Sentence:  **f appeared to be well suited for the study of these matters , since it is a normal paramagnet , with three unpaired electrons on the chromium , its <WSD>crystal</WSD> structure is very simple , and the unknown position of the hydrogen in the strong **f bond provides structural interest . 
Meaning of the word marked with <WSD> tags:  a solid formed by the solidification of a chemical and having a highly regular atomic structure