Hi Adal,
It is mostly related to multiple valence states of the ions. Ti has valence 3+ and 4+, molybdum has valences 3,4,6. In mixed ion oxides it is known (e.g. spinels like Fe3O4) you have both Fe2+ as Fe3+. An electron can jump from Fe2+ to a Fe3+ ion and this is one of the conduction mechanisms. If you add Mo you even have more possibilities and electrons can also jump from Ti3+ to Mo3+. It becomes soon very complicated since some of these processes are competing with each other. Furthermore some ions like to sit either on an tetraeder or octaeder position in the crystal structure which makes them energetically different. There is an abundant literature on e.g. Spinels regarding these conduction mechanisms. Some are n-type conduction, some are p-type like conduction, some switch from N to P-type depending on the temperature range (meaning a competing mechanism takes over above or below a certain temperature). This defect chemistry conduction is a very interesting research field. I cannot say what is the mechanism in your specific case but that also is determined by the way you make it. If you have for instance sub-stoichiometric oxygen content you will change the Ti3+/Ti4+ ratio and that has an affect on the electrical behaviour. Hope this answer will help you further.
kind regards,
Peter
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