  The theoretical discovery of new and stable 2D penta materials has stimulated
the technological advancement due to the anticipated exotic properties of such
structure, including the recent $\alpha$ phase and $\beta$ phase of penta-NiPS
based on first-principles calculations. Inspired by the similarity between the
theoretically proposed penta-NiPS and the experimentally synthesized ($\alpha$
phase) of penta-PdPSe, we preposed herein the $\beta$ phase penta-PdPSe as a
new member of the penta-2D materials. Comprehensive analysis indicated that the
$\beta$ phase penta-PdPSe is thermodynamically, dynamically, mechanically, and
thermally stable, similar to the NiPS analog. It was found that $\beta$
penta-PdPSe is a wide band gap semiconductor with an indirect band gap of 1.58
eV, significantly lower than 2.15 eV for the $\alpha$ phase. Moreover, the two
polymorphs of penta-PdPSe are a soft material with 2D Young's modulus of $E_a$
= 151 Nm$^-1$ and $E_b$ = 123 Nm$^-1$ for the $\beta$ phase, to be compared
with $E_a$ = 155 Nm$^-1$ and $E_b$ = 113 Nm$^-1$ for the $\alpha$ phase. The
calculated adsorption coefficient shows that $\beta$ phase penta-PdPSe is
acceptable for electronic and optical nanodevice.
