YbGaGe: Zero Thermal Expansion as a Result of an Electronic Valence Transition?

In contrast to classic ZTE materials (ZTE=zero thermal expansion), the ZTE effect in YbGaGe (see crystal structure) is caused by the change in the valency of the ytterbium ion from +2.6 to +2 with decreasing temperature. This change in the valence state compensates for the contraction of the net as the temperature decreases.