Single-crystal x-ray diffraction (XRD) measurements were conducted on quasi-two-dimensional organic salt ${\alpha }^{\prime }\text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{IBr}}_2$ to investigate the charge ordering (CO) pattern, which remained unresolved by local probes, and the anomalous absence of second-harmonic generation (SHG) signal at intermediate temperature ($160\text{K}<T<200\text{K}$). Our structural analysis at 110 K reveals a noncentrosymmetric horizontal stripe type CO pattern with a charge separation ($\mathrm{\Delta }\rho =|{\rho }_{\text{rich}}-{\rho }_{\text{poor}}|$) of $\mathrm{\Delta }\rho \approx 0.8e$. These findings are consistent with results from ${}^{13}\mathrm{C}\text{−}\mathrm{NMR}$, vibrational spectroscopy, and the observation of finite SHG signal. At intermediate temperature, SHG signal was absent despite the presence of CO, indicating a superstructure with antiparallel polarization. However, there was no direct structural evidence, including phase transition and CO pattern at 200 K. A superlattice structure with lattice doubling along the $c$ axis was identified in the intermediate temperature range ($160\text{K}<T<200\text{K}$). The structural analysis at 180 K confirms that the charge ordered layers are related by an inversion center and exhibit a horizontal stripe type CO pattern, explaining the absence of SHG signal in this phase. Those findings establish three distinct structural and electronic phases in ${\alpha }^{\prime }\text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{IBr}}_2$. Furthermore, the suppression of dynamic ethylene group disorder below 200 K was observed, suggesting a possible contribution to the previously reported Raman spectral broadening of the anion. This offers an alternative explanation to the previous model based on dynamic charge fluctuations.

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• Charge
• Dielectric properties
• Ferroelectricity
• Charge-transfer insulators
• Crystal structures
• Ferroelectrics
• Organic compounds
• Single crystal materials
• Two-dimensional electron system
• Crystallography
• Optical second-harmonic generation
• X-ray diffraction