The crystalline bis(n-undecylammonium)tetrachlorocadmiumate (C₁₁H₂₃NH₃)₂CdCl₄(s) [abbreviated as C₁₁Cd(s)] was synthesized. The crystal structure and composition of the complex were determined by single crystal X-ray diffraction, chemical analysis and elementary analysis. The lattice potential energy was calculated to be U_POT＝908.18 kJ·mol^-1 from crystallographic data. Low-temperature heat capacities of the complex were measured by a precision automatic adiabatic calorimeter over the temperature range from 78 to 395 K. The experimental results show that there are two continuous phase changes appearing in the temperature region. The peak temperatures, molar enthalpies and entropies of two phase transitions for the complex were determined to be: T_trs,1＝(321.88±0.07) K, Δ_trsH_m,1＝(37.59±0.17) kJ·mol^-1 and Δ_trsSm,1＝ (117.24±0.12) J·K^-1·mol^-1 for the first peak; T_trs,2＝(323.81±0.30) K, Δ_trsH_m,2＝(12.42±0.02) kJ· mol^-1 and Δ_trsSm,2＝(38.36±0.09) J·K^-1·mol^-1 for the second peak. Two polynomial equations of the heat capacities as a function of the temperature were fitted by least square method. The smoothed heat capacities and thermodynamic functions relative to 298.15 K of the complex were calculated with an interval of 5 K based on the fitted polynomials.

Key words: bis(n-undecylammonium)tetrachlorocadmiumate, crystal structure, lattice potential energy, low-temperature heat capacity, solid-solid phase transition, thermodynamic function