Hidden tellurium speciation resolves the paradox of infinite Te chains in LaCu x Te2.

Low-dimensional polytellurides are of broad interest because their flexible bonding gives rise to unusual electronic states, structural instabilities, and transport phenomena, yet their average crystal structures can obscure the local tellurium motifs that actually control these properties. The electronic character of LnCu x Te2 (Ln = lanthanide) remains unresolved because electronic structure calculations based on the average crystal structure predict metallic behavior, whereas experiments consistently show semiconducting transport. Here we examine this discrepancy in LaCu x Te2 by combining low-temperature single-crystal X-ray diffraction, total-scattering analysis, electrical transport measurements, high-pressure studies, and first-principles calculations. Single-crystal diffraction at 100 K reveals an incommensurately modulated structure that is absent at room temperature, and X-ray total scattering indicates local unit-cell tripling along the direction of the apparent linear Te chains. These results show that the crystallographic infinite chain is not a real chemical entity but an average motif that masks locally distorted Te arrangements. In this picture, the anomalous transport of LaCu x Te2 is linked to chain distortion and structural disorder rather than to an idealized delocalized Te chain electronic state. Consistent with this interpretation, LaCu x Te2 exhibits semiconducting behavior over the full temperature range studied, variable-range hopping at low temperature, and quasi-linear nonsaturating magnetoresistance attributable to spatially inhomogeneous carrier mobility. Under pressure, the room-temperature structure remains unchanged and the charge transport activation energy decreases, yet metallic behavior does not emerge. Together, these results reconcile the apparent conflict between ideal-chain calculations and experiment, demonstrating that hidden local distortion within an average one-dimensional tellurium motif governs the electronic behavior of LaCu x Te2.