Reinvestigation of β-Li₃TaO₄

The structure of β-Li₃TaO₄ has been reinvestigated with an image plate diffractometer. It crystallizes with a cation-ordered, distorted rock-salt-type lattice structure, composed of distorted TaO₆ and LiO₆ octahedra. Some evidence of a superlattice was observed, but it had no impact on the established structural model.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](Ta-O) = 0.003 Å
• R factor = 0.033
• wR factor = 0.026
• Data-to-parameter ratio = 32.7

checkCIF results

No syntax errors found


 Alert Level B:



PLAT_112  Alert B ADDSYM Detects Additional (Pseudo) Symm. Elem.         -4

Author response: Li3TaO4 has an ordered but distorted rock salt structure. Rock salt has cubic symmetry. It is therefore not suprising that pseudo symmetry elements exist. The symmetry element in question is a possible -4 fold axis about a [1 0 2] vector of the lattice we report here. That vector constitutes the c-axis of a pseudo-tetragonal cell of dimensions 8.508x8.516x16.66 \b=90.232 where a and b are vectors identical to those we report here. However, the authors are confident that a!=b so that tetragonality can be excluded. The authors are also confident that \b=90 so that orthorhombicity can be excluded. In addition the structural geometries along a and b differ to an appreciable extent.

 Alert Level C:



GOODF_01  Alert C The least squares goodness of fit parameter lies
          outside the range 0.80 <> 2.00
          Goodness of fit given =      3.034


 0 Alert Level A = Potentially serious problem

 1 Alert Level B = Potential problem

 1 Alert Level C = Please check

---

S1. Comment

Miao and Toradi (1999) explored the X-ray luminescence properties of the Li₃Ta_{1 − x}Nb_xO₄ phase diagram and identified a promising phosphor candidate for medical X-ray and UV imaging detectors. When the β-Li₃TaO₄ lattice is doped with Nb in the composition range 0.001 < x < 0.01, the normal Li₃TaO₄ broadband blue luminescence becomes concentrated and peaks at around 415 nm with an intensity comparable to that of some commercial phosphors. The luminescence efficiency rapidly declines with increasing Nb concentration and presumably increased lattice strain, which changes the photonic coupling of the substrate to the Nb ion fluoresence centres.

β-Li₃TaO₄ crystallizes in a structure resembling that of common rock salt (Fig. 1). It contains a well ordered Li and Ta cation sublattice (Mather et al., 2000) organized as edge-sharing LiO₆ and TaO₆ octahedra. The TaO₆ octahedra form distinct continuous chains, each edge sharing with two other TaO₆ octahedra (Fig. 2 and 3). The octahedral chains zigzag back and forth with a spatial repetition period of four octahedral units. The Li cations then occupy the considerably distorted LiO₆ octahedra which encloak and separate the Ta containing chains. Twofold axes pass through the midpoints of one subset of TaO₆ shared edges, while inversion centres are to be found on the other shared TaO₆ edges, as well as on shared LiO₆ octahedral edges.

Martel and Roth (1981) observed two phase transitions in the Li₃TaO₄ system using differential thermal analysis, one at 1173 K and a second at around 1700 K. Zocchi et al. (1983) ascribed the low temperature β-form to a cell of dimensions a = 8.500 (3), b = 8.500 (3), c = 9.3443 (3) Å, β = 117.05 (2)° and space group C2/c. The high temperature α-form was ascribed to a = 6.027 (2), b = 6.004 (2), c = 12.822 (4) Å, β = 103.60 (2)° and space group P2. Subsequently the α-phase was revised to P2/n (Zocchi et al., 1984). Roth (1984) suggested that there may be several intermediate polymorphs, including both disordered and metastable variants, though as yet these remain uncharacterized. Earlier stuctural reports of Li₃TaO₄ were made by Lapicky and Simanov (1953), Blasse (1964), and Grenier et al. (1964); although those authors basically concur on the general lattice morphology, the actual symmetry and cell choices are invariably close approximations. Zocchi et al. (1983) explored the cation ordering with greater precision using X-ray and neutron powder diffraction to study the β form and single-crystal X-ray diffraction to study a quenched sample of the α-phase.

Reinvestigation of a single-crystal of the β-phase on an image plate diffractometer reveals a weakly scattering superlattice (see refinement details). We suspect that local structural disorder, for example in which the two-level zigzag chains in the (1 0 0) plane become locally three-level chains, with a lattice repeat of eight TaO₆ units, can easily be accommodated by the lattice. However, the weak intensity and limited number of the superlattice reflections preclude a more quantitative analysis. The refined structural model indicates that the Li2-centred octahedra is strongly distorted, with two Li2—O bonds longer than 2.4 Å. However, bond valence sums calculated for the present model were 0.94, 0.97, and 0.95 for Li1, Li2, and Li3, respectively, and 4.97 for Ta, which appear quite consistent with formal Li⁺¹ and Ta⁺⁵ cation configurations.

The refined atomic parameters converge to a sensible model that differs from the results of Zocchi et al. (1983) by up to the order of 15 of their s.u.s, which are typically larger than those herein by a factor of at least 3. In addition we report anisotropic displacement parameters for all atoms for the first time. The enhanced precision could be useful for future theoretical property calculations.

S2. Experimental

In a Pt crucible, Li₂SO₄·H₂O (0.574 mmol) and Ta₂O₅ (0.191 mmol) with an additional 37.8 mmol of Li₂SO₄·H₂O flux were heated at 275 K h⁻¹ to 1373 K, which was sustained for 5 h before cooling at 5 K h⁻¹ to 723 K. The sample was discharged into room conditions and rinsed under water, revealing transparent colourless crystal blocks with sizes up to around 0.24 × 0.24 × 0.28 mm and a fine white powdery residue of LiTaO₃.

S3. Refinement

A weakly scattering superlattice was identified that was fully indexed on a unit cell of dimensions a = 8.5, b = 8.5, and c = 16.6 Å with α = γ = 90.0° and β = 90.2°. Apart from a small number of reflections (36 for the whole sphere) all located on the k=+/-1 plane (e.g. 011, 211, 013 etc), the superlattice exhibits nearly perfect B and C centring. The exceptions were all of low scattering angle, very weak, but significant and with reasonable Friedel pair agreement. The a and c lattice vectors used above match those reported by Zocchi et al. (1983) and the vector from the origin to the superlattice pseudo B-centre matches their c axis. The precise transformation we used to convert between them was a' = −a, b' = −b, c' = a/2 + c/2 =9.338 Å with β = 116.874°.

In adopting the smaller sublattice we have assumed that the small number of observed weak, pseudo-B-centred reflections of the superlattice arose as artifacts of a minor degree of structural disorder, leading to some locally doubled c axis. Although complicated twinning modes cannot be categorically excluded, the c = 16.6 Å superlattice is pseudo-tetragonal so simple twinning operations, such as the interchange of a and b (of the reduced cell), or twofold or mirror twinning operations about a and b lead to pseudo-merohedral reflection superpositions and therefore should not give rise to the extra half c* (of the reduced cell) superlattice reflections.

Although the wR(F)-factor of 0.026 is mostly acceptable, some high and low angle weakly scattering reflection measurements were considerably stronger than their modeled values. Presumably this was associated with the twin or disorder component which we opted to ignore.

Computing details

Data collection: RAPID-AUTO (Rigaku, 1999); cell refinement: RAPID-AUTO (Rigaku, 1999); data reduction: RAPID-AUTO and DIFDAT ADDREF SORTRF in Xtal3.7 (Hall et al., 200o); program(s) used to solve structure: ICSD Retrieve; program(s) used to refine structure: CRYLSQ in Xtal3.7; molecular graphics: ATOMS (Dowty, 1999); software used to prepare material for publication: BONDLA CIFIO in Xtal3.7.

Figures

	Fig. 1. 90% probability level ORTEP (Xtal3.7; Hall et al., 2001) plot of extended assymetric unit. [Symmetry codes: (i) −x,+y,1/2 − z; (ii) −x,-y,-z; (iii) 1 − x,+y,1/2 − z; (iv) −1/2 + x,1/2 + y,+z; (v) 1/2 − x,-1/2 − y,-z; (vi) +x,-y,-1/2 + z; (vii) 1 − x,-y,1 − z; (viii) 1/2 − x,-1/2 − y,1 − z; (ix) −1 + x,+y,-1 + z; (x) −1 + x,-y,-1/2 + z.]
	Fig. 2. An ATOMS (Dowty, 1999) polyhedral view projected along the crystallographic c axis (O atoms yellow; Li red and pink). Ellipsoids rendered at 90% probability.
	Fig. 3. Modulated TaO6 octahedral chains (ATOMS; Dowty, 1999) in the (100) plane. Ellipsoids rendered at 90% probability.

(I)

Crystal data

Li3TaO4	F(000) = 912
Mr = 265.77	Dx = 5.85 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -c 2yc	Cell parameters from 63939 reflections
a = 8.508 (1) Å	θ = 3.6–69.9°
b = 8.516 (1) Å	µ = 36.24 mm^−1
c = 9.338 (1) Å	T = 293 K
β = 116.869 (10)°	Irregular block, colourless
V = 603.54 (13) Å^3	0.13 × 0.12 × 0.10 mm
Z = 8

Data collection

Rigaku Rapid image plate diffractometer	2422 independent reflections
Radiation source: Mo Kα	2269 reflections with F > 0.00σ(F)
Graphite monochromator	Rint = 0.038
Detector resolution: 0.1 mm pixels mm^-1	θmax = 45.3°, θmin = 3.6°
ω scans	h = −16→16
Absorption correction: numerical Gaus. Integ. on 8× 8× 8 grid	k = −16→16
Tmin = 0.066, Tmax = 0.237	l = −13→18
10260 measured reflections

Refinement

Refinement on F	0 constraints
Least-squares matrix: full	Weighting scheme based on measured s.u.'s
R[F^2 > 2σ(F^2)] = 0.033	(Δ/σ)max = 0.001
wR(F^2) = 0.026	Δρmax = 4.69 e Å^−3
S = 3.03	Δρmin = −3.41 e Å^−3
2422 reflections	Extinction correction: Zachariasen, Eq22 p292 "Cryst. Comp." Munksgaard 1970
74 parameters	Extinction coefficient: 138 (7)
0 restraints

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ta1	0.078942 (14)	−0.140811 (13)	0.124844 (12)	0.00618 (4)
Li1	0.3164 (10)	−0.1184 (9)	0.6182 (9)	0.012 (2)
Li2	0.5425 (12)	−0.1073 (9)	0.1263 (11)	0.017 (2)
Li3	0.8205 (12)	−0.1201 (9)	0.6388 (11)	0.017 (3)
O1	0.1643 (3)	−0.1228 (3)	0.3608 (2)	0.0079 (6)
O2	0.4394 (3)	−0.1389 (3)	0.8822 (3)	0.0092 (6)
O3	0.6961 (3)	−0.1258 (3)	0.3660 (3)	0.0090 (6)
O4	0.9436 (3)	−0.1106 (3)	0.8886 (3)	0.0084 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ta1	0.00648 (5)	0.00666 (4)	0.00567 (4)	0.00038 (3)	0.00297 (3)	0.00036 (3)
Li1	0.010 (3)	0.016 (3)	0.014 (2)	0.005 (2)	0.010 (2)	0.008 (2)
Li2	0.026 (3)	0.005 (2)	0.025 (4)	0.001 (2)	0.015 (3)	−0.005 (2)
Li3	0.022 (4)	0.009 (3)	0.021 (3)	−0.005 (2)	0.010 (3)	−0.005 (2)
O1	0.0074 (7)	0.0114 (8)	0.0037 (6)	0.0005 (5)	0.0016 (5)	−0.0001 (5)
O2	0.0102 (8)	0.0064 (6)	0.0110 (7)	−0.0019 (6)	0.0048 (6)	−0.0033 (6)
O3	0.0088 (7)	0.0093 (7)	0.0087 (7)	0.0004 (5)	0.0037 (6)	−0.0027 (5)
O4	0.0069 (7)	0.0101 (6)	0.0067 (6)	−0.0005 (5)	0.0018 (5)	0.0013 (5)

