Titanium crystal structures, lattice parameters

It has been established that under intense milling in a planetary mill in a nitrogen atmosphere, titanium hydride may decompose with formation of defected structure characterized by partly non-occupied tetrahedron voids in a face-centered cubic sublattice of titanium atoms. This effect is accompanied by decrease in the crystal lattice parameter of titanium hydride subjected to intense milling for 60 min. 

The titanium aluminide TiAl - often designated as y phase - crystallizes with the tetragonal LIq structure (CuAu-type) which is shown in Fig. 1. The LI o structure results from ordering in the f.c.c. lattice (Al), i.e. it is basically a cubic structure which is tetragonally distorted because of the particular stacking of the atom planes, as is seen in Fig. 1. The ratio of the lattice parameters c and a is cja = 1.015 at the stoichiometric composition and the density is 3.76 g/cm (Kim and Dimiduk, 1991), whereas for TiAl-base alloys the range 3.7-3.9 g/cm is given (see Table 2). This density is still lower than that of TijAl and has made the titanium aluminides most attractive for materials developments. 

Its crystal structure has varying proportions of both titanium and oxygen vacancies. Density and y-ray lattice parameter measurements have shown that a third of the oxygen sites are vacant in TiO, a quarter of the titanium sites are vacant in TiOj j, and even in stoichiometric TiO about 15% of both sites are vacant. Above 990 C, the vacancies are arranged randomly, giving rise to diffraction patterns typical of the cubic NaCl-type structure. 

In the X-ray diffraction powder pattern of the R = 3 compound (in aqueous medium) most of main lines fall at similar 26 values as those of TiOF2 published by Vorres and Donohue. These authors successfully synthesized this compound using two different routes hydrolysis of titanium tetrafluoride or titanium trifluorochloride or reaction of aqueous or anhydrous HF with Ti02. The oxyfluoride crystallizes in the Pm-3m space group with a lattice parameter of 3.80 A, and a powder density of 3.06 g.cm with one TiOF2 molecule per unit cell. The crystal structure is built up of comer-sharing Ti(X)g (X = O, F) octahedra, where the O and F atoms are randomly distributed. 

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