Sentence:  The **f layers are stacked normal to the [ 111 ] axis with the lower oxygens of one layer directly above the upper <WSD>oxygens</WSD> of the neighboring lower layer , in such a manner that the repeat is every three layers . 
Meaning of the word marked with <WSD> tags:  a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust