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The cooperative effect of the ammonias in X(N03)26NH3 (X = Ni,Zn.Cd) was investigated by EPR.In Ni(N0 3 h 6NH 3 • the line width of the single EPR absorption line undergoes a sudden broadening at T c = 243 K. This effect is probably a result of a cooperative freezing of the degrees of freedom of rotation of the ammonias. giving the appearance of a crystal field on the Ni + + ions. The crystal field does not split the single line. probably because strong exchange effects assemble the lines. The crystal field parameter D = 0.425 cm-1 was evaluated by applying the Anderson and Weiss theory for the exchange narrowing of the linewidth. Small quantities of the Ni + + ions in Zn and Cd salts do not exhibit strong exchange effects. So the single EPR absorption line above T c goes to a split line below T c' For Ni: Zn(N03)26NH3. T c = 231 K and D = 0.606 em-I. For Ni: Cd(N03)26NH3. T c = 198 K and D ~ 0.3 em-I. Hysteresis in T c was also observed for all salts.

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