Monoclinic modification of polymorphic TbB₃O₆

Terbium triborate, TbB₃O₆, is confirmed to adopt at least two different structural modifications. Its monoclinic modification represents the terminal member of the isostructural series of REB₃O₆ with RE = La-Tb, and crystallizes in space group I2/a. The structure consists of chains of [B₆O₁₂]_n^6- building units, that run parallel to the c axis, and tenfold coordinated Tb³⁺ which link the borate chains to give a three-dimensional framework.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](O-B) = 0.005 Å
• R factor = 0.036
• wR factor = 0.082
• Data-to-parameter ratio = 33.4

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

PLATON alerts of the form PLAT_7?? have been detected for an inorganic
structure. These tests are under development  for inorganics and
comments are welcomed. It is not necessary to supply a data
validation response form for these alerts at this time.


 Alert Level A:



PLAT_702  Alert A Angle   Calc   149.01(8), Rep  137.26(16), Dev.    146.88 Sigma
                     O3   -TB1  -O3      3.555   1.555   4.555

PLAT_702  Alert A Angle   Calc    82.79(8), Rep   76.68(11), Dev.     76.38 Sigma
                     O3   -TB1  -O2      4.555   1.555   1.555

PLAT_702  Alert A Angle   Calc    34.19(8), Rep   54.95(10), Dev.    259.50 Sigma
                     O3   -TB1  -O1      4.555   1.555   5.666

PLAT_702  Alert A Angle   Calc    76.24(6), Rep   88.12(10), Dev.    198.00 Sigma
                     O3   -TB1  -O1      4.555   1.555   1.555


 4 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 0 Alert Level C = Please check

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S1. Comment

Among the binary rare earth oxoborates of the general composition REB₃O₆ only the compounds LaB₃O₆ (Ysker & Hoffmann, 1970; Abdullaev et al., 1981), NdB₃O₆ (Pakhomov et al., 1972), SmB₃O₆ and GdB₃O₆ (Abdullaev et al., 1975) are fully structurally characterized. They form an isostructural series and crystallize in monoclinic space group I2/a. For REB₃O₆ with RE = Dy—Lu only a somewhat doubtful indication of their existence can be found in the literature (Tananaev et al., 1975). In earlier works on cell parameters of TbB₃O₆ (Bambauer et al., 1969, Weidelt, 1970), the compound is described as crystallizing with monoclinic symmetry, similar to REB₃O₆ with RE = La—Gd, while a later structural analysis on single crystals of TbB₃O₆ reveals an orthorhombic symmetry Pbnm or Pbn2₁ of the compound (Pakhomov et al., 1971). Very recently the crystal structure of orthorhombic TbB₃O₆ has been solved by Nikelski & Schleid (2003), the results of our own structure determination of the orthorhombic structure of TbB₃O₆ are in good agreement with the data of these authors. Orthorhombic TbB₃O₆ crystallizes with symmetry Pnma (No. 62) and a = 15.9770 (7) Å, b = 7.4136 (3) Å, c = 12.2905 (6) Å and Z = 16.

During our systematic investigations of the crystal chemistry and crystal-growth conditions of binary rare earth borates, methods of synthesis from ternary systems were established that led to single crystals of REB₃O₆ with RE= La—Tb. Depending on the composition of the ternary system used, orthorhombic as well as monoclinic crystals of TbB₃O₆ were grown. The crystal structure of the monoclinic modification of TbB₃O₆ is presented here for the first time. Monoclinic TbB₃O₆ is isostructural with REB₃O₆ with RE = La, Nd, Sm and Gd, and crystallizes in space group I2/a (No. 15). The structure consists of infinite chains of [B₆O₁₂]_n⁶⁻ running along the c axis. Tenfold coordinated Tb atoms link the borate chains to a three-dimensional framework. The complex borate polyanion (4D2T:D<DTDT>D; Becker, 2001) is composed of [BO₄] tetrahedra that are linked via two [BO₃] triangles at a time to the adjacent [BO₄] tetrahedra on both sides. Each [BO₃] is connected to two [BO₄], the bridging O atoms belong also to the coordination polyhedron of one Tb. The nonbridging O atoms of the [BO₃] groups coordinate two Tb simultaneously, each. The irregular [TbO₁₀] coordination polyhedra are connected via edges to infinite chains along the c axis.

The mean B—O distances of 1.370 Å for [BO₃] and of 1.465 Å for [BO₄] fit well into the range of B—O distances found for many other borate structures [see, for comparison, Zobetz (1982) and Zobetz (1990)]. However, the [BO₃] triangles are substantially distorted with a B—O distance of non-bridging atoms O2 that is significantly shorter than the B—O distances of bridging atoms O1 and O3 (see Table 1).

According to the results of the structure work on TbB₃O₆, the compound seems to play the role of a transient point within the series of REB₃O₆ with RE = La—Lu. TbB₃O₆ shows a structural flexibility that allows to be on one hand the terminal member of the isostructural monoclinic series of REB₃O₆ with RE = La—Tb, but as well to be probably the starting point of an assumed orthorhombic series for the smaller lanthanides Dy—Lu, that still has to be synthesized.

The found structural variability of TbB₃O₆ is further corroborated by a structural phase transition at about 143 K that was recently discovered in our group.

S2. Experimental

Crystals of monoclinic TbB₃O₆ were grown in the ternary system Tb₂O₃–B₂O₃–PbO. A homogenized powder mixture of Tb₄O₇ (99.9%, Alfa Aesar), H₃BO₃ (99.8%, Merck) and PbO (99%, Riedel de Haën), in a mol% ratio of 1.1:87.9:11.0, was heated in a covered platinum crucible to 1213 K and subsequently cooled at a rate of about 3.4 K h⁻¹ to 943 K. Transparent single crystals of the title compound were separated from the lead borate flux using hot diluted HCl.

Computing details

Data collection: MACH3 (Enraf-Nonius, 1993); cell refinement: MACH3; data reduction: MolEN (Fair, 1990); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 2002) and ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Projection of the structure of the title compound along [100]. Tb atoms are shown as red spheres, O atoms as small blue spheres, [BO4] groups (olive) and [BO3] (green) are represented as polyhedra.
	Fig. 2. ORTEPIII projection (Burnett & Johnson, 1996) of the tenfold Tb coordination and the main features of the borate chains of title compound with atom-numbering scheme (projection along [100]). Atoms are shown as 50% probability ellipsoids.

Terbium(III) trioxoborate

Crystal data

TbB3O6	F(000) = 512
Mr = 287.35	Dx = 4.910 Mg m^−3
Monoclinic, I2/a	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -I 2ya	Cell parameters from 25 reflections
a = 6.2147 (4) Å	θ = 12.3–19.1°
b = 8.0225 (5) Å	µ = 18.13 mm^−1
c = 7.8111 (7) Å	T = 293 K
β = 93.44 (1)°	Parallelepiped, colourless
V = 388.74 (5) Å^3	0.15 × 0.13 × 0.11 mm
Z = 4

Data collection

Nonius MACH3 diffractometer	1321 reflections with I > 2σ(I)
Radiation source: fine-focus sealed X-ray tube	Rint = 0.094
Graphite monochromator	θmax = 44.9°, θmin = 3.6°
ω/2θ scans	h = −12→12
Absorption correction: ψ scan MolEN (Fair, 1990)	k = −15→15
Tmin = 0.082, Tmax = 0.136	l = −15→15
5729 measured reflections	3 standard reflections every 60 min
1602 independent reflections	intensity decay: 4.5%

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)] = 0.036	w = 1/[σ^2(Fo^2) + (0.0215P)^2 + 2.4096P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.082	(Δ/σ)max < 0.001
S = 1.07	Δρmax = 2.91 e Å^−3
1602 reflections	Δρmin = −2.45 e Å^−3
48 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0023 (4)

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
Tb1	0.7500	0.70532 (4)	0.5000	0.00547 (7)
O1	0.6011 (4)	0.3864 (5)	0.3960 (3)	0.0095 (4)
O2	0.6961 (5)	0.5921 (4)	0.2056 (4)	0.0108 (5)
O3	0.6040 (4)	0.3171 (4)	0.1002 (4)	0.0083 (4)
B1	0.6367 (6)	0.4370 (6)	0.2267 (5)	0.0060 (5)
B2	0.7500	0.2756 (10)	0.5000	0.0093 (10)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Tb1	0.00585 (8)	0.00450 (11)	0.00611 (9)	0.000	0.00080 (5)	0.000
O1	0.0120 (8)	0.0101 (13)	0.0064 (8)	0.0032 (10)	0.0008 (7)	0.0025 (10)
O2	0.0148 (9)	0.0049 (13)	0.0128 (10)	−0.0034 (9)	0.0009 (8)	0.0002 (10)
O3	0.0059 (6)	0.0091 (12)	0.0102 (9)	−0.0016 (8)	0.0019 (7)	−0.0060 (9)
B1	0.0062 (9)	0.0051 (15)	0.0064 (11)	−0.0012 (10)	−0.0009 (9)	0.0000 (11)
B2	0.0065 (14)	0.015 (3)	0.0069 (17)	0.000	0.0022 (13)	0.000

Geometric parameters (Å, º)

Tb1—O2^i	2.323 (3)	B1—O2	1.311 (6)
Tb1—O3^ii	2.460 (3)	B1—O3	1.385 (5)
Tb1—O2	2.477 (3)	B2—O3^iv	1.440 (5)
Tb1—O1^iii	2.485 (3)	B2—O3^v	1.440 (5)
Tb1—O1	2.823 (4)	B2—O1^vi	1.489 (6)
B1—O1	1.414 (5)	B2—O1	1.489 (6)
O1^iii—Tb1—O1^vii	145.55 (18)	O2—Tb1—O1^vii	70.14 (9)
O1—Tb1—O1^vi	50.04 (11)	O2^i—Tb1—O1	140.75 (10)
O2^i—Tb1—O2^viii	91.21 (17)	O2^viii—Tb1—O1	119.57 (10)
O2—Tb1—O2^vi	136.98 (17)	O3^ii—Tb1—O1	133.89 (10)
O3^ii—Tb1—O3^ix	137.26 (16)	O3^ix—Tb1—O1	88.12 (10)
O2^i—Tb1—O3^ii	71.84 (10)	O2—Tb1—O1	51.46 (10)
O2^viii—Tb1—O3^ii	78.57 (11)	O2^vi—Tb1—O1	87.63 (10)
O2^i—Tb1—O2	151.81 (11)	O1^iii—Tb1—O1	63.03 (12)
O2^viii—Tb1—O2	68.38 (14)	O1^vii—Tb1—O1	85.23 (8)
O3^ii—Tb1—O2	119.84 (9)	O2—B1—O3	126.8 (4)
O3^ix—Tb1—O2	76.68 (11)	O2—B1—O1	116.8 (4)
O2^viii—Tb1—O1^vii	79.36 (11)	O3—B1—O1	116.3 (4)
O2^viii—Tb1—O1^iii	126.79 (10)	O3^iv—B2—O3^v	117.9 (6)
O3^ii—Tb1—O1^iii	142.20 (9)	O3^iv—B2—O1	102.33 (16)
O3^ix—Tb1—O1^iii	54.95 (10)	O3^v—B2—O1	113.77 (17)
O2—Tb1—O1^iii	97.04 (10)	O1^vi—B2—O1	106.7 (6)

Symmetry codes: (i) x, −y+3/2, z+1/2; (ii) x+1/2, y+1/2, z+1/2; (iii) −x+1, −y+1, −z+1; (iv) x, −y+1/2, z+1/2; (v) −x+3/2, −y+1/2, −z+1/2; (vi) −x+3/2, y, −z+1; (vii) x+1/2, −y+1, z; (viii) −x+3/2, −y+3/2, −z+1/2; (ix) −x, y+1/2, −z+1/2.