inorganic compounds
Redetermination of terbium scandate, revealing a defecttype perovskite derivative
^{a}LeibnizInstitute for Crystal Growth, MaxBornStrasse 2, 12489 Berlin, Germany, and ^{b}University of Innsbruck, Institute of Mineralogy and Petrography, Innrain 52, 6020 Innsbruck, Austria
^{*}Correspondence email: velickov@ikzberlin.de
The _{3}, has been reported previously on the basis of powder diffraction data [Liferovich & Mitchell (2004). 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. Siteoccupancy 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}(□_{0.04}Tb_{0.96})^{B}ScO_{2.94}. In the title compound, Tb occupies the eightfoldcoordinated sites (site symmetry m) and Sc the centres of cornersharing [ScO_{6}] octahedra (site symmetry ). The mean bond lengths and site distortions fit well into the data of the remaining lanthanoid scandates in the series from DyScO_{3} to NdScO_{3}. A linear structural evolution with the size of the lanthanoid from DyScO_{3} to NdScO_{3} can be predicted.
of terbium(III) scandate(III), with ideal formula TbScORelated literature
Rietvelt refinements on powders of LnScO_{3} with Ln = La^{3+}–Ho^{3+} were reported by Liferovich & Mitchell (2004). The crystal structures of the Dy, Gd, Sm and Nd members, refined from singlecrystal diffraction data, have been recently provided by Veličkov et al. (2007). Geometrical parameters have been calculated by means of atomic coordinates following the concept of Zhao et al. (1993). A more detailed description of the growth procedure of the Ln scandates is given by Uecker et al. (2006). For the applications of Ln scandates, see: Choi et al. (2004); Haeni et al. (2004).
Experimental
Crystal data

Refinement

Data collection: XAREA (Stoe & Cie, 2006); cell XAREA; data reduction: XRED32 (Stoe & Cie, 2006); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXS97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 2004); software used to prepare material for publication: SHELXL97.
Supporting information
10.1107/S1600536808033394/wm2190sup1.cif
contains datablocks I, global. DOI:Structure factors: contains datablock I. DOI: 10.1107/S1600536808033394/wm2190Isup2.hkl
TbScO_{3} was grown as a bulk crystal (Ø = 20 mm) from a melt by conventional Czochralski technique with an automatic diameter control. The starting materials Tb_{4}O_{7 }and Sc_{2}O_{3} (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 RFheated 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 Tb^{3+} can be assumed. A part of the singlecrystal material was crushed and irregular fragments were screened using a polarizing light microscope to find a sample of good optical quality for diffraction experiments.
The ICP OES (inductively coupled plasma optical emission spectrometry) investigation of this sample resulted in a compostion of Tb_{2}O_{3} = 48.79 mol% and Sc_{2}O_{3 }= 51.21 mol%, indicating a nonstoichiometric chemical composition. Site occupancy refinements based on diffraction data support the idea of the Tbdeficiency on the Asite coupled with Odefects on the O2position. The calculated chemical compositions provided by structure
agree very well with the data of the ICP OES study. The crystallochemical formula of the investigated sample may thus be written as ^{A}(□_{0.04}Tb_{0.96})^{B}ScO_{2.94.}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
cycle a constraint ensuring charge neutrality was included. In contrast to the previous powder performed with the setting Pbnm of no. 62, the standard setting in Pnma was used for the present redetermination.Data collection: XAREA (Stoe & Cie, 2006); cell
XAREA (Stoe & Cie, 2006); data reduction: XRED32 (Stoe & Cie, 2006); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXS97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 2004); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).Tb_{0.96}ScO_{2.94}  F(000) = 427 
M_{r} = 244.56  D_{x} = 6.55 Mg m^{−}^{3} 
Orthorhombic, Pnma  Mo Kα radiation, λ = 0.71073 Å 
Hall symbol: P 2ac 2n  Cell parameters from 1947 reflections 
a = 5.7233 (8) Å  θ = 2.6–29.1° 
b = 7.9147 (12) Å  µ = 29.58 mm^{−}^{1} 
c = 5.4543 (7) Å  T = 298 K 
V = 247.07 (6) Å^{3}  Plate, colourless 
Z = 4  0.14 × 0.12 × 0.02 mm 
Stoe IPDSII diffractometer  353 independent reflections 
Radiation source: finefocus sealed tube  328 reflections with I > 2σ(I) 
Graphite monochromator  R_{int} = 0.065 
Detector resolution: 6.67 pixels mm^{1}  θ_{max} = 29.1°, θ_{min} = 4.5° 
ω scans  h = −7→7 
Absorption correction: analytical (Alcock, 1970)  k = −9→10 
T_{min} = 0.088, T_{max} = 0.278  l = −7→7 
2143 measured reflections 
Refinement on F^{2}  Primary atom site location: structureinvariant direct methods 
Leastsquares matrix: full  Secondary atom site location: difference Fourier map 
R[F^{2} > 2σ(F^{2})] = 0.024  w = 1/[σ^{2}(F_{o}^{2}) + (0.0165P)^{2} + 1.3905P] where P = (F_{o}^{2} + 2F_{c}^{2})/3 
wR(F^{2}) = 0.047  (Δ/σ)_{max} = 0.015 
S = 1.20  Δρ_{max} = 2.15 e Å^{−}^{3} 
353 reflections  Δρ_{min} = −1.12 e Å^{−}^{3} 
31 parameters  Extinction correction: SHELXS97 (Sheldrick, 2008), Fc^{*}=kFc[1+0.001xFc^{2}λ^{3}/sin(2θ)]^{1/4} 
1 restraint  Extinction coefficient: 0.158 (6) 
Tb_{0.96}ScO_{2.94}  V = 247.07 (6) Å^{3} 
M_{r} = 244.56  Z = 4 
Orthorhombic, Pnma  Mo Kα radiation 
a = 5.7233 (8) Å  µ = 29.58 mm^{−}^{1} 
b = 7.9147 (12) Å  T = 298 K 
c = 5.4543 (7) Å  0.14 × 0.12 × 0.02 mm 
Stoe IPDSII diffractometer  353 independent reflections 
Absorption correction: analytical (Alcock, 1970)  328 reflections with I > 2σ(I) 
T_{min} = 0.088, T_{max} = 0.278  R_{int} = 0.065 
2143 measured reflections 
R[F^{2} > 2σ(F^{2})] = 0.024  31 parameters 
wR(F^{2}) = 0.047  1 restraint 
S = 1.20  Δρ_{max} = 2.15 e Å^{−}^{3} 
353 reflections  Δρ_{min} = −1.12 e Å^{−}^{3} 
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 F^{2} against ALL reflections. The weighted Rfactor wR and goodness of fit S are based on F^{2}, conventional Rfactors R are based on F, with F set to zero for negative F^{2}. The threshold expression of F^{2} > σ(F^{2}) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. Rfactors based on F^{2} are statistically about twice as large as those based on F, and R factors based on ALL data will be even larger. 
x  y  z  U_{iso}*/U_{eq}  Occ. (<1)  
