Structural, in-situ Raman spectroscopy and XPS studies on ZrW2-Mo O8 ceramics with tunable negative thermal expansion properties

This study presents an analysis of the characterization and crystal structure of a series of materials, namely ZrW2-xMoxO8 (0 ≦ x ≦ 1). The isotropic coefficient of thermal expansion of these materials exhibited a systematic and progressive approach from negative values to zero thermal expansion, which is dependent on the increasing value of x. The range of the linear coefficient of thermal expansion, αl, for the entire series was observed to be between −8.44 × 10−6/°C and approximately −4.0 × 10−6/°C. In-situ X-ray diffraction and In-situ Raman spectroscopy were used to study the different structures of ZrW2-xMoxO8 (0 ≦ x ≦ 1), elucidating the changing rule of the thermal expansion coefficient and the mechanism of the ordered-disordered phase transition. According to Rietveld refinement results, Mo doping causes a decrease in the lattice parameters of the entire system and slows down the transverse motion of the bridge oxygen atoms in the small cell volume, which reduces the negative thermal expansion. As the interaction between W (Mo) and a single coordination oxygen atom weakened, the phase transition temperature decreased. This is because the W(Mo)1O4 and W(Mo)2O4 polyhedron can be flipped more easily via the "ratchet" process due to the lower electronegativity of the Mo atom (2.16 Pauling) compared to that of the W atom (2.36 Pauling). Furthermore, the impact of Mo substitution for W sites on the electronic structure was initially reported through X-ray photoelectron spectroscopy.