Redetermination of synthetic warwickite, Mg₃TiO₂(BO₃)₂

Abstract

Single crystals of warwickite, trimagnesium titanium(IV) dioxide bis­(borate), Mg₃TiO₂(BO₃)₂, were prepared by slow cooling of the melt. The title compound is isotypic with Co₃TiO₂(BO₃)₂. In contrast to the previous refinement of warwickite [Moore & Araki (1974 ▶). Am. Mineral. 59, 985–1004], that reported only isotropic atomic displacement parameters for all atoms, anisotropic displacement parameters of all atoms were refined during the current redetermination. All atoms are situated on special positions (site symmetry .m.). One of the two Mg sites is statistically disordered with Ti atoms (ratio 1:1), while the other is fully occupied by Mg atoms. The occupancy ratio of the Mg and Ti atoms is similar to that reported in the previous study. Metal atoms (M) at the Ti/Mg and Mg sites are coordinated by six O atoms in form of distorted octa­hedra. Four edge-sharing MO₆ octa­hedra form M ₄O₁₈ units, which are connected by common corners into layers parallel to (010). Adjacent layers are linked along [010] into a framework structure by sharing common edges. The B atoms are located in the triangular prismatic tunnels of the framework.

Related literature

For the structure determination of natural warwickite, Mg₃TiO₂(BO₃)₂, see: Takéuchi et al. (1950 ▶); Moore & Araki (1974 ▶). For the synthesis and crystal structure analysis of Co₃ MO₂(BO₃)₂ (M = Ti, Zr), see: Utzolino & Bluhm (1995 ▶). For the synthesis of Mg₅TiO₄(BO₃)₂ and Mg₃ZrO₂(BO₃)₂, see: Konijnendijk & Blasse (1985 ▶). For the structure of Mg₅TiO₄(BO₃)₂, see: Kawano & Yamane (2010 ▶). For bond-valence-sum calculations, see: Brown & Altermatt (1985 ▶). For bond-valence parameters, see: Brese & O’Keeffe (1991 ▶). For structure standardization, see: Gelato & Parthé (1987 ▶).

Experimental

Crystal data

• Mg₃TiO₂(BO₃)₂

• M _r = 270.45

• Orthorhombic,

• a = 9.3013 (5) Å

• b = 3.10080 (14) Å

• c = 9.3914 (6) Å

• V = 270.86 (3) ų

• Z = 2

• Mo Kα radiation

• μ = 1.94 mm⁻¹

• T = 293 K

• 0.17 × 0.17 × 0.12 mm

Data collection

• Rigaku R-AXIS RAPID II diffractometer

• Absorption correction: numerical (NUMABS; Higashi, 1999 ▶) T _min = 0.791, T _max = 0.839

• 2510 measured reflections

• 364 independent reflections

• 348 reflections with I > 2σ(I)

• R _int = 0.018

Refinement

• R[F ² > 2σ(F ²)] = 0.026

• wR(F ²) = 0.070

• S = 1.20

• 364 reflections

• 44 parameters

• Δρ_max = 0.36 e Å⁻³

• Δρ_min = −0.59 e Å⁻³

Data collection: PROCESS-AUTO (Rigaku/MSC, 2005 ▶); cell refinement: PROCESS-AUTO; data reduction: PROCESS-AUTO; program(s) used to solve structure: SIR2004 (Burla et al., 2005 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: VESTA (Momma & Izumi, 2008 ▶); software used to prepare material for publication: SHELXL97.

Supplementary Material

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

Table 1. Selected geometric parameters (Å, °), M = (Mg, Ti).

M1—O4i	1.9702 (12)
M1—O4ii	1.9989 (19)
M1—O2	2.0730 (18)
M1—O1i	2.1565 (13)
Mg2—O3iii	2.0043 (12)
Mg2—O4iv	2.0698 (19)
Mg2—O1	2.1387 (19)
Mg2—O2v	2.1522 (13)
B1—O3	1.353 (3)
B1—O2	1.393 (3)
B1—O1	1.395 (3)

O3—B1—O2	119.1 (2)
O3—B1—O1	120.7 (2)
O2—B1—O1	120.3 (2)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) .

supplementary crystallographic information

Comment

Crystal structure determinations of the mineral warwickite, Mg₃TiO₂(BO₃)₂, were reported by Takéuchi et al. (1950) and Moore & Araki (1974). The natural samples contained a few amount of Fe and Al. The crystal structure of synthetic Mg₃TiO₂(BO₃)₂ has not been analyzed up to now. We obtained single crystals of this compound during the preparation of Mg₅TiO₄(BO₃)₂ (Kawano & Yamane, 2010). Anisotropic atomic displacement parameters (U_ij) of Mg, Ti, B and O atoms were refined in the present study. Moore & Araki (1974) refined isotropic atomic displacement parameters (B_iso) of Mg, Ti, B and O atoms; neither U nor B-values were reported by Takéuchi et al. (1950).

Synthetic warwickite-type oxyborates with general composition M^II₃M^IVO₂(BO₃)₂ are known for Co₃MO₂(BO₃)₂ [M = Ti, Zr (Utzolino & Bluhm, 1995)] and Mg₃ZrO₂(BO₃)₂ (Konijnendijk & Blasse, 1985). However, only the crystal structures of Co₃MO₂(BO₃)₂ (M = Ti, Zr) were analyzed. The title compound Mg₃TiO₂(BO₃)₂ is isotypic with Co₃MO₂(BO₃)₂ [M = Ti, Zr (Utzolino & Bluhm, 1995)].

Figs. 1 and 2 show the coordination environments of the Mg, Ti, B and O atoms, and the crystal structure of Mg₃TiO₂(BO₃)₂, respectively. In the asymmetric unit, there is one Ti/Mg mixed site (M1) with occupancies of 0.5/0.5 and one Mg site (M2). Moore and Araki (1974) refined the site occupancy factors of Mg and Ti atoms at the M1 and M2 sites, ignoring Al and Fe atoms due to their similarities with the scattering profiles of Mg²⁺ and Ti⁴⁺, respectively. Refined occupancy factors were M1 = Mg_0.96 (1)/Ti_0.04 (1) and M2 = Mg_0.62 (2)/Ti_0.38 (2) and an ideal formula of Mg(Mg_0.5Ti_0.5)O₂[BO₃] was suggested (Moore & Araki, 1974). Our refinement (M1 = Mg1 and M2 = Mg_0.5/Ti_0.5) is consistent with the ideal formula. Although Takéuchi et al. (1950) reported the atomic coordinates of natural warwickite, site occupancy factors of the M1 and M2 sites were not reported.

All atoms are at special positions (x, 1/4, z), 4c, with site symmetries of (.m.). Mg and Ti atoms occupy six-coordinated oxygen-octahedral sites, forming layers composed of M₄O₁₈ (M = Ti/Mg, Mg) units. The layers are connected by edge-sharing O4 atoms of (Ti1/Mg1)O₆ and Mg2O₆ octahedra into a three-dimensional framework. B1 atoms are located in triangular prismatic tunnels of the framework.

Bond valence sums (BVS; Brown & Altermatt, 1985) of the Mg, Ti and B atoms were calculated with the bond valence parameters of 1.693 Å for Mg²⁺, 1.815 Å for Ti⁴⁺ and 1.371 Å for B³⁺ (Brese & O'Keeffe, 1991). The BVS values of the Mg2 and B1 atoms were 2.1 and 2.9, respectively. Those of the Ti1 and Mg1 atoms at the Ti1/Mg1 site were 3.22 and 2.31, respectively. The average of these value is 2.8 and close to the expected mean valence (+3) of Mg²⁺ and Ti⁴⁺. The B1—O distances of 1.353 (3)–1.395 (3) Å agree well with those of isotypic Co₃MO₂(BO₃)₂ (M = Ti, Zr): 1.36 (2)–1.39 (2) Å (Utzolino & Bluhm, 1995).

Warwickite-type Mg₃TiO₂(BO₃)₂ did not emit visible light under ultraviolet excitation at room temperature, while ludwigite-type Mg₅TiO₄(BO₃)₂ shows broad blue emission (435 nm) attributed to charge transfer transitions between Ti⁴⁺ and O^2– (Konijnendijk & Blasse, 1985).

Experimental

Starting materials were powders of MgO (99.9%, Rare Metallic), TiO₂ (99.9%, Rare Metallic) and H₃BO₃ (99.99%, Sigma-Aldrich). MgO was heated at 1173–1273 K for 6–12 h in air before weighing. The powders were weighed with a molar ratio of MgO: TiO₂: H₃BO₃ = 5: 1: 2.7 and mixed in an agate mortar with a pestle. The mixture was pressed into a pellet, placed in a Pt boat and heated at 1623 K for 6 h in air. Heating and cooling rates were 200 and 900 K/h, respectively. About 400 mg of the sample and 100 mg of H₃BO₃ were weighed and mixed. The mixture in the Pt boat was heated at 1723 K for 3 h in air and cooled to room temperature at a cooling rate of 900 K/h. The obtained sample was crushed into fragments and a colourless and transparent single-crystal of about 0.12–0.17 mm was picked up under an optical microscope.

Refinement

The crystal structures of natural warwickites were described in the space group Pnam (no. 62) in the previous studies (Takéuchi et al., 1950; Moore & Araki, 1974). The original single-crystal X-ray diffraction data in the present study were indexed in a different setting in space group Pmnb and unit-cell parameters of a = 3.10080 (14), b = 9.3013 (5) and c = 9.3914 (6) Å. Structure parameters were eventually standardized based on the standard setting of the space group Pnma using the STRUCTURE TIDY program (Gelato & Parthé, 1987). In the final refinement, site occupation factors (s.o.f.'s) of the Ti and Mg atoms at the Ti1/Mg1 and Mg2 sites were fixed to 0.5/0.5 and 1.0, respectively, since the freely refined s.o.f.'s of the Ti and Mg atoms at the Ti1/Mg1 site were close to 1/2, and the s.o.f. of the Mg atom at the Mg2 site was about 1.0. The highest peak in the difference electron density map is 0.36 Å from O2 while the deepest hole is -0.59 Å from Ti1/Mg1.

Figures

Fig. 1.

The atomic arrangement around Mg, Ti, B and O atoms in the structure of Mg3TiO2(BO3)2. Displacement ellipsoids are drawn at the 95% probability level. Symmetry codes: (i) –x + 1/2, –y, z–1/2; (ii) –x + 1/2, –y + 1, z–1/2; (iii) x–1/2, y, –z + 1/2; (iv) –x, –y, –z + 1; (v) –x, –y + 1, –z + 1; (vi) x–1/2, y, –z + 3/2; (vii) –x + 1/2, –y + 1, z + 1/2; (viii) –x + 1/2, –y, z + 1/2.

Fig. 2.

The crystal structure of Mg3TiO2(BO3)2 in a representation using cation-centred oxygen polyhedra.

Crystal data

Mg3TiO2(BO3)2	F(000) = 264
Mr = 270.45	Dx = 3.316 Mg m−3
Orthorhombic, Pnma	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2n	Cell parameters from 2288 reflections
a = 9.3013 (5) Å	θ = 3.1–27.5°
b = 3.10080 (14) Å	µ = 1.94 mm−1
c = 9.3914 (6) Å	T = 293 K
V = 270.86 (3) Å3	Block, colourless
Z = 2	0.17 × 0.17 × 0.12 mm

Data collection

Rigaku R-AXIS RAPID II diffractometer	364 independent reflections
Radiation source: fine-focus sealed tube	348 reflections with I > 2σ(I)
graphite	Rint = 0.018
Detector resolution: 10.0 pixels mm-1	θmax = 27.5°, θmin = 3.1°
ω scans	h = −12→11
Absorption correction: numerical (NUMABS; Higashi, 1999)	k = −3→3
Tmin = 0.791, Tmax = 0.839	l = −12→12
2510 measured reflections

Refinement

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.026	w = 1/[σ2(Fo2) + (0.0347P)2 + 0.3981P] where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.070	(Δ/σ)max < 0.001
S = 1.20	Δρmax = 0.36 e Å−3
364 reflections	Δρmin = −0.59 e Å−3
44 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraints	Extinction coefficient: 0.039 (7)

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)
Ti1	0.11388 (7)	0.2500	0.07167 (7)	0.0115 (3)	0.50
Mg1	0.11388 (7)	0.2500	0.07167 (7)	0.0115 (3)	0.50
Mg2	0.10160 (8)	0.2500	0.68497 (9)	0.0055 (3)
B1	0.1708 (3)	0.2500	0.3719 (3)	0.0061 (5)
O1	0.24045 (19)	0.2500	0.50344 (18)	0.0088 (4)
O2	0.25030 (18)	0.2500	0.24622 (19)	0.0078 (4)
O3	0.0255 (2)	0.2500	0.36441 (19)	0.0100 (4)
O4	0.5095 (2)	0.2500	0.61439 (18)	0.0100 (4)

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Ti1	0.0148 (4)	0.0079 (4)	0.0119 (4)	0.000	−0.0015 (2)	0.000
Mg1	0.0148 (4)	0.0079 (4)	0.0119 (4)	0.000	−0.0015 (2)	0.000
Mg2	0.0040 (4)	0.0045 (5)	0.0079 (4)	0.000	0.0001 (3)	0.000
B1	0.0069 (12)	0.0038 (13)	0.0077 (12)	0.000	−0.0013 (9)	0.000
O1	0.0071 (8)	0.0139 (10)	0.0054 (7)	0.000	−0.0006 (6)	0.000
O2	0.0085 (8)	0.0090 (9)	0.0058 (7)	0.000	0.0004 (6)	0.000
O3	0.0063 (9)	0.0095 (10)	0.0143 (9)	0.000	−0.0017 (7)	0.000
O4	0.0102 (9)	0.0128 (10)	0.0071 (8)	0.000	−0.0020 (6)	0.000

Geometric parameters (Å, °)