Tb1  0.06029 (6)  0.25  0.01672 (6)  0.0087 (2)  0.9591 (13) 
Sc2  0  0  0.5  0.0082 (3)  
O1  0.4455 (10)  0.25  0.8761 (9)  0.0114 (10)  
O2  0.1946 (7)  0.9357 (5)  0.8100 (6)  0.0108 (8)  0.9693 (10) 
U^{11}  U^{22}  U^{33}  U^{12}  U^{13}  U^{23}  
Tb1  0.0074 (3)  0.0106 (3)  0.0080 (2)  0  0.00053 (12)  0 
Sc2  0.0085 (6)  0.0085 (7)  0.0075 (5)  −0.0003 (7)  −0.0002 (4)  0.0002 (4) 
O1  0.013 (3)  0.010 (2)  0.012 (2)  0  0.0018 (19)  0 
O2  0.0078 (19)  0.014 (2)  0.0111 (15)  −0.0024 (14)  −0.0037 (13)  0.0025 (13) 
Tb1—O1^{i}  2.241 (5)  Sc2—O2^{ii}  2.088 (3) 
Tb1—O2^{ii}  2.277 (4)  Sc2—O2^{xii}  2.088 (3) 
Tb1—O2^{iii}  2.277 (4)  Sc2—O2^{xiii}  2.095 (4) 
Tb1—O1^{iv}  2.334 (5)  Sc2—O2^{vi}  2.095 (4) 
Tb1—O2^{v}  2.586 (4)  Sc2—O1^{xiv}  2.1141 (19) 
Tb1—O2^{vi}  2.586 (4)  Sc2—O1^{x}  2.1141 (18) 
Tb1—O2^{vii}  2.837 (4)  Sc2—Tb1^{xv}  3.2026 (4) 
Tb1—O2^{viii}  2.837 (4)  Sc2—Tb1^{i}  3.2026 (4) 
Tb1—Sc2^{ix}  3.2026 (4)  Sc2—Tb1^{xvi}  3.3140 (4) 
Tb1—Sc2^{x}  3.2026 (4)  Sc2—Tb1^{xvii}  3.4608 (4) 
Tb1—Sc2^{xi}  3.3140 (4)  Sc2—Tb1^{xviii}  3.4608 (4) 
Tb1—Sc2  3.3140 (4)  
O1^{i}—Tb1—O2^{ii}  102.07 (14)  O2^{xii}—Sc2—O2^{xiii}  89.16 (7) 
O1^{i}—Tb1—O2^{iii}  102.07 (14)  O2^{ii}—Sc2—O2^{vi}  89.16 (7) 
O2^{ii}—Tb1—O2^{iii}  80.4 (2)  O2^{xii}—Sc2—O2^{vi}  90.84 (7) 
O1^{i}—Tb1—O1^{iv}  87.86 (12)  O2^{xiii}—Sc2—O2^{vi}  180 
O2^{ii}—Tb1—O1^{iv}  137.88 (11)  O2^{ii}—Sc2—O1^{xiv}  87.26 (17) 
O2^{iii}—Tb1—O1^{iv}  137.88 (11)  O2^{xii}—Sc2—O1^{xiv}  92.74 (17) 
O1^{i}—Tb1—O2^{v}  138.63 (11)  O2^{xiii}—Sc2—O1^{xiv}  86.91 (18) 
O2^{ii}—Tb1—O2^{v}  117.25 (8)  O2^{vi}—Sc2—O1^{xiv}  93.09 (18) 
O2^{iii}—Tb1—O2^{v}  73.97 (9)  O2^{ii}—Sc2—O1^{x}  92.74 (17) 
O1^{iv}—Tb1—O2^{v}  72.00 (13)  O2^{xii}—Sc2—O1^{x}  87.26 (17) 
O1^{i}—Tb1—O2^{vi}  138.63 (11)  O2^{xiii}—Sc2—O1^{x}  93.09 (18) 
O2^{ii}—Tb1—O2^{vi}  73.97 (9)  O2^{vi}—Sc2—O1^{x}  86.91 (18) 
O2^{iii}—Tb1—O2^{vi}  117.25 (8)  O1^{xiv}—Sc2—O1^{x}  180 
O1^{iv}—Tb1—O2^{vi}  72.00 (13)  Sc2^{xix}—O1—Sc2^{xv}  138.8 (3) 
O2^{v}—Tb1—O2^{vi}  69.28 (17)  Sc2^{xix}—O1—Tb1^{xx}  105.22 (14) 
O1^{i}—Tb1—O2^{vii}  72.51 (9)  Sc2^{xv}—O1—Tb1^{xx}  105.22 (14) 
O2^{ii}—Tb1—O2^{vii}  76.86 (13)  Sc2^{xix}—O1—Tb1^{xviii}  91.96 (15) 
O2^{iii}—Tb1—O2^{vii}  154.79 (10)  Sc2^{xv}—O1—Tb1^{xviii}  91.96 (15) 
O1^{iv}—Tb1—O2^{vii}  67.26 (9)  Tb1^{xx}—O1—Tb1^{xviii}  126.2 (2) 
O2^{v}—Tb1—O2^{vii}  126.67 (6)  Sc2^{xxi}—O2—Sc2^{xxii}  141.9 (2) 
O2^{vi}—Tb1—O2^{vii}  66.45 (5)  Sc2^{xxi}—O2—Tb1^{ii}  98.72 (15) 
O1^{i}—Tb1—O2^{viii}  72.51 (9)  Sc2^{xxii}—O2—Tb1^{ii}  119.09 (16) 
O2^{ii}—Tb1—O2^{viii}  154.79 (10)  Sc2^{xxi}—O2—Tb1^{xxii}  85.81 (12) 
O2^{iii}—Tb1—O2^{viii}  76.86 (13)  Sc2^{xxii}—O2—Tb1^{xxii}  89.52 (13) 
O1^{iv}—Tb1—O2^{viii}  67.26 (9)  Tb1^{ii}—O2—Tb1^{xxii}  103.74 (15) 
O2^{v}—Tb1—O2^{viii}  66.45 (5)  Sc2^{xxi}—O2—Tb1^{xxiii}  87.91 (13) 
O2^{vi}—Tb1—O2^{viii}  126.67 (6)  Sc2^{xxii}—O2—Tb1^{xxiii}  79.43 (12) 
O2^{vii}—Tb1—O2^{viii}  122.50 (15)  Tb1^{ii}—O2—Tb1^{xxiii}  103.14 (13) 
O2^{ii}—Sc2—O2^{xii}  180  Tb1^{xxii}—O2—Tb1^{xxiii}  153.02 (16) 
O2^{ii}—Sc2—O2^{xiii}  90.84 (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. 
