What is that silver layer at the bottom part of the arctube of my R7s 70W green thallium iodide lamp?
It don't gets liquid when the lamp is hot, and it don't moves when I put the lamp in other positions.
What it is?

Does it also not evaporate/sublimate with the lamp warming up?
To me it look strange for something like this to be present in the main path of the light output during normal operation, but I would not be surprised too much if the fill will condense in a similar way when the lamp cools down. But in such case I would expect it to evaporate/sublimate out, so in other words disappear, with the lamp warming up to normal operating temperature.

During warm up, I can see how the thallium evaporates, but this silver layer don't evaporates or melting at all during warm up.

Thallium bromide lamps seem to blacken far more than other metal halide lamps, and usually in the coldest spots behind the electrodes.

I have never analysed the coating to see what it is, but suspect it may be tungsten.  There is perhaps a transport reaction that causes tungsten migration from the electrodes.  Or that the thallium becomes dissociated and leaves a metallic deposit, whose vapour pressure is too low to re-evaporate.  Silicon is very often involved in these reactions, and may be present.  Although free silicon usually dissolves into the electrodes (lowering their melting point and exacerbating the problem), it would be unusual to see that on the wall.

I can think of only one man who may know the answer : Dr Wim van Erk, formerly of Philips Eindhoven.  His career was spent studying chemical transport mechanisms in metal halide lamps and he has written countless papers on the subject - but I do not see him any more since retirement.  Maybe he could be contacted via other means.

James, do you know the reason bromide is used and not iodide?

Here is a closeup video of the lamp warming up.

The bromides are more chemically active than iodides and help to protect the quartz from attack by recombining with corrosive metal vapours further away from the wall.  They are also more active in accelerating the halogen cycle to keep the wall clean from sputtered tungsten in lamps that are sufficiently highly loaded - but also have the disadvantage of being more active in causing beavering of the electrode shank at the cold spot where it passes through the arc tube wall, which can reduce life.

The bromides also have different vapour pressures than iodides, and depending on the metal may be more or less effective in bringing their vapour into the plasma.

Many metal halide lamps contain a finely balanced mix of bromides, iodides, and in highly loaded lamps also the chlorides, to balance each of these processes for optimum initial performance, lumen and colour maintenance, and life.  With some elements it is not easy to obtain the bromides, or it is desired to have a bromine-iodine mixture.  In this case they are added as iodides, and part of the metallic mercury mix is eliminated and dosed as mercuric bromide to get that halogen into the fill via an easier mechanism.  The same is done for lamps where iodine is consumed too rapidly during life, by replacing part of the mercury fill with mercury iodide.  Those are often easy to spot thanks to the intense reddish-orange colour of that salt when cold.