The search for high-performance thermoelectrics is increasingly focused on quasi-one-dimensional (Q-1D) materials, which leverage quantum confinement effects to intrinsically decouple transport properties. Herein we report a series of Q-1D phosphidosilicates with systematically tunable structural distortions, stereochemically active lone pairs, anharmonic chains, and distinct bonding hierarchies. This unique structural combination induces strong phonon anharmonicity, resulting in intrinsic ZT values above 1, including a peak of $\sim 2.32$ along the $a$ direction in $\mathrm{B}{\mathrm{a}}_4{\mathrm{Si}}_3{\mathrm{P}}_8$. Quantifying the influence of structural distortion reveals two distinct trends. Electronically, increased distortion promotes valley convergence and drives bands toward degenerate Q-1D flat bands, enhancing power factors. This effect is most pronounced in $\mathrm{C}{\mathrm{a}}_3{\mathrm{Si}}_2{\mathrm{P}}_4$, which exhibits superior carrier mobility and high power factor under both doping types, matching state-of-the-art NbFeSb. Phononically, $\mathrm{C}{\mathrm{a}}_3{\mathrm{Si}}_2{\mathrm{P}}_4$ exhibits a lattice thermal conductivity nearly three times higher than ${\mathrm{K}}_2\mathrm{Si}{\mathrm{P}}_2$ despite its heavier and more complex structure, violating conventional Slack's rule. Moreover, structural complexity shifts thermal transport toward wavelike behavior, culminating in $\mathrm{B}{\mathrm{a}}_4{\mathrm{Si}}_3{\mathrm{P}}_8{}^{\prime }\mathrm{s}$ ultralow phonon conductivity without nanostructuring. These phosphidosilicates demonstrate how structural distortion can simultaneously suppress thermal transport and enhance electronic performance via dimensional tailoring, offering a promising platform for advanced thermoelectrics.

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• Electrical conductivity
• Lattice dynamics
• Thermal conductivity
• Thermal transport
• Thermoelectric transport
• Crystal structures
• Functional materials
• Thermoelectrics
• Band structure methods
• Boltzmann theory
• Density functional theory
• First-principles calculations