Synthesis and structural elucidation of Ba4Nd3F17: A powder XRD study [PDF]

Synthesis and structural elucidation of Ba4 Nd3 F17 : A powder XRD study V. Grover, S. N. Achary, S. J. Patwe, and A. K. Tyagia) Applied Chemistry Division, Bhabha Atomic Research Centre, Mumbai (400 085), India

共Received 10 June 2002; accepted 2 October 2002兲 The compound Ba4 Nd3 F17 was prepared by heating predried BaF2 and EuF3 共4:3兲 at 900 °C for 8 h in static vacuum. The polycrystalline sample obtained was characterized by Rietveld refinement of the observed powder diffraction data with the starting model of Ba4 Y3 F17 . This compound crystallizes in a rhombohedral lattice with unit-cell parameters, a⫽11.2818(3) and c ⫽20.7788(11) Å, Z⫽6, 共Space group R ¯3 , No. 148兲. The R p , R wp and R exp factors were 9.5%, 12.9% and 10.9%, respectively. © 2002 International Centre for Diffraction Data. 关DOI: 10.1154/1.1523873兴 Key words: anion-excess fluorite, barium neodymium fluoride, Rietveld refinement

I. INTRODUCTION

The phase relations between fluorite-type metal fluorides (M F 2 ) and rare-earth fluorides (M ⬘ F3 ) have been reported under high temperature quenched conditions 共Greis and Haschke, 1982; Sobolev, 1992兲. In these studies stabilization of several new compounds and crystal chemistry of the anion-rich fluorides have been explained. Recently, several phase relations in M F2 – M ⬘ F3 systems, under slow cooled and short anneal conditions, have also been reported 共Achary et al., 1999a, b; Patwe et al., 2001兲. In all these studies, several different kinds of ordered phases, namely, rhombohedral 共Rh␣, ␤, ␥兲, tetragonal, cubic, etc., were observed in the anion excess fluorite lattice depending on the radius of the metal ions, heat treatment and cooling protocols. Under high temperature, long anneal conditions, Ba4 M 3 F17 compositions in BaF2 – M F3 systems were shown to have Rh␣-type rhombohedral lattice 共Keiser and Greis, 1980兲. The detailed structural analysis of the Rh␣ type rhombohedral ordered phase Ba4 M 3 F17 共for M ⫽Y, Yb 共Maksimov et al., 1996兲, Er 共Tyagi and Koehler, 2001兲 and Eu 共Achary et al., 2002兲 has been reported. In this manuscript the structural details of Ba4 Nd3 F17 , which has the Rh␣ type lattice is being reported.

II. EXPERIMENTAL

BaF2 was prepared by treating BaCO3 共Aldrich兲 with 40% aqueous HF solution. The product was dried over a hot water bath, and further dried at 800 °C, under flowing argon atmosphere. NdF3 was synthesized by heating Nd2 O3 and NH4 HF2 repeatedly at 450 °C, with excess addition of NH4 HF2 each time. A homogeneous mixture of appropriate amounts of BaF2 and NdF3 was prepared and pelletized. The pellet was wrapped in a platinum foil and encapsulated in an evacuated (⬃10⫺6 Torr) sealed quartz tube. This tube was heated at 900 °C for 8 h, with heating and cooling rates of 4 °C/min and 2 °C/min, respectively. The product obtained was analyzed by powder XRD recorded on a Philips X-ray a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected]

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diffractometer 共Model PW1710兲 with Ni filtered Cu K␣ radiation. Silicon was used as an external standard. Rietveld refinements were carried out using Fullprof 2K 共RodriguezCarvajal, 2000兲.

