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A single crystal of Sn1.59Nb1.84O6.35 was grown at 1273 K from a mixture of sodium niobate and tin(II) chloride. The structure is of pyrochlore type A2B2O7. The tin is partially oxidized to tin(IV) and competes with niobium for the occupation of site B. The stereoactivity of the Sn2+ lone pair induces displacement of tin towards the O atoms of the tunnel.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270101010320/br1329sup1.cif
Contains datablocks snnbo, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270101010320/br1329Isup2.hkl
Contains datablock I

Comment top

Ferroelectric compounds are of great interest in many applications (Kung et al., 1977; Horowitz et al., 1979; Bouhard & Rogers, 1975). Many of them crystallize in the perovskite structure. One of the cations presenting the highest effect and incorporated in these compounds is lead, as in Pb(Zr0.55Ti0.45)O3 (Auciello et al., 1994), (Pb,Ln)(Zr,Ti)O3 (Zhang et al., 1999) or Pb(Mg1/3Nb2/3)O3 (Tavernor & Thomas, 1994). The important problem of environment pollution motivates research aimed at the substitution of less toxic metals. Since many physical properties and structural features of lead(II) compounds are due to the lone electron pair (Lp), this metal must be replaced by divalent cations with a lone electron pair, for example, tin(II). The products of the synthesis of niobates are often contaminated with pyrochlore-type compounds. In the case of lead niobates, compounds PbxNb2O6 + x with the pyrochlore structure are known (Sreedhar & Mitra, 1999), while for tin, only two niobates are known, viz. foordite, SnNb2O6, which crystallizes in the monoclinic system and Sn2Nb2O7 which has the pyrochlore structure (Bodiot, 1968; Birchall & Sleight, 1975; Trunov et al. 1963). This study reports the structure of a tin niobate with a lacunar pyrochlore-type structure. Two formula are proposed for pyrochlore compounds, i.e. AB2O6 or (A2O')B2O6. In both cases, the skeleton is made of corner-sharing octahedra (B2O6) assembled in a tetrahedron. In such a structure, tunnels are developed in the [110], [101] and [011] directions. At their intersection, huge cavities are formed. The A atom occupies preferentially the center of the cavity (CC) in AB2O6 compounds and the center of the tunnel (CT) in (A2O')B2O6 compounds, the CC being occupied by O'. The structure determination confirms what was assumed from chemical analysis. Tin is partially located in the niobium (B) site and is assumed to be tin(IV). This site is of regular octahedral shape with B–O distances of 1.981 (1) Å, a value which corresponds to a weighted average between pure Nb—O (1.97 Å; Shannon, 1976) and Sn—O (2.04 Å; given in Bergerhoff & Brown, 1987) distances: d(B—O) = (1.6 × 1.97) + (0.4 × 2.04) = 1.984 Å. Valence calculations (Brown & Altermatt, 1985) confirm the occupation of this site by a mixture of Nb and Sn [valence(Nb) = 4.96 and valence(Sn) = 4.88]. The remaining tin is found outside the classical CT site, which was already expected by Birchall & Sleight (1975). The two possible coordination polyhedra of tin in the tunnel are presented in Figs. 1 and 2. In both cases, an Sn shift of 0.39 (1) Å from CT is found. The average Sn—O distances [Sn21 polyhedron 2.6 (3) Å and Sn22 polyhedron 2.6 (3) Å] are shorter than that observed in foordite [2.7 (5) Å; Trunov et al., 1963]; however, the shortest distance is observed in foordite (Sn–O1 = 2.177 Å), which allows an Sn valence of 1.98. The corresponding valences calculated using the whole set of Sn—O bonds are 1.86 and 1.87 for Sn21 and Sn22, respectively. The distortion of the tin(II) polyhedron has already been used to determine lone pair stereoactivity (Galy et al., 1975); these authors locate the electronic Lp in the center of the coordination polyhedron. In the title compound, this center coincides with CT so that the Sn—Lp distance is equal to the Sn offset [0.386 (13) and 0.395 (15) Å for Sn21 and Sn22, respectively]. Such a distance is shorter than the Sn—Lp distance proposed by Galy et al. (1975) of 0.95 Å. So the tin environment cannot be 6O1 + 2O2. Moreover, since the O2 content in CC is less than expected, being half the tin(II) content, one of the O2 atoms is probably missing from the coordination sphere of Sn, i.e. some CC are empty. Galy et al. (1975) have proposed that the sphere of interaction of an Lp has the same volume as an oxygen anion. So competition between Lp and O2 occurs in the occupation of a cavity and O2 cannot occupy a site at which a lone pair is pointing. Still, they have shown that lone pairs can point at each other. In this compound, at most four lone pairs can point at CC. The pyrochlore formula of a compound which includes this restraint is then M2+1.6Nb2O6.6. Such a formula is close to the observed lead niobate formula of Pb1.5Nb2O6.5. But cancelling one O2 around Sn gives bond valences that are unrealistic (1.45 and 1.46 for Sn21 and Sn22, respectively). A similar observation can be made for the lead pyrochlore compound. Although the agreement index (R) is good, vacancies in the CT and CC sites may induce changes of symmetry or commensurate or incommensurate arrangements and the use of a cubic cell with the Fd3 m space group may result in finding only the average location of tin in the tunnels.

The main result of this study is the chemical and structural behavior of tin(II); it is easily oxidized and exhibits strong lone-pair stereoactivity. Such behavior could be the reason for the difficulty in inserting tin(II) in the A site of ABO3 perovskite-type compounds.

Experimental top

The title compound was synthesized by the flux method using NaCl as the melting salt. A mixture of NaNbO3 and an excess of SnCl2 was heated with NaCl at 1273 K in a quartz tube under vacuum. Red crystals of regular octahedral shape were obtained. Elemental analysis for Sn and Nb suggested the formula Sn1.6Nb1.8O6.3. Mossbauer spectroscopy indicated that tin(II) was partially oxidized to tin(IV). The ionic formula which complies with electroneutrality is SnII1.4NbV1.8SnIV0.2O6.3.

Refinement top

The atomic positions were taken from the pyrochlore-type structure (Jona et al., 1955; Sleight, 1968). The second setting, with the origin at 3 m of the Fd3 m space group, was chosen. As expected, some of the niobium site (16c) is occupied by Sn atoms. The occupation factors of Sn and Nb were constrained in order to fill the site. Surprisingly, the atomic displacement parameter when tin is placed at CT, which is normally occupied by A atoms, indicates that no tin can occupy this site. Similarly, the CT site which contains either the A atom of the AB2O6 pyrochlore or the O atom of the A2B2O7 pyrochlore cannot contain these remaining Sn atoms. A difference Fourier synthesis shows that Sn is delocalized around CT, and two sites, with coordinates 0, y, -y and x, x, z, were chosen in order to simulate the corresponding electron density. As in Pb1.5Nb2O6.5 (Bernotat-Wulf & Hoffman, 1982), the residue which appeared at CC was attributed to oxygen. Because strong correlations appear between the Sn atomic parameters around CT, cycles of refinement with alternating optimization of coordinate parameters and atomic displacement parameters of both Sn21 and Sn22 were performed until convergence occurred. A general constraint of electroneutrality was applied to all occupation factors.

Computing details top

Data collection: COLLECT (Nonius BV, 1997-2000); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL SCALEPACK and DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: maXus (Mackay et al., 1999) and SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXL97.

Figures top
[Figure 1] Fig. 1. View of the observed surrounding of Sn21. Ellipsoids are shown at the 60% probability level.
[Figure 2] Fig. 2. View of the observed surrounding of Sn22. Ellipsoids are shown at the 60% probability level.
Tin niobate top
Crystal data top
Sn1.59Nb1.84O6.35Melting point: 1373 K
Mr = 461.08Mo Kα radiation, λ = 0.71073 Å
Cubic, Fd3mCell parameters from 654 reflections
Hall symbol: -F 4vw 2vw 3θ = 1.0–39.4°
a = 10.539 (2) ŵ = 10.19 mm1
V = 1170.5 (4) Å3T = 293 K
Z = 8Octahedral, clear intense red
F(000) = 16450.06 × 0.06 × 0.06 mm
Dx = 5.233 Mg m3
Data collection top
KappaCCD
diffractometer
198 independent reflections
Radiation source: fine-focus sealed tube172 reflections with I > 2σ(I)
Vertically mounted graphite crystal monochromatorRint = 0.047
Detector resolution: 9 pixels mm-1θmax = 39.2°, θmin = 3.4°
CCD scansh = 018
Absorption correction: numerical
(Coppens et al., 1965)
k = 014
Tmin = 0.52, Tmax = 0.54l = 017
640 measured reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.027 w = 1/[σ2(Fo2) + 13.1226P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.060(Δ/σ)max = 0.035
S = 0.94Δρmax = 0.94 e Å3
198 reflectionsΔρmin = 0.74 e Å3
24 parametersExtinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
4 restraintsExtinction coefficient: 0.0021 (2)
Crystal data top
Sn1.59Nb1.84O6.35Z = 8
Mr = 461.08Mo Kα radiation
Cubic, Fd3mµ = 10.19 mm1
a = 10.539 (2) ÅT = 293 K
V = 1170.5 (4) Å30.06 × 0.06 × 0.06 mm
Data collection top
KappaCCD
diffractometer
198 independent reflections
Absorption correction: numerical
(Coppens et al., 1965)
172 reflections with I > 2σ(I)
Tmin = 0.52, Tmax = 0.54Rint = 0.047
640 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0274 restraints
wR(F2) = 0.060 w = 1/[σ2(Fo2) + 13.1226P]
where P = (Fo2 + 2Fc2)/3
S = 0.94Δρmax = 0.94 e Å3
198 reflectionsΔρmin = 0.74 e Å3
24 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Nb10.00000.00000.00000.0150 (2)0.920 (8)
Sn10.00000.00000.00000.0150 (2)0.082 (8)
O10.3140 (3)0.12500.12500.0128 (5)
O20.37500.37500.37500.053 (9)0.352 (14)
Sn210.4849 (14)0.4849 (14)0.5297 (12)0.011 (3)0.0594 (5)
Sn220.00000.2765 (14)0.2765 (14)0.037 (7)0.0594 (5)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Nb10.0150 (2)0.0150 (2)0.0150 (2)0.00133 (11)0.00133 (11)0.00133 (11)
Sn10.0150 (2)0.0150 (2)0.0150 (2)0.00133 (11)0.00133 (11)0.00133 (11)
O10.0134 (12)0.0126 (8)0.0126 (8)0.0000.0000.0052 (9)
O20.053 (9)0.053 (9)0.053 (9)0.0000.0000.000
Sn210.011 (6)0.011 (6)0.011 (4)0.003 (4)0.0026 (15)0.0026 (15)
Sn220.027 (10)0.042 (6)0.042 (6)0.015 (5)0.015 (5)0.008 (4)
Geometric parameters (Å, º) top
Nb1—O1i1.9813 (10)Sn21—O2xiii2.317 (14)
