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Acta Crystallogr Sect E Struct Rep Online. 2008 November 1; 64(Pt 11): i79.
Published online 2008 October 31. doi:  10.1107/S1600536808033394
PMCID: PMC2959553

Redetermination of terbium scandate, revealing a defect-type perovskite derivative

Abstract

The crystal structure of terbium(III) scandate(III), with ideal formula TbScO3, has been reported previously on the basis of powder diffraction data [Liferovich & Mitchell (2004 [triangle]). J. Solid State Chem. 177, 2188–2197]. The current data were obtained from single crystals grown by the Czochralski method and show an improvement in the precision of the geometric parameters. Moreover, inductively coupled plasma optical emission spectrometry studies resulted in a nonstoichiometric composition of the title compound. Site-occupancy refinements based on diffraction data support the idea of a Tb deficiency on the A site (inducing O defects on the O2 position). The crystallochemical formula of the investigated sample thus may be written as A(□0.04Tb0.96)BScO2.94. In the title compound, Tb occupies the eightfold-coordinated sites (site symmetry m) and Sc the centres of corner-sharing [ScO6] octa­hedra (site symmetry An external file that holds a picture, illustration, etc.
Object name is e-64-00i79-efi1.jpg). The mean bond lengths and site distortions fit well into the data of the remaining lanthanoid scandates in the series from DyScO3 to NdScO3. A linear structural evolution with the size of the lanthanoid from DyScO3 to NdScO3 can be predicted.

Related literature

Rietvelt refinements on powders of LnScO3 with Ln = La3+–Ho3+ were reported by Liferovich & Mitchell (2004 [triangle]). The crystal structures of the Dy, Gd, Sm and Nd members, refined from single-crystal diffraction data, have been recently provided by Veličkov et al. (2007 [triangle]). Geometrical parameters have been calculated by means of atomic coordinates following the concept of Zhao et al. (1993 [triangle]). A more detailed description of the growth procedure of the Ln scandates is given by Uecker et al. (2006 [triangle]). For the applications of Ln scandates, see: Choi et al. (2004 [triangle]); Haeni et al. (2004 [triangle]).

Experimental

Crystal data

  • Tb0.96ScO2.94
  • M r = 244.56
  • Orthorhombic, An external file that holds a picture, illustration, etc.
Object name is e-64-00i79-efi2.jpg
  • a = 5.7233 (8) Å
  • b = 7.9147 (12) Å
  • c = 5.4543 (7) Å
  • V = 247.07 (6) Å3
  • Z = 4
  • Mo Kα radiation
  • μ = 29.58 mm−1
  • T = 298 (2) K
  • 0.14 × 0.12 × 0.02 mm

Data collection

  • Stoe IPDS-II diffractometer
  • Absorption correction: analytical (Alcock, 1970 [triangle]) T min = 0.088, T max = 0.278
  • 2143 measured reflections
  • 353 independent reflections
  • 328 reflections with I > 2σ(I)
  • R int = 0.065

Refinement

  • R[F 2 > 2σ(F 2)] = 0.024
  • wR(F 2) = 0.047
  • S = 1.20
  • 353 reflections
  • 31 parameters
  • 1 restraint
  • Δρmax = 2.15 e Å−3
  • Δρmin = −1.12 e Å−3

Data collection: X-AREA (Stoe & Cie, 2006 [triangle]); cell refinement: X-AREA; data reduction: X-RED32 (Stoe & Cie, 2006 [triangle]); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 [triangle]); program(s) used to refine structure: SHELXS97 (Sheldrick, 2008 [triangle]); molecular graphics: ATOMS (Dowty, 2004 [triangle]); software used to prepare material for publication: SHELXL97.

