New cubic superstructure of titanium monoxide with double structure imperfection

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Abstract

A cubic model of Ti5O5 (Ti5■O5□=Ti9018O9018) superstructure of nonstoichiometric titanium monoxide TixOz with double structure imperfection is proposed for the first time on the basis of symmetry analysis. The unit cell of the cubic superstructure Ti5O5 has a triple lattice constant as compared with that of the basic disordered B1 structure of TixOz monoxide and belongs to the space group Pm3¯m with regard to symmetry. The disorder–order TixOz (space group Fm3¯m)—Ti5O5 (space group Pm3¯m) phase transition channel includes 75 superstructure vectors of seven stars {k10}, {k7}, {k6(1)}, {k6(2)}, {k4(1)}, {k4(2)}, and {k1}. The distribution functions of Ti and O atoms on the sites of the model cubic superstructure Ti5O5 were calculated. Diffraction investigation of annealed ordered titanium monoxide TiO1.087 confirms the existence of the cubic (space group Pm3¯m) ordered phase Ti5O5. The cubic (space group Pm3¯m) superstructure Ti5O5 is shown to be a high-temperature structure with respect to the known monoclinic (space group C2/m) superstructure of the same type.

Graphical abstract

A cubic (space group Pm3m) model of new Ti5O5 (Ti5■O5□=Ti9018O9018) ordered phase of nonstoichiometric titanium monoxide TixOz with double structure imperfection.

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Highlights

► Cubic model of Ti5O5 (Ti5■O5□=Ti9018O9018) superstructure. ► The disorder–order TixOz (space group Fm3¯m)—Ti5O5 (space group Pm3¯m) phase transition channel. ► Distribution of Ti and O atoms and vacancies ■ and □ in the unit cell of cubic Ti5O5 ordered phase. ► Admissible sequences of transformaions connected with the formation of cubic and monoclinic Ti5O5 ordered phases.

Introduction

The nonstoichiometric cubic titanium monoxide TiOy with the basic cubic B1 structure belongs to the group of strongly nonstoichiometric interstitial compounds [1], [2], [3], [4] and has a wide homogeneity interval from TiO0.80 to TiO1.25. This unique compound contains a large number of structural vacancies (crystal lattice sites which are not occupied by atoms) both in nonmetallic and metallic sublattices. With allowance for the content of structural vacancies in each sublattice, the composition of titanium monoxide should be written as TixOz≡TiOy or Tix1−xOz1−z≡TiOy, where y=z/x, □ and ■ are structural vacancies in nonmetallic (oxygen) and metallic (titanium) sublattices, respectively.

Depending on the oxygen content and heat treatment conditions, the distribution of atoms and vacancies in the crystal lattice of TiOy can be disordered or ordered [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. The most interesting superstructure of titanium monoxide is the ordered phase Ti5O5 (Ti5■O5□) with the same number of vacancies in the titanium and oxygen sublattices. The authors [5], [6], [7], [9], [10] proposed a monoclinic (space group C2/m (A12/m1) (C2h3)) structure model for this ordered phase, although already in one of the first works [8] a cubic structure was suggested, which is stable at temperatures between 1250 and 1520 K and belongs to the space groups Fm3¯, F432, F4¯3m, or Fm3¯m. However no cubic model of the Ti5■O5□ superstructure has been actually proposed.

In view of prospective application in photocatalysis and possible semiconductor-metal transitions, in the last 3 or 5 years the interest in such phases of the TiO system as titanium di- and monoxide rose immensely, and their electronic structure calculations were reported. For titanium monoxide, the experimental and theoretical studies were and are performed either for the basic cubic structure B1 without consideration of vacancies in the titanium and oxygen sublattices [12], [13], [14], [15], [16], or for disordered monoxide TiOy with double structure imperfection [17], [18], [19], [20], [21], or for the hypothetical ordered titanium monocarbide Ti0.75O0.75 [22], or for the monoclinic superstructure Ti5O5 [17], [23]. These authors are apparently unaware of other possible types of ordering of titanium monoxide.

In this connection, in the present work a cubic model of the Ti5O5 (Ti5■O5□) superstructure of nonstoichiometric titanium monoxide TixOz with double structure imperfection is proposed for the first time on the basis of symmetry analysis and notion of the disorder–order transition channel.

Section snippets

Model of the cubic superstructure Ti5O5

It is convenient to model the cubic Ti5O5 (Ti5■O5□) superstructure with vacancies ■ and □ in metallic (titanium) and nonmetallic (oxygen) sublattices, respectively, for each sublattice separately. In compounds with a basic B1 structure, both sublattices are face-centered cubic (fcc), they are displaced relative to one other by a vector [½ ½ ½]B1 and have a point symmetry group m3¯m.

Let us consider first the A5B-type superstructure in the metallic sublattice. Suppose that in the disordered solid

Relationship between monoclinic and cubic Ti5O5 superstructures

Analysis showed that in the disordered nonstoichiometric titanium monoxide TiO1.0 (Ti0.833O0.833) with an equal number of vacancies in the titanium and oxygen sublattices, along with the familiar monoclinic (space group C2/m) ordered phase Ti5■O5□, a cubic (space group Pm3¯m) ordered phase of the same composition can be formed. A question arises whether these ordered phases are alternative or an order–order transition is possible between them. If such a transition is possible, then in what

Conclusion

Simulation of the cubic superstructure Ti5O5 (Ti9018O9018) of the titanium monoxide and analysis of experimental data on X-ray and electron diffraction of annealed TiOy samples showed that the cubic (space group Pm3¯m) ordered phase Ti5O5 exists in the temperature range from ∼1200 to 1300 K. The unit cell lattice constant of the cubic superstructure Ti5O5 is equal to the triple lattice constant of the basic disordered phase TiOy with a B1 structure. The channel of the disorder–order transition

Acknowledgments

The work is supported by the Russian Foundation for Basic Research (Grants nos. 13-03-00077а and 13-03-00164а) and the Ural Division of the RAS (interregional project 12-C-3-1002).

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