Tunable uniaxial, area, and volume negative thermal expansion in quartz-like and diamond-like metal-organic frameworks

This paper proposes that it will be an effective way to discover and explore organic negative thermal expansion (NTE) materials based on the specific topologies in inorganic NTE materials. Various NTE behaviors from the uniaxial, area, and volume-NTE can be achieved by adjusting the topology, for instance, quartz-like and diamond-like. Zn(ISN)₂ and InH(BDC) metal-organic frameworks (MOFs) with quartz-like topology have been studied by first principles calculations. The calculated area-NTE of Zn(ISN)₂ and uniaxial-NTE of InH(BDC) within quasi-harmonic approximation (QHA) agree well with the experimental data. Through the calculation of Grüneisen parameters, it is shown that low-frequency optical phonons appear dominant resulting in their NTE, but the coupling to high-frequency phonons is of greater ultimate importance. The lattice vibrational modes of great contribution to area-NTE of Zn(ISN)₂ and uniaxial-NTE of InH(BDC) are analyzed in detail. Also, four MOFs with diamond-like topology are predicted to exhibit volume-NTE behavior. Moreover, it is found that there is a bulk modulus anomaly in some studied MOFs with the quartz-like and diamond-like framework, where the temperature dependence of bulk modulus does not follow the inverse dependence on that of volume. These specific topologies provide key geometric frameworks for various NTE behaviors of MOFs, and meanwhile, the local structure and bond environment in MOFs can lead to abnormal interatomic force, i.e., bulk modulus anomaly. This abnormal elastic property also deserves more attention.

Adjustable uniaxial zero thermal expansion and zero linear compressibility in unique hybrid semiconductors: the role of the organic chain

Zero thermal expansion (ZTE) and zero linear compressibility (ZLC) are unique and rare properties. Materials combining ZTE and ZLC will have promising prospects. A novel route is proposed in this work to design the coexistence of uniaxial-ZTE and ZLC based on layered hybrid semiconductors [ZnTe(L)0.5] [L = N2H4, ethylenediamine (en), propyldiamine (pda)]. In the framework of [ZnTe(L)0.5], the organic chain contains the attractive and repulsive interactions that arise from the different organic components. It is demonstrated that changing the length of the organic chain can effectively regulate the interaction between different organic components and then achieve the uniaxial-ZTE and ZLC or the desired thermal expansion and compression behaviors. The origin of the coexistence of abnormal axial responses has been traced from thermodynamic formalisms, model Grüneisen parameters and specific vibration modes. It is found that low-energy phonons play an important internal role in realizing the multi-peculiar properties.

Black phosphorene exhibiting negative thermal expansion and negative linear compressibility

Positive expansion on heating, and positive contraction on hydrostatic compression are our general understanding. Actually, the abnormal negative thermal expansion (NTE) or negative linear compressibility (NLC) behavior is permissible, but exists in very few materials. Interestingly, we find that the NTE and NLC behaviors can coexist in the low-dimensional black phosphorene using first-principles calculations. In the temperature-field, the volume-NTE of black phosphorene can be exhibited below about 200 K, while the axial-NTE exists in the whole studied temperature range. In the hydrostatic pressure-field, the NLC behavior can occur along a zigzag direction or armchair direction in different pressure ranges. The phonon abnormality in black phosphorene is detected. It is found that acoustic phonons, especially the lowest-frequency TA _z mode, play a very important role. The re-entrant honeycomb network in black phosphorene, as the specific topology, will facilitate the occurrence of NTE and NLC behaviors. This work can provide some foundations and clues for exploring the abnormal properties in other 2D materials.