SOLID-STATE REACTIONS - RIGOROUS TEST OF THE HEDVALL EFFECT

The experimental testing of the comparatively rapid diffusion of ions during a transient period of disorder (the Hedvall effect) has met with mixed results and conclusions because in specific cases alternative explanations for the observed beginnings or increases in reactivity in the temperature region of a solid1-solid2 transition could be advanced. The criticisms were generally based on the possibility of a liquid phase, either due to moisture or to a eutectic phase. Sodium and calcium carbonate react below 500°C to form Na2Ca(CO3)2, which has multiple transitions in the temperature range of reaction. Na2CO3 also has two transitions. If disorder during a crystallographic phase transition enhances the reaction rate because of easy diffusion, these multiple transitions should enable a specimen cycled through the transitions to react to a greater extent than a specimen held just above the highest transition temperature for the same length of time. For several preparations, the isothermally heated material reacted to a greater extent than the cycled material, so the hypothesis of increased reactivity due to diffusion during solid-solid transitions is disproven. © 1979 American Chemical Society.