Phosphorus, elemental hydrogen halides

It is easy to reduce anhydrous rare-earth halides to the metal by reaction of more electropositive metals such as calcium, lithium, sodium, potassium, and aluminum. Electrolytic reduction is an alternative in the production of the light lanthanide metals, including didymium, a Nd—Pr mixture. The rare-earth metals have a great affinity for oxygen, sulfur, nitrogen, carbon, silicon, boron, phosphorus, and hydrogen at elevated temperature and remove these elements from most other metals. 

The Group lA Elements 20.9 The Chemistry of Phosphorus Hydrogen Halides... 

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