Nb1—O1ii1.9813 (10)Sn22—O1xiv2.375 (17)
Nb1—O1iii1.9813 (10)Sn22—O1xv2.375 (17)
Nb1—O1iv1.9813 (10)Sn22—O1xvi2.733 (10)
Nb1—O1v1.9813 (10)Sn22—O1xvii2.733 (10)
Nb1—O1vi1.9813 (10)Sn22—O1i3.049 (13)
Sn21—O1vii2.323 (17)Sn22—O1iv3.049 (13)
Sn21—O1viii2.536 (14)Sn22—O2xviii2.316 (4)
Sn21—O1ix2.536 (14)Sn22—O2xix2.316 (4)
Sn21—O1x2.914 (14)Sn21—Sn21xx0.386 (15)
Sn21—O1xi2.914 (14)Sn21—Sn22xxi0.202 (8)
Sn21—O1xii3.087 (14)Sn22—Sn22xxii0.395 (15)
Sn21—O22.311 (14)
O1ii—Nb1—O1v89.39 (10)O1viii—Sn21—O1xii110.7 (5)
O1i—Nb1—O1iii90.61 (10)O1ix—Sn21—O1x165.8 (6)
O1i—Nb1—O1ii90.61 (10)O1ix—Sn21—O1xi61.0 (3)
O1ii—Nb1—O1iii90.61 (10)O1ix—Sn21—O1xii110.7 (5)
O1i—Nb1—O1iv89.39 (10)O1x—Sn21—O1xi105.4 (4)
O1ii—Nb1—O1iv180.00 (1)O1x—Sn21—O1xii55.3 (2)
O1iii—Nb1—O1iv89.39 (10)O1xi—Sn21—O1xii55.3 (2)
O1i—Nb1—O1v180.00 (1)O2xviii—Sn22—O2xix160.4 (10)
O1iii—Nb1—O1v89.39 (10)O2xviii—Sn22—O1xv107.4 (4)
O1iv—Nb1—O1v90.61 (10)O2xviii—Sn22—O1xiv88.7 (3)
O1i—Nb1—O1vi89.39 (10)O2xviii—Sn22—O1xvi96.6 (1)
O1ii—Nb1—O1vi89.39 (10)O2xviii—Sn22—O1xvii80.5 (4)
O1iii—Nb1—O1vi180.00 (1)O2xviii—Sn22—O1i88.5 (4)
O1iv—Nb1—O1vi90.61 (10)O2xviii—Sn22—O1iv73.9 (3)
O1v—Nb1—O1vi90.61 (10)O2xix—Sn22—O1xiv107.4 (6)
O2—Sn21—O2xiii160.8 (6)O2xix—Sn22—O1xv88.7 (3)
O2—Sn21—O1vii90.0 (5)O2xix—Sn22—O1xvi80.5 (4)
O2—Sn21—O1viii102.4 (5)O2xix—Sn22—O1xvii96.6 (1)
O2—Sn21—O1ix102.4 (5)O2xix—Sn22—O1i73.9 (3)
O2—Sn21—O1x76.7 (4)O2xix—Sn22—O1iv88.5 (4)
O2—Sn21—O1xi76.7 (4)O1xiv—Sn22—O1xv71.9 (4)
O2—Sn21—O1xii87.7 (4)O1xiv—Sn22—O1xvi65.7 (2)
O2xiii—Sn21—O1vii109.1 (5)O1xiv—Sn22—O1xvii130.2 (6)
O2xiii—Sn21—O1viii84.8 (4)O1xiv—Sn22—O1i171.3 (4)
O2xiii—Sn21—O1ix84.8 (4)O1xiv—Sn22—O1iv116.9 (2)
O2xiii—Sn21—O1x91.8 (5)O1xv—Sn22—O1xvi130.2 (6)
O2xiii—Sn21—O1xi91.8 (5)O1xv—Sn22—O1xvii65.7 (2)
O2xiii—Sn21—O1xii73.1 (4)O1xv—Sn22—O1i116.9 (2)
O1vii—Sn21—O1viii69.8 (4)O1xv—Sn22—O1iv171.3 (4)
O1vii—Sn21—O1ix69.8 (4)O1xvi—Sn22—O1xvii163.4 (6)
O1vii—Sn21—O1x124.1 (6)O1xvi—Sn22—O1i106.4 (5)
O1vii—Sn21—O1xi124.1 (6)O1xvi—Sn22—O1iv57.3 (2)
O1vii—Sn21—O1xii177.7 (6)O1xvii—Sn22—O1i57.3 (2)
O1viii—Sn21—O1ix132.1 (5)O1xvii—Sn22—O1iv106.4 (5)
O1viii—Sn21—O1x61.0 (3)O1i—Sn22—O1iv54.4 (3)
O1viii—Sn21—O1xi165.8 (6)
Symmetry codes: (i) y, z+1/4, x+1/4; (ii) y, x+1/4, z+1/4; (iii) x+1/4, y+1/4, z; (iv) y1/4, x1/4, z; (v) z, y1/4, x1/4; (vi) x1/4, z, y1/4; (vii) y+1/4, z+1/4, x+1; (viii) x, y+1/2, z+1/2; (ix) z+3/4, x, y+3/4; (x) y+1/4, x+1, z+1/4; (xi) x+1, z+1/4, y+1/4; (xii) y+3/4, z+3/4, x; (xiii) y+1/4, z+1, x+1/4; (xiv) z, y+1/4, x3/4; (xv) y, x+3/4, z1/4; (xvi) x1/2, y, z1/2; (xvii) x+1/2, z+1/4, y1/4; (xviii) z+1/4, x, y+1/4; (xix) y1/4, z+1/2, x3/4; (xx) y+1, z+1, x+1; (xxi) z+3/4, x+1/2, y+1/4; (xxii) z+1/4, x+1/4, y.

