Two-Step Structure Phase Transition, Dielectric Anomalies, and Thermochromic Luminescence Behavior in a Direct Band Gap 2D Corrugated Layer Lead Chloride Hybrid of [(CH ₃ ) ₄ N] ₄ Pb ₃ Cl ₁₀

A direct band gap 2D corrugated layer lead chloride hybrid, [(CH ₃ ) ₄ N] ₄ Pb ₃ Cl ₁₀ (1), shows analogous topology to the {Mg ₃ F ₁₀ ⁴⁻ } _∞ layer in Cs ₄ Mg ₃ F ₁₀ , and with the (CH ₃ ) ₄ N ⁺ cations locating in the inorganic layer voids and between the interlayers. Two reversible structural phase transitions occur in 1 at 225/210 K and 328/325 K upon heating/cooling, respectively. On going from the low- to intermediate-temperature phase, the space group changes from P2 ₁ /c to Cmca, and the crystallographic axis perpendicular to the layers is doubled with the order–disorder transformation of (CH ₃ ) ₄ N ⁺ cations between the interlayers. The intermediate- and high-temperature phases are isomorphic with similar cell parameters and packing structure; their main difference concerns the disorder degree of the (CH ₃ ) ₄ N ⁺ cations between the interlayers. The two-step structural phase transitions lead to dielectric anomalies around the corresponding T _c . Interestingly, 1 shows multiband emission, originating from the recombination of exciton and emission of defects. Moreover, 1 exhibits divergent thermochromic luminescent features around the T _c on the intermediate to low temperature transition.