Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Ternary Suboxides Ti7Ga2O6, Ti3GaO, and Ti5Ga3O Hisanori Yamane* and Shinsaku Amano Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
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S Supporting Information *
ABSTRACT: Single crystals of the new suboxides Ti7Ga2O6 and Ti3GaO were prepared using a Bi flux. Ti7Ga2O6 has a novel monoclinic crystal structure (a = 17.3111(8) Å, b = 2.97660(10) Å, c = 7.2563(3) Å, β = 105.836(2)°, space group C2/m), in which three O sites and one Ga site are separately coordinated by Ti atoms. Ti3GaO crystallizes in an orthorhombic cell (a = 3.0952(2) Å, b = 10.6440(7) Å, c = 8.3206(5) Å, space group Cmcm) with a filled-Re3B type (anti-CaIrO3 type or antipostperovskite type) structure. The electrical resistivities of a single crystal of Ti3GaO as measured in the c-axis direction were 1.6 × 10−6 Ωm at 300 K and 0.15 × 10−6 Ωm at 10 K. Single crystals of Ti5Ga3Ox (x = 1) with a filled-Mn5Si3 type structure (hexagonal, a = 7.5882(2) Å, c = 5.30170(10) Å, space group P63/mcm) were also obtained.
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INTRODUCTION Suboxides, having a metal/oxygen (M/O) ratio of less than 1, have often been recognized as interstitial compounds in which the interstitial sites of the metal or alloy structures are filled with O atoms as well as with B, C, and N atoms. Thus, the structures are named with a prefix of “filled-”, for example, the filled-AuCu3 type and the filled-Ti2Ni type.1 The filled-AuCu3 type is widely referred to the anti-perovskite type. The “anti-” prefix means the interchange of cation sites and anion (O2−) sites of the perovskite-type structure. Many anti-perovskitetype compounds and some anti-post-perovskite-type compounds have been studied in order to clarify their superconductivity,2,3 negative thermal expansion,4 and mechanical,5 magnetic,6−8 and thermoelectric properties.9 However, the interstitial compounds of O are not so common compared to those of C and N. Within the series of Ti−Ga−O ternary compounds TiGa2O5, TiGa4O8, and Ti5Ga4O16, and the homologous series having the general formula Tim−4Ga4O2m−2 (15 ≤ m ≤ 23, 25, 31),10−15 Ti and Ga atoms are coordinated by oxide anions with formal ionic charges of +4 for Ti, +3 for Ga, and −2 for O atoms. The other compound reported in this system is Ti5Ga3Ox, which contains O atoms at the interstitial sites of a Mn5Si3-type structure (filled-Mn5Si3 type, a = 7.6936(4) Å, c = 5.2792(8) Å, space group P63/mcm).16 The atomic positions and O atom occupancy in the structure of Ti5Ga3Ox have not yet been analyzed. Recently, we found single crystals of a new © XXXX American Chemical Society
polymorph of TiO (ε phase, hexagonal, a = 4.9980(3) Å, c = 2.87716(17) Å, space group P6̅2m) in a Ti−O sample prepared using a Bi flux.17 According to the Ti−Bi binary phase diagram reported by Maruyama et al.18 Ti metal (melting point 1670 °C) dissolves in a Bi melt up to apporximately 30 atom % at 900 °C. Thus, the Bi flux could be expected to grow single crytals of compounds containing Ti. Using a Bi flux, single crystals of the new Ti-containing suboxides Ti8BiO7,19 Ti8(SnxBi1−x)O7, Ti11.17(Sn0.85Bi0.15)3O10,20 and Ti12−δGaxBi3−xO1021 have been synthesized. These new suboxides are composed of O atom centered Ti polyhedra (oxide parts) and Bi-, Sn-, and/or Gaatom centered Ti polyhedra (intermetallic parts). The O content of these suboxides are larger, and the coordination numbers (4−5) are smaller than those of the conventional interstitial compounds of oxygen. The present paper reports the Bi flux synthesis of new suboxides Ti7Ga2O6 and Ti3GaO and known suboxide Ti5Ga3Ox (x = 1) in single crystal forms and the analysis of the crystal structures of these substances by X-ray diffraction (XRD).
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EXPERIMENTAL SECTION
Synthesis. Bi powder (Mitsuwa Chemicals, 99.999%), Ti powder (Mitsuwa Chemicals, 99.99%), TiO2 powder (rutile type, Rare Received: April 22, 2018
A
DOI: 10.1021/acs.inorgchem.8b01109 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Metallic, 99.99%), and Ga shot (DOWA, 99.9999%) were employed as starting materials. The procedure used to synthesize Ti7Ga2O6 single crystals was the same as that reported in a previous study on Ti12−δGaxBi3−xO1021 except for the Ga content and the introduction and mixing of the raw materials. Ga (0.25 mmol), Ti (0.5 mmol), Bi (3 mmol), and TiO2 (0.5 mmol) were successively transferred into an Al2O3 crucible (Japan Fine Ceramics, 99.5%, 8.5 mm outside diameter, 6.5 mm inside diameter, 18 mm depth), followed by gentle manual mixing with a spatula. The crucible was subsequently sealed in a stainless-steel tube with one end welded shut (SUS316, 12.7 mm outer diameter, 10.5 mm inner diameter, 70 mm length) with a stainless-steel cap in a glovebox filled with Ar gas (Taiyo Nippon Sanso, 99.9999%). The tube was heated to 900 °C at a rate of 7.5 °C/ min and maintained at this temperature for 2 h in an electric box furnace, then cooled to 200 °C at a rate of −10 °C/h. After cooling to room temperature in the furnace by shutting off the power to the heater, the stainless-steel tube was cut open in air to give sample 1. Single crystals of Ti3GaO and Ti5Ga3O were obtained by successively transferring Ga (0.3 mmol), Ti (0.5 mmol), Bi (3 mmol), and TiO2 (0.5 mmol) into an aluminum boat (Nikkato, 99.5%, SSA-S #5). The boat was covered with a Ta foil and placed in a quartz glass tube sealed at one end (21 mm inside diameter). The tube was capped with an O-ring flange attached to a valve, and connected to a vacuum line, after which the air in the tube was replaced with Ar. Subsequently, the valve was shut and the boat in the tube was heated to 900 °C at 220 °C/h in an electric tube furnace and maintained at this temperature for 2 h. The boat was cooled first to 600 °C at −25 °C/h and then to room temperature by shutting off the power to the heater (sample 2). A single crystal of Ti3GaO for electrical resistivity measurements was obtained from a sample prepared by heating a combination of Ga (0.5 mmol), Ti (1.8 mmol), Bi (6 mmol), and TiO2 (0.2 mmol) in the alumina boat to 930 °C at 460 °C/h. This temperature was maintained for 1 h after which the material was cooled first to 630 °C at −25 °C/h, and then to room temperature by shutting off the power to the heater (sample 3). The samples in the alumina crucible and boats were washed with a nitric acid aqueous solution (HNO3, ∼6 mol/L) to dissolve away the Bi flux, and then the remaining product was washed with water. Characterization. The morphologies of the single crystals obtained from the residual samples were observed with a scanning electron microscope (SEM, KEYENCE, VE-8800SP1614) and an optical microscope. Chemical compositions were determined with an electron probe microanalyzer (EPMA, JEOL, JXA-8200) at the flat surfaces of single crystals embedded in In metal. GaAs (JEOL, 99.9%) and TiO2 (JEOL, 99.99%) were employed as calibration standards. X-ray diffraction (XRD) data were acquired from single crystals with a Bruker D8 QEST diffractometer employing monochromatic Mo Kα radiation. Data acquisition and unit-cell refinement were performed with the APEX3 software package,22 and multiscan absorption correction was applied using the SADABS software.23 An initial structure model of Ti7Ga2O6 was obtained with the Intrinsic Phasing program installed in APEX3. The structure parameters of the crystals were refined using the full-matrix least-squares method with F2, using the SHELXL-2014 program.24 The crystal structures were drawn with the VESTA program.25 The electrical resistivities of a Ti3GaO single crystal between 10 and 300 K were measured by the direct-current four-terminal method with Ag paste electrodes connected to Au wire leads.
Figure 1. Scanning electron micrographs of Ti7Ga2O6 single crystals (a) and Ti5Ga3O single crystal (b), and an optical micrograph of a Ti3GaO single crystal (c).