Experimental details

Crystal data
Chemical formulaSn1.59Nb1.84O6.35
Mr461.08
Crystal system, space groupCubic, Fd3m
Temperature (K)293
a (Å)10.539 (2)
V3)1170.5 (4)
Z8
Radiation typeMo Kα
µ (mm1)10.19
Crystal size (mm)0.06 × 0.06 × 0.06
Data collection
DiffractometerKappaCCD
diffractometer
Absorption correctionNumerical
(Coppens et al., 1965)
Tmin, Tmax0.52, 0.54
No. of measured, independent and
observed [I > 2σ(I)] reflections
640, 198, 172
Rint0.047
(sin θ/λ)max1)0.890
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.027, 0.060, 0.94
No. of reflections198
No. of parameters24
No. of restraints4
w = 1/[σ2(Fo2) + 13.1226P]
where P = (Fo2 + 2Fc2)/3
Δρmax, Δρmin (e Å3)0.94, 0.74

Computer programs: COLLECT (Nonius BV, 1997-2000), HKL SCALEPACK (Otwinowski & Minor, 1997), HKL SCALEPACK and DENZO (Otwinowski & Minor, 1997), SHELXS97 (Sheldrick, 1997), maXus (Mackay et al., 1999) and SHELXL97 (Sheldrick, 1997), ORTEPIII (Burnett & Johnson, 1996), SHELXL97.

Selected geometric parameters (Å, º) top
Nb1—O1i1.9813 (10)Sn22—O1ix2.375 (17)
Sn21—O1ii2.323 (17)Sn22—O1x2.375 (17)
Sn21—O1iii2.536 (14)Sn22—O1xi2.733 (10)
Sn21—O1iv2.536 (14)Sn22—O1xii2.733 (10)
Sn21—O1v2.914 (14)Sn22—O1xiii3.049 (13)
Sn21—O1vi2.914 (14)Sn22—O1xiv3.049 (13)
Sn21—O1vii3.087 (14)Sn22—O2xv2.316 (4)
Sn21—O22.311 (14)Sn22—O2xvi2.316 (4)
Sn21—O2viii2.317 (14)
O1i—Nb1—O1xvii89.39 (10)O1v—Sn21—O1vi105.4 (4)
O1xiii—Nb1—O1i90.61 (10)O1v—Sn21—O1vii55.3 (2)
O1i—Nb1—O1xviii90.61 (10)O2xv—Sn22—O2xvi160.4 (10)
O1i—Nb1—O1xiv180.00 (1)O1ix—Sn22—O1x71.9 (4)
O2—Sn21—O2viii160.8 (6)O1ix—Sn22—O1xi65.7 (2)
O1ii—Sn21—O1iii69.8 (4)O1ix—Sn22—O1xii130.2 (6)
O1ii—Sn21—O1v124.1 (6)O1ix—Sn22—O1xiii171.3 (4)
O1ii—Sn21—O1vii177.7 (6)O1ix—Sn22—O1xiv116.9 (2)
O1iii—Sn21—O1iv132.1 (5)O1xi—Sn22—O1xii163.4 (6)
O1iii—Sn21—O1v61.0 (3)O1xi—Sn22—O1xiii106.4 (5)
O1iii—Sn21—O1vi165.8 (6)O1xi—Sn22—O1xiv57.3 (2)
O1iii—Sn21—O1vii110.7 (5)O1xiii—Sn22—O1xiv54.4 (3)
Symmetry codes: (i) y, x+1/4, z+1/4; (ii) y+1/4, z+1/4, x+1; (iii) x, y+1/2, z+1/2; (iv) z+3/4, x, y+3/4; (v) y+1/4, x+1, z+1/4; (vi) x+1, z+1/4, y+1/4; (vii) y+3/4, z+3/4, x; (viii) y+1/4, z+1, x+1/4; (ix) z, y+1/4, x3/4; (x) y, x+3/4, z1/4; (xi) x1/2, y, z1/2; (xii) x+1/2, z+1/4, y1/4; (xiii) y, z+1/4, x+1/4; (xiv) y1/4, x1/4, z; (xv) z+1/4, x, y+1/4; (xvi) y1/4, z+1/2, x3/4; (xvii) z, y1/4, x1/4; (xviii) x+1/4, y+1/4, z.
 

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