inorganic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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ISSN: 2056-9890

Apatite-type SrPr4(SiO4)3O

aCeramics Research Laboratory, Nagoya Institute of Technology, Asahigaoka, Tajimi 507-0071, Japan
*Correspondence e-mail: ishizawa@nitech.ac.jp

(Received 10 August 2010; accepted 18 August 2010; online 25 August 2010)

Single crystals of the title compound, strontium tetra­praseo­dymium tris­(silicate) oxide, SrPr4(SiO4)3O, have been grown by the self-flux method using SrCl2. The structure is isotypic with the apatite supergroup family having the generic formula IXM12VIIM23(IVTO4)3X, where M = alkaline earth and rare earth metals, T = Si and X = O. The M1 site (3.. symmetry) is occupied by Pr and Sr atoms with almost even proportions and is surrounded by nine O atoms forming a tricapped trigonal prism. The M2 site (m.. symmetry) is almost exclusively occupied by Pr and surrounded by seven O atoms, forming a distorted penta­gonal bipyramid. The Si atom (m.. symmetry) is surrounded by two O (m.. symmetry) and two O atoms in general positions, forming an isolated SiO4 tetra­hedron. Another O atom at the inversion centre ([\overline{6}].. symmetry) is surrounded by three M2 sites, forming an equilateral triangle perpendicular to the c axis.

Related literature

Náray-Szabó (1930[Náray-Szabó, S. (1930). Z. Kristallogr. 75, 387-398.]) and Mehmel (1930[Mehmel, M. (1930). Z. Kristallogr. 75, 323-331.]) independently determined the structure of fluorapatite for the first time. Since then, a large number of apatite supergroup minerals have been reported, as classified recently by Pasero et al. (2010[Pasero, M., Kampf, A. R., Ferraris, C., Pekov, I. V., Rakovan, J. & White, T. J. (2010). Eur. J. Mineral. 22, 163-179.]). The synthetic rare-earth-bearing orthosilicates belonging to the apatite supergroup have attracted considerable attention since high oxide ionic conductivity was found (Nakayama et al., 1995[Nakayama, S., Kageyama, T., Aono, H. & Sadaoka, Y. (1995). J. Mater. Chem. 5, 1801-1805.]). The title compound is isostructural with CaLa4(SiO4)3O (Schroeder & Mathew, 1978[Schroeder, L. W. & Mathew, M. (1978). J. Solid State Chem. 26, 383-387.]).

Experimental

Crystal data
  • SrPr4(SiO4)3O

  • Mr = 943.53

  • Hexagonal, P 63 /m

  • a = 9.5999 (1) Å

  • c = 7.1388 (1) Å

  • V = 569.76 (1) Å3

  • Z = 2

  • Mo Kα radiation

  • μ = 21.82 mm−1

  • T = 296 K

  • 0.14 × 0.05 × 0.03 mm

Data collection
  • Bruker APEXII CCD diffractometer

  • Absorption correction: multi-scan (SADABS; Sheldrick, 1996[Sheldrick, G. M. (1996). SADABS. University of Göttingen, Germany.]) Tmin = 0.240, Tmax = 0.546

  • 31664 measured reflections

  • 3151 independent reflections

  • 2815 reflections with I > 2σ(I)

  • Rint = 0.030

Refinement
  • R[F2 > 2σ(F2)] = 0.018

  • wR(F2) = 0.047

  • S = 1.07

  • 3151 reflections

  • 42 parameters

  • 1 restraint

  • Δρmax = 2.45 e Å−3

  • Δρmin = −1.75 e Å−3

Table 1
Selected bond lengths (Å)

Pr1—O1 2.4617 (8)
Pr1—O2i 2.5258 (9)
Pr1—O3i 2.8509 (12)
Pr2—O4 2.2809 (1)
Pr2—O3i 2.4296 (8)
Pr2—O2ii 2.4568 (12)
Pr2—O3iii 2.5519 (10)
Pr2—O1 2.6876 (15)
Si1—O1 1.6172 (12)
Si1—O2 1.6251 (13)
Si1—O3 1.6324 (9)
Symmetry codes: (i) x-y, x, -z; (ii) -x+y, -x+1, z; (iii) -y, x-y, z.

Data collection: APEX2 (Bruker, 2008[Bruker (2008). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SAINT (Bruker, 2008[Bruker (2008). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]) and WinGX (Farrugia, 1999[Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837-838.]); molecular graphics: ATOMS for Windows (Dowty, 2006[Dowty, E. (2006). ATOMS for Windows. Shape Software, Kingsport, Tennessee, USA.]); software used to prepare material for publication: PLATON (Spek, 2009[Spek, A. L. (2009). Acta Cryst. D65, 148-155.]) and publCIF (Westrip, 2010[Westrip, S. P. (2010). J. Appl. Cryst. 43, 920-925.]).

Supporting information


Comment top

Náray-Szabó (1930) and Mehmel (1930) independently determined the structure of fluorapatite for the first time. Since then, a large number of apatite supergroup minerals and synthetic compounds have been reported, as classified recently by Pasero et al. (2010). The apatite supergroup has a general formula IXM12VIIM23(IVTO4)3X, where M = alkali, alkaline earth and rare earth metals, T = P, As, V, Si, S, B and X = F, Cl, OH, O, in hexagonal or pseudo-hexagonal symmetries. After the discovery of high oxide-ion conductivity in rare-earth orthosilicate oxyapatites (T = Si, X = O) by Nakayama et al. (1995), considerable attention has been drawn to related compounds for potential industrial applications.

In the title compound, the M1 site (site symmetry 3..) is occupied by Pr and Sr with almost even proportions, and surrounded by three O1, three O2 and three O3 atoms, forming a tri-capped trigonal prism. The M site (m.. symmetry) is almost exclusively occupied by Pr, and surrounded by one O1, one O2, four O3 and one O4 forming a distorted pentagonal bipyramid. The isolated SiO4 tetrahedra are composed of one O1, one O2 and two O3 atoms. They show a slight angular distortion and exhibit m.. symmetry. The O atom at the X site with 6.. symmetry is located at the centre of the M2 triangle and its anisotropic displacement ellipsoid is more than two times prolate along the c axis, a feature also observed for other members of the rare-earth orthosilicate oxyapatite family. The present compound is isostructural with CaLa4(SiO4)3O (Schroeder & Mathew, 1978).

Related literature top

Náray-Szabó (1930) and Mehmel (1930) independently determined the structure of fluorapatite for the first time. Since then, a large number of apatite supergroup minerals have been reported, as classified recently by Pasero et al. (2010). The synthetic rare-earth-bearing orthosilicates belonging to the apatite supergroup have attracted considerable attention since high oxide ionic conductivity was found (Nakayama et al., 1995). The title compound is isostructural with CaLa4(SiO4)3O (Schroeder & Mathew, 1978).

Experimental top

All chemicals used were of analytical grade (3 N) from Kojundo Chemical Laboratory Co. Ltd., Japan. Powders of Pr2O3 (1.643 g), SiO2 (0.359 g) and SrCl2 (3.006 g) were mixed together and put into a platinum crucible (30 ml). The crucible capped with a platinum lid was then placed on alumina powder in an alumina crucible. The double crucible was heated in air to 1373 K at the rate of 100 K/h, held for 6 h at 1373 K, cooled at the rate of 20 K/h to 923 K, and then furnace-cooled by turning off the power. The flux components were washed away by distilled water in an oven at 373 K. Crystals were hexagonal prismatic with the longest dimension of 150 µm. Energy dispersive spectroscopy indicated that the crystals contained no elements other than Sr, Pr, Si and O. The Sr:Pr ratio was also estimated from eight samples to be 1.04:3.96 with an estimated standard uncertainty of ±0.06. Therefore, the stoichiometric composition of SrPr4Si3O13 was assumed for the refinement of X-ray data.

Refinement top

Possibility of lower symmetries like P63 or P3 for the crystal was discarded in the course of refinements because no significant improvement was obtained. Defects at the O sites were not observed from the preliminary population analysis. Presence of interstitial O atoms were not detected in the difference Fourier maps. Populations of Sr and Pr at the M1 and M2 sites were refined with a restraint to satisfy the charge neutrality for the stoichiometric composition SrPr4Si3O13. The positional and atomic displacement parameters of Sr and Pr at both sites were constrained to unity, taking into account the respective site symmetries. The highest remaining peak is 0.45 Å from M2 and the deepest hole is 0.04 Å from M1.

Structure description top

Náray-Szabó (1930) and Mehmel (1930) independently determined the structure of fluorapatite for the first time. Since then, a large number of apatite supergroup minerals and synthetic compounds have been reported, as classified recently by Pasero et al. (2010). The apatite supergroup has a general formula IXM12VIIM23(IVTO4)3X, where M = alkali, alkaline earth and rare earth metals, T = P, As, V, Si, S, B and X = F, Cl, OH, O, in hexagonal or pseudo-hexagonal symmetries. After the discovery of high oxide-ion conductivity in rare-earth orthosilicate oxyapatites (T = Si, X = O) by Nakayama et al. (1995), considerable attention has been drawn to related compounds for potential industrial applications.

In the title compound, the M1 site (site symmetry 3..) is occupied by Pr and Sr with almost even proportions, and surrounded by three O1, three O2 and three O3 atoms, forming a tri-capped trigonal prism. The M site (m.. symmetry) is almost exclusively occupied by Pr, and surrounded by one O1, one O2, four O3 and one O4 forming a distorted pentagonal bipyramid. The isolated SiO4 tetrahedra are composed of one O1, one O2 and two O3 atoms. They show a slight angular distortion and exhibit m.. symmetry. The O atom at the X site with 6.. symmetry is located at the centre of the M2 triangle and its anisotropic displacement ellipsoid is more than two times prolate along the c axis, a feature also observed for other members of the rare-earth orthosilicate oxyapatite family. The present compound is isostructural with CaLa4(SiO4)3O (Schroeder & Mathew, 1978).

Náray-Szabó (1930) and Mehmel (1930) independently determined the structure of fluorapatite for the first time. Since then, a large number of apatite supergroup minerals have been reported, as classified recently by Pasero et al. (2010). The synthetic rare-earth-bearing orthosilicates belonging to the apatite supergroup have attracted considerable attention since high oxide ionic conductivity was found (Nakayama et al., 1995). The title compound is isostructural with CaLa4(SiO4)3O (Schroeder & Mathew, 1978).

Computing details top

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) and WinGX (Farrugia, 1999); molecular graphics: ATOMS for Windows (Dowty, 2006); software used to prepare material for publication: PLATON (Spek, 2009) and publCIF (Westrip, 2010).

Figures top
[Figure 1] Fig. 1. The crystal structure of SrPr4(SiO4)3O in projection along [001]. Pr and Sr atoms are given as red, Si as dark blue, and O atoms as light blue ellipsoids at the 50% probability level.
Strontium tetrapraseodymium tri(silicate) oxide top
Crystal data top
SrPr4(SiO4)3ODx = 5.5 Mg m3
Mr = 943.53Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63/mCell parameters from 9966 reflections
Hall symbol: -P 6cθ = 4.3–61.7°
a = 9.5999 (1) ŵ = 21.82 mm1
c = 7.1388 (1) ÅT = 296 K
V = 569.76 (1) Å3Prism, colourless
Z = 20.14 × 0.05 × 0.03 mm
F(000) = 840
Data collection top
Bruker APEXII CCD
diffractometer
3151 independent reflections
Radiation source: fine-focus sealed tube2815 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.030
φ and ω scansθmax = 62.0°, θmin = 3.8°
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)
h = 2223
Tmin = 0.240, Tmax = 0.546k = 2323
31664 measured reflectionsl = 1717
Refinement top
Refinement on F22 constraints
Least-squares matrix: full w = 1/[σ2(Fo2) + (0.0249P)2 + 0.3254P]
where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.018(Δ/σ)max = 0.001
wR(F2) = 0.047Δρmax = 2.45 e Å3
S = 1.07Δρmin = 1.75 e Å3
3151 reflectionsExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
42 parametersExtinction coefficient: 0.0052 (3)
1 restraint
Crystal data top
SrPr4(SiO4)3OZ = 2
Mr = 943.53Mo Kα radiation
Hexagonal, P63/mµ = 21.82 mm1
a = 9.5999 (1) ÅT = 296 K
c = 7.1388 (1) Å0.14 × 0.05 × 0.03 mm
V = 569.76 (1) Å3
Data collection top
Bruker APEXII CCD
diffractometer
3151 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)
2815 reflections with I > 2σ(I)
Tmin = 0.240, Tmax = 0.546Rint = 0.030
31664 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.01842 parameters
wR(F2) = 0.0471 restraint
S = 1.07Δρmax = 2.45 e Å3
3151 reflectionsΔρmin = 1.75 e Å3
Special details top

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Pr10.33330.66670.000387 (15)0.00860 (2)0.5358 (8)
Sr10.33330.66670.000387 (15)0.00860 (2)0.4641 (8)
Pr20.010753 (8)0.242790 (8)0.25000.00742 (2)0.9761 (5)
Sr20.010753 (8)0.242790 (8)0.25000.00742 (2)0.0239 (5)
Si10.39915 (5)0.37061 (5)0.25000.00683 (5)
O10.31811 (17)0.48323 (16)0.25000.01330 (16)
O20.59455 (14)0.47298 (15)0.25000.01224 (14)
O30.34191 (14)0.25210 (11)0.06770 (12)0.01501 (14)
O40.00000.00000.25000.0152 (3)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Pr10.00951 (3)0.00951 (3)0.00678 (3)0.00476 (1)0.0000.000
Sr10.00951 (3)0.00951 (3)0.00678 (3)0.00476 (1)0.0000.000
Pr20.00721 (2)0.00712 (2)0.00745 (2)0.00323 (2)0.0000.000
Sr20.00721 (2)0.00712 (2)0.00745 (2)0.00323 (2)0.0000.000
Si10.00702 (11)0.00647 (11)0.00747 (11)0.00371 (10)0.0000.000
O10.0168 (4)0.0127 (4)0.0153 (4)0.0110 (4)0.0000.000
O20.0079 (3)0.0099 (3)0.0171 (4)0.0031 (3)0.0000.000
O30.0243 (4)0.0119 (3)0.0100 (2)0.0098 (3)0.0065 (2)0.00319 (19)
O40.0107 (4)0.0107 (4)0.0242 (10)0.00537 (19)0.0000.000
Geometric parameters (Å, º) top
Pr1—O1i2.4617 (8)Pr2—O3vi2.4296 (8)
Pr1—O1ii2.4617 (8)Pr2—O3iii2.4296 (8)
Pr1—O12.4617 (8)Pr2—O2i2.4568 (12)
Pr1—O2iii2.5258 (9)Pr2—O3vii2.5519 (10)
Pr1—O2iv2.5258 (9)Pr2—O3viii2.5519 (10)
Pr1—O2v2.5258 (9)Pr2—O12.6876 (15)
Pr1—O3iv2.8509 (12)Si1—O11.6172 (12)
Pr1—O3iii2.8509 (12)Si1—O21.6251 (13)
Pr1—O3v2.8509 (12)Si1—O31.6324 (9)
Pr2—O42.2809 (1)Si1—O3ix1.6324 (9)
O1—Si1—O2113.04 (7)O1—Si1—O3ix111.05 (5)
O1—Si1—O3111.05 (5)O2—Si1—O3ix107.82 (5)
O2—Si1—O3107.82 (5)O3—Si1—O3ix105.73 (7)
Symmetry codes: (i) x+y, x+1, z; (ii) y+1, xy+1, z; (iii) xy, x, z; (iv) y, x+y+1, z; (v) x+1, y+1, z; (vi) xy, x, z+1/2; (vii) y, xy, z; (viii) y, xy, z+1/2; (ix) x, y, z+1/2.

Experimental details

Crystal data
Chemical formulaSrPr4(SiO4)3O
Mr943.53
Crystal system, space groupHexagonal, P63/m
Temperature (K)296
a, c (Å)9.5999 (1), 7.1388 (1)
V3)569.76 (1)
Z2
Radiation typeMo Kα
µ (mm1)21.82
Crystal size (mm)0.14 × 0.05 × 0.03
Data collection
DiffractometerBruker APEXII CCD
Absorption correctionMulti-scan
(SADABS; Sheldrick, 1996)
Tmin, Tmax0.240, 0.546
No. of measured, independent and
observed [I > 2σ(I)] reflections
31664, 3151, 2815
Rint0.030
(sin θ/λ)max1)1.243
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.018, 0.047, 1.07
No. of reflections3151
No. of parameters42
No. of restraints1
Δρmax, Δρmin (e Å3)2.45, 1.75

Computer programs: APEX2 (Bruker, 2008), SAINT (Bruker, 2008), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008) and WinGX (Farrugia, 1999), ATOMS for Windows (Dowty, 2006), PLATON (Spek, 2009) and publCIF (Westrip, 2010).

Selected bond lengths (Å) top
Pr1—O12.4617 (8)Pr2—O3iii2.5519 (10)
Pr1—O2i2.5258 (9)Pr2—O12.6876 (15)
Pr1—O3i2.8509 (12)Si1—O11.6172 (12)
Pr2—O42.2809 (1)Si1—O21.6251 (13)
Pr2—O3i2.4296 (8)Si1—O31.6324 (9)
Pr2—O2ii2.4568 (12)
Symmetry codes: (i) xy, x, z; (ii) x+y, x+1, z; (iii) y, xy, z.
 

Acknowledgements

This work was supported by the Grant-in-Aids for Scientific Research No. 18206071 and No. 22360272 from the Japan Society for the Promotion of Science. TS and JW appreciate research assistant scholarships from the Institute of Ceramics Research and Education, Nagoya Institute of Technology.

References

First citationBruker (2008). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.  Google Scholar
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First citationPasero, M., Kampf, A. R., Ferraris, C., Pekov, I. V., Rakovan, J. & White, T. J. (2010). Eur. J. Mineral. 22, 163–179.  Web of Science CrossRef CAS Google Scholar
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First citationWestrip, S. P. (2010). J. Appl. Cryst. 43, 920–925.  Web of Science CrossRef CAS IUCr Journals Google Scholar

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