III. RESULTS AND DISCUSSIONS

The powder XRD pattern of the title compound appears to be very closely similar to the parent fluorite lattice of BaF2 , but with a shift to the higher angle side. In general, the incorporation of rare-earth fluorides to the BaF2 lattice in the fluorite type solid solution causes a decreases in the unit cell volume, owing to the considerable decrease in the average ionic radii at the metal ion sites. Thus the shift in position of the Bragg reflection at composition Ba4 Nd3 F17 , which is observed up to the saturation limit of the fluorite type solid solution, is expected. A careful analysis of the observed XRD pattern indicates the splitting of some reflections at 2 ␪ ⬇42°, 50° in addition to some weak reflections at 2 ␪ ⬇39°, 41°, 44°, 45°. It is known that up to certain concentration of M ⬘ 3⫹ in M F2 – M ⬘ F3 system, anion-excess fluorite type solid solutions are formed. The excess anions and guest cations form clusters beyond a limiting concentration of M ⬘ 3⫹ ion, which often lead to the fluorite related phases. Bevan et al. 共1980兲 have explained such fluorite related structures with the M n F2n⫹5 or M n F2n⫹6 (n⫽13, 14, etc.兲 clusters. The present composition Ba4 Nd3 F17 is also such an anion-excess fluorite related phase, representing n ⫽14 homologous of fluorite related phase. The detailed phase equlibria study in this system, which has been communicated 共Grover et al., 2002兲 elsewhere, revealed this phase to have a narrow homogeneity width. The present composition is observed to be isotypic to Ba4 Eu3 F17 共Achary et al., 2002兲 and Ba4 Y3 F17 共Maksimov et al., 1996兲. The intense fluorite reflections including the split reflections at 2 ␪ ⬇42°, 50° could be indexed on a rhombohedral lattice with hexagonal unit cell parameters: a⫽4.265(1) and c⫽10.394(7) Å, V⫽163.8(1) Å 3 , the corresponding rhombohedral unit cell parameters are a⫽4.250(2) Å and ␣ ⫽60.24(4)°, V⫽54.58(4) Å 3 . However, this unit cell could not account for the observed weak reflections. All the promi-

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TABLE I. Crystallographic data and parameters of data collection and structure refinements of Ba4 Nd3 F17 . Empirical formula, molecular weight Space group; Z

Ba4 Nd3 F17 , 1305.1 R ¯3 共No.148兲; 6 11.2818共3兲 and 20.7788共11兲Å V⫽2290.4(1) Å 3 5.676 gm/cm3 K␣ 1 ⫽1.5406 Å and K␣ 2 ⫽1.5444 Å PW 1710 关Bragg Brentano, 共␪ –2␪兲 vertical兴 Step scan Curved graphite 2.3° 1° 0.2 mm Proportional counter 10° – 105° 0.02, 3.05 s 300 K, 1 atm. Profile refinements, Rietveld 共Fullprof-2K兲 共Rodriguez-Caravjal, 2000兲 26 Pseudo-Voigt 0.4858, ⫺0.1015, 0.0452 and 0.49 0.0139, 0.0099 ⫺0.086 0.080 1.41 9.5%, 12.9% and 10.9% 6.1% and 5.9%

Unit cell dimensions 共Å兲: a, c, v Density 共X-ray兲 Radiation 共Cu-K␣兲 Diffractometer, scan mode Diffracted beam monochromator Soller slit Divergent slit Receiving slit Detector 2␪ range Scan-width and scan speed Temperature and pressure Refinement No. of free parameters Profile Profile parameters (U, V, W and ␩兲 Asymmetry parameter A1 and A2 Displacement Transparency Goodness-of-fit ( ␹ 2 ) R p , R wp and R exp R B and R F

nent observed reflections including the weak reflections could be indexed on a larger rhombohedral lattice with unit cell parameters: a⫽11.281(2), c⫽20.783(7) Å, V ⫽2290.5(9) Å 3 共hexagonal settings兲, which represents the rhombohedral super-structure. The corresponding rhombohedral unit cell parameters are a⫽9.509(2) Å and ␣ ⫽72.76(2)°, V⫽763.4(2) Å 3 . The structure is very closely related to the parent fluorite unit cell, wherein the cations and anions are arranged in an ordered manner. The fluorite unit cell parameters and the ordered rhombohedral unit cell parameters are related as a H⫽ 冑 (7/2)⫻a F and c H⫽2⫻ 冑 3⫻a F 共the subscript F is used to indicate the parent fluorite unit cell兲. The crystal structure of this phase was obtained by refining the observed profile with the initial model of Ba4 Y3 F17 and the above mentioned rhombohedral super-structure pa-

rameters. The profile was refined with Fullprof-2K 共Rodriguez-Carvajal, 2000兲. The data collection parameters are given in Table I. Initially the scale and background parameters were adjusted. The background was fitted with a fifth order polynomial and the profile was fitted using a pseudo-Voigt profile function. Subsequently, the unit cell parameters and the overall thermal parameters were refined. The sample displacement and transparency parameters (cos ␪ and sin ␪) were also refined. The refined profile parameters along with the other crystallographic parameters are also given in Table I. The final refined unit cell parameters are a⫽11.2818(3) and c⫽20.7788(11) Å, V⫽2290.4(1) Å 3 .