Ti1—O4i	1.9702 (12)	Mg2—O4x	2.0698 (19)
Ti1—O4ii	1.9702 (12)	Mg2—O1	2.1387 (19)
Ti1—O4iii	1.9989 (19)	Mg2—O2xi	2.1522 (13)
Ti1—O2	2.0730 (18)	Mg2—O2xii	2.1522 (13)
Ti1—O1ii	2.1565 (13)	Mg2—Mg2vi	3.10080 (14)
Ti1—O1i	2.1565 (13)	Mg2—Mg2vii	3.10080 (14)
Ti1—Ti1iv	2.9502 (11)	Mg2—Ti1xi	3.2465 (9)
Ti1—Mg1iv	2.9502 (11)	Mg2—Mg1xi	3.2465 (9)
Ti1—Ti1v	2.9502 (11)	Mg2—Ti1xii	3.2465 (9)
Ti1—Mg1v	2.9502 (11)	Mg2—Mg1xii	3.2465 (9)
Ti1—Ti1vi	3.10080 (14)	B1—O3	1.353 (3)
Ti1—Ti1vii	3.10080 (14)	B1—O2	1.393 (3)
Mg2—O3viii	2.0043 (12)	B1—O1	1.395 (3)
Mg2—O3ix	2.0043 (12)
O4i—Ti1—O4ii	103.80 (9)	Mg2—O1—Ti1xi	98.20 (6)
O4i—Ti1—O4iii	83.98 (6)	B1—O1—Ti1xii	123.71 (9)
O4ii—Ti1—O4iii	83.98 (6)	Mg2—O1—Ti1xii	98.20 (6)
O4i—Ti1—O2	101.28 (6)	Mg1xi—O1—Ti1xii	91.94 (7)
O4ii—Ti1—O2	101.28 (6)	Ti1xi—O1—Ti1xii	91.94 (7)
O4iii—Ti1—O2	171.31 (8)	B1—O1—Mg1xii	123.71 (9)
O4i—Ti1—O1ii	172.87 (6)	Mg2—O1—Mg1xii	98.20 (6)
O4ii—Ti1—O1ii	82.00 (5)	Mg1xi—O1—Mg1xii	91.94 (7)
O4iii—Ti1—O1ii	92.60 (6)	Ti1xi—O1—Mg1xii	91.94 (7)
O2—Ti1—O1ii	81.39 (6)	B1—O2—Ti1	110.19 (15)
O4i—Ti1—O1i	82.00 (5)	B1—O2—Mg2ii	124.49 (8)
O4ii—Ti1—O1i	172.87 (6)	Ti1—O2—Mg2ii	100.40 (6)
O4iii—Ti1—O1i	92.60 (6)	B1—O2—Mg2i	124.49 (8)
O2—Ti1—O1i	81.39 (6)	Ti1—O2—Mg2i	100.40 (6)
O1ii—Ti1—O1i	91.94 (7)	Mg2ii—O2—Mg2i	92.17 (7)
O3viii—Mg2—O3ix	101.34 (9)	B1—O3—Mg2viii	126.96 (6)
O3viii—Mg2—O4x	88.08 (6)	B1—O3—Mg2ix	126.96 (6)
O3ix—Mg2—O4x	88.08 (6)	Mg2viii—O3—Mg2ix	101.34 (9)
O3viii—Mg2—O1	99.89 (7)	Mg1xii—O4—Mg1xi	103.80 (9)
O3ix—Mg2—O1	99.89 (7)	Ti1xii—O4—Mg1xi	103.80 (9)
O4x—Mg2—O1	167.30 (8)	Mg1xii—O4—Ti1xi	103.80 (9)
O3viii—Mg2—O2xi	175.34 (6)	Ti1xii—O4—Ti1xi	103.80 (9)
O3ix—Mg2—O2xi	83.24 (5)	Mg1xii—O4—Mg1xiii	96.02 (6)
O4x—Mg2—O2xi	91.24 (6)	Ti1xii—O4—Mg1xiii	96.02 (6)
O1—Mg2—O2xi	80.01 (6)	Mg1xi—O4—Mg1xiii	96.02 (6)
O3viii—Mg2—O2xii	83.24 (5)	Ti1xi—O4—Mg1xiii	96.02 (6)
O3ix—Mg2—O2xii	175.34 (6)	Mg1xii—O4—Ti1xiii	96.02 (6)
O4x—Mg2—O2xii	91.24 (6)	Ti1xii—O4—Ti1xiii	96.02 (6)
O1—Mg2—O2xii	80.01 (6)	Mg1xi—O4—Ti1xiii	96.02 (6)
O2xi—Mg2—O2xii	92.17 (7)	Ti1xi—O4—Ti1xiii	96.02 (6)
O3—B1—O2	119.1 (2)	g1xii—O4—Mg2xiv	115.24 (6)
O3—B1—O1	120.7 (2)	Ti1xii—O4—Mg2xiv	115.24 (6)
O2—B1—O1	120.3 (2)	Mg1xi—O4—Mg2xiv	115.24 (6)
B1—O1—Mg2	115.18 (15)	Ti1xi—O4—Mg2xiv	115.24 (6)
B1—O1—Mg1xi	123.71 (9)	Mg1xiii—O4—Mg2xiv	126.50 (10)
Mg2—O1—Mg1xi	98.20 (6)	Ti1xiii—O4—Mg2xiv	126.50 (10)
B1—O1—Ti1xi	123.71 (9)

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