We want to deposit Gd in MBE by effusion cells (HTEZ - MBE componenten). We tried to use Al2O3 and PBN crucibles. Al2O3 reacts with molten Gd and effusion cell evaporates Al at 1440 C. Evaporation of Al instead of Gd was confirmed by X-ray photoelectron spectroscopy. Pressure is in the limit of 1e-7 Torr. As for PBN, we observe rising of pressure up to 1e-7 near at 1320 C which is close to the melting temperature of Gd. We did not observe any flux, fortunately.
Probably, these materials are OK if the melting temperature is not reached but in our case we do not obtain the necessary flux because of a large source to sample distance.
I just want to ask whether someone had an experience of deposition of Gd by effusion cells. Whar crucible materials were used? Do anyone have any ideas about using Ta or W crucibles to evaporate Gd at temperatures larger than 1320 C?
Dear A. V. Davydenko,
I am surprised that you found Al in the evaporated from Al2O3 crucibles. The melting point of Alumina is higher than 2000 degree C. The alumina crucible for Gd evaporation is highly recommended.
Whether someone used the crucible for Al evaporation before you used for Gd evaporation?
I strongly suggest that you use a brand new one. You need to consider two factors to choose the size of your crucible and current rate and make sure that these parameters fit into your source power rating.
Dear Xuhua Wang,
We were surprised as well. We have obtained steady but nonmagnetic flux. Probably Gd melts with Al2O3, destroys its structure, which leads to rise in O2 presure and evaporation of Al. The crucible was new and clean. It was ordered from manufacter and annealed prior to be filled in with Gd.
If molten Gd comes in contact with alumina it is not particularly surprising that alumina is reduced, because Al reduction potential is much less negative than that of Gd. At 1300C and with a fluid phase present, materials will tend to do what thermochemistry dictates.
Best bet in terms of likely chemical interactions would be gadolinium oxide, but probably you'd have to find somebody skilled in sintering rare earth oxides and place a special order ($$$$). Next best in my opinion would be an yttrium oxide crucible (multiple commercial sources). Yttrium has about the same reduction potential as gadolinium, the oxide is more refractory than alumina, and yttrium is less volatile than gadolinium (or aluminum). If you find unacceptable yttrium contamination you can approach the yttrium oxide crucible supplier and ask about making you a gadolinium oxide crucible because the technology should be similar.
Dear Emanuel,
Thank you for your advice! I have found a scientific group in which the reseachers deposited Gd from Ta crucible. I think we will try Ta (or maybe W) bazed on their successful experience. What do you think about these refractory materials from the position of thermochemistry?
Dear Dr. Davidenko,
I think Ta is a very good choice, better than the oxides we discussed or any others we didn't.
Ta appears to have no significant interaction with Gd - see attachment - and has extremely low vapor pressure.
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