Crystal structure of Ti8Bi9O0.25 containing interstitial oxygen atoms

Single crystals of Ti8Bi9O0.25 were grown by using a Bi flux and were investigated in order to clarify the structural changes by O atom occupation at the interstital site in the Ti4 tetrahedron.


Chemical context
The crystal structure of Ti 8 Bi 9 , having the tetragonal space group P4/nmm, a = 10.277 (1) Å , c = 7.375 (1) Å , Z = 2, was determined by Richter and Jeitschko (1997). This compound was identified in Ti-Bi binary phase diagrams (Okamoto, 2010(Okamoto, , 2015, and was also confirmed by powder X-ray diffraction (PXRD) in a study of the Ti-Bi phase diagram (Maruyama et al., 2013). Recently, the use of a Bi flux has enabled single-crystal growth of a new polymorph of TiO ("phase; Amano et al., 2016) and some new suboxides: Ti 8 (Sn x Bi 1-x )O 7 , Ti 11.17 (Sn 0.85 Bi 0.15 ) 3 O 10 and Ti 12-Ga x Bi 3-x O 10 . While exploring new suboxides containing Ti using a Bi flux, we also found the title compound, Ti 8 Bi 9 O 0.25 where interstitial O sites are partly occupied.
In the present communication, details of single-crystal growth of Ti 8 Bi 9 O 0.25 and its comparison with the crystal structure of Ti 8 Bi 9 (Richter & Jeitschko, 1997) are reported.

Structural commentary
Reflections from a single crystal of Ti 8 Bi 9 O 0.25 could be indexed with a primitive tetragonal cell similar to that of the oxygen-free compound Ti 8 Bi 9 (Richter & Jeitschko, 1997). The differences in the lengths of the a and c axes and in the cell volume from those of Ti 8 Bi 9 were +1.0%, À0.09% and +0.74%, respectively. The reflection conditions observed for the new compound were the same as for Ti 8 Bi 9 , revealing space group P4/nmm.
The crystal structure and atomic arrangement for Ti 8 Bi 9 O 0.25 are depicted in Figs. 1 and 2, respectively. In the crystal structure of Ti 8 Bi 9 (Richter & Jeitschko, 1997), the Ti2 site is in a trigonal antiprism (point group symmetry. .2/m) made up from Bi atoms with Bi-Ti distances of 2.848 (1) and 2.931 (1) Å (Table 1). The Ti3 and Ti4 sites are situated in square antiprisms in which the Bi-Ti distances range from 2.937 (5) to 3.144 (6) Å . The Ti3-and Ti4-centered Bi1 4 Bi2 4 ISSN 2056-9890 square antiprisms both exhibit point group symmetry 4mm and are arranged along the c axis by sharing the square planes. The Bi1Bi2 2 triangle plane is shared by the Ti2-centered Bi1 2 Bi2 4 trigonal antiprism and the Ti3-centered Bi1 4 Bi2 4 square antiprism. In the crystal structure of Ti 8 Bi 9 , only the Ti1 site forms a Ti polyhedron. The Ti1-Ti1 distances of the Ti1 4 tetrahedron are 2.934 (6) and 3.074 (3) Å . In addition to the three Ti1 sites, each Ti1 site is surrounded by six Bi atoms at distances of 2.945 (4)-3.074 (5) Å , and by two Ti2 sites at a distance of 3.017 (2) Å . The O atom of Ti 8 Bi 9 O 0.25 is located in the Ti1 4 tetrahedron at a site with symmetry 4m2 and with a site occupancy of 0.25 (4). The partial occupation by the O atoms changes the Ti1-Ti1 distances in the tetrahedron to 2.992 (2) and 3.1142 (19) Å , representing increases of 1.9% and 1.3%, respectively. The Ti1-Bi2 distance is also increased by 1.4%, although the changes in the Ti3-Bi and Ti4-Bi distances are both less than 0.4%.
The O1-Ti1 distance of 1.8824 (11) Å is intermediate between the sums of ionic radii for Ti 3+ and O 2À (1.91 Å ) and Ti 2+ -O 2À (1.845 Å ), based on ionic radii of 0.67 and 0.605 Å for Ti 3+ and Ti 2+ , respectively, in sixfold coordination, and an O 2À radius of 1.24 Å in fourfold coordination (Shannon, 1976). The bond-valence sums (BVSs) calculated for the O1 site in the Ti1 4 tetrahedron using bond-valence parameters (R 0 ) for Ti 4+ (1.815 Å ), Ti 3+ (1.815 Å ) and Ti 2+ (1.734 Å ) and B = 0.37 (Brese & O'Keeffe, 1991; are 3.33, 3.12 and 2.68 valence units (v.u.), respectively. All of these values are considerably greater than the expected valence value of 2 for an O atom, which may suggest that the O1 site is not fully occupied, or that bond-valence parameters for titanium in lower oxidation states (and/or tetrahedral coordination) need revision. Complete occupation of O atoms in tetrahedral sites surrounded by Ti atoms has been reported for the crystal structures of Ti 12-Ga x Bi 3-x O 10 . In these structures, the Ti-O distances range from 1.957 (3) to 2.291 (3) Å , all of which exceed the value of 1.8824 (11) Å for O1-Ti1 in Ti 8 Bi 9 O 0.25 . The BVSs calculated for the O sites in Ti 12-Ga x Bi 3-x O 10 using the parameters for Ti 3+ and Ti 2+ were found to be in the ranges 2.18-2.21 and 1.87-1.89 v.u., respectively .
The O concentration was greater than the expected values, indicating that some oxidation had occurred while transferring the specimens to the EPMA instrument. In addition to fragments with a Ti:Bi atomic ratio of approximately 8:9, some Birich (>85%) portions and fragments with a Ti:Bi ratio of approximately 3:2 were also identified.

Special details
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.