allotrope
Concise Oxford English Dictionary © 2008 Oxford University Press:
allotrope /ˈalətrəʊp/
▶noun Chemistry each of two or more different physical forms in which an element can exist (e.g. graphite, charcoal, and diamond as forms of carbon).
– derivatives
allotropic adjective,
allotropy /əˈlɒtrəpi/ noun.
allotropic adjective,
allotropy /əˈlɒtrəpi/ noun.
– origin C19: from Gk allotropos ‘of another form’, from allo- ‘other’ + tropos ‘manner’.
'allotrope' also found in these Oxford entries:

