Negative Thermal Expansion Properties of Two Metal-Organic Perovskite Frameworks

作者：Feng Guo-Qiang^*; Ma Jun; Gui Di; Li Zhi-Hua^*; Li Wei

来源：Chinese Journal of Inorganic Chemistry, 2017, 33(6): 932-938.

DOI：10.11862/CJIC.2017.133

• metal-organic perovskite
• negative thermal expansion
• "hinge-strut" like structure model
• hydrogen bonding

Lattice parameters and crystal structures of two metal-organic perovskites [C(NH2)(3)] [Mn (HCOO)(3)] (1) and [(CH2)(3)NH2][Mn(HCOO)(3)] (2) are investigated via variable-temperature single-crystal (VT-SCRD) X-ray diffraction and transmission electron microscopy (TEM) experiments. The significant negative thermal expansion (NTE) phenomena along their c-axes are revealed. The average NTE coefficients of frameworks 1 and 2 are alpha(c1)= -1.2(1)x10(-5) K-1 and alpha(c2)=-6.1(11)x10(-5) K-1 respectively, obtained via linear fits using the Pascal software. A hinge-strut like structure model is used to explain the NTE mechanism of these two frameworks, the perovskite frameworks 1 and 2 can be considered as a simple hinge-strut like structure which is a prototypic motif for anisotropic thermal expansion, the formate ligand and MAO stand for the hinge and strut, respectively, the hinge angles are represented as theta and phi. The results indicate that the hydrogen bond lengths and angles between the anion framework and the A-site amine cations change slightly with increasing temperature, which induce the distortion of the perovskite frameworks and give rise to the increase of the hinge angle theta but decrease of phi with increasing temperature. These structural rearrangements result in the NTE along the diagonal of the pseudocubic perovskite framework. Furthermore, the two frameworks show significantly different thermal expansion properties due to their distinct modes of hydrogen-bonding between the framework hosts and A-site amine cations. In framework 2, each [(CH2)(3)NH2](+) is aligned with the ac plane and bonded to the two opposite edges within the same face of the pseudocubic unit cell by four hydrogen bonds. However in framework 1, each [C (NH2)(3)](+) is cross linked to two perpendicular edges from two opposite faces of each pseudocubic unit cell by six hydrogen bonds and tilted with respect to the ac plane, which consequently gives more cross -linking constraints to the cubic unit cell along three orthogonal orientations. As a result of the constraint, the amine cation and anionic framework of 1 are bonded more tightly. Therefore, it is concluded that the number and orientation of hydrogen bonds have a marked impact on the thermal expansion properties of these two frameworks.

• 出版日期2017-6-10
• 单位华中科技大学; 湖北第二师范学院