Chemical bonding in phase-change chalcogenides.

Almost all phase-change memory materials (PCM) contain chalcogen 
atoms, and their chemical bonds have been denoted both as 
``electron-deficient'' [sometimes referred to as 
``metavalent''] and ``electron-rich'' [``hypervalent'', multicentre]. The latter 
involve lone-pair electrons. We have 
performed calculations that can discriminate unambiguously between 
these two classes of bond and have shown that PCM have electron-rich, 
3c-4e (``hypervalent'') bonds. Plots of charge transferred between 
($ET$) and shared with ($ES$) neighbouring atoms cannot on their own 
distinguish between ``metavalent'' and ``hypervalent'' bonds, both of 
which involve single-electron bonds. PCM do not exhibit ``metavalent'' 
bonding and are not electron-deficient; the bonding is electron-rich of the 
``hypervalent'' or multicentre type.&#xD.