Ti 5 O 5 superstructures of cubic titanium monoxide
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Ti5O5 Superstructures of Cubic Titanium Monoxide A. I. Gusev Institute of SolidState Chemistry, Ural Branch, Russian Academy of Sciences, ul. Pervomaiskaya 91, Yekaterinburg, 620990 Russia email: [email protected] Received January 28, 2013
Abstract—A cubic model is proposed for the Ti5O5 (Ti5䊏O5䊐 ≡ Ti90䊏18O90䊐18) superstructure of nonsto ichiometric titanium monoxide TixOz with double imperfection. The unit cell of the cubic Ti5O5 superstruc ture has the threefold lattice parameter of the unit cell of the basis disordered B1 structure of TixOz monoxide and belongs to space group Pm 3 m. The channel of the disorder–order transition, i.e., TixOz (space group Fm 3 m)–Ti5O5 (space group Pm 3 m), 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 over the sites of the cubic Ti5O5 superstructure are calculated. A comparison of the Xray and electron diffraction data obtained for ordered TiO1.087 monoxide with the theoretical simulation results supports the existence of the cubic Ti5O5 super structure. The cubic (space group Pm 3 m) Ti5O5 superstructure is shown to be a hightemperature structure relative to the wellknown monoclinic (space group C2/m) superstructure of the same type. DOI: 10.1134/S1063776113080141
INTRODUCTION Nonstoichiometric titanium monoxide TiOy with basis cubic structure B1 enters in the group of strongly nonstoichiometric compounds and has a wide homo geneity area from TiO0.80 to TiO1.25 [1–5]. This unique compound contains a large number of structural vacancies (lattice sites free of atoms) in both the non metallic and metallic sublattices. With allowance for the vacancy content in each sublattice, the titanium monoxide composition should be written in the form TixOz ≡ TiOy or Tix䊏1 – xOz䊐1 – z ≡ TiOy, where y = z/x and 䊐 and 䊏 are structural vacancies in the nonmetal lic (oxygen) and metallic (titanium) sublattices, respectively. Under standard conditions, titanium monoxide, which formally has equiatomic composi tion TiO, contains about 16.7 at % vacancies in each of the Ti and O sublattices. Therefore, its real composi tion is approximately Ti0.833O0.833 with allowance for the sublattice imperfection. The distribution of atoms and vacancies in the monoxide TiOy lattice can be both disordered and ordered depending on the oxygen content and heat treatment conditions [3–12]. The ordered Ti5O5 phase (Ti5䊏O5䊐) with the same vacancy content in the tita nium and oxygen sublattices is most interesting among the titanium monoxide superstructures. The authors of [6–8, 10, 11] proposed a monoclinic model (space 3 group C2/m (A12/m1) ( C 2h )) for the structure of this ordered phase, and it was repeatedly supported in experiments. However, in one of the first works dealing
with titanium monoxide, Hilti [9] briefly noted the existence of a cubic ordered Ti5O5 phase and did not support this statement experimentally. According to [9], the cubic Ti5O5 phase exists in the temperatur
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