Fig. 2: Thermally induced hydrocarbon order–disorder transitions in (DA)₂MnCl₄ and (NA)₂CuBr₄ at ambient pressure.

Differential scanning calorimetry (DSC) traces for powder samples of (a) (DA)₂MnCl₄ and (b) (NA)₂CuBr₄ at 1 bar with heating (red) and cooling (blue) rates of 2 K min⁻¹. Thermal hysteresis (ΔT_hys) is indicated by the vertical gray bars. Note that ΔT_hys is calculated as the difference between heating and cooling transition onset temperatures, with ΔT_hys = T_tr,heating – T_tr,cooling. Specific volumes obtained from variable-temperature powder X-ray diffraction and He pycnometry measurements are used to determine the volume changes, ΔV, that accompany the order–disorder transition for (c) (DA)₂MnCl₄ and (d) (NA)₂CuBr₄. Variable-temperature crystal structures of the low-temperature (LT) and high-temperature (HT) phases of (e) (DA)₂MnCl₄ and (f) (NA)₂CuBr₄. Note that the LT crystal structures were both obtained at 270 K, while the HT crystal structures were obtained at 330 K and 335 K for (DA)₂MnCl₄ and (NA)₂CuBr₄, respectively. Purple, orange, green, brown, gray, and blue spheres represent Mn, Cu, Cl, Br, C, and N atoms, respectively. H atoms are omitted for clarity. Note that DA chains are disordered over a special position in both the LT and HT phases, while NA chains are modeled with two-part disorder in the LT phase and disordered over a special position in the HT phase. Geometric parameters obtained from single-crystal X-ray diffraction experiments are summarized in Supplementary Tables 14–22.