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I have rubidium vapor inside of the vacuum chamber. Inside the vacuum chamber, there are two flat stainless steel coated with aluminum oxide at temperature ~100 °C.

Since rubidium is active, I am worrying that it will interact with the aluminum oxide $\ce{Al2O3}$ coating. Any ideas?

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    $\begingroup$ Though I can't answer definitively, Al2O3 is used in sodium-vapor lamps that have a long lifetime. A guess would be that the surface does react, forming rubidium aluminate, but that it doen't penetrate rapidly. $\endgroup$ – DrMoishe Pippik Feb 26 at 0:37
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    $\begingroup$ I don't know about rubidium aluminate but a certain rubidium tetranitroaluminate does exist. $\endgroup$ – Nilay Ghosh Feb 26 at 4:25
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    $\begingroup$ Rubidium aluminate with formula $\ce{Rb_6Al_2O_6}$ $\endgroup$ – TheSimpliFire Feb 26 at 7:32
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As suggested in the comments, rubidium aluminates do exist. Aluminates such as $\ce{RbAlO2}$ and $\ce{Rb6Al2O6}$ are prepared at higher temperatures (above 550 °C [1]) or even from the melt.

However, alkali metal vapors at elevated temperatures act as reducing agent, so I wouldn't expect aluminates anyway. On the other hand, aluminides such as $\ce{RbAl}$ (Zintl phase) are formed from the melt at much higher temperatures (above melting point of aluminium) and pressures exceeding atmospheric.

Since the corundum-coated plate is supposed to perform in vacuum under mild temperatures, I think it shouldn't be affected by rubidium vapors. There is a possibility of coating embrittlement over time as rubidium diffusion occurs, but I wouldn't expect it to be an issue.

References

  1. Schläger, M.; Hoppe, R. Darstellung und Kristallstruktur von $\ce{K6[Al2O6]}$ und $\ce{Rb6[Al2O6]}$. Zeitschrift für anorganische und allgemeine Chemie 1994, 620 (5), 882–887. https://doi.org/10.1002/zaac.19946200522.
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    $\begingroup$ Maybe not aluminates alone, but you could get aluminates + intermetallics, as, $\ce{4Rb + 2Al2O3->3RbAlO2 + RbAl}$. $\endgroup$ – Oscar Lanzi Feb 28 at 10:42

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