TABLE II. Position coordinates of various atoms of Ba4 Nd3 F17 . Atom

Site

x

Y

z

Ba

Occ.

Ba1 Ba2 Nd1 F1 F2 F3 F4 F5 F6 F7 F8

6c 18f 18f 18f 18f 18f 18f 18f 6c 3a 18f

0 0.2286共8兲 0.0838共8兲 0.046共5兲 0.437共6兲 0.471共5兲 0.207共4兲 0.250共4兲 0 0 0.948共35兲

0 0.0321共7兲 0.6132共7兲 0.790共5兲 0.278共6兲 0.092共7兲 0.504共4兲 0.345共4兲 0 0 0.916共13兲

0.2642共5兲 0.0848共4兲 0.0833共4兲 0.032共2兲 0.116共2兲 0.040共2兲 0.049共2兲 0.175共2兲 0.143共4兲 0 0.493共14兲

0.038 0.038 0.038 0.038 0.038 0.038 0.038 0.038 0.038 0.038 0.038

1 1 1 1 1 1 1 1 1 1 0.167

a

Fixed during refinement.

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Figure 1. Experimental 共䊊兲 and calculated 共continuous line兲 patterns of Ba4 Nd3 F17 . The difference profile is given at the bottom. The Bragg positions are indicated by the vertical marker below the observed pattern. Synthesis and structural elucidation

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TABLE III. Motifs of mutual adjunction and coordination numbers 共C.N.兲 for Ba4 Nd3 F17 . 18 F1 18 F2 18 F3 18 F4 18 F5 6 F6 3 F7 3 F8 C.N. 6 Ba1 18 Ba2 18 Nd1 C.N.

Figure 2. A typical Nd6 F36⫹1 cluster (NdF8 polyhedra are shown兲. Nd 共small spheres兲 and F共8兲 共smallest unconnected spheres兲.

The isotropic thermal parameters were initially fixed to zero, and subsequently the overall thermal parameter was assigned to all the atoms. The position coordinates of all atoms except the F8 fluorine were sequentially refined. The acceptable R-values (R p⫽9.5%, R wp⫽12.9% and R exp⫽10.9%) could be obtained after this refinement. Finally only the F8 positions were refined. The refined position coordinates are given in Table II. The refinement of F8 position does not significantly improve the R-values. The final R-values are R p ⫽9.5% and R wp⫽12.9%. The goodness of refinement is signified by the ␹ 2 ⫽1.41 and the corresponding R B⫽6.1%. The observed and calculated XRD patterns along with the difference plot are shown in Figure 1. The Nd atoms have square antiprismatic coordination. Six of the NdF8 square antiprisms share their corners forming a cluster of Nd6 F36 cluster. A cubo-octahedron of fluo-

0/0 3/3 1/1 4

3/1 2/2 1/1 4

3/1 1/1 2/2 4

3/1 1/1 2/2 4

0/0 2/2 2/2 4

1/1 1/3 0/0 4

0/0 1/6 0/0 6

0/0 0/0 1/3 3

10 11 8⫹1

rides formed inside such a cluster. The F8 fluoride ion ( 61 occupied兲 exists inside the cubo-octahedron, making the cluster Nd6 F37 , i.e., Nd6 F36⫹1 . A representative cluster observed for this structure is shown in Figure 2. The Ba1 and Ba2 have 10- and 11-fold coordination with fluorine atoms. The mutual adjunction table indicating various coordination numbers and motif 共Hoppe, 1979兲 are shown in Table III. The typical metal ion polyhedrons along with their typical bond lengths are shown in Figure 3. The crystal structure of Ba4 Nd3 F17 is similar to that of Ba4 Eu3 F17 共Achary et al., 2002兲 or Ba4 Y3 F17 共Maksimov et al., 1996兲 and Ba4 Er3 F17 共Tyagi and Kohler, 2001兲. The Nd6 F37 clusters share their external edges to form the total crystal structure. The Ba1 and Ba2 atoms are occupied in the interstices of such frame. A list of the observed reflections and the reflections calculated from the refined unit cell parameters are given in Table IV.

