Opto-lattice coupling and thermally switchable dielectric transition in [N(CH3)3H]2CdCl4

Abstract

We establish a coherent structure–property framework for the hybrid halide [N(CH₃)₃H]₂CdCl₄ by integrating crystallography, thermal analysis, broadband optics, photodynamics, and dielectric spectroscopy. Powder X-ray diffraction confirms an orthorhombic, non-centrosymmetric phase (Pna2₁) supported by STEM–EDS and vibrational fingerprints of [CdCl₄]²⁻ units and trimethylammonium cations. Thermogravimetry shows no mass loss up to ∼533 K, underscoring robust stability well above the phase-transition window. Differential scanning calorimetry resolves two reversible transitions at 253/263 K and 290/300 K with ∼10 K hysteresis and order–disorder entropies. Diffuse-reflectance UV–Vis treated via the Kubelka–Munk transform (α/S vs hν) reveals a direct band gap E_g (298 K) = 4.18 eV, narrowing to ∼4.10 eV at 350 K, accompanied by a modest red shift and an emergent Urbach tail indicative of strengthened exciton–phonon coupling. Consistently, steady-state PL (peak ∼472 nm) red-shifts, broadens (FWHM ∼105 → ∼125 nm), and quenches by ∼22 % on heating, while TRPL lifetimes contract (⟨τ⟩ ≈ 22 → ∼9 ns), signaling thermally activated non-radiative channels. Temperature-dependent permittivity exhibits step-like switching with ∼10 K hysteresis and minimal dispersion across 20–10⁶ Hz, mirroring the calorimetric transitions and consolidating an opto-lattice coupling scenario in which lattice reorganizations regulate both band-edge and emissive dynamics. These cross-validated correlations position [N(CH₃)₃H]₂CdCl₄ as a promising platform for stimuli-responsive dielectrics and UV–Vis–NIR photonic functions.