A Computational Analysis of ZrSe₂O₆ and HfSe₂O₆ Oxychalcogenides: Wide-Bandgap Semiconductors with Strain-Tunable Properties

We present a comprehensive first-principles investigation of strain-induced polarization mechanisms in recently discovered monoclinic ZrSe2O6 and HfSe2O6, integrating full tensorial Born effective charge (BEC) analysis, symmetry-mode decomposition, and strain-tunable electronic–optical response. Both compounds crystallize in the centrosymmetric P21/c space group and yet reveal emergent polar instabilities when subjected to hydrostatic strain. ZrSe2O6 and HfSe2O6 oxychalcogenide materials are wide-band indirect semiconductors having electronic bandgaps of 5.1 and 5.2 eV, respectively. Moreover, these materials also depict birefringence (Δn) values of 0.146 and 0.137 at wavelengths of 1064 nm for incident light. These values determined from the first-principles showed excellent agreement with the previously determined experimental values. The elastic constants in the ground states of both monoclinic unit cell compounds were also determined for the first time. Displacive mode analysis reveals asymmetric strain–mode coupling in both materials: ZrSe2O6 exhibits a peak total distortion amplitude (∼2.6 Å) under compressive strain (−8%), while HfSe2O6 shows maximal mode activation (∼3.1 Å) under tensile strain (+8%). These dominant distortions occur primarily in A1- and A3-type symmetry channels, reflecting polarization components along the c- and b-axes, respectively. The BEC tensors exhibit strong coupling with these lattice distortions. In ZrSe2O6, the longitudinal charge Zzz of Zr declines significantly from 7.36 to 5.28 across the strain range, tracking the suppression of polar displacements. Off-diagonal terms such as Zzy and Zzx undergo polarity reversals, indicating a rehybridization of local bonding and anisotropic charge redistribution. Conversely, in HfSe2O6, Zzz of Hf increases from 5.18 to 6.65 under tensile strain, along with a concurrent rise in cross-axis charge coupling (Zyz, Zzx), signaling enhanced dynamical polarization aligned with structural softening. These trends directly correlate with strain-modulated bond lengths, orbital hybridization, and optical properties. Tensile strain elongates Se–O and Zr/Hf–O bonds, shifts Se 4p and Zr/Hf d PDOS peaks toward lower energies, and narrows the bandgap. Optical absorption edges red shift under tension and the blue shift under compression, while the refractive index follows the density trend, rising under compression and falling under expansion. These optical responses stem from enhanced polarizability and modified interband transitions, emphasizing the critical role of structural distortions in the dielectric behavior. Collectively, our findings reveal that Born charge tensors and symmetry-adapted lattice modes offer a predictive framework for understanding and controlling strain-responsive polarization phenomena. The asymmetry in their evolution highlights the importance of directionality, symmetry modification, and anharmonic coupling in the governing functional behavior.