The crystal structure of bis-(methyltriphenylphosphonium) tetrabromocobaltate (II), [(C 19 H 18 P) 2 CoBr 4 ] is determined: M r = 933.203, monoclinic, P2 1 , a = 9. 6977 (3) ร
, b = 12.5547 (4)ร
, c = 16.4503 (6)ร
, ฮฒ = 105.603 (2)ยฐ, V = 1929.04 (11)ร
3 , Z = 2, D x = 1.607 Mg m -3 , T = 298 K. Differential thermal analysis at high temperatures shows three endothermic peaks characterizing four phases, with onset temperatures at T 1 = 313ยฑ2 K, T 2 = 320ยฑ4 K and T 3 = 360ยฑ1 K. The structural instability detected via the temperature dependence of permittivity at T 1 is ascribed to order-disorder transition associated with cation dipole reorientation. Permittivity and ac conductivity studies as a function of temperature (295 K-375 K) and frequency (0.11 kHz < f <100 kHz) are presented. The results indicate the importance of the cation size and shape on the phase transitions in the system. Bulk conductivity behavior is thermally activated. The associated activation energies are in the range 2.9 to 1.0 eV depending on the temperature regime. Two contributions to the ac conductivity, one dominating at low temperatures and high frequencies which are characterized by superlinear frequency exponent and the second dominates at high temperatures characterized by a sublinear frequency exponent. The behavior is interpreted in terms of the jump relaxation model.