Experimental details
Crystal data  
Chemical formula  Tb_{0.96}ScO_{2.94} 
M_{r}  244.56 
Crystal system, space group  Orthorhombic, Pnma 
Temperature (K)  298 
a, b, c (Å)  5.7233 (8), 7.9147 (12), 5.4543 (7) 
V (Å^{3})  247.07 (6) 
Z  4 
Radiation type  Mo Kα 
µ (mm^{−}^{1})  29.58 
Crystal size (mm)  0.14 × 0.12 × 0.02 
Data collection  
Diffractometer  Stoe IPDSII diffractometer 
Absorption correction  Analytical (Alcock, 1970) 
T_{min}, T_{max}  0.088, 0.278 
No. of measured, independent and observed [I > 2σ(I)] reflections  2143, 353, 328 
R_{int}  0.065 
(sin θ/λ)_{max} (Å^{−}^{1})  0.685 
Refinement  
R[F^{2} > 2σ(F^{2})], wR(F^{2}), S  0.024, 0.047, 1.20 
No. of reflections  353 
No. of parameters  31 
No. of restraints  1 
Δρ_{max}, Δρ_{min} (e Å^{−}^{3})  2.15, −1.12 
Computer programs: XAREA (Stoe & Cie, 2006), XRED32 (Stoe & Cie, 2006), SHELXS97 (Sheldrick, 2008), ATOMS (Dowty, 2004), SHELXL97 (Sheldrick, 2008).
Tb1—O1^{i}  2.241 (5)  Tb1—O2^{v}  2.837 (4) 
Tb1—O2^{ii}  2.277 (4)  Sc2—O2^{ii}  2.088 (3) 
Tb1—O1^{iii}  2.334 (5)  Sc2—O2^{vi}  2.095 (4) 
Tb1—O2^{iv}  2.586 (4)  Sc2—O1^{vii}  2.1141 (19) 
Symmetry codes: (i) x−1/2, y, −z+1/2; (ii) −x, −y+1, −z+1; (iii) x, y, z−1; (iv) −x+1/2, y−1/2, z−1/2; (v) x, y−1, z−1; (vi) x−1/2, y−1, −z+3/2; (vii) x−1/2, y, −z+3/2. 
Acknowledgements
The authors thank M. Bernhagen for technical support in carrying out the growth experiments.
References
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The lanthanoid scandates, LnScO_{3}, with Ln = La^{3+} to Ho^{3+} 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 BaTiO_{3} and SrTiO_{3} films (Choi et al., 2004; Haeni et al., 2004).
Liferovich & Mitchell (2004) studied the crystal structure of lanthanoid scandates, including TbScO_{3}, by Rietveld analysis from powder diffraction data. Crystallographic data of DyScO_{3}, GdScO_{3}, SmScO_{3} and NdScO_{3} 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 Lnsubstitution: Veličkov et al. (2007) assumed linear trends, whereas Liferovich & Mitchell (2004) observed no obvious continious evolution. Especially the TbScO_{3} and EuScO_{3} compounds seemed to exhibit an anomalous behaviour in the latter study. The present paper provides first results on TbScO_{3}, redetermined from singlecrystal data. Investigations on EuScO_{3} are in preparation.
The orthorhombic distorted perovskite structure of TbScO_{3} (Fig.1) is confirmed from our refinements. Whereas the lattice parameters for TbScO_{3} 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 Asite 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 ^{A}Δ_{8} = 8.78x10^{3 }(Δ_{n}=1/nΣ{(r_{i}r)/r}^{2}). The Bsite shows bond lengths typical for octahedrally coordinated scandium (<B—O> = 2.101 Å) and is rather distorted with ^{B}Δ_{6} = 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 DyScO_{3} to NdScO_{3} in dependence on the Lnsubstitution. Consequently, an anomalous behaviour of TbScO_{3} in Lnscandate series could not be confirmed.