Coordination-Driven Color Tuning in Non-Centrosymmetric Ba M Te2O7 Solid Solutions for High Near-Infrared Reflective Inorganic Pigments

Designing inorganic pigments that are capable of offering adjustable color with high near-infrared (NIR) reflectance is still a major challenge for energy-efficient coating technologies. In this study, a series of BaMTe2O7-based solid solutions, namely BaCu1–xNixTe2O7, BaZn1–xCoxTe2O7, and BaZn1–xCuxTe2O7, were prepared using a conventional solid-state reaction method to understand how transition-metal substitution affects their crystal structure, optical response, magnetic properties, and thermal performance as pigments. Powder X-ray diffraction and neutron Rietveld refinement confirmed that all compositions formed single-phase orthorhombic structures with the Ama2 space group. The substituted Co2+, Cu2+, and Ni2+ ions were found to occupy non-centrosymmetric square-pyramidal sites within the structure. By systematically changing the composition, the pigments displayed a broad palette of colors, including cyan blue, turquoise green, pistachio green, lime green, and yellow-green. The color variations originate from crystal-field-controlled d-d transitions, while magnetic analysis confirmed the divalent metal ions and their electronic configurations. The pigments also exhibited excellent acid stability and retained high NIR reflectance. This was particularly seen in the BaZn1–x(Co/Cu)xTe2O7 compositions, emphasizing their potential for cool pigments in energy saving coatings. Overall, BaMTe2O7 provides a robust platform for developing color-tunable, structurally stable, and highly solar-reflective multifunctional inorganic pigments.