Pressure-induced modulation of structural, mechanical, electronic, optical, and thermodynamic properties of KZn4As3 and CsCd4As3 zintl compounds: implications for thermal and optoelectronic applications.

In this work, using DFT, we systematically investigate the structural, dynamic, elastic, thermodynamic, electronic, and optical properties of ternary arsenide Zintl phases KZn4As3 and CsCd4As3 under hydrostatic pressures of 0-15 GPa. The optimized lattice parameters at ambient pressure agree closely with the available experimental data, validating the computational approach. Both compounds undergo pronounced anisotropic lattice compression, with the c-axis contracting more than the a-axis, and reduction in unit-cell volumes by approximately 22-24% over the studied pressure range. Phonon dispersion calculations confirm that both phases remain dynamically stable, with no imaginary frequency modes across the Brillouin zone at any pressure. The compounds satisfy the Born mechanical stability criteria under ambient and applied pressures, confirming elastic stability throughout the studied range. Poisson's ratio increases from 0.29 to 0.36 for KZn4As3 and from 0.286 to 0.36 for CsCd4As3, confirming that both compounds remain ductile across the entire pressure range. Thermodynamic parameters such as average sound velocity, Debye temperature, and melting temperature, all derived from the elastic constants, increase with pressure, with KZn4As3 consistently exhibiting stronger interatomic bonding and higher thermal robustness than CsCd4As3. GGA-PBE predicts KZn4As3 to be metallic across all pressures, while HSE06 reveals a narrowing semiconducting gap of 0.921-0.557 eV, reflecting the known underestimation of GGA-PBE. For CsCd4As3, both functionals show pressure-induced gap reduction, with HSE06 consistently predicting larger gaps. Strong optical absorption, conductivity, and energy-loss features in the near-UV to UV region (4-6 eV), combined with low static reflectivity, highlight both compounds as mechanically robust, pressure-tunable Zintl phases suitable for infrared and UV optoelectronic devices, high-pressure structural applications, and thermoelectric energy conversion.