Table 1
Selected bond lengths (Å)

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536808033394/wm2190sup1.cif

Structure factors: contains datablocks I. DOI: 10.1107/S1600536808033394/wm2190Isup2.hkl

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Acknowledgments

The authors thank M. Bernhagen for technical support in carrying out the growth experiments.

supplementary crystallographic information

Comment

The lanthanoid scandates, LnScO3, with Ln = La3+ to Ho3+ are known to adopt an orthorhombic derivative of the perovskite structure. Their lattice dimensions are suitable to use them as substrates for the epitaxial growth of strain engineered BaTiO3 and SrTiO3 films (Choi et al., 2004; Haeni et al., 2004).

Liferovich & Mitchell (2004) studied the crystal structure of lanthanoid scandates, including TbScO3, by Rietveld analysis from powder diffraction data. Crystallographic data of DyScO3, GdScO3, SmScO3 and NdScO3 obtained from single crystals were recently reported by Veličkov et al. (2007). However, in the literature there are disagreements concerning some structural characteristics and their dependence on the Ln-substitution: Veličkov et al. (2007) assumed linear trends, whereas Liferovich & Mitchell (2004) observed no obvious continious evolution. Especially the TbScO3 and EuScO3 compounds seemed to exhibit an anomalous behaviour in the latter study. The present paper provides first results on TbScO3, redetermined from single-crystal data. Investigations on EuScO3 are in preparation.

The orthorhombic distorted perovskite structure of TbScO3 (Fig.1) is confirmed from our refinements. Whereas the lattice parameters for TbScO3 compare well with the data of Liferovich & Mitchell (2004), the atomic coordinates show deviations of up to 0.008 in the fractional atomic coordinates, resulting in slightly different geometrical parameters. The A-site is occupied by Tb and has an average bond length in an eightfold coordination of [8]<A—O> = 2.499 Å with a polyhedral bond length distortion of AΔ8 = 8.78x10-3 n=1/nΣ{(ri-r)/r}2). The B-site shows bond lengths typical for octahedrally coordinated scandium (<B—O> = 2.101 Å) and is rather distorted with BΔ6 = 0.025x10-3 and a bond angle variance of δ = 3.23°. The tilting of the corner sharing octahedra calculated after Zhao et al. (1993) are θ = 20.64° in [110] and Ø = 12.97° in [001] directions. From our data we can establish linear trends for the crystallochemical parameters from DyScO3 to NdScO3 in dependence on the Ln-substitution. Consequently, an anomalous behaviour of TbScO3 in Ln-scandate series could not be confirmed.

Experimental

TbScO3 was grown as a bulk crystal (Ø = 20 mm) from a melt by conventional Czochralski technique with an automatic diameter control. The starting materials Tb4O7 and Sc2O3 (Alfa Aesar) with 99.99% purity were dried, mixed in a stoichiometric ratio, sintered and pressed to pellets easing the melting procedure. An iridium crucible (40 x 40 mm) was used as melt container combined with an iridium afterheater both RF-heated with a 25 kW mf generator. The crystal was withdrawn with a pulling rate of 1 mm/h under flowing nitrogen atmosphere. The grown crystal was colourless, so that a valence state of Tb3+ can be assumed. A part of the single-crystal material was crushed and irregular fragments were screened using a polarizing light microscope to find a sample of good optical quality for diffraction experiments.

Refinement

The ICP OES (inductively coupled plasma optical emission spectrometry) investigation of this sample resulted in a compostion of Tb2O3 = 48.79 mol% and Sc2O3 = 51.21 mol%, indicating a non-stoichiometric chemical composition. Site occupancy refinements based on diffraction data support the idea of the Tb-deficiency on the A-site coupled with O-defects on the O2-position. The calculated chemical compositions provided by structure refinement agree very well with the data of the ICP OES study. The crystallochemical formula of the investigated sample may thus be written as A(□0.04Tb0.96)BScO2.94.

The highest peak and deepest hole are located 0.59 and 1.42 Å from Tb1. Site occupation refinements indicated deviations from full occupancy on the Tb1 and the O2 sites. For the final refinement cycle a constraint ensuring charge neutrality was included. In contrast to the previous powder refinement, performed with the setting Pbnm of space group no. 62, the standard setting in Pnma was used for the present redetermination.

Figures

Fig. 1.
The orthorhombic perovskite structure of TbScO3 characterized by a tilted corner sharing ScO6 framework and the 8-fold coordinated Tb sites. The ScO6 octahedra are brownish and translucent, the Tb atoms are grey and the O atoms are red.
Fig. 2.
Projection of the TbScO3 structure along [010], showing the Tb atoms and the Sc coordination with displacement ellipsoids at the 80% probability level.

Crystal data

Tb0.96ScO2.94F(000) = 427
Mr = 244.56Dx = 6.55 Mg m3
Orthorhombic, PnmaMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2nCell parameters from 1947 reflections
a = 5.7233 (8) Åθ = 2.6–29.1°
b = 7.9147 (12) ŵ = 29.58 mm1
c = 5.4543 (7) ÅT = 298 K
V = 247.07 (6) Å3Plate, colourless
Z = 40.14 × 0.12 × 0.02 mm

Data collection

Stoe IPDS-II diffractometer353 independent reflections
Radiation source: fine-focus sealed tube328 reflections with I > 2σ(I)
graphiteRint = 0.065
Detector resolution: 6.67 pixels mm-1θmax = 29.1°, θmin = 4.5°
ω scansh = −7→7
Absorption correction: analytical (Alcock, 1970)k = −9→10
Tmin = 0.088, Tmax = 0.278l = −7→7
2143 measured reflections

Refinement

Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.024w = 1/[σ2(Fo2) + (0.0165P)2 + 1.3905P] where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.047(Δ/σ)max = 0.015
S = 1.20Δρmax = 2.15 e Å3
353 reflectionsΔρmin = −1.11 e Å3
31 parametersExtinction correction: SHELXS97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
1 restraintExtinction coefficient: 0.158 (6)

Special details

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

xyzUiso*/UeqOcc. (<1)
Tb10.06029 (6)0.250.01672 (6)0.0087 (2)0.9591 (13)
Sc2000.50.0082 (3)
O10.4455 (10)0.250.8761 (9)0.0114 (10)
O20.1946 (7)0.9357 (5)0.8100 (6)0.0108 (8)0.9693 (10)

Atomic displacement parameters (Å2)

U11U22U33U12U13U23
Tb10.0074 (3)0.0106 (3)0.0080 (2)00.00053 (12)0
Sc20.0085 (6)0.0085 (7)0.0075 (5)−0.0003 (7)−0.0002 (4)0.0002 (4)
O10.013 (3)0.010 (2)0.012 (2)00.0018 (19)0
O20.0078 (19)0.014 (2)0.0111 (15)−0.0024 (14)−0.0037 (13)0.0025 (13)

Geometric parameters (Å, °)

