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

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). 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(□_0.04Tb_0.96)^BScO_2.94. In the title compound, Tb occupies the eightfold-coordinated sites (site symmetry m) and Sc the centres of corner-sharing [ScO₆] octa­hedra (site symmetry ). 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.

Key indicators

• Single-crystal X-ray study
• T = 298 K
• Mean [sigma](Sc-O) = 0.002 Å
• Disorder in main residue
• R factor = 0.024
• wR factor = 0.047
• Data-to-parameter ratio = 11.4

checkCIF/PLATON results

No syntax errors found



Alert level A
PLAT301_ALERT_3_A Main Residue  Disorder .........................      51.00 Perc.

Author Response: The current compount represents a defect structure with vacancies on the cation and anion sites.

Alert level C
            Value of measurement temperature given =   298.000
            Value of melting point given =     0.000
PLAT041_ALERT_1_C Calc. and Rep. SumFormula Strings    Differ ....          ?
PLAT045_ALERT_1_C Calculated and Reported Z Differ by ............       0.25 Ratio
PLAT068_ALERT_1_C Reported F000 Differs from Calcd (or Missing)...          ?
PLAT077_ALERT_4_C Unitcell contains non-integer number of atoms ..          ?

Alert level G
ABSTM02_ALERT_3_G  The ratio of expected to reported Tmax/Tmin(RR) is > 2.00
            Tmin and Tmax reported:      0.088     0.278
            Tmin and Tmax expected:      0.026     0.553
            RR =      6.766
            Please check that your absorption correction is appropriate.
PLAT860_ALERT_3_G Note: Number of Least-Squares Restraints .......          1

   1 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   4 ALERT level C = Check and explain
   2 ALERT level G = General alerts; check

   3 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   0 ALERT type 2 Indicator that the structure model may be wrong or deficient
   3 ALERT type 3 Indicator that the structure quality may be low
   1 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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S1. 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 (Δ_n=1/nΣ{(r_i-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.

Related literature

Rietvelt refinements on powders of LnScO₃ with Ln = La³⁺–Ho³⁺ were reported by Liferovich & Mitchell (2004). 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). 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).

S2. 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.

S3. 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(□_0.04Tb_0.96)^BScO_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 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.

Computing details

Data collection: X-AREA (Stoe & Cie, 2006); cell refinement: X-AREA (Stoe & Cie, 2006); data reduction: X-RED32 (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).

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.

terbium(III) scandate(III)

Crystal data

Tb0.96ScO2.94	F(000) = 427
Mr = 244.56	Dx = 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

Data collection

Stoe IPDS-II diffractometer	353 independent reflections
Radiation source: fine-focus sealed tube	328 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.065
Detector resolution: 6.67 pixels mm^-1	θmax = 29.1°, θmin = 4.5°
ω scans	h = −7→7
Absorption correction: analytical (Alcock, 1970)	k = −9→10
Tmin = 0.088, Tmax = 0.278	l = −7→7
2143 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.024	w = 1/[σ^2(Fo^2) + (0.0165P)^2 + 1.3905P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.047	(Δ/σ)max = 0.015
S = 1.20	Δρmax = 2.15 e Å^−3
353 reflections	Δρmin = −1.12 e Å^−3
31 parameters	Extinction correction: SHELXS97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
1 restraint	Extinction 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 (Ų)

	x	y	z	Uiso*/Ueq	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)

Atomic displacement parameters (Ų)

	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)

Geometric parameters (Å, º)

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.