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Most known compounds with five-coordinated Ti4+ are natural and synthetic titanosilicates. The crystal structure of natural fresnoite, Ba2TiSi2O8 [tetragonal, space group P4bm, a = 8.510 (1) Å, c = 5.197 (1) Å, V = 376.4 (1) Å3, Z = 2], has been refined to R = 0.011 on the basis of 807 unique single-crystal reflections with I > 2σ(I). Titanium has fivefold coordination with one short (`titanyl') bond of 1.692 (5) Å. Bonds in the TiO5 polyhedron are discussed in comparison to analogous coordination polyhedra in other minerals and compounds. A review of all known compounds with Ti4+O5 polyhedra shows that most of them are titanosilicates in which titanium forms a short Ti—O bond (∼1.61 to ∼1.77 Å). Poor Gladstone–Dale compatibility between chemical composition, optical characteristics and density of these compounds is explained by the anomalous contribution of [5]Ti4+ to the optical properties as shown by calculations based on the relationship between electronic polarizabilities and refractive indices. An improved Gladstone–Dale coefficient of 0.29 is suggested for TiO2 with [5]Ti4+. A negative correlation between `titanyl' bond lengths and wavenumbers of the bands of Ti—O stretching vibrations (in the range of 890–830 cm−1) in infrared and Raman spectra is observed.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S2052520622012045/ra5120sup1.cif
Contains datablock I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S2052520622012045/ra5120Isup2.hkl
Contains datablock I

CCDC reference: 2150917

Computing details top

Program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2018).

(I) top
Crystal data top
Ba2O8Si2TiDx = 4.471 Mg m3
Mr = 506.76Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P4bmCell parameters from 11109 reflections
a = 8.5104 (12) Åθ = 3.4–33.4°
c = 5.1975 (10) ŵ = 11.71 mm1
V = 376.44 (13) Å3T = 293 K
Z = 2Parallelepiped, yellow
F(000) = 4520.25 × 0.14 × 0.13 mm
Data collection top
Xcalibur, Eos
diffractometer
θmax = 33.4°, θmin = 3.4°
25512 measured reflectionsh = 1313
807 independent reflectionsk = 1313
806 reflections with I > 2σ(I)l = 88
Rint = 0.033
Refinement top
Refinement on F2 w = 1/[σ2(Fo2) + (0.0047P)2 + 0.9597P]
where P = (Fo2 + 2Fc2)/3
Least-squares matrix: full(Δ/σ)max = 0.001
R[F2 > 2σ(F2)] = 0.012Δρmax = 0.87 e Å3
wR(F2) = 0.027Δρmin = 0.65 e Å3
S = 1.20Extinction correction: SHELXL-2018/3 (Sheldrick 2018), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
807 reflectionsExtinction coefficient: 0.0165 (7)
40 parametersAbsolute structure: Refined as an inversion twin.
1 restraintAbsolute structure parameter: 0.01 (3)
Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refined as a 2-component inversion twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Ba10.32715 (2)0.82715 (2)0.00051 (5)0.00865 (7)
Ti20.0000000.0000000.53608 (18)0.00667 (19)
Si10.12793 (9)0.62793 (9)0.5130 (4)0.00767 (17)
O10.0000000.0000000.2106 (9)0.0149 (9)
O20.0000000.5000000.6300 (10)0.0250 (13)
O30.1260 (3)0.6260 (3)0.2067 (6)0.0088 (5)
O40.2922 (3)0.5761 (4)0.6451 (5)0.0236 (7)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Ba10.00940 (8)0.00940 (8)0.00715 (9)0.00491 (6)0.00086 (9)0.00086 (9)
Ti20.0071 (2)0.0071 (2)0.0057 (5)0.0000.0000.000
Si10.0087 (2)0.0087 (2)0.0055 (4)0.0013 (3)0.0013 (4)0.0013 (4)
O10.0178 (14)0.0178 (14)0.0091 (18)0.0000.0000.000
O20.035 (2)0.035 (2)0.0056 (18)0.022 (3)0.0000.000
O30.0098 (8)0.0098 (8)0.0067 (12)0.0013 (11)0.0002 (7)0.0002 (7)
O40.0142 (11)0.0462 (19)0.0105 (10)0.0150 (12)0.0035 (9)0.0075 (11)
Geometric parameters (Å, º) top
Ba1—O32.647 (3)Ba1—Si13.5838 (17)
Ba1—O3i2.789 (2)Ti2—O11.692 (5)
Ba1—O3ii2.789 (2)Ti2—O4viii1.967 (3)
Ba1—O4iii2.840 (3)Ti2—O4ix1.967 (3)
Ba1—O4iv2.840 (3)Ti2—O4x1.967 (3)
Ba1—O2v2.835 (3)Ti2—O4xi1.967 (3)
Ba1—O4vi2.989 (4)Si1—O31.592 (4)
Ba1—O4vii2.989 (4)Si1—O4xii1.619 (3)
Ba1—Si1iv3.4885 (16)Si1—O41.619 (3)
Ba1—Si1vii3.6220 (15)Si1—O21.655 (2)
Ba1—Si1v3.6220 (15)
O3—Ba1—O3i121.77 (10)O4ix—Ti2—Ba1xiv112.91 (10)
O3—Ba1—O3ii121.77 (10)O4x—Ti2—Ba1xiv109.48 (9)
O3i—Ba1—O3ii65.89 (12)O4xi—Ti2—Ba1xiv42.28 (9)
O3—Ba1—O4iii73.10 (9)Ba1xiii—Ti2—Ba1xiv68.276 (14)
O3i—Ba1—O4iii162.00 (10)O1—Ti2—Ba1xv127.473 (13)
O3ii—Ba1—O4iii116.94 (8)O4viii—Ti2—Ba1xv112.91 (10)
O3—Ba1—O4iv73.10 (9)O4ix—Ti2—Ba1xv109.48 (9)
O3i—Ba1—O4iv116.94 (8)O4x—Ti2—Ba1xv42.28 (9)
O3ii—Ba1—O4iv162.00 (9)O4xi—Ti2—Ba1xv46.84 (11)
O4iii—Ba1—O4iv54.51 (11)Ba1xiii—Ti2—Ba1xv105.05 (3)
O3—Ba1—O2v161.09 (10)Ba1xiv—Ti2—Ba1xv68.276 (14)
O3i—Ba1—O2v73.36 (8)O1—Ti2—Ba1xvi127.473 (13)
O3ii—Ba1—O2v73.36 (8)O4viii—Ti2—Ba1xvi109.48 (9)
O4iii—Ba1—O2v90.16 (10)O4ix—Ti2—Ba1xvi42.28 (9)
O4iv—Ba1—O2v90.16 (10)O4x—Ti2—Ba1xvi46.84 (11)
O3—Ba1—O4vi119.26 (6)O4xi—Ti2—Ba1xvi112.91 (10)
O3i—Ba1—O4vi116.50 (7)Ba1xiii—Ti2—Ba1xvi68.276 (14)
O3ii—Ba1—O4vi68.87 (9)Ba1xiv—Ti2—Ba1xvi105.05 (3)
O4iii—Ba1—O4vi54.30 (10)Ba1xv—Ti2—Ba1xvi68.276 (14)
O4iv—Ba1—O4vi95.49 (8)O3—Si1—O4xii115.48 (13)
O2v—Ba1—O4vi52.32 (6)O3—Si1—O4115.48 (13)
O3—Ba1—O4vii119.26 (6)O4xii—Si1—O4106.9 (2)
O3i—Ba1—O4vii68.87 (9)O3—Si1—O2110.7 (2)
O3ii—Ba1—O4vii116.50 (7)O4xii—Si1—O2103.48 (17)
O4iii—Ba1—O4vii95.49 (8)O4—Si1—O2103.48 (17)
O4iv—Ba1—O4vii54.30 (10)O3—Si1—Ba1xvii137.41 (13)
O2v—Ba1—O4vii52.32 (6)O4xii—Si1—Ba1xvii53.45 (12)
O4vi—Ba1—O4vii95.74 (11)O4—Si1—Ba1xvii53.45 (12)
O3—Ba1—Si1iv70.46 (7)O2—Si1—Ba1xvii111.87 (18)
O3i—Ba1—Si1iv142.36 (7)O3—Si1—Ba1xviii133.90 (8)
O3ii—Ba1—Si1iv142.36 (7)O4xii—Si1—Ba1xviii54.53 (13)
O4iii—Ba1—Si1iv27.26 (6)O4—Si1—Ba1xviii110.02 (13)
O4iv—Ba1—Si1iv27.26 (6)O2—Si1—Ba1xviii49.28 (11)
O2v—Ba1—Si1iv90.63 (8)Ba1xvii—Si1—Ba1xviii77.53 (4)
O4vi—Ba1—Si1iv74.36 (6)O3—Si1—Ba1xiii133.90 (8)
O4vii—Ba1—Si1iv74.36 (6)O4xii—Si1—Ba1xiii110.02 (13)
O3—Ba1—Si1vii143.94 (5)O4—Si1—Ba1xiii54.53 (13)
O3i—Ba1—Si1vii67.00 (7)O2—Si1—Ba1xiii49.28 (11)
O3ii—Ba1—Si1vii94.10 (5)Ba1xvii—Si1—Ba1xiii77.53 (4)
O4iii—Ba1—Si1vii95.03 (8)Ba1xviii—Si1—Ba1xiii70.11 (3)
O4iv—Ba1—Si1vii72.44 (7)O3—Si1—Ba142.82 (13)
O2v—Ba1—Si1vii26.27 (3)O4xii—Si1—Ba192.03 (13)
O4vi—Ba1—Si1vii74.52 (6)O4—Si1—Ba192.03 (13)
O4vii—Ba1—Si1vii26.17 (5)O2—Si1—Ba1153.54 (18)
Si1iv—Ba1—Si1vii83.49 (4)Ba1xvii—Si1—Ba194.59 (3)
O3—Ba1—Si1v143.94 (5)Ba1xviii—Si1—Ba1143.548 (14)
O3i—Ba1—Si1v94.10 (5)Ba1xiii—Si1—Ba1143.548 (14)
O3ii—Ba1—Si1v67.00 (7)O3—Si1—Ba1xix43.69 (7)
O4iii—Ba1—Si1v72.44 (7)O4xii—Si1—Ba1xix91.58 (11)
O4iv—Ba1—Si1v95.03 (8)O4—Si1—Ba1xix158.05 (12)
O2v—Ba1—Si1v26.27 (3)O2—Si1—Ba1xix82.98 (14)
O4vi—Ba1—Si1v26.17 (5)Ba1xvii—Si1—Ba1xix143.696 (15)
O4vii—Ba1—Si1v74.52 (6)Ba1xviii—Si1—Ba1xix90.22 (2)
Si1iv—Ba1—Si1v83.49 (4)Ba1xiii—Si1—Ba1xix130.33 (3)
Si1vii—Ba1—Si1v50.31 (4)Ba1—Si1—Ba1xix75.19 (4)
O3—Ba1—Si124.13 (7)O3—Si1—Ba1xx43.69 (7)
O3i—Ba1—Si1102.32 (6)O4xii—Si1—Ba1xx158.05 (12)
O3ii—Ba1—Si1102.32 (6)O4—Si1—Ba1xx91.58 (11)
O4iii—Ba1—Si194.48 (7)O2—Si1—Ba1xx82.98 (14)
O4iv—Ba1—Si194.48 (7)Ba1xvii—Si1—Ba1xx143.696 (15)
O2v—Ba1—Si1174.78 (8)Ba1xviii—Si1—Ba1xx130.33 (3)
O4vi—Ba1—Si1129.34 (5)Ba1xiii—Si1—Ba1xx90.22 (2)
O4vii—Ba1—Si1129.34 (5)Ba1—Si1—Ba1xx75.19 (4)
Si1iv—Ba1—Si194.59 (3)Ba1xix—Si1—Ba1xx68.13 (3)
Si1vii—Ba1—Si1154.718 (15)Si1xxi—O2—Si1136.9 (3)
Si1v—Ba1—Si1154.718 (15)Si1xxi—O2—Ba1xiii104.45 (9)
O1—Ti2—O4viii106.75 (9)Si1—O2—Ba1xiii104.45 (9)
O1—Ti2—O4ix106.75 (9)Si1xxi—O2—Ba1xviii104.45 (9)
O4viii—Ti2—O4ix85.24 (5)Si1—O2—Ba1xviii104.45 (9)
O1—Ti2—O4x106.75 (9)Ba1xiii—O2—Ba1xviii94.41 (15)
O4viii—Ti2—O4x146.51 (18)Si1—O3—Ba1113.05 (17)
O4ix—Ti2—O4x85.24 (5)Si1—O3—Ba1xix113.09 (11)
O1—Ti2—O4xi106.75 (9)Ba1—O3—Ba1xix109.98 (8)
O4viii—Ti2—O4xi85.24 (5)Si1—O3—Ba1xx113.09 (11)
O4ix—Ti2—O4xi146.51 (18)Ba1—O3—Ba1xx109.98 (8)
O4x—Ti2—O4xi85.24 (5)Ba1xix—O3—Ba1xx96.49 (10)
O1—Ti2—Ba1xiii127.473 (13)Si1—O4—Ti2xxii138.08 (18)
O4viii—Ti2—Ba1xiii42.28 (9)Si1—O4—Ba1xvii99.29 (14)
O4ix—Ti2—Ba1xiii46.84 (11)Ti2xxii—O4—Ba1xvii109.95 (13)
O4x—Ti2—Ba1xiii112.91 (10)Si1—O4—Ba1xiii99.30 (15)
O4xi—Ti2—Ba1xiii109.48 (9)Ti2xxii—O4—Ba1xiii104.49 (13)
O1—Ti2—Ba1xiv127.473 (13)Ba1xvii—O4—Ba1xiii99.60 (9)
O4viii—Ti2—Ba1xiv46.84 (11)
Symmetry codes: (i) y, x+1, z; (ii) y+1, x+1, z; (iii) y1/2, x+1/2, z1; (iv) x, y, z1; (v) y+1, x+1, z1; (vi) x+1/2, y+1/2, z1; (vii) y, x+1, z1; (viii) x+1/2, y1/2, z; (ix) y+1/2, x+1/2, z; (x) x1/2, y+1/2, z; (xi) y1/2, x1/2, z; (xii) y1/2, x+1/2, z; (xiii) y+1, x, z+1; (xiv) x, y1, z+1; (xv) y1, x, z+1; (xvi) x, y+1, z+1; (xvii) x, y, z+1; (xviii) y1, x+1, z+1; (xix) y1, x+1, z; (xx) y+1, x, z; (xxi) x, y+1, z; (xxii) x+1/2, y+1/2, z.
 

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