Reversible thermochromic property of Cr, Mn, Fe, Co-doped Ca14Zn6Ga10O35

Abstract

Thermochromic materials are one of the most important types of intelligent materials that show controllable thermal indicator performance in many potential applications. In this paper, we report a host structure for a new family of reversible intermediate-to-high temperature thermochromic materials based on four transition metal doped Ca₁₄Zn₆Ga_10−xM_xO₃₅ (M = Cr, Mn, Fe, Co) materials. The relationship between phase purity and doping level for different dopants was examined using powder X-ray diffraction and high-resolution transmission electron microscopy. The crystal structure of Ca₁₄Zn₆Ga₁₀O₃₅ is composed of anion-oxygen-centered OZn₄-derived four ZnO₄ tetrahedral units and isolated (Ga,Zn)O₆ octahedral units, which provide versatile possibilities for site-selective doping of the transition metal chromophore ions in the crystal lattice. All of the four sets of dopants could clearly induce the thermochromic property of the materials, with the colors of reddish-brown, greenish-yellow, pale-yellow, and pale-green, for Cr, Mn, Fe, and Co doped samples, respectively. Among the four series of samples, Mn-doped samples show the best thermochromic performance, due to the co-contribution of Mn⁴⁺ and Mn⁵⁺ in the tetrahedral sites. The thermochromic phenomenon for all of the materials is reversible in the temperature range from 25 to 460 °C with the highest chromatic aberration value of 51 (Ca₁₄Zn₆Ga_10−xMn_xO₃₅, x = 0.8). The absorbance spectra of the materials indicate that Cr and Fe occupy the octahedral sites, while Mn and Co occupy the tetrahedral sites in the structure. The thermochromic mechanism of the materials could be ascribed to thermally induced lattice expansion, which changes the positions and intensities of the absorption bands of chromophore ions in different crystal fields. The presented new family of intermediate-to-high temperature thermochromic materials may find promising applications in various thermal indicator fields in the future.