正交相Fe2(MoO4)3的制备与表征及其负膨胀行为的第一性原理研究

Preparation and characterization of orthorhombic Fe_2(MoO_4)_3 and first-principle study of its negative thermal expansion properties

采用水热合成法制备出Fe_2(MoO_4)_3样品,并用高温X-射线衍射、热重和差示扫描量热同步热分析仪对其进行表征,发现样品在510℃附近发生低温单斜相和高温正交相之间的可逆相变,且正交相表现出负膨胀特征.采用第一性原理计算了正交相Fe_2(MoO_4)_3的原子、电子结构以及声子谱、声子态密度,并和可获得的实验结果进行了系统的比较.结果显示正交相Fe_2(MoO_4)_3中MoO_4四面体较之FeO_6八面体具有更强的刚性.发现最低频的光学支处具有最负的格林乃森(Gr(u|¨)neisen)系数,MoO_4四面体和FeO_6八面体相连的桥氧原子的横向振动、FeO_6八面体柔性扭曲转动以及MoO_4四面体的刚性翻转共同导致了Fe_2(MoO_4)_3负膨胀现象的发生.

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 FeC6 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 Gr（u｜¨）neisen parameters for phonon branches at Γ point,the optical branch with the lowest vibration frequency is believed to have the largest negative Griineisen 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,.