J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 1 9, 2 0 0 0, 2075 – 2076

New orthorhombic titanium oxide, TiO1.94
I. TSUYUMOTO, H. UCHIKAWA
Department of Environmental Systems Engineering, Kanazawa Institute of Technology, 7-1 Ohgigaoka,Nonoichi, Ishikawa 921-8501, Japan
E-mail: tsuyu@neptune.kanazawa-it.ac.jp

The structure and electrical properties of the titaniumoxides, having the composition of Tin O2n−1 wheren is 2–10, have been the subject of much previous
research [1–3]. Most of these compounds exhibit asemiconductor-metal transition at a critical temperature
Tc . This transition accompanies a significant change in
optical properties, i.e., light is transmitted through asemiconductor phase below Tc and reflected by a metal
phase above Tc . The Tc s of Tin O2n−1 have been reported
to be 660 K, 450 K, 150 K, 125 K, and 133 K for n = 2, 3,4, 5, 6, respectively. Materials, having Tc around room
temperature, can be applied to thermochromic windows
which shade radiation from the sun above Tc . Thus, titanium oxide, having Tc around room temperature, ishighly desirable. As for some other compounds suchas VO2 and V1−x Mx O2 (M = W or Mo), trial applications to thermochromic windows have been reported[4, 5]. However, these include deleterious heavy metal,
vanadium and may cause environmental pollution. Titanium oxide is known to act as a photocatalyst which
decomposes deleterious chemical species. It can be expected that thermochromic windows using titanium oxide would be an environment-friendly material whichkeeps mild room temperature automatically and at thesame time decomposes harmful chemical species. In
this study, we focus on synthesis of new titanium oxidewith reduced form TiO2−δ , aiming at Tc around room
temperature. We present here a new orthorhombic titanium oxide with the composition of TiO1.94 .
Starting material was TiO2 powder with anatase form
(reagent grade). After grinding in an agate mortar, thesample powder was transferred in an alumina boat andplaced in a quartz tube. After purging the tube completely with N2 gas, H2 gas was introduced. Heatingwas performed in a H2 100% stream at 600 ◦ C, 800 ◦ C,and 950 ◦ C for 1 h, respectively. The temperature wasincreased at 15 ◦ C/min. For comparison, the same experiment was performed in a N2 100% stream.
Crystalline phases were determined using an X-ray
diffractometer with monochromatized Cu Kα radiation.
T A B L E I Compounds derived from anatase titanium oxide
Heating conditions
H2 100% 1 h

N2 100% 1 hProducts
600 ◦ C
800 ◦ C950 ◦ C

AnataseAnataseDark blue TiO1.94 (New)600 ◦ C
800 ◦ C950 ◦ C

AnataseAnataseRutile

C 2000 Kluwer Academic Publishers
0261–8028 °As shown in Table I, a dark blue titanium oxide wasformed in H2 at 950 ◦ C. We found that this is a new crystalline phase, which has not been reported so far. Because the products at 800 ◦ C and 600 ◦ C were the sameas the starting materials, the transition of the crystal
structure is considered to occur from 800 ◦ C to 950 ◦ C.
In contrast, the rutile TiO2 was obtained in N2 at 950 ◦ C.
This indicates that the removal of oxygen atoms inT A B L E I I X-ray powder diffraction data for orthorhombic TiO1.94 .
Lattice parameters a = 0.938 nm, b = 0.919 nm, c = 0.588 nmhkl

dobs (nm)dcal (nm)I /I0 obs

201021
121
220
300
030
002
131
230
202
022
400
040
222
330

0.36730.3617
0.33540.32660.31760.30800.29340.26210.25590.2506
0.2492
0.23600.23220.22040.21770.21200.20860.20630.20100.17830.17000.16850.16370.15320.14710.14510.14380.13890.13820.13710.13620.13270.12500.12370.12080.11940.11750.11590.11510.11000.10800.10430.1035

0.36670.36190.33770.32820.3126
0.30630.29380.26090.25640.24900.24750.23440.22970.21890.2188

21
18
61
21
71
60
14
29
33
65
61
25
22
67
34
18
23
15
12
14
91
100
72
13
45
20
40
38
36
34
35
20
17
15
13
16
22
20
19
30
27
31
26

420
240
501
422
242
440
060
620
004
260
204
024
602
062
224
404
044
424
244
800
080
444
660
624
264

0.20880.20630.17870.17020.16880.16410.15310.14800.14690.14560.14020.13990.13800.13580.13410.12450.12380.12020.11970.11720.11490.1094
0.10940.10430.10342075

Figure 1 Powder X-ray diffraction pattern of new orthorhombic titanium oxide, TiO1.94 .

reducing atmosphere is essential for the formation of
the new crystalline phase. Powder XRD pattern of thenew crystalline phase is shown in Fig. 1. As shownin Table II, the crystal system of the new compound
was found to be orthorhombic and the lattice parameters were calculated as a = 0.938 nm, b = 0.919 nm,
c = 0.588 nm using the least square method. Although
the similar XRD pattern has not been reported so far,it seems that this structure arises from the lack of symmetry in the rutile structure [6].
The composition of the new compound was determined by using thermogravimetry (TG) and differential thermal analysis (DTA). In heating the sample in air
from room temperature to 1000 ◦ C at 10 ◦ C/min, an oxidation from the TiO2−δ to TiO2 took place in the rangeof 430 ◦ C to 570 ◦ C and the weight of the sample wasincreased by 1.17%. This weight change corresponds tothe incorporation of 0.06 mol of oxygen atoms. Therefore, the composition of the new compound was esti-2076

mated as TiO1.94 . Furthermore, the possibility of formation of hydrogen-doped TiO2 (Hδ TiO2 ) can be ruled
out, because the transition from the reduced form to theoxidized form accompanied the weight increase.
Resistivity of the pellet was measured by the DC twoprobe technique. The product was pressed into powder
compact with diameter of 2.5 mm, reheated in H2 at950 ◦ C for 1 h, and used for the measurement. The resistivity was ca. 16 Ä cm. This value includes possible
ambiguities due to its strong dependence on the relative
density of pellet. It can be considered that this compound shows electronic conductivity due to the mixed
valence of titanium.
In conclusion, new titanium oxide with orthorhombic structure was formed in hydrogen atmosphere at
950 ◦ C. The composition was found to be TiO1.94 by TG
and DTA measurement. Electronic conductivity can beexpected from the preliminary conductometry.

References
1. R . M A R C H A N D , L . B R O H A N and M . T O U R N O U X , Mat.
Res. Bull. 15 (1980) 1129.
2. R . F . B A R T H O L O M E W and D . R . F R A N K L , Phys. Rev. 187(1969) 828.
3. J . M . H O N I G and T . B . R E E D , ibid. 174 (1968) 1020.
4. I . T A K A H A S H I , M . H I B I N O and T . K U D O , Jpn. J. Appl. Phys.
35 (1996) L438.
5. P . J I N G and S . T A N E M U R A , ibid. 33 (1994) 1478.
6. Joint Committee of Powder Diffraction Standard 03-1122.

Received 3 Mayand accepted 8 June 2000