bi0 , sof = 0.44(2). Mn/Bi0 is in octahedral coordination with O8 at opposite vertices. The irregularity of the Mn/Bi0 –O bond lengths clearly is due to the mixed site occupancy. When the atomic coordinates of Mn/Bi0 are idealized at the position 1/2, 1/2, 1/2 the two bond distances Mn– ˚ , the two Mn–O7 = 1.955(13) A ˚ , and the two Mn–O9 = 2.276(13) A ˚ . The long Mn–O9 bond may O8 = 1.88(3) A well be due to the admixture of Bi.
Fig. 2. A view of the crystal structure of Bi6(Mn0.6Bi0.4)P2O15 parallel to [0 1 0]. The Mn/Bi site occupancy is shown with only Mn and its location has been idealized at 0, 0, 0.
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Fig. 3. Rietveld refinement of the X-ray diffraction pattern of Bi6(Mn0.6Bi0.4)P2O15.
˚ . The chains are The structure consists of chains of edge sharing (OBi4) tetrahedra parallel to [1 0 1], 16.68 A ˚ , by linked PO4 tetrahedra and (Mn/Bi)O6 octahedra parallel to [1 0 1], into a bridged parallel to [1 0 1], 14.95 A three-dimensional structure. The lone-pair electrons of adjacent Bi atoms along the chain point in opposite directions along the b-axis. Rietveld refinement was carried out using the data obtained from the previously mentioned powder X-ray diffraction pattern [11]. The motivation for this refinement was the fact that the single-crystal data came from a complexly twinned crystal. The background was modeled with a 17 term shifted Chebyschev polynomial; a modified Pseudo-Voigt function was used to describe the diffraction line profiles; data below 208 2u were eliminated. The atomic coordinates obtained from the single-crystal structure determination, Table 2, were used for the refinement with similar constraints on some of the parameters. Anisotropic displacement parameters for the heavy atoms did not affect R significantly so only isotropic refinements were used. The displacement parameters for the oxygen atoms were constrained to be the same. The final x2 = 5.85, Nobs = 1453, 67 variables, Rwp = 0.097, R(F 2) = 0.067. The final atomic coordinates did not differ significantly from those derived from the single-crystal data and the fitted powder diffraction data are shown in Fig. 3. We have synthesized nominal Bi6CuP2O15 and Bi6ZnP2O15 and both compounds are isostructural with Bi6TiP2O16. The X-ray diffraction powder patterns of these phases also display the splitting of the (3 1 0) and (0 1 3) lines indicative of the inequality of the a- and c-axes of the unit cells. It is also likely that the octahedral site has mixed M2+/ M3+ occupancy. We have also prepared the nominal phase Bi6PbP2O15 but it has a different crystal structure. Acknowledgments HS and RID gratefully acknowledge the support of the Robert A. Welch Foundation of Houston, Texas for this research under grant F-273. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.materresbull.2006.01.023. References [1] [2] [3] [4]
Y.–C. Jie, Heidelberger geowissenschaftliche Abhandlungen 84 (1995) 1. Y.C. Jie, W. Eysel, Powder Diffraction 10 (1995) 76. H. Steinfink, V. Lynch, J. Solid State Chem. 177 (2004) 1412. DENZO-SMN, Z. Otwinowski, W. Minor, in: C.W. Carter Jr., R.M. Sweets (Eds.), Methods in Enzymology, 276: Macromolecular Crystallography, Part A, Academic Press, 1997, pp. 307–326.
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[5] G.M. Sheldrick, SHELXL 97-2, a program for crystal structure refinement, University of Go¨ttingen, Germany 1997 as incorporated in L.J. Farrugia, WinGX v. 1.70.01, J. Appl. Cryst. 32 (1999) 837. [6] Theo Hahn (Ed.), International Tables for Crystallography, vol. A, 2nd ed., Kluver Academic Publishers, 1989. [7] J. de Meulenaar, H. Tompa, Acta Crystallogr. 19 (1965) 1014 (as incorporated in the WINGX program suite [5]). [8] R.I. Cooper, R.O. Gould, S. Parsons, D.J. Watkin, J. Appl. Crystallogr. 35 (2002) 168 (as incorporated in the WINGX program suite [5]). [9] E. Morin, G. Wallez, S. Jaulmes, J.C. Couturier, M. Quarton, J. Solid State Chem. 137 (1998) 283. [10] O. Labidi, J.P. Wignacourt, P. Roussel, M. Drache, P. Conflant, H. Steinfink, Solid State Sci. 6 (2004) 383. [11] H.M. Rietveld, Acta Crystallogr. 25 (1992) 589.
Variants of the Bi6TiP2O16 structure: The preparation and crystal structure of the isomorph Bi6(Mn0.6Bi0.4)P2O15 R.I. Dass a, V. Lynch b, R.L. Harlow c, H. Steinfink a,* a
Department of Chemical Engineering, Texas Materials Institute, University of Texas at Austin, 1 University Station C0400, Austin, TX 78712, United States b Department of Chemistry, University of Texas at Austin, Austin, TX 78712, United States c R. L. Harlow & Co. Inc. Silver Springs, MD 20906, United States Received 11 November 2005; accepted 18 January 2006 Available online 28 February 2006
Abstract Compounds with the composition Bi6(Bi1yMy)X2O16z, M = transition metal or Pb, X = P, V, As, display pseudo-tetragonal crystal systems. They are, however, monoclinic with space group I2 and the heavy atom positions mimic the d-Bi2O3 structure. The ˚ , b = 5.4259(11) A ˚ , c = 11.112(2) A ˚ , b = 96.25(3)8, I2, Z = 2. Least-squares title compound is monoclinic, a = 11.284(2) A refinement of single-crystal X-ray diffraction data on F 2 converged to R1 = 0.050, wR2 = 0.130. The crystal is twinned by two-fold rotation about [0 1 0] and each twin consists of its inverted component forming a racemate. The structure consists of chains of edge sharing (OBi4) tetrahedra parallel to [1 0 1]. The chains are bridged parallel to [1 0 1] by linked PO4 tetrahedra and (Mn/ Bi)O6 octahedra parallel to [1 0 1], into a three-dimensional structure. The lone-pair electrons of adjacent Bi atoms along the chain point in opposite directions along the b-axis. The Bi atoms are in distorted trigonal prismatic coordination that has one or two ˚ . The Mn/Bi atoms are disordered around the two-fold axis. faces capped. The Bi–O bond lengths vary from 2.08(5) to 3.05(2) A Three oxygen atom sites contain vacancies. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; Crystal growth; C. X-ray diffraction; D. Crystal structure
1. Introduction The ternary phase diagrams PbO–Bi2O3–X2O5, X = P, V, As, were studied in detail by Jie [1,2]. He reported compounds with the compositions PbBi6X2O15 although their crystal structures were unknown. During the synthesis of the recently reported compound Bi6TiP2O16 [3] we also synthesized Bi6MnP2Oy, grew single crystals and determined its crystal structure. It is isomorphous with the Ti phase. The Mn is in the Ti position. These structures might bear a relationship to the previously reported orthorhombic compounds Bi6PbP2O15 and the vanadate and arsenate analogues [1].
* Corresponding author. Tel.: +1 512 4715233; fax: +1 512 4717060. E-mail address: [email protected] (H. Steinfink). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.01.023
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2. Experimental Nominal MnBi6P2Oy was prepared by grinding analytical grade, stoichiometric quantities of Bi2O3, MnCO3, and (NH4)H2PO4 and reacting them in an alumina crucible. Prior to weighing, the Bi2O3 powder was dried in air at 150 8C for 24 h. The mixture was initially heated in air at 195 8C for 5 h to decompose (NH4)H2PO4 and then at 600 8C for 12 h. The product, a gray-brown powder, was ground, pelletized into 1/2 inch diameter pellets 3 mm thick, and annealed in air at 800 8C for 8 h. This sample was then ground, repelletized, and annealed in 8 h intervals at 800 8C in air for a total of 40 h with intermediate regrinding and repelletizing. The cooling rate was 2 8C/min. It was finally annealed in air at 850 8C for 8 h and cooled at a rate of 2 8C/min. The pellets were teal in color and were very dense. The thermal behavior of nominal Bi6MnP2Oy was determined by heating a small amount of the powder in air to 1000 8C with a heating and cooling rate of 10 8C/min in a Perkin Elmer Series 7 Differential Thermal Analyzer (DTA). Two endotherms at 873(4) and 917(6) 8C were observed on heating the sample while two exotherms at 852(2) and 794(7) 8C were observed on cooling. The double thermal signatures may be indicative of a phase transition prior to melting, incongruent melting, or an impurity component. Single crystals were obtained by melting a small amount of the powder in a gold tube of inner diameter 2.7 mm that was crimped at both ends. The encapsulated powder was heated in air to 825 8C at a rate of 2 8C/min and finally at a rate of 0.2 8C/min to 925 8C where the melt was kept for 1.5 h. The melt was slowly cooled at a rate of 1 8C/h to 825 8C and then quenched in air to room temperature by removing the tube from the furnace. Examination of the product under a polarizing binocular microscope revealed dark green-blue crystals with platy habits. Single crystals were selected and analyzed quantitatively by inductively coupled plasma optical emission (ICP) spectroscopy yielding Bi 88.04 wt.%, Mn 3.05 wt.%, P 3.84 wt.% with a worst-case error limit of 10%. With the assumption of two P in the formula this yields Bi6.5Mn0.5P2 or Bi6(Bi0.5Mn0.5)P2. An X-ray diffraction powder pattern was obtained with a Philips X-pert diffractometer, Cu Ka radiation, equipped with a diffracted beam graphite monochromator, 45 KV, 40 mA, scan range 4–1608 2u, 0.0258 2u step size and 5 s/step. The sample holder was a zero-background quartz plate with a diameter of 32 mm. This powder pattern was eventually used in a Ritveld analysis. The powder pattern indicated that the compound was isostructural with Bi6TiP2O16 [3]. Several single crystals were selected using a polarizing optical microscope. The crystals displayed inclusions and multiple extinctions. Several crystals were mounted on a Weissenberg camera and photographs were obtained. The crystals displayed considerable mosaicity. The best crystal was transferred to a Nonius k automated CCD ˚ ). A total of 398 frames of data diffractometer using a graphite monochromator with Mo Ka radiation (l = 0.71073 A were collected using v-scans with a scan range of 18 and a counting time of 165 s per frame. The intensities were ˚ , b = 14.9068(11) A ˚ , c = 16.6672(13) A ˚ . Data reduction and scaling collected using lattice parameters a = 5.4219(4) A were performed using DENZO-SMN [4]. Details of crystal data, data collection and structure refinement are listed in Table 1. After processing of the data the linear overall R-merge was 0.133. The lattice parameters were transformed to ˚ , b = 5.422 A ˚, an I-centered monoclinic cell by the matrix (0 1/2 1/2, 1 0 0, 0 1/2 1/2) yielding a = 11.180 A ˚ c = 11.180 A, b = 96.388 similar to the parameters of Bi6TiP2O16 [3]. The most intense feature of the powder diffraction pattern of Bi6TiP2O16 is the superimposed (3 1 0) and (0 1 3), ˚ , line. A striking difference between this pattern and that of the Mn phase is that this line splits in the latter 3.07 A indicating that the a- and c-axes are no longer equal. Lattice parameters were derived from the Rietveld refinement of ˚ , b = 5.426(1) A ˚ , c = 11.114(2) A ˚, the above mentioned powder pattern. The lattice parameters are a = 11.282(2) A b = 96.257(1)8. 3. Structure solution The direct method using the SHELX suite of programs contained in WINGX [5] yielded Bi positional parameters. The Bi atoms were used for phasing the structure factors to calculate difference electron density maps that revealed the Mn, P and the oxygen atoms that are in tetrahedral interstices formed by four Bi atoms. It became evident that this compound was isostructural with Bi6TiP2O16 [3]. The Mn atom, similar to Ti, was situated in the 2a position of I2, (0, y, 0) [6], and a variable site occupancy factor (sof) in the least-squares calculation resulted in a larger value than required for this special position. Analogous to the Ti compound, Bi and Mn were placed into this position and the positional and atomic displacement parameters (ADP) were restrained in the refinement to be
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Table 1 Crystal data and structure refinement for Bi6(Mn0.6Bi0.4)O15 Empirical formula Formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system, space group ˚) Unit cell dimensions (A a b c b ˚3 Volume A Z, calculated density (g cm1) Absorption coefficient (mm1) Crystal size (mm) Color Theta range for data collection Data completeness (%) Limiting indices Reflections collected/unique Absorption correction Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I>2sigma(I)] R indices (all data) Absolute structure parameter Extinction coefficient ˚ 3 Largest diff. peak and hole e A
Bi12.85Mn1.15O30.24P4 Bi6(Mn0.6Bi0.4)P2O15 1672.4 293(2) 0.71073 Monoclinic, I2 11.284(2) 5.4259(11) 11.112(2) 96.25(3)8 676.3(2) 2, 8.22 84.12 0.038 0.02 0.04 Dark green-blue 2.91–27.488 99.4 0 h 14, 6 k 6, 14 l 14 15325/1806 [R(int) = 0.133] Face indexed Full-matrix least-squares on F2 1294/21/84 1.108 R1 = 0.0503, wR2 = 0.1297 R1 = 0.0503, wR2 = 0.1297 0.00 0.00035(11) 5.13 and 3.67
equal; the sum of the sof for the two atoms was restrained to 1. The difference electron density map indicated that the Mn/Bi position was slightly displaced from the 2a equipoint and the x and z parameters were also permitted to vary in the least-squares refinements. The PO4 moiety was restrained to tetrahedral parameters. At this point of the refinement R1 was 0.092. An analytical absorption calculation using indexed crystal faces was carried out [7]. After sorting and merging of 3506 intensities R(merge) = 0.076 was obtained. The program TwinRotMat [8] indicated that pseudo-merohedral twinning was present by two-fold rotation about the b-axis and introduction of twin refinement lowered R to 0.057. Each twin fraction 0.22(6), 0.28(5) has also an inverted component 0.22(6), and 0.28(5). It was noted that Bi2 had a large value of U22 and this atom was split into two sites. The final cycle of least-squares refinement with anisotropic displacement parameters for the Bi atoms converged to the stoichiometry Bi6(Mn0.6Bi0.4)P2O15.2 and R = 0.050. This is in good agreement with the analytical determination of the cations. On the assumption that Mn is divalent the oxygen stoichiometry is 15 in agreement with the value from the structure determination. The single-crystal data collection and results of least-squares refinements are shown in Table 1, atomic parameters in Table 2 and selected bond lengths in Table 3. The program HYBRIDE [9,10] was used to calculate the lone-pair electron positions for Bi1, Bi2 and Bi3 and are listed in Table 4. 4. Discussion The crystal structure of this compound is isostructural with that of Bi6TiP2O16 [3] The heavy atom positions in these compounds mimic the d-Bi2O3 structure and impart features of pseudo-centrosymmetry. Small changes in the atomic coordinates occur because of the change in valence of Mn2+ for Ti4+. The oxygen coordination polyhedra around bismuth are similar to those in the Ti phase, consisting of trigonal prisms with an oxygen atom capping either one or two faces, Fig. 1(a)–(c). The lone-pair positions force distortions of the prisms as becomes evident from the O–O ˚ . In Fig. 1(b) O5–O9 is 4.64 A ˚ and O3–O9 is 4.03 A ˚ while all other O–O distances. In Fig. 1(a) O5–O5 is 5.426 A
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Table 2 ˚ 2 103) for Bi6(Mn0.6Bi0.4)P2O15 Atomic coordinates (104) and equivalent isotropic displacement parameters (A sof Bi(1) Bi(2) Bi(20 ) Bi(3) Mn Bi0 P O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) a
0.57(2) 0.43(1) 0.288(9) 0.212(6)
0.87(6) 0.86(6) 0.83(7)
x
y
z
U(eq)a
3309(2) 1710(10) 1720(6) 153(2) 4829(4) 4829(4) 1383(8) 0 0 1430(30) 1660(30) 2614(13) 3554(17) 660(30) 1460(20) 629(18)
279(11) 4461(10) 5701(1) 275(12) 4780(20) 4780(20) 4730(30) 2840(80) 2250(100) 2280(50) 2890(130) 4940(50) 620(50) 2520(50) 330(110) 2870(40)
58(2) 1591(9) 1654(6) 3364(1) 4874(5) 4874(5) 1483(7) 5000 5000 3290(20) 3290(30) 741(14) 2167(15) 990(30) 990(20) 1270(20)
27(1) 34(2) 17(1) 28(1) 23(1) 23(1) 20 15(3) 15(3) 4(6) 55(14) 16(6) 44(7) 66(17) 46(10) 21(6)
U(eq) is defined as one-third of the trace of the orthogonalized Uij tensor.
