Large Positive Thermal Expansion and Small Band Gap in Double-ReO3-Type Compound NaSbF6

Size and crystal symmetry breaking effects on negative thermal expansion in ScF3 nanostructures

Nowadays, one of the most typical and important potential applications of negative thermal expansion (NTE) materials is to prepare zero thermal expansion or controllable coefficient thermal expansion materials by compounding them with positive thermal expansion materials. The research on NTE properties at the nanoscales is the basis and premise for the realization of high-quality composites. Here, using first-principles calculations, we take a typical open framework material ScF₃ as an example to study a new NTE mechanism at the nanoscale, which involves edge and size effects, as well as crystal symmetry breaking. By analyzing the vibrational modes in ultrathin ScF₃ films, three effects contributing to the NTE properties are identified, namely, the acoustic mode (ZA mode) induced by surface truncation, the enhanced rotations of ScF₆ octahedra in the surface layer and the suppressed rotations of ScF₆ octahedra in the inner layer due to crystal symmetry breaking. With increasing thickness, the effect of the ZA mode vibration gradually weakens, while the rotations of the ScF₆ octahedra in the surface and inner layers are enhanced. Ultimately, the approximately mutual compensation of these three effects makes the NTE coefficients of different thicknesses almost unchanged. Finally, we simply generalize our conclusions to zero dimensional nanoparticles. This work reveals a new NTE mechanism in low-dimensional open framework materials, which serves as a guide in designing NTE materials at the nanoscale.

Perovskite-related ReO3-type structures

Materials with the perovskite ABX3 structure play a major role across materials chemistry and physics as a consequence of their ubiquity and wide range of useful properties. ReO3-type structures can be described as ABX3 perovskites in which the A-cation site is unoccupied, giving rise to the general composition BX3, where B is typically a cation and X is a bridging anion. The chemical diversity of such structures is extensive, ranging from simple oxides and fluorides, such as WO3 and AlF3, to complex structures in which the bridging anion is polyatomic, such as in the Prussian blue-related cyanides Fe(CN)3 and CoPt(CN)6. The same ReO3-type structure is found in metal-organic frameworks, for example, ln (im)3(im = imidazolate) and the well-known MOF-5 structure, where the B-site cation is polyatomic. The extended 3D connectivity and openness of this structure type leads to compounds with interesting and often unusual properties. Notable among these properties are negative thermal expansion (for example, ScF3), photocatalysis (for example, CoSn(OH)6), thermoelectricity (for example, CoAs3) and superconductivity in a phase that is controversially described as SH3 with a doubly interpenetrating ReO3 structure. We present an account of this exciting family of materials and discuss future opportunities in the area.