Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4 THE SOLID-SOLID INTERFACE 255Any solid state reaction, however complex, must resolve itself into interactionsbetween pairs of solid phases, the elementary processes occurring successively orsimultaneously to give a variety of intermediate and final products. Because theentropy change is small, all solid state reactions are exothermic. This propertyforms the basis of the ‘heating curve’ method for detecting reactivity in solidmixtures.Difision in solids is necessarily linked with reaction in solids; this topic hasbeen reviewed by BarrerJJ3 and by J ~ s t General ~ ~ ~ reviews . of solid state reactionshave been given by Huttig335, CohnJJ6 and by WelchJ3’.4.2 X-RAY CRYSTALLOGRAPHYThe technique of determining the extent of a solid state reaction by X-ray powderphotographs was pioneered by Jander’ 38. Basically, the method consists of detectingthe presence of a crystalline phase by its characteristic X-ray pattern. Theamount of the phase present is determined from the intensity of the X-ray lines.Its disadvantage is that many products in solid state reactions are poorly crystallisedand consequently difficult to detect. Furthermore, the presence of poorlycrystallised material is likely to cause excessive scattering, thus preventing thedetection of phases present in small concentration during the initial stages ofreaction.A more direct application of X-ray diffraction is the measurement of changesin X-ray patterns at elevated temperatures, whilst the reaction is occurring. Apreliminary examination at a number of temperatures serves to pinpoint thetemperature of reaction and to determine the change in the cell dimensions causedby thermal expansion. The rate of reaction can then be followed from the rate ofchange in intensity of the diffraction lines at the reaction temperature. Weiss andRowlandJJg9 340 have devised a technique for oscillating the goniometer acrossa selected X-ray diffraction maximum. This scanning technique permits repetitiveautomatic examination of the beam, and changes in the intensity due to latticeexpansion can be detected. The limits of the goniometer travel are set as narrowlyas possible, taking into account any change in the angle of the diffracted beamwhich occurs due to lattice expansion.4.3 MICROSCOPYThe techniques of opticalJ4’ and electron microscopy have been applied tosolid state reactions, but it is difficult to obtain more than semi-quantitative results.Wagenblast et ~ 1 . have j ~ studied ~ the formation and growth of carbon precipitationin iron by electron microscopy. The polymerisation of crystalline trioxaneReferences pp. 270-278