MOSSBAUER STUDY OF THE EFFECT OF IRON SUBSTITUTION BY NICKEL ON THE (1)A(1)REVERSIBLE-ARROW-T-5(2) SPIN TRANSITION IN FE(4-AMINO-1,2,4-TRIAZOLE)(3)(NO3)(2)

Solid phases of FexNi1-x,(ATr)(3)(NO3)(2) (0.2 less than or equal to x 1), having a polymer chain structure, were studied by Mossbauer spect roscopy at 295 and 78 K. The dependences of the fraction of the high-s pin form and of the area and parameters of the spectra on the relative content of nickel in the samples were measured As the iron atoms are substituted by the nickel atoms the fraction of the high-spin form, th e ionicity of Fe-N chemical bonds, di; and the degree of distortion of the octahedral environment of iron atoms in the low-spin form increas e. A model explaining these effects is suggested. When the iron atoms are substituted by the nickel atoms an effect of ''softening'' of the vibrational spectrum of the crystal lattice for the low-spin form of t he complex fakes place.