# Does aluminum oxide react with rubidium?

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?

• 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. – DrMoishe Pippik Feb 26 '19 at 0:37
• I don't know about rubidium aluminate but a certain rubidium tetranitroaluminate does exist. – Nilay Ghosh Feb 26 '19 at 4:25
• Rubidium aluminate with formula $\ce{Rb_6Al_2O_6}$ – TheSimpliFire Feb 26 '19 at 7:32

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.
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.
• Maybe not aluminates alone, but you could get aluminates + intermetallics, as, $\ce{4Rb + 2Al2O3->3RbAlO2 + RbAl}$. – Oscar Lanzi Feb 28 '19 at 10:42