(1)A(1)REVERSIBLE-ARROW-T-5(2) SPIN TRANSITION IN THE SOLID-PHASES OFFEXNI1-X(ATR)(3)(NO3)(2) (ATR = 4-AMINO-1,2,4-TRIAZOLE)

The effect of Fe2+ substitution by Ni2+ in the complex of iron(II) nit rate with 4-amino-1,2,4-triazole Fe(ATr)(3)(NO3)(2) on the character o f the (1)A(1) reversible arrow(5)T(2) spin transition (ST) is studied by magnetic susceptibility and calorimetry methods. Solid phases of Fe xNi1-x(ATr)(3)(NO3)(2) (0.1 less than or equal to x less than or equal to 0.9) were synthesized. The temperature dependences of the effectiv e magnetic moment were measured in the range of 78-360 K. Heat capacit ies wee measured in the range of 210-340 K for 0.1 less than or equal to x less than or equal to 0.5 and in the range of 230-340 K for 0.6 l ess than or equal to x less than or equal to 0.90. As x decreases, the transition temperature (T-C), hysteresis (Delta T-C), and transition enthalpy (Delta H) decrease and the ST is leveled. The results are com pared with the data obtained previously for the solid phases of FexZn1 -x(ATr)(3)(NO3)(2) (0.01 less than or equal to x less than or equal to 0.8). The dependence mu(eff)(T) is analyzed theoretically in terms of both the domain model and the spin equilibrium model.