MATS 3011 Lecture : Lecture 492014.pdf

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15 Jul 2014
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If %ic > 50% or xa - xb > 1. 7, then mostly ionic. 2 factors that control structure for ionic-bonded ceramics: charge neutrality, ratio of ionic radii of cation and anion (rc/ra) ex: caf2 ---> 20ca: 1s22s22p63s23p64s2. Ca wants to give 2 electrons and f wants to take 1 electron need 1 ca2+ and 2 f- -------> caf2; Some elements are happy w/ multiple valencies: ceramics composed of fe and o. Feo (fe2+ and o2-) fe2o3 (fe3+ and o2-) fe3o4 (has mix of 2+ and 3+ fe and o2-) 2nd factor 2) relative size of ions (rc/ra) Matsci page 1 larger anions smaller cations play around w/ geometry. @ which coordination = 4 if go beyond first one, can change coordination # to 5 (increase) 2nd = critical configuration to figure out coordination # for given radius ratio @ which cation -anion contact is established certain ratios of radii have certain corresponding coordination # (rc/ra)

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