Zero and controllable thermal expansion in HfMgMo_3-xW_xO₁₂

HfMgMo_3-xW_xO₁₂ with x=0.5, 1.0, 1.5, 2.0, and 2.5 are developed with a simple solid state method. With increasing the content of W, solid solutions of HfMgMo_3-xW_xO₁₂ crystallize in an orthorhombic structure for x≤2.0 and a monoclinic structure for x>2.0. A near-zero thermal expansion (ZTE) is realized for HfMgMo_2.5W_0.5O₁₂ and negative coefficients of thermal expansion (NCTE) are achieved for other compositions with different values. The ZTE and variation of NCTE are attributed to the difference in electronegativity between W and Mo and incorporation of a different amount of W, which cause variable distortion of the octahedra and softening of the MoO₄ tetrahedra, and hence an enhanced NCTE in the a- and c-axis and reduced CTE in the b-axis as revealed by Raman spectroscopy and x-ray diffraction.