Magnetic and electric studies of a new Co(II) perovskite-like material

Structural phase transitions in the perovskite-like material [(CH4)(12)(NH3)(2)]COCl4 have been observed using differential thermal scanning. The material shows an order-disorder transition at T-1 = 396 +/- 5 K with entropy, (DeltaS(1)) = 12.8 J/mole/K. A "chain melting" transition with a major endothermic peak at T-2 = 337 +/- 3 K and a minor one at T' = 316 +/- 2K, has total entropy DeltaS = 28 J/mole/K. At low temperatures, the transitions at T-3 = 298 +/- 3k and at T-4= 188 +/- 3K, have entropies of DeltaS(3) = 14.4 J/mole/K and DeltaS(4) = 2.6 J/mole/K respectively. AC magnetic susceptibility in the temperature range 78-290 K, in a magnetic field of 160 A/m and at a frequency of 320 Hz is presented. The results indicate changes in symmetry at 188 K. Dielectric permittivity has been studied as a function of temperature in the range 300-430 K and frequency range (60 Hz-100 kHz), confirming the observed transitions. The dielectric permittivity reflects rotational and conformational transition for the material. The variation of the real part of the conductivity with temperature is thermally activated with different activation energies in the range of ionic hopping. The temperature dependence of the dc conductivity and that of the ions hopping rate have indicated that the concentration of mobile ions is independent of temperature. The dependence of the conductivity on frequency follows the universal power law, sigma(T) = sigma(dc) + A(1)(T)omega(s1(T)) + A(2)(T)omega(s2(T)) in the temperature range 340 K < T < 390 K. Values 0 < s(1) < 1 dominate at low frequency and correspond to translational hopping motion and values 1 < s(2) < 2 dominate at high frequencies and correspond to well localized hopping and/or reorientational motion. For T > 396 K, the AC conductivity was fitted to sigma(T) = sigma(dc) + A(1)(T)omega(s1(T)) with 0 < s < 1. Comparison with the corresponding Cu-containing material is discussed.