NATURE OF THE OMEGA TRANSFORMATION* K E. COOK? The symmetry change associated with the b.c.c. to omega transformation is examined using the Landau criterion for a second order reaction. The ordering parameter 11 is defined in terms of the transformation displacements such that the structure defined by + rj is omega. The structure given by -rl, however, is not omega. As a result the reaction cannot occur by a second order reaction aecordmg to the Landau criterion. The -Q structure, called anti-omega, is described by a collapse of the nearest neighbor (Ill> planes to every fourth {ill ) plane of the b.o.e. parent lattice. The resulting free energy expression as a function of n is AF = Aq2+ Bq3 + Crf + .s., where B < 0 since 9 > 0 is the stable low temperature phase. Above the transformation temperature the term B < 0 creates a metsstable state in the phonon structure at the omega wave vector, k' = (2/a&)e where e is a unit vector in the (111) direction. A consequence is that a dynamic phonon having this wave vector oan interact with itself and become "pinned" in the static omega position. The transformation of the b.c.c structure to omega follows as a result of the anharmonic softening given by the B < 0 term. Tbe equilibrium separation between the collapsing pairs of (I 11 f planes should decrease as the temperature is lowered. SATURE DE LA TRA~SFOR~~A~O~ OMEGA Le changement de symetrie associe 8, la transformation b.c.c. --+ omega est 6tudie a l'aide du critere de Landau pour une reaction du second ordre. Le parametre de mise en ordre q est defini 8, l'aide des d&placements au cours de la transformation de telle facon que la structure d&inie par +r) soit omega. La structure donn6e par --)I, cependant, n'est pas omega. 11 en resulte d'aprits le critere de Landau que la reaction ne peut pas se produire par une reaction du deuxieme ordre. La structure -?I, appelee antiomega, peut etre d&rite par un rapprochement de deux plans {III} cons&utifs tous les quatre plans {III} du reseau C.C. d'origine. L'expression de I'energie libre en fox&ion de 17, est alors: