Negative linear compressibility of molecular and ionic-molecular crystals

The compressibility of crystalline tetrabromophthalic anhydride (TBPA) and 1-ethyl-3-methylimidazolium nitrate (EMN) was studied based on density functional theory including dispersion interactions at pressures below 1 GPa. It is found for the first time that EMN demonstrates negative linear compressibility (NLC) up to ∼0.15 GPa, whereas TBPA shows significant NLC at pressures higher than ∼0.2 GPa. Mechanisms of the negative linear compressibility of TBPA and EMN at the microscopic (molecular) level have been found for the first time. It was shown that NLC correlates with a baric change of spatial orientation (rotation) and linear dimensions of molecular structural units relative to crystallographic axes, as well as with a baric increase of intermolecular distances along the NLC direction. Quantum topological analysis of electron density was used to study intermolecular interactions. It has been established that TBPA and EMN crystals are optically transparent for visible light at pressures up to 1 GPa.

Managing negative linear compressibility and thermal expansion through steric hindrance: a case study of 1,2-bis(4'-pyridyl)ethane cocrystals

Multicomponent crystals have great scientific potential because of their amenability to crystal engineering in terms of composition and structure, and hence their properties can be easily modified. More and more research areas are employing the design of multicomponent materials to improve the known or induce novel physicochemical properties of crystals, and recently they have been explored as materials with abnormal pressure behaviour. The cocrystal of 1,2-bis(4'-pyridyl)ethane and fumaric acid (ETYFUM) exhibits a negative linear compressibility behaviour comparable to that of framework and metal-containing materials, but overcomes many of their deficiencies restricting their use. Herein ETYFUM was investigated at low temperature to reveal negative thermal expansion behaviour. Additionally, a cocrystal isostructural with ETYFUM, based on 1,2-bis(4'-pyridyl)ethane and succinic acid (ETYSUC), was exposed to high pressure and low temperature, showing that its behaviour is similar in nature to that of ETYFUM, but significantly differs in the magnitude of both effects. It was revealed that the minor structural difference between the acid molecules does not significantly affect the packing under ambient conditions, but has far-reaching consequences when it comes to the deformation of the structure when exposed to external stimuli.

Hydrogen-bond-modulated negative linear compressibility in a V-shaped molecular crystal

A material with the "hidden" negative linear compressibility (NLC) will expand along a specific crystal direction upon uniformly compression to a critical pressure; such materials are thought to be promising candidates for non-linear actuators, switches and sensors. Herein, we use density functional theory (DFT) calculations to uncover the hidden NLC in a V-shaped molecular crystal, bis(5-amino-1,2,4-triazol-3-yl)methane (BATZM). The calculations indicate that the crystal is normally compressed over the pressure range of 0-3 GPa while it expands along the b-axis when the external hydrostatic pressure exceeds 3 GPa. The compressive behavior of the BATZM crystal is modulated by inter-molecular hydