Curie Temperature and Single-Crystal Properties of Glycine Doped with Four Chloride Salts: A Landau–Devonshire Theoretical Study

Abstract

Chloride-doped glycine single crystals are an emerging class of semi-organic materials. These crystals feature the polar γ-polymorph of the simplest amino acid, integrated with chloride salt units. To explain the experimentally observed Curie temperature (Tc) for γ-glycine single crystals doped with LiCl, NaCl, KCl, and NH4Cl, we have developed a unified theoretical model. This model combines Landau-Devonshire phenomenology, the mean-field pseudospin approach to the hydrogen-bond network, and elastic-mismatch doping prescriptions. The parent compound benchmark is the theoretical dielectric anomaly of γ-glycine near 210 °C. The model predicts second-order ferroelectric transitions at Tc = 477.0 K (Gly-NaCl), 472.3 K (Gly-LiCl), 467.9 K (Gly-KCl), and 465.3 K (Gly-NH4Cl) for a nominal dopant mole fraction of 0.05. The spontaneous polarisations (2.49-2.58 μC cm-2), pyroelectric coefficients (73-76 μC m-2 K-1 at 300 K), and mean-field specific heat jump (≈2 J mol-1 K-1) align well with experimental data for triglycine sulfate. Additionally, the Curie-Weiss constants (≈2.3 × 103 K) are expected to be comparable to those of triglycine sulfate. These findings indicate that chloride-doped glycine crystals have potential for use in uncooled pyroelectric sensors, biocompatible piezoelectric actuators, and nonlinear optical devices. Among the systems studied, Gly-NaCl and Gly-LiCl are the most promising candidates for immediate experimental investigation.