IV. CONCLUSION

The detailed powder XRD data of a new compound Ba4 Nd3 F17 was obtained and analyzed in this study. Ba4 Nd3 F17 phase exists in a rhombohedral lattice 共Space group R-3, No 148兲 with unit cell parameters 11.2818共3兲 and 20.7788共11兲 Å, V⫽2290.4 Å 3 . The crystal structure is isotypic to that of Ba4 Y3 F17 and Ba4 Er3 F17 .

Figure 3. Coordination polyhedra of 共a兲 NdF8 , 共b兲 Ba(1)F10 , and 共c兲 Ba(2)F11 in Ba4 Nd3 F17 . 328

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TABLE IV. A list of observed d values and the corresponding miller indices for the composition Ba4 Nd3 F17 . 2␪ (°) 共observed兲

⌬2␪

25.55

⫺0.030

28.12 29.66 35.32 38.91 41.17 42.30 42.52 44.44

⫺0.054 0.002 ⫺0.037 0.011 0.003 0.042 ⫺0.002 0.018

1 0 10 317 309

44.63 45.26 48.31

0.082 ⫺0.014 0.030

1.8392 1.8156

421 416

49.56 50.18

0.034 ⫺0.024

36 2 10 1 8 11

1.8146 1.7865 1.7398 1.6553 1.5049 1.3833 1.3771 1.3480

67.97 69.74

⫺0.054 0.033

23

1.3310

1

1.3307

3 9 4 12 8 2 2 14 4 4 9

0.036 0.001 0.026 0.034 0.037 ⫺0.048

8 8

3 2 2 1 2 3 0 1 3 0 0

50.27 51.08 52.59 55.50 61.61 67.63

1.3781 1.3474

3 2 4 1 4 5 7 2 5 7 6

70.72

⫺0.018

24

1.3238

2

1.3231

1 2 9

1.2857 1.2684 1.2295

73.62 74.79 77.60

0.004 0.000 0.014

28

1.1623

4

1.1616

5 5 11 14 8 8 15

⫺0.042

1.2856 1.2684 1.2293

3 0 1 0 3 0 1

71.17

25 26 27

5 7 5 4 5 7 4

83.02

⫺0.061

29

1.1587

6

1.1586

30

1.0186

4

31

1.0152

32

S. No.

d 共Å兲 共observed兲

I/I 0 共%兲

d 共Å兲a 共calculated兲

1

3.4836

100

2 3 4 5 6 7 8 9

3.1713 3.0096 2.5394 2.3130 2.1909 2.1347 2.1246 2.0369

6 43 1 3 2 43 37 5

3.4796 3.4631 3.1654 3.0098 2.5368 2.3136 2.1911 2.1367 2.1245 2.0377

2 0 2 2 2 2 3 1 2 4

10 11 12

2.0289 2.0019 1.8824

1 1 1

2.0324 2.0013 1.8835

13 14

1.8380 1.8164

2 40

15 16 17 18 19 20

1.8134 1.7865 1.7390 1.6544 1.5041 1.3842

21 22

h k l 1 0 0 1 0 1 2 1 1 1

2 6 5 4 7 7 2 9 8 3

83.33

⫺0.006

1.0189

7 0 10 5 3 10 826

98.27

0.033

5

1.0151

4 1 18

98.71

⫺0.009

1.0036

3

1.0033

6 3 12

100.27

⫺0.045

33

0.9819

1

0.9824

909

103.35

0.075

34 35 36 37

0.9715 0.9511 0.9400 0.9203

1 2 2 1

0.9717 0.9511 0.9401 0.9201

7 3 10 10 0 5 1 0 22 6 3 15

104.91 108.17 110.06 113.65

0.024 0.000 0.019 ⫺0.029

38

0.9188

2

0.9187

113.94

⫺0.025

39 40

0.9173 0.9074

2 1

0.9173 0.9073

6 9 5 6

114.23 116.19

0.008 ⫺0.018

5 1 0 6

10 10 20 6

h k lb 2 1 ⫺1 222 311 310 331 421 3 ⫺2 1 432 431 4 ⫺1 0 3 2 ⫺2 433 430 522 441 3 ⫺3 1 4 3 ⫺1 510 4 ⫺2 1 531 4 2 ⫺2 543 620 4 ⫺3 ⫺3 5 ⫺3 0 653 6 ⫺1 ⫺1 5 ⫺3 2 5 5 ⫺1 711 6 ⫺2 1 4 4 ⫺3 650 662 7 2 ⫺1 5 5 ⫺2 843 762 811 7 4 ⫺1 8 ⫺2 0 6 4–4 873 954 8 5 ⫺1 930 6 6 ⫺3 900 9 2 ⫺1 5 5 ⫺5 877 10 4 1 960 7 6 ⫺3 9 3 ⫺2 10 5 5 8 ⫺4 2

Calculated from the unit cell that was refined from the powder data: a⫽11.2818, c⫽20.7788 Å, ␥ ⫽120° and V⫽2290.4 Å 3 . b Corresponding rhombohedral indices 共calculated from the rhombohedral transformed unit cell of the previous hexagonal unit cell: a⫽9.5079 Å and ␣ ⫽72.781°, V⫽763.46 Å 3 ). a

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ACKNOWLEDGMENT

Dr. N. M. Gupta, Head, Applied Chemistry Division is thanked for his keen interest on this work. Achary, S. N., Patwe, S. J., and Tyagi, A. K. 共1999a兲. ‘‘Synthesis and characterization of mixed fluorides Y1⫺x Ca2⫹1.5x F7 , (⫺1.33⭐x⭐1.0),’’ Mater. Res. Bull. 34, 2093–2100. Achary, S. N., Mohapatra, S., Patwe, S. J., and Tyagi, A. K. 共1999b兲. ‘‘Synthesis and characterization of mixed fluorides Y1⫺x Ba2⫹1.5x F7 (⫺1.33 ⬍x⬍1.0),’’ Mater. Res. Bull. 34, 1535–1543. Achary, S. N., Patwe, S. J., and Tyagi, A. K. 共2002兲. ‘‘Powder XRD study of Ba4 Eu3 F17 : A new anion rich fluorite related mixed fluoride,’’ Powder Diffr. 17, 225–229. Bevan, D. J. M., Greis, O., and Strahle, J. 共1980兲. ‘‘A new structural principle in anion-excess fluorite related superlattices,’’ Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. A36, 889– 890. Greis, O. and Haschke, J. M. 共1982兲. ‘‘Rare earth fluorides’’ in Handbook on the Physics and Chemistry of Rare Earths, edited by K. A. Gschneidner and L. Eyring 共North-Holland, Amsterdam兲, p. 387. Grover, V., Achary, S. N., Patwe, S. J., and Tyagi, A. K. 共2002兲. ‘‘Synthesis and characterization of Ba1⫺x Ndx F2⫹x (0.00⭐x⭐1.00),’’ Mater. Res. Bull. 共communicated兲.

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Hoppe, R. 共1979兲. ‘‘Effective coordination number and mean fictive ionic radius,’’ Z. Kristallogr. 150, 23. Keiser, V. M. and Greis, O. 共1980兲. ‘‘Preparation and properties of Fluorite related super-structure Phases Ba4 RE3 F17 . . . ,’’ Z. Anorg. Allg. Chem. 469, 164 –171. Maksimov, B. A., Solans, K., Dudka, A. P., Genkina, E. A., Font-Badria, M., Buchinskaya, I. I., Loshmanov, A. A., Golubev, A. M., Simonov, V. I., Font-Altaba, M., and Sobolev, B. P. 共1996兲. ‘‘Fluorite-matrix-based Ba4 R3 F17 (R⫽Y,Yb) Crystal Structure . . . ,’’ Crystallogr. Rep. 41, 56 – 64. Patwe, S. J., Achary, S. N., and Tyagi, A. K. 共2001兲. ‘‘Synthesis and characterization of Y1⫺x Pb2⫹1.5x F7 (⫺1.33⭐x⭐1.0),’’ Mater. Res. Bull. 36, 597– 605. Rodriguez-Carvajal, J. 共2000兲. ‘‘Multi pattern Rietveld refinement program, Fullprof.2K. Version 1.6, July 2000.’’ Sobolev, B. P. 共1992兲. ‘‘Multi-component fluoride single crystals 共Current status of their synthesis and prospects兲,’’ Growth Crystal. 18, 197–211. Tyagi, A. K. and Koehler, J. 共2001兲. ‘‘Preparation and Structural elucidation of the new anion-excess fluorite variant Ba4 Er3 F17 ,’’ Solid State Sci. 3, 689.

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