The work, published in the journal Results in Chemistry by A.M. Metha and S. Sumathi, addresses a persistent dilemma in pigment chemistry. Traditional dark pigments, including many iron oxide and chromium-based compounds, absorb broadly across both the visible and near-infrared regions, converting absorbed sunlight into surface heat. This contributes to the urban heat island effect, in which cities become substantially warmer than their surroundings because of heat-absorbing construction materials, and it drives up the energy demand for air conditioning. Many classic pigments also carry a toxic burden, relying on cadmium, selenium, chromium, or lead, all of which pose risks to human health and the environment. A benign pigment that combines strong color with high near-infrared reflectance has therefore become a prized target in materials research.

The Indian team chose strontium molybdate, SrMoO4, as the host lattice, and the choice is chemically shrewd. The compound adopts the tetragonal scheelite structure, an arrangement in which isolated tetrahedral molybdate groups sit among strontium ions. Its wide band gap of about 4.2 electronvolts means the material itself is essentially colorless and nearly transparent to visible light, and its low intrinsic absorption in the near-infrared makes it an excellent reflector in that band. Because of its forgiving framework, foreign ions can be slipped into the lattice without destroying the structure, which is precisely what doping experiments require. Previous studies had already exploited this versatility, producing yellow pigments with cerium doping and luminescent materials with europium, dysprosium, and terbium.

To generate color, the researchers turned to iron, a proven chromophore for warm tones. They synthesized a complete series of compounds with the formula Sr1-xFexMoO4, where the iron fraction ranged from zero to half of the strontium content, using a hydrothermal method that is remarkably gentle by pigment standards. Aqueous solutions of strontium nitrate, ferric nitrate, and ammonium molybdate were mixed, the pH was adjusted to neutral with ammonia, and the mixture was sealed in a Teflon-lined autoclave at 180 degrees Celsius for fifteen hours. Conventional reddish-brown pigments often require calcination at 600 to 1200 degrees Celsius or beyond, so a low-temperature, water-based route carries both energy and cost advantages for industrial scale-up.

Structural analysis confirmed the doping strategy worked cleanly. Powder X-ray diffraction showed that every composition, from pure SrMoO4 through the highest iron loading, crystallized in the single tetragonal scheelite phase with no secondary impurities, and Rietveld refinement verified that the lattice remained intact with high goodness-of-fit values. Interestingly, the lattice parameters increased slightly with iron content rather than contracting, even though iron ions are much smaller than strontium. The authors attribute this to local structural relaxation, a mixed iron valence state, and charge-compensation effects within the crystal. Fourier-transform infrared spectroscopy supported the same conclusion, showing the characteristic molybdate vibrations but no separate iron oxide signals, which would have indicated undissolved impurity phases.

Where the chemistry gets truly interesting is in the electronic structure. X-ray photoelectron spectroscopy of the optimal composition, Sr0.8Fe0.2MoO4, revealed a mixed-valence world at the surface: iron present as both Fe2+ and Fe3+, and molybdenum coexisting as Mo5+ and Mo6+. This charge redistribution, together with iron-induced oxygen vacancies, introduces new energy levels inside the wide band gap of the host. The result is dramatic in the optical data. The undoped compound absorbs only in the ultraviolet, through a ligand-to-metal charge-transfer transition within the molybdate tetrahedra, and its band gap of 4.22 electronvolts falls steadily to 1.87 electronvolts at the highest iron loading. The new absorption channels include oxygen-to-iron charge transfer, iron d-d transitions, and intervalence charge transfer between neighboring iron ions, all of which soak up visible light and paint the powder in warm brown tones.

Color measurements in the CIE Lab* system traced this evolution quantitatively. The pure compound is nearly white, with a lightness value of 93.67. Iron doping drags the lightness down to about 45 at the highest loading while pushing the red and yellow coordinates positive, and the hue angle slides from 63.6 to 44.5 degrees, crossing into the 40-to-60-degree window that defines true reddish-brown shades. Crucially, the near-infrared solar reflectance, calculated against the ASTM G173-03 standard solar spectrum, stayed impressively high: 82.4 percent for the undoped host, 70.0 percent at ten percent iron, and 69.8 percent at the optimal twenty percent loading, only falling to 58.4 percent at the heaviest doping. That trade-off between color intensity and reflectance is the central challenge of cool pigment design, and the molybdate framework held the line better than most alternatives.

The comparison with published reddish-brown pigments makes this performance striking. Lanthanum ferrite and related orthoferrite pigments, typically fired at 1200 degrees Celsius, manage reflectances of only 33 to 43 percent. Bismuth ferrite variants reach about 52 percent, iron-substituted yttrium aluminate 45 percent, and cerium-praseodymium oxide 58 percent. The iron-doped strontium molybdate made here at 180 degrees Celsius beats them all, delivering comparable color with 60 to 70 percent reflectance. The authors credit the molybdate lattice itself, which retains its near-infrared reflectivity even after iron is introduced, effectively decoupling the visible coloration from the thermal performance.

Real-world testing put the numbers to work. In a laboratory setup, metal sheets coated with a PVDF-based paint containing the Sr0.8Fe0.2MoO4 pigment were irradiated with a 250-watt infrared lamp thirty centimeters away. The bare sheet climbed from 31.8 to a scorching 62.3 degrees Celsius within ten minutes, while the coated sheet peaked at 57.1 degrees, and the coated surface stayed roughly 3.6 degrees cooler on average over an hour of continuous irradiation. Under genuine outdoor sunlight on the Vellore campus in early September, with ambient temperatures near 37 degrees and solar irradiance approaching 1000 watts per square meter, the pigment-coated sheet ran about 4 to 5 degrees cooler than bare metal, and a version layered over a white titanium dioxide base coat performed best of all, reaching 45.2 degrees when the bare surface hit 61.2 degrees. Commercial iron oxide paint, by contrast, provided only about half that benefit.

The pigment also passed the durability gauntlet that determines whether a laboratory curiosity can become a commercial coating. Thermogravimetric analysis showed only about 6 percent weight loss up to 800 degrees Celsius, with the crystal structure surviving intact, and exposure to dilute hydrochloric acid, nitric acid, and sodium hydroxide produced total color differences between 1.84 and 4.96, all within the accepted threshold of 5 for chemical resistance. Taken together, the results sketch a coherent recipe for the next generation of cool coatings: a benign, iron-based chromophore locked inside a highly reflective molybdate framework, manufactured at low temperature, and capable of shaving several degrees off the surfaces that bake under the sun. For a warming world covered in dark roofs, that modest temperature difference, multiplied across entire cities, could translate into meaningful savings in cooling energy and a measurable dent in the urban heat island effect.

Subject of Research: Iron-doped SrMoO4 near-infrared reflective cool pigments for energy-efficient coatings

Article Title: Design of reddish-brown iron-doped SrMoO 4 pigments with high NIR reflectance for cool coating applications

Article References: Metha, A., & Sumathi, S. (2026). Design of reddish-brown iron-doped SrMoO4 pigments with high NIR reflectance for cool coating applications. Results in Chemistry, 31, Article 103958. https://doi.org/10.1016/j.rechem.2026.103958

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103958

Keywords: cool pigments, near-infrared reflectance, strontium molybdate, iron doping, urban heat island, scheelite structure, hydrothermal synthesis, passive cooling, inorganic pigments, band gap engineering, thermal management, PVDF coatings