Table 3 ˚ ) and angles (deg) for Bi6(Mn0.6Bi0.4)O15 Bond lengths (A Bi(1)–O(4)#1 Bi(1)–O(3)#2 Bi(1)–O(1)#1 Bi(1)–O(2)#3 Bi(1)–O(8)#4 Bi(1)–O(6)#4 Bi(1)–O(5)#5 Bi(1)–O(5)
2.24(5) 2.26(2) 2.33(2) 2.34(3) 2.43(3) 2.524(16) 2.83(3) 3.04(3)
Bi(2)–O(4) Bi(2)–O(3)#7 Bi(2)–O(8) Bi(2)–O(4)#8 Bi(2)–O(3)#9 Bi(2)–O(9)#4 Bi(2)–O(5)#9
2.08(5) 2.29(3) 2.35(6) 2.61(6) 2.63(3) 2.76(3) 2.91(2)
Bi(20 )–O(3)#9 Bi(20 )–O(4)#8 Bi(20 )–O(4) Bi(20 )–O(8)#5 Bi(20 )–O(3)#7 Bi(20 )–O(7)#5 Bi(20 )–O(5)#9 Bi(20 )–O(8) Bi(20 )–O(9)#4
Mn–O(8)#1 Mn–O(7)#15 Mn–O(7)#1 Mn–O(8)#15 Mn–O(9)#15 Mn–O(9)#1
1.65(3) 2.05(2) 2.05(2) 2.11(3) 2.183(19) 2.22(2)
P–O(6)#12 P–O(5) P–O(7) P–O(9)#6
1.521(16) 1.540(15) 1.580(18) 1.587(17)
O(6)#12–P–O(5) O(6)#12–P–O(7) O(5)–P–O(7) O(6)#12–P–O(9)#6 O(5)–P–O(9)#6 O(7)–P–O(9)#6
2.18(3) 2.18(5) 2.38(5) 2.63(6) 2.64(3) 2.88(3) 2.968(19) 3.01(6) 3.05(2)
Bi(3)–O(3)#4 Bi(3)–O(2) Bi(3)–O(1) Bi(3)–O(4) Bi(3)–O(9)#4 Bi(3)–O(5)#11 Bi(3)–O(6)#12 Bi(3)–O(7)
2.27(3) 2.27(3) 2.28(3) 2.50(5) 2.72(3) 2.810(16) 2.95(3) 3.04(3)
113.6(10) 109.3(11) 109.7(10) 109.2(10) 109.2(10) 105.5(9)
Symmetry transformations used to generate equivalent atoms: #1 x + 1/2, y + 1/2, z + 1/2; #2 x + 1/2, y 1/2, z + 1/2; #3 x + 1/2, y 1/2, z + 1/ 2; #4 x, y, z; #5 x, y 1, z; #6 x, y + 1, z; #7 x 1/2, y 1/2, z 1/2; #8 x 1/2, y 1/2, z 1/2; #9 x, y 1,z; #10 x 1/2, y + 1/ 2, z 1/2; #11 x + 1/2, y 1/2, z 1/2; #12 x + 1/2, y + 1/2, z 1/2; #13 x + 1/2, y 1/2, z 1/2; #14 x, y, z 1; #15 x + 1/2, y + 1/2, z + 1/2; #16 x + 1/2, y + 3/2, z + 1/2; #17 x 1/2, y + 1/2, z 1/2; #18 x + 1/2, y + 1/2, z 1/2; #19 x, y, z 1; #20 x, y + 1, z; #21 x, y, z + 1; #22 x 1/2, y 1/2, z + 1/2; #23 x 1/2, y + 1/2, z + 1/2; #24 x 1/2, y + 1/2, z + 1/2; #25 x 1/2, y 1/2, z + 1/2.
Table 4 Electron lone-pair localization in Bi6(Mn0.6Bi0.4)P2O15
Bi1 Bi2 Bi3
x
y
z
˚ d, A
0.317 0.837 0.0313
0.959 0.482 0.940
0.0151 0.869 0.685
0.275 0.506 0.336
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Fig. 1. The oxygen environments around: (a) Bi1, (b) Bi2, (c) Bi3 in Bi6(Mn0.6Bi0.4)P2O15. The large spheres represent the lone-pair electrons. The interatomic oxygen–oxygen distances are shown in (c) to illustrate distortions of the coordination polyhedron to accommodate the lone-pair electrons.
˚ or less. All O–O distances are labeled in Fig. 1(c) to illustrate the severe distortion occurring there distances are 3 A (Fig. 2). Mn/Bi0 is slightly displaced from the 2a position (0 y 0), of space group I2 and is disordered. This site is nearly equally occupied by Mn, sof = 0.56(2) and Bi0 , sof = 0.44(2). Mn/Bi0 is in octahedral coordination with O8 at opposite vertices. The irregularity of the Mn/Bi0 –O bond lengths clearly is due to the mixed site occupancy. When the atomic coordinates of Mn/Bi0 are idealized at the position 1/2, 1/2, 1/2 the two bond distances Mn– ˚ , the two Mn–O7 = 1.955(13) A ˚ , and the two Mn–O9 = 2.276(13) A ˚ . The long Mn–O9 bond may O8 = 1.88(3) A well be due to the admixture of Bi.
Fig. 2. A view of the crystal structure of Bi6(Mn0.6Bi0.4)P2O15 parallel to [0 1 0]. The Mn/Bi site occupancy is shown with only Mn and its location has been idealized at 0, 0, 0.
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Fig. 3. Rietveld refinement of the X-ray diffraction pattern of Bi6(Mn0.6Bi0.4)P2O15.
˚ . The chains are The structure consists of chains of edge sharing (OBi4) tetrahedra parallel to [1 0 1], 16.68 A ˚ , by linked PO4 tetrahedra and (Mn/Bi)O6 octahedra parallel to [1 0 1], into a bridged parallel to [1 0 1], 14.95 A three-dimensional structure. The lone-pair electrons of adjacent Bi atoms along the chain point in opposite directions along the b-axis. Rietveld refinement was carried out using the data obtained from the previously mentioned powder X-ray diffraction pattern [11]. The motivation for this refinement was the fact that the single-crystal data came from a complexly twinned crystal. The background was modeled with a 17 term shifted Chebyschev polynomial; a modified Pseudo-Voigt function was used to describe the diffraction line profiles; data below 208 2u were eliminated. The atomic coordinates obtained from the single-crystal structure determination, Table 2, were used for the refinement with similar constraints on some of the parameters. Anisotropic displacement parameters for the heavy atoms did not affect R significantly so only isotropic refinements were used. The displacement parameters for the oxygen atoms were constrained to be the same. The final x2 = 5.85, Nobs = 1453, 67 variables, Rwp = 0.097, R(F 2) = 0.067. The final atomic coordinates did not differ significantly from those derived from the single-crystal data and the fitted powder diffraction data are shown in Fig. 3. We have synthesized nominal Bi6CuP2O15 and Bi6ZnP2O15 and both compounds are isostructural with Bi6TiP2O16. The X-ray diffraction powder patterns of these phases also display the splitting of the (3 1 0) and (0 1 3) lines indicative of the inequality of the a- and c-axes of the unit cells. It is also likely that the octahedral site has mixed M2+/ M3+ occupancy. We have also prepared the nominal phase Bi6PbP2O15 but it has a different crystal structure. Acknowledgments HS and RID gratefully acknowledge the support of the Robert A. Welch Foundation of Houston, Texas for this research under grant F-273. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.materresbull.2006.01.023. References [1] [2] [3] [4]
Y.–C. Jie, Heidelberger geowissenschaftliche Abhandlungen 84 (1995) 1. Y.C. Jie, W. Eysel, Powder Diffraction 10 (1995) 76. H. Steinfink, V. Lynch, J. Solid State Chem. 177 (2004) 1412. DENZO-SMN, Z. Otwinowski, W. Minor, in: C.W. Carter Jr., R.M. Sweets (Eds.), Methods in Enzymology, 276: Macromolecular Crystallography, Part A, Academic Press, 1997, pp. 307–326.
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[5] G.M. Sheldrick, SHELXL 97-2, a program for crystal structure refinement, University of Go¨ttingen, Germany 1997 as incorporated in L.J. Farrugia, WinGX v. 1.70.01, J. Appl. Cryst. 32 (1999) 837. [6] Theo Hahn (Ed.), International Tables for Crystallography, vol. A, 2nd ed., Kluver Academic Publishers, 1989. [7] J. de Meulenaar, H. Tompa, Acta Crystallogr. 19 (1965) 1014 (as incorporated in the WINGX program suite [5]). [8] R.I. Cooper, R.O. Gould, S. Parsons, D.J. Watkin, J. Appl. Crystallogr. 35 (2002) 168 (as incorporated in the WINGX program suite [5]). [9] E. Morin, G. Wallez, S. Jaulmes, J.C. Couturier, M. Quarton, J. Solid State Chem. 137 (1998) 283. [10] O. Labidi, J.P. Wignacourt, P. Roussel, M. Drache, P. Conflant, H. Steinfink, Solid State Sci. 6 (2004) 383. [11] H.M. Rietveld, Acta Crystallogr. 25 (1992) 589.