ferric vs unyielding

ferric

adj
  • Of compounds of iron in which it has a valence or oxidation number of 3 

  • Pertaining to, derived from, or containing iron. 

unyielding

adj
  • Not giving in; not bending; stubborn. 

How often have the words ferric and unyielding occurred in a corpus of books? (source: Google Ngram Viewer )