Hi...Laghrib,.....well as known that TiO2 anatase has a tetragonal structure (a=b but not c,  and TiO2 brookite has a orthorhombic structure (a is not equal to b and c). Both structure is changeable (based on their thermodynamic point of view). So, doped by Ni (using organic or inorganic compounds ? and operational condition : pH etc.,)  means that the Ni cations inserted to TiO2 structure or layer (calcined at 500oC) and then the cell-parameter a,b, and c changed to minimize the steric effect so that tetragonal (anatase) changed into orthorhombic (brookite).

Hope this information will be useful

Rudy 

Hi...Laghrib,.....well as known that TiO2 anatase has a tetragonal structure (a=b but not c,  and TiO2 brookite has a orthorhombic structure (a is not equal to b and c). Both structure is changeable (based on their thermodynamic point of view). So, doped by Ni (using organic or inorganic compounds ? and operational condition : pH etc.,)  means that the Ni cations inserted to TiO2 structure or layer (calcined at 500oC) and then the cell-parameter a,b, and c changed to minimize the steric effect so that tetragonal (anatase) changed into orthorhombic (brookite).

Hope this information will be useful

Rudy 

Dear colleague Laghrib Souad ,

Have a nice time and good day.

Herein fined an attached review article about TiO2 it may help you and be useful.

Yours

A. EL-Denglawey 

In general anatase (and brookite too) is a metastable phase which will form the equilibrium rutile phase once sufficient energy is given to the system. Dopants tend to significantly effect this transition either by enhancing it or impeding (kinetically stabilising the anatase). The formation of anatase alongside brookite has been reported to occur with brookite-TiO2 as a secondary phase I believe, but I think the transformation between the two metastable phases usually proceeds brookite to anatase or brookite directly to rutile(it might be worth searching for more reports in the literature). If you are able to completely transform bulk anatase to a single phase brookite by including a nickel dopant, this would be quite an interesting finding and, I believe, a very publishable result. For the case of thin films(which might be your case) there is likely to be some lattice strain caused by the interface with the substrate, which may play an important role in forming brookite, which consists I believe of distorted TiO6 octahedra sharing edges (this is reported more accurately somewhere I am sure). Perhaps as you heat your film, decomposition of residuals causes it to become thinner and more distorted, causing the formation of brookite. The role of Ni in all of this would need to be considered by evaluating lattice position of the dopant (e.g. interstital vs substitutional) and coordination. The formation of fully dense single phase brookite films might be a novel finding.

Hope that helps. I am curious to see where your research takes you.

Best regards,

Dorian