Geometric parameters (Å, º)

Ta1^i—O2	1.881 (2)	Li2—O3	2.025 (9)
Ta1^i—O3	1.881 (3)	Li2—O2	2.057 (10)
Ta1^i—O1^i	1.990 (2)	Li2—O3^ii	2.069 (12)
Ta1^i—O4	1.991 (2)	Li2—O2	2.107 (8)
Ta1^i—O1^ii	2.139 (3)	Li2—O4	2.412 (8)
Ta1^i—O4	2.148 (2)	Li2—O1^ii	2.446 (12)
Li1—O2	2.086 (10)	Li3—O1^ii	2.072 (8)
Li1—O3	2.091 (8)	Li3—O4	2.082 (9)
Li1—O1	2.155 (8)	Li3—O4	2.136 (12)
Li1—O4	2.185 (10)	Li3—O2	2.136 (12)
Li1—O2	2.207 (8)	Li3—O3	2.168 (8)
Li1—O1^ii	2.212 (8)	Li3—O3	2.274 (10)
O2—Ta1^i—O3	97.84 (11)	Ta1^ii—O1^ii—Li3	94.0 (3)
O2—Ta1^i—O1^i	95.67 (10)	Ta1^ii—O1^ii—Ta1^i	100.64 (8)
O2—Ta1^i—O4	95.32 (10)	Ta1^ii—O1^ii—Li1^ii	166.2 (3)
O2—Ta1^i—O1^ii	90.36 (11)	Ta1^ii—O1^ii—Li1	89.8 (2)
O2—Ta1^i—O4	171.14 (9)	Ta1^ii—O1^ii—Li2	85.0 (2)
O3—Ta1^i—O1^i	95.16 (10)	Li3—O1^ii—Ta1^i	97.6 (3)
O3—Ta1^i—O4	96.23 (11)	Li3—O1^ii—Li1^ii	88.6 (3)
O3—Ta1^i—O1^ii	170.25 (9)	Li3—O1^ii—Li1	172.8 (3)
O3—Ta1^i—O4	89.82 (10)	Li3—O1^ii—Li2	83.4 (4)
O1^i—Ta1^i—O4	162.94 (10)	Ta1^i—O1^ii—Li1^ii	92.4 (3)
O1^i—Ta1^i—O1^ii	78.71 (9)	Ta1^i—O1^ii—Li1	87.6 (3)
O1^i—Ta1^i—O4	88.04 (9)	Ta1^i—O1^ii—Li2	174.2 (2)
O4—Ta1^i—O1^ii	88.21 (10)	Li1^ii—O1^ii—Li1	86.3 (3)
O4—Ta1^i—O4	79.34 (9)	Li1^ii—O1^ii—Li2	81.9 (3)
O1^ii—Ta1^i—O4	82.45 (10)	Li1—O1^ii—Li2	90.9 (3)
O2—Li1—O3	99.5 (4)	Ta1^i—O2—Li2^iii	99.0 (2)
O2—Li1—O1	95.3 (4)	Ta1^i—O2—Li1	98.7 (2)
O2—Li1—O4	176.6 (4)	Ta1^i—O2—Li2	176.1 (3)
O2—Li1—O2	91.4 (3)	Ta1^i—O2—Li3	90.3 (2)
O2—Li1—O1^ii	83.3 (3)	Ta1^i—O2—Li1	92.9 (2)
O3—Li1—O1	94.1 (3)	Li2^iii—O2—Li1	93.7 (4)
O3—Li1—O4	83.6 (3)	Li2^iii—O2—Li2	84.8 (4)
O3—Li1—O2	90.8 (3)	Li2^iii—O2—Li3	89.8 (4)
O3—Li1—O1^ii	171.4 (5)	Li2^iii—O2—Li1	167.7 (3)
O1—Li1—O4	83.0 (2)	Li1—O2—Li2	80.0 (4)
O1—Li1—O2	170.9 (5)	Li1—O2—Li3	169.7 (3)
O1—Li1—O1^ii	93.7 (3)	Li1—O2—Li1	87.8 (3)
O4—Li1—O2	89.9 (4)	Li2—O2—Li3	90.7 (4)
O4—Li1—O1^ii	93.8 (3)	Li2—O2—Li1	83.4 (3)
O2—Li1—O1^ii	81.0 (3)	Li3—O2—Li1	86.7 (4)
O3—Li2—O2	162.5 (6)	Ta1^i—O3—Li2	101.0 (4)
O3—Li2—O3^ii	96.3 (5)	Ta1^i—O3—Li2^i	175.9 (3)
O3—Li2—O2	95.6 (3)	Ta1^i—O3—Li1	98.5 (3)
O3—Li2—O4	87.8 (3)	Ta1^i—O3—Li3	89.3 (3)
O3—Li2—O1^ii	78.9 (3)	Ta1^i—O3—Li3	90.1 (3)
O2—Li2—O3^ii	95.4 (3)	Li2—O3—Li2^i	83.0 (4)
O2—Li2—O2	95.2 (4)	Li2—O3—Li1	88.4 (3)
O2—Li2—O4	79.2 (3)	Li2—O3—Li3	92.9 (3)
O2—Li2—O1^ii	87.8 (5)	Li2—O3—Li3	167.8 (4)
O3^ii—Li2—O2	99.6 (4)	Li2^i—O3—Li1	80.8 (3)
O3^ii—Li2—O4	90.0 (3)	Li2^i—O3—Li3	91.2 (4)
O3^ii—Li2—O1^ii	171.4 (4)	Li2^i—O3—Li3	85.8 (4)
O2—Li2—O4	169.3 (7)	Li1—O3—Li3	171.6 (5)
O2—Li2—O1^ii	88.0 (4)	Li1—O3—Li3	85.0 (3)
O4—Li2—O1^ii	82.7 (3)	Li3—O3—Li3	92.1 (3)
O1^ii—Li3—O4	87.7 (3)	Ta1^i—O4—Li3	169.3 (3)
O1^ii—Li3—O4	84.3 (4)	Ta1^i—O4—Li3	91.3 (2)
O1^ii—Li3—O2	97.8 (4)	Ta1^i—O4—Ta1^iv	100.66 (10)
O1^ii—Li3—O3	179.0 (6)	Ta1^i—O4—Li1^ii	95.7 (2)
O1^ii—Li3—O3	91.2 (3)	Ta1^i—O4—Li2^iii	85.2 (2)
O4—Li3—O4	96.2 (3)	Li3—O4—Li3	83.6 (4)
O4—Li3—O2	94.7 (5)	Li3—O4—Ta1^iv	89.3 (2)
O4—Li3—O3	93.3 (3)	Li3—O4—Li1^ii	88.6 (4)
O4—Li3—O3	177.6 (6)	Li3—O4—Li2^iii	84.8 (3)
O4—Li3—O2	169.0 (5)	Li3—O4—Ta1^iv	95.4 (2)
O4—Li3—O3	95.2 (4)	Li3—O4—Li1^ii	171.5 (3)
O4—Li3—O3	81.6 (4)	Li3—O4—Li2^iii	83.2 (4)
O2—Li3—O3	82.4 (3)	Ta1^iv—O4—Li1^ii	88.1 (2)
O2—Li3—O3	87.6 (3)	Ta1^iv—O4—Li2^iii	174.0 (3)
O3—Li3—O3	87.9 (3)	Li1^ii—O4—Li2^iii	92.5 (3)

Symmetry codes: (i) −x+1/2, y+1/2, −z+1/2; (ii) x+1/2, y+1/2, z; (iii) x, −y, z+1/2; (iv) x+1/2, −y+1/2, z+1/2.