Ti, Ga, and O were found in these single crystals by EPMA analysis. The analyzed chemical composition of a Ti7Ga2O6 single crystal was determined to be Ti 60.0(3), Ga 22(2), and O 17(1) mass % (total 99(2) mass %), which is almost consistent with the ideal composition of Ti7Ga2O6 (Ti 58.7, Ga 24.4, O 16.8 mass %). The compositions of the Ti3GaO and Ti5Ga3O single crystals were Ti 64.1(2), Ga 26.5(8), and O 6.1(1) mass % (total 96.8(8) mass %) and Ti 52.9(8), Ga 40.3(4), and O 3.4(7) mass % (total 97(1) mass %), respectively. These are also close to the ideal compositions of 62.6, 30.4, and 7.0 mass % and 51.5, 45.0, and 3.4 mass %, respectively. The results of the crystal structure analyses for Ti7Ga2O6, Ti3GaO, and Ti5Ga3O single crystals are presented in Tables 1 and 2, and the anisotropic displacement parameters are summarized in Table S1. Tables 3 and S2 provide selected interatomic distances. The XRD reflections from the Ti7Ga2O6 crystal were indexed with the monoclinic cell parameters, a = 17.3111(8) Å, b = 2.97660(10) Å, c = 7.2563(3) Å, and β = 105.836(2)°. The candidate space groups based on systematic extinction were C2, Cm, and C2/m, and the initial structure model was constructed using the Intrinsic Phasing program22 with the space group C2/m. No compound having a crystal structure of the same type was found in the Inorganic Crystal Structure Data Base (ICSD). The crystal structure of Ti3GaO corresponds to the filled-Re3B type (anti-CaIrO3 type, or antipost-perovskite type, space group Cmcm). The refined cell parameters of Ti3GaO are a = 3.0952(2) Å, b = 10.6440(7) Å, and c = 8.3206(5) Å. Ti5Ga3O crystallizes in a hexagonal cell
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RESULTS AND DISCUSSION Black prismatic single crystals of Ti7Ga2O6 with lengths of 50− 100 μm were included in sample 1, while 50−100 μm wide, black, rectangular prismatic single crystals of Ti3GaO and black hexagonal platelet single crystals of Ti5Ga3O were obtained from sample 2. SEM micrographs of Ti7Ga2O6 and Ti5Ga3O single crystals are shown in Figure 1a,b. An optical micrograph of a flat prismatic crystal of Ti3GaO (having a size of 0.1 × 0.2 × 0.3 mm) that was grown in sample 3 is shown in Figure 1c. B
DOI: 10.1021/acs.inorgchem.8b01109 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Crystal Data and Refinement Results for Ti7Ga2O6, Ti3GaO, and Ti5Ga3Oa chemical formula formula weight, Mr (g mol−1) crystal form, color crystal size (mm) temperature, T (K) crystal system space group unit-cell dimensions a (Å) b (Å) c (Å) β (deg) unit-cell volume, V (Å3) Z calculated density, Dcal (Mg m−3) radiation wavelength, l (Å) absorption correction absorption coefficient, m (mm−1) limiting indices
F000 θ range for date collection (deg) reflections collected/unique Rint date/restraints/parameters weight parameters, a, b extinction collection extinction coefficient, c goodness-of-fit on F2, S R1, wR2 (I > 2σ(I)) R1, wR2 (all date) largest diff. peak and hole, Δρ (e Å−3)
Ti7Ga2O6 570.74 prism, black 0.057 × 0.022 × 0.019 301(2) monoclinic C2/m
Ti3GaO 229.42 prism, black 0.054 × 0.050 × 0.029 301(2) orthorhombic Cmcm
Ti5Ga3O 464.66 platelet, black 0.061 × 0.048 × 0.032 301(2) hexagonal P63/mcm
17.3111(8) 2.97660(10) 7.2563(3) 105.836(2) 359.71(3) 2 5.269 0.71073 multiscan (SADABS)14) 14.687 −26 ≤ h ≤ 26 −4 ≤ k ≤ 4 −11 ≤ l ≤ 11 528 2.446−34.34 7896/878 0.0394 878/0/48 0.0128, 0.1406 SHELXL15 0.0041(3) 1.070 0.0177, 0.0305 0.0281. 0.0321 0.769, −0.648
3.0952(2) 10.6440(7) 8.3206(5)
7.5882(2) 5.30170(10)
274.12(3) 4 5.559
264.377(15) 2 5.837
17.820 −2 ≤ h ≤ 4 −13 ≤ k ≤ 14 −11 ≤ l ≤ 11 420 3.829−29.098 2137/226 0.0481 226/0/20 0.0179, 0
22.039 −10 ≤ h ≤ 11 −11 ≤ k ≤ 11 −8 ≤ l ≤ 8 422 6.209−32.995 2806/200 0.0370 200/0/14 0.0112, 0.0138
0.0069(7) 1.082 0.0181, 0.0344 0.0253, 0.0359 0.703, −0.701
0.0102(14) 1.215 0.0121, 0.0265 0.0125, 0.0268 0.394, −0.562
a R1 = Σ||F0| − |Fc||/Σ|F0|. wR2 = [Σw(F02 − Fc2)2/Σ(wF02)2]1/2, w = 1/[σ2(F02) + (aP)2 + bP], where F0 is the observed structure factor, Fc is the calculated structure factor, σ is the standard deviation of Fc2, and P = (F02 + 2Fc2)/3. S = [Σw(F02 − Fc2)2/(n − p)]1/2, where n is the number of reflections and p is the total number of parameters refined.
Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters for Ti7Ga2O6, Ti3GaO, and Ti5Ga3O z
Ueq (Å2)a
0 0 0 0 0 0 0 0
0.15323(5) 0.53696(5) 0.12128(5) 1/2 0.18713(3) 0.6484(2) 0.0423(2) 0.3847(2)
0.00290(8) 0.00358(8) 0.00306(8) 0.00448(11) 0.00437(6) 0.0036(3) 0.0039(3) 0.0036(3)
0 0 0 0
0.36892(5) 0.03948(8) 0.74560(5) 0
0.04006(8) 1/4 1/4 0
0.00339(19) 0.0030(2) 0.00435(19) 0.0041(7)
0.22990(5) 0.3333 0.59559(3) 0
0 0.6667 0 0
1/4 0 1/4 0
0.00585(10) 0.00723(12) 0.00582(10) 0.0072(5)
atom
site
occ.
x
y
Ti7Ga2O6
Ti1 Ti2 Ti3 Ti4 Ga1 O1 O2 O3
4i 4i 4i 2c 4i 4i 4i 4i
1 1 1 1 1 1 1 1
0.21118(2) 0.33774(2) 0.42598(2) 0 0.06238(2) 0.23498(8) 0.31121(8) 0.41964(8)
Ti3GaO
Ti1 Ti2 Ga1 O1
8f 4c 4c 4a
1 1 1 1
Ti5Ga3O
Ti1 Ti2 Ga1 O1
6g 4d 6g 2b
1 1 1 1
Ueq = (∑i ∑j Uij ai*ja* ai·aj)/3.
a
(a = 7.5882(2) Å and c = 5.3017(10) Å) with a filled-Mn5Si3 type structure (space group P63/mcm). These hexagonal cell
parameters are 1.4% shorter and 0.4% longer in the cases of the a- and c-axis lengths, respectively, compared to the parameters C
DOI: 10.1021/acs.inorgchem.8b01109 Inorg. Chem. XXXX, XXX, XXX−XXX
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Table 3. Selected Bond Lengths (Å), Bond Valence Sums (BVSs), and Bond Valence Parameters (R0) for Ti7Ga2O6, Ti3GaO, and Ti5Ga3Oa Ti7Ga2O6
O1Ti1 (2×) O1Ti2 (1×) O1Ti2 (2×) ∑BVS(Ti2+−O2−) ∑BVS(Ti3+−O2−) ∑BVS(Ti4+−O2−) R0(Ti1/2−O12−)
2.1032(9) 2.1456(13) 2.1644(9) 1.691 1.973 2.105 1.796
O3Ti3 (1×) O3Ti2 (1×) O3Ti4 (2×) ∑BVS(Ti2+−O2−) ∑BVS(Ti3+−O2−) ∑BVS(Ti4+−O2−) R0(Ti2/3/4−O32−)
1.9443(13) 2.0213(12) 2.0531(8) 1.871 2.182 2.329 1.750
O2Ti3 (1×) O2Ti1 (2×) O2Ti1 (1×) ∑BVS(Ti2+−O2−) ∑BVS(Ti3+−O2−) ∑BVS(Ti4+−O2−) R0(Ti1/3−O22−)
1.9116(13) 2.0195(11) 2.0993(12) 1.916 2.235 2.385 1.759
Ga1Ti1 (1×) Ga1Ti2 (2×) Ga1Ti3 (2×) Ga1Ti3 (2×) Ga1Ti4 (1×) R0(Ti1/2/3/4−Ga15−)
2.6536(5) 2.7062(3) 2.7208(4) 2.7404(4) 2.7674(3) 2.544
Ti3GaO
O1Ti1 (4×) O1Ti1 (2×) ∑BVS(Ti2+−O2−) ∑BVS(Ti3+−O2−) ∑BVS(Ti4+−O2−) R0(Ti1−O12−)
2.1101(5) 2.1221(3) 2.148 2.505 2.673 1.708
Ga1Ti1 (4×) Ga1Ti2 (2×) Ga1Ti1 (2×) Ga1Ti2 (1×) R0(Ti1/2−Ga15−)
2.6776(6) 2.6849(9) 2.7038(7) 3.1281(11) 2.498
Ti5Ga3O
O1Ti1 (6×) ∑BVS(Ti2+−O2−) ∑BVS(Ti3+−O2−) ∑BVS(Ti4+−O2−) R0(Ti1−O12−)
2.19092(4) 1.745 2.036 2.172 1.784
Ga1Ti2 (4×) Ga1Ti1 (2×) Ga1Ti1 (1×) Ga1Ti1 (2×) R0(Ti1/2−Ga15−)
2.66107(18) 2.6659(4) 2.77493(5) 2.96320(5) 2.506
Bond valence parameters R0(Ti2+−O2−) = 1.734, R0(Ti3+−O2−) = 1.791,35 R0(Ti4+−O2−) = 1.815 Å.35
a
previously reported for Ti5Ga3Ox (a = 7.6936(4) Å and c = 5.2792(8) Å).16 The occupancies of O1, O2, and O3 sites in Ti7Ga2O6 were refined to be 1.019(5), 1.033(5), and 1.039(5), respectively. The refined occupancies of O1 sites in Ti3GaO and Ti5Ga3O were 1.006(11) and 0.985(15), respectively. Thus, the occupancies of all O sites were fixed at a value of 1 in the final refinement of the structure analysis for these three compounds. The R1(2σ)-value was 1.77% for Ti7Ga2O6, 1.81% for Ti3GaO, and 1.21% for Ti5Ga3O. The crystal structures of these suboxides are shown in Figures 2−4. Figure 3. Crystal structure of Ti3GaO illustrated with O- and Gacentered Ti polyhedra.
Figure 4. Crystal structure of Ti5Ga3O illustrated with O- and Gacentered Ti polyhedra. Figure 2. Crystal structure of Ti7Ga2O6 illustrated with O- and Gacentered Ti polyhedra. D
