Preparation and characterization of orthorhombic Fe2(MoO4)3 and first-principle study of its negative thermal expansion properties

Monoclinic Fe2(MoO4)3 sample is synthesized by the hydrothermal method, and characterized via high temperature X-ray diffraction and thermogravimetric-differential scanning calorimetry. It is observed that the reversible phase transition between the low-temperature monoclinic and high-temperature orthorhombic phases occurs at about 510 ℃. The cell parameters at different temperatures are calculated by the Rietveld refinement method. In a temperature range from 25 ℃ to 400 ℃, the a, b and c crystallographic axes with the monoclinic phase gradually expand. On the other hand, in a temperature range from 530 ℃ to 710 ℃, the orthorhombic phase exhibits a negative thermal expansion (NTE) behavior, in which the b and c axes gradually contract but the a axis first contracts and then expands a little. Atomic and electronic structures are investigated using first-principle calculation. Results indicate that the Mo-O bonds are much stronger than the Fe-O bonds in Fe2(MoO4)_{3} and the MoO4 tetrahedrons are more rigidly than FeO6 octahedrons. To reveal the relationship between NTE and polyhedral distortion, the phonon density of state of Fe2(MoO4)3 is calculated using the ab initio method. The experimental Raman spectrum positions can be identified in the calculated dispersion of the total phonon density of states (DOS). Meanwhile, by calculating the Grneisen parameters for phonon branches at point, the optical branch with the lowest vibration frequency is believed to have the largest negative Grneisen parameter. Furthermore, we analyze the vibrational behaviors of atoms, and find that oxygen atoms have different vibrational eigenvectors from Fe or Mo atoms. and more obvious amplitudes than Fe or Mo atoms. Therefore, it is concluded that the transverse vibration of the oxygen bridge atom between the MoO4 tetrahedron and FeO6 octahedron, the soft distortion of FeO6 octahedrons, and the rigid rotation of MoO4 tetrahedrons jointly lead to the negative thermal expansion of Fe2(MoO4)3,.