Tb1—O1i2.241 (5)Sc2—O2ii2.088 (3)
Tb1—O2ii2.277 (4)Sc2—O2xii2.088 (3)
Tb1—O2iii2.277 (4)Sc2—O2xiii2.095 (4)
Tb1—O1iv2.334 (5)Sc2—O2vi2.095 (4)
Tb1—O2v2.586 (4)Sc2—O1xiv2.1141 (19)
Tb1—O2vi2.586 (4)Sc2—O1x2.1141 (18)
Tb1—O2vii2.837 (4)Sc2—Tb1xv3.2026 (4)
Tb1—O2viii2.837 (4)Sc2—Tb1i3.2026 (4)
Tb1—Sc2ix3.2026 (4)Sc2—Tb1xvi3.3140 (4)
Tb1—Sc2x3.2026 (4)Sc2—Tb1xvii3.4608 (4)
Tb1—Sc2xi3.3140 (4)Sc2—Tb1xviii3.4608 (4)
Tb1—Sc23.3140 (4)
O1i—Tb1—O2ii102.07 (14)O2xii—Sc2—O2xiii89.16 (7)
O1i—Tb1—O2iii102.07 (14)O2ii—Sc2—O2vi89.16 (7)
O2ii—Tb1—O2iii80.4 (2)O2xii—Sc2—O2vi90.84 (7)
O1i—Tb1—O1iv87.86 (12)O2xiii—Sc2—O2vi180
O2ii—Tb1—O1iv137.88 (11)O2ii—Sc2—O1xiv87.26 (17)
O2iii—Tb1—O1iv137.88 (11)O2xii—Sc2—O1xiv92.74 (17)
O1i—Tb1—O2v138.63 (11)O2xiii—Sc2—O1xiv86.91 (18)
O2ii—Tb1—O2v117.25 (8)O2vi—Sc2—O1xiv93.09 (18)
O2iii—Tb1—O2v73.97 (9)O2ii—Sc2—O1x92.74 (17)
O1iv—Tb1—O2v72.00 (13)O2xii—Sc2—O1x87.26 (17)
O1i—Tb1—O2vi138.63 (11)O2xiii—Sc2—O1x93.09 (18)
O2ii—Tb1—O2vi73.97 (9)O2vi—Sc2—O1x86.91 (18)
O2iii—Tb1—O2vi117.25 (8)O1xiv—Sc2—O1x180
O1iv—Tb1—O2vi72.00 (13)Sc2xix—O1—Sc2xv138.8 (3)
O2v—Tb1—O2vi69.28 (17)Sc2xix—O1—Tb1xx105.22 (14)
O1i—Tb1—O2vii72.51 (9)Sc2xv—O1—Tb1xx105.22 (14)
O2ii—Tb1—O2vii76.86 (13)Sc2xix—O1—Tb1xviii91.96 (15)
O2iii—Tb1—O2vii154.79 (10)Sc2xv—O1—Tb1xviii91.96 (15)
O1iv—Tb1—O2vii67.26 (9)Tb1xx—O1—Tb1xviii126.2 (2)
O2v—Tb1—O2vii126.67 (6)Sc2xxi—O2—Sc2xxii141.9 (2)
O2vi—Tb1—O2vii66.45 (5)Sc2xxi—O2—Tb1ii98.72 (15)
O1i—Tb1—O2viii72.51 (9)Sc2xxii—O2—Tb1ii119.09 (16)
O2ii—Tb1—O2viii154.79 (10)Sc2xxi—O2—Tb1xxii85.81 (12)
O2iii—Tb1—O2viii76.86 (13)Sc2xxii—O2—Tb1xxii89.52 (13)
O1iv—Tb1—O2viii67.26 (9)Tb1ii—O2—Tb1xxii103.74 (15)
O2v—Tb1—O2viii66.45 (5)Sc2xxi—O2—Tb1xxiii87.91 (13)
O2vi—Tb1—O2viii126.67 (6)Sc2xxii—O2—Tb1xxiii79.43 (12)
O2vii—Tb1—O2viii122.50 (15)Tb1ii—O2—Tb1xxiii103.14 (13)
O2ii—Sc2—O2xii180Tb1xxii—O2—Tb1xxiii153.02 (16)
O2ii—Sc2—O2xiii90.84 (7)

Symmetry codes: (i) x−1/2, y, −z+1/2; (ii) −x, −y+1, −z+1; (iii) −x, y−1/2, −z+1; (iv) x, y, z−1; (v) −x+1/2, y−1/2, z−1/2; (vi) −x+1/2, −y+1, z−1/2; (vii) x, y−1, z−1; (viii) x, −y+3/2, z−1; (ix) x+1/2, −y+1/2, −z+1/2; (x) −x+1/2, −y, z−1/2; (xi) −x, y+1/2, −z+1; (xii) x, y−1, z; (xiii) x−1/2, y−1, −z+3/2; (xiv) x−1/2, y, −z+3/2; (xv) −x+1/2, −y, z+1/2; (xvi) −x, −y, −z+1; (xvii) −x, −y, −z; (xviii) x, y, z+1; (xix) x+1/2, −y+1/2, −z+3/2; (xx) x+1/2, y, −z+1/2; (xxi) x, y+1, z; (xxii) −x+1/2, −y+1, z+1/2; (xxiii) x, y+1, z+1.

Footnotes

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2190).

References

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