DOI: 10.1021/acs.inorgchem.8b01109 Inorg. Chem. XXXX, XXX, XXX−XXX
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similar Ti octahedra containing O atoms.30−32 As an example, the O−Ti interatomic distances of Ti4Fe2O and Ti4Cu2O are 2.10631 and 2.11850(13) Å,32 respectively, which are close to the O−Ti distances of Ti3GaO and Ti5Ga3O. The Ti−O distances expected from the ionic radii reported by Shannon33 vary with the ionic charge of the Ti atoms and the coordination numbers of the Ti and O atoms. The Tin+(VI)−O2−(IV) distances (where (IV) and (VI) indicate 4and 6-fold coordinations, respectively) are 2.24 Å for n = 2, 2.05 Å for n = 3, and 1.985 Å for n = 4, while the Tin+(VI)− O2−(VI) distances are 2.26 Å for n = 2, 2.07 Å for n = 3, and 2.01 Å for n = 4. These Ti−O distances are in reasonably good agreement with the values observed for Ti7Ga2O6, Ti3GaO, and Ti5Ga3O. However, the Ti charges in these compounds cannot be unambiguously determined from the Ti−O distances. Since O atoms are only coordinated by Ti atoms in the suboxides of Ti7Ga2O6, Ti3GaO, and Ti5Ga3O, bond valence sums (BVSs) can be calculated for each O site based on the bond valence parameter R0 using a general equation of BVS:
Ternary suboxides Zr3FeOx, Zr3CoOx, Zr3NiOx, Hf3CoOx, and Hf3NiOx, all of which have filled-Re3B type structures have been reported.26−28 These compounds were prepared by arc melting, and the crystal structure of Zr3NiO was analyzed by single crystal XRD.27 To the best of our knowledge, Ti3GaO is the first filled-Re3B type suboxide containing Ti. Details of the crystal structures have not been determined for the filledMn5Si3 type compounds containing Ti and O, although the cell parameters of Ti5Ga3Ox were reported.16 In the crystal structures of Ti7Ga2O6, Ti3GaO, and Ti5Ga3O, O and Ga atoms are separately surrounded by Ti atoms, forming O-centered Ti polyhedra (oxide parts) and Gacentered Ti polyhedra (intermetallic parts), respectively. There are three O sites in the Ti7Ga2O6 structure (Figure 5). The O1 site is in a Ti pseudosquare pyramid (5-fold coordination), and the O2 and O3 sites are in distorted Ti tetrahedra (4-fold coordination). The O−Ti interatomic distances in Ti7Ga2O6 are in the range of 1.9116(13)−2.1644(9) Å. Similar square pyramidal Ti coordination environments around O atoms are seen in the structures of ε-TiO17 and Ti8BiO7,19 with O−Ti distance ranges of 1.9745(8)−2.1025(2) Å and 2.0088(17)− 2.0480(12) Å, respectively. Examples of O atoms in Ti tetrahedra have been reported in the case of Ti8BiO7 (O−Ti: 2.002(3)−2.0919(4) Å)19 and Ti12−δGaxBi3−xO10 (O−Ti: 1.948(3)−2.291(3) Å).21 The three-dimensional framework of Ti7Ga2O6 is constructed by sharing of the edges and vertexes of the Ti tetrahedra and pyramids, forming a large space running along the b axis, with the Ga atoms located in this space (Figure 2). In the structure of Ti3GaO, each O1 site is surrounded by four Ti1 sites and two Ti2 sites and is located at the center of an elongated Ti14Ti22 octahedron with the O1−Ti1 and O1− Ti2 interatomic distances of 2.1101(5) and 2.1221(3) Å, respectively (Figure 5). The Ti14Ti22 octahedra extend to the a-axis direction via Ti1−Ti1 edge sharing and in the a-axis direction by sharing Ti2 vertexes (Figure 3). Ga1 sites are located between the O1-centered Ti14Ti22 octahedral layers. The O1 site in Ti5Ga3O is in a Ti16 regular-trigonal antiprism (distorted Ti16 octahedra, O1−Ti2: 2.19092(4) Å × 6) (Figure 5). The trigonal antiprisms align along the c axis by sharing a regular triangle base (Figure 4). Each Ga 1 site is between the one-dimensional Ti16 trigonal antiprism columns. The O−Ti distances reported for filled-Mn5Si3 type Ti5Si3Ox and Ti5Si3O are 2.2105(4) and 2.183(3) Å, respectively.29 Suboxides of Ti3M3O and Ti4M2O (M = Mn, Fe, Co, Ni, Cu) known as the η phases of filled-Ti2Ni type structures have
Vi = Σjvij = Σj exp[(R 0 − R ij)/0.37]
(1)
where Rij is the length of the bond between atoms i and j and vij is its experimental (apparent) bond valence.34,35 Values of R0(Ti3+−O2−) = 1.791 Å and R0(Ti4+−O2−) = 1.815 Å were presented by Brese and O’Keeffe.35 Because R0(Ti2+−O2−) has not been reported, a value of 1.734 Å was calculated based on the O−Ti2+ distances in the crystal structure of ε-TiO.17 The BVSs calculated from the O−Ti bond lengths and these parameters are provided in Table 3. Among the BVSs calculated for the O sites of Ti7Ga2O6 with these parameters, the values of 1.973 calculated from R0(Ti3+− O2−) for the O1 site and 1.916 and 1.871 with R0(Ti2+−O2−) for the O2 and O3 sites, respectively, are closest to the expected oxide anion charge of 2. In Ti3GaO, the BVS of the O1 site, 2.148 as calculated from R0(Ti2+−O2−), is also close to 2, while the other BVSs of 2.505 and 2.673 obtained from R0(Ti3+−O2−) and R0(Ti4+−O2−) are much larger. The BVS for the O1 site (2.036) from R0(Ti3+−O2−) is almost 2 for Ti5Ga3O. These results suggest that the state of the Ti cations in these suboxides is not Ti4+, but rather Ti3+ or Ti2+. As shown in Figure 6, the Ga1 site in Ti7Ga2O6 is in a distorted bicapped trigonal prism of eight Ti atoms with Ti− Ga distances of 2.6536(5)−2.7674(3) Å (avg. 2.7195 Å). The Ga1 in Ti3GaO is also in a similar distorted Ti bicapped trigonal prism (Ti−Ga: 2.6776(6)−2.7038(7) Å, avg. 2.6860 Å) when the Ga1−Ti2 bonding with a distance of 3.1281(11) Å is excluded. When this bond is added, the Ga1 site is in a distorted tricapped trigonal prism of Ti 9-fold coordination, similar to that observed in Ti5Ga3O. The Ti−Ga distances in Ti5Ga3O are in the range of 2.6659(4)−2.96320(5) Å (avg. 2.7419 Å). The distances between the Ga and apical Ti atoms in the Ti polyhedra, with the exception of the Ga1−Ti2 distance (3.1281(11) Å), are comparable to the values of 2.671−2.762 Å reported for Ti5Ga3 (W5Si3-type, tetragonal I4/ mcm)36 and 2.613−2.955 Å of Ti2Ga (Ni2In-type, hexagonal P63/mmc).37 All Ti atoms in Ti7Ga2O6 are in 6-fold coordination as shown in Figure 7. The Ti1, Ti2, and Ti4 sites in Ti7Ga2O6 are located in O12O23Ga1, O13O3Ga12, and O34Ga12 distorted octahedra, respectively, and the Ti3 site is in a distorted O2O3Ga14 trigonal prism. The crystal structure of Ti7Ga2O6 is illustrated with Ti-centered O and Ga polyhedra in Figure S1
Figure 5. Coordination environments around the O sites in Ti7Ga2O6, Ti3GaO, and Ti5Ga3O. Displacement ellipsoids are drawn at 99% probability. E
DOI: 10.1021/acs.inorgchem.8b01109 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 6. Coordination environments around the Ga sites in Ti7Ga2O6, Ti3GaO, and Ti5Ga3O. Displacement ellipsoids are drawn at 99% probability.
Figure 7. Coordination environments around Ti sites in Ti7Ga2O6, Ti3GaO, and Ti5Ga3O. Displacement ellipsoids are drawn at 99% probability.
(−2) × 1 = 5e−). These results suggest the metallic character of these suboxides. In fact, metallic electrical resistivities of 1.6 × 10−6 Ωm at 300 K and 0.15 × 10−6 Ωm at 10 K were measured for the Ti3GaO single crystal shown in Figure 1c in the c-axis direction (Figure 8). If these residual electrons remain at Ti atoms, then 0.60e− valence electron will be distributed to each Ti atom in Ti5Ga3O, such that the formal charge of the Ti atom is +3.40 (that is Ti3.40+). Similarly, the residual electrons per Ti atom in Ti7Ga2O6 and Ti3GaO are
for reference purposes. The Ti1 and Ti2 sites in Ti3GaO are situated in a distorted O12Ga13 pyramid of 5-fold coordination and a distorted O12Ga13 trigonal bipyramid of 5-fold coordination, respectively. In Ti5Ga3O, the Ti1 site is in a O12Ga15 polyhedron and the Ti2 site is at the centers of a Ga16 regular trigonal prism. A total of 175 filled-Mn5Si3 type compounds, having the general formula A5B3Z, and in which various kinds of atoms (such as B, C, N, P, As, and Sb), occupy the interstitial Z sites, are recorded in the ICSD. Only two filled-Mn5Si3 type suboxides, Nb5Pt3O38 and Zr5Sn3O0.92,39 analyzed by powder and single crystal XRD, respectively, are listed. Ti5Si3O and Ti5Si3Ox20 are not included in the database. Corbett et al. reviewed the crystal structures and stabilities of the filledMn5Si3 type compounds,40 and explained the occupancy limit of the interstitial Z sites with a simple 8 − n rule. As an example, the valence electron number is 3 × 5 + (−4) × 3 = 3e− for the frame of La5Ge3P, and the occupancy limit for P3− at the interstitial Z sites is estimated to be 1. Thus, La5Ge3P is a valence (Zintl) compound which exhibits semiconducting behavior. In the case of Ti5Ga3O, the number of frame valence electrons is 4 × 5 + (−5) × 3 = 5e− and three electrons (3e−) remain upon complete O2− occupation of the Z sites, resulting in metallic conduction. Similar calculations, including the numbers of electrons received by O2− at the fully occupied interstitial Z sites, were carried out for Ti7Ga2O6 (4 × 7 + (−5) × 2 + (−2) × 6 = 6e− and Ti3GaO (4 × 3 + (−5) × 1 +
Figure 8. Electrical resistivity of a Ti3GaO single crystal measured in the c-axis direction. F
DOI: 10.1021/acs.inorgchem.8b01109 Inorg. Chem. XXXX, XXX, XXX−XXX
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of Ti3GaO and Ti5Ga3O are in octahedral 6-fold sites. The electrical resistivity of a single crystal of Ti3GaO exhibited a temperature dependence similar to that of a metal.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01109. Anisotropic displacement parameters, selected Ti−O/ Ga bond lengths (Å), bond valence sums (BVSs) calculated from the bond valence parameters, Madelung potentials (MP), Madelung part of the lattice energies per formula unit (MAPLE) for Ti7Ga2O6, Ti3GaO, and Ti5Ga3O, crystal structure and coordination environments around the Ti1, Ti2, Ti3, and Ti4 sites of Ti7Ga2O6 (PDF)
(2)
The parameters obtained from the observed interatomic distances Rij are shown in Table 3. The R0(Ti−O2−) value for each O site are in the range from 1.750 to 1.796 Å for Ti7Ga2O6 and Ti5Ga3O and are close to the values of R0(Ti2+− O2−) = 1.734 Å and R0(Ti3+−O2−) = 1.791 Å. However, the R0(Ti−O2−) value of Ti3GaO (1.708 Å) are smaller than those. The R0(Ti−Ga5−) values for Ti3GaO (2.498 Å) and Ti5Ga3O (2.506 Å) are nearly equal and the value for Ti7Ga2O6 (2.544 Å) is a little larger. The BVSs calculated for the Ti sites of the three suboxides using these bond valence parameters are shown in Table S2. The formulas of Ti7Ga2O6, Ti3GaO, and Ti5Ga3O expressed with formal charges of the atoms obtained by the BVSs are ( T i 1 2 . 9 7 + ) 2 ( Ti 2 2 . 9 1 + ) 2 ( T i 3 3 . 6 7 + ) 2 (Ti4 2 . 9 0 + ) ( G a 1 5 − ) 2 (O12−)2(O22−)2(O32−)2, (Ti12.48+)2Ti22.04+Ga15−O12−, and (Ti13.03+)3(Ti23.95+)2(Ga15−)3O12−, with average Ti formal charges of +3.14, +2.33, and +3.40, respectively. The Madelung potentials calculated from these formal charges are consistent with the formal charges in the order (Table S3). As similar to the case of oxides and other inorganic ionic solid state compounds,41 the Madelung part of lattice energies (MAPLEs) for Ti7Ga2O6, Ti3GaO, and Ti5Ga3O were almost the sums of those for binary titanium oxides and gallides within differences of 2−3% (Table S4). The O sites of filled-Re3B type and filled-Mn5Si3 type structures have generally been thought to be interstitial sites in which O atoms are octahedrally coordinated by six Ti atoms. The O contents of Ti7Ga2O6 are greater than of these conventional suboxides, and its O atoms are found at the 4and 5-fold Ti coordination sites. Similar O-rich suboxides, including Ti8BiO7,19 Ti8(SnxBi1−x)O7, Ti11.17(Sn0.85Bi0.15)3O10,20 and Ti12−δGaxBi3−xO10,21 which have recently been synthesized, also contain O atoms at Ti 4- and 5-fold coordination sites. It will be of interest to determine whether such O-rich suboxides exist in other systems involving an early transition metal, a post transition metal and oxygen.
Accession Codes
CCDC 1838807, 1838809, and 1838816 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing
[email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*Phone/Fax: +81 22 217 5813. E-mail: hisanori.yamane.a1@ tohoku.ac.jp. ORCID
Hisanori Yamane: 0000-0002-7931-5210 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Takashi Kamaya for performing the EPMA analyses. This work was supported in part by a Grant-in-Aid for Scientific Research (B) (no.16H04494) from the Ministry of Education, Culture, Sports, and Technology (MEXT), Japan.
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REFERENCES
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CONCLUSIONS Single crystals of a new suboxide, Ti7Ga2O6, having a new structure type, were synthesized with a Bi flux, in addition to the single crystals of filled-Re3B type Ti3GaO and filled-Mn5Si3 type Ti5Ga3O suboxides, all within the Ti−Ga−O ternary system. In the crystal structures of these compounds, O and Ga atoms are separately contained in Ti polyhedra. The O atoms are in distorted pyramidal 5-fold and tetrahedral 4-fold coordination sites in Ti7Ga2O6, which has the highest O content among the suboxides of this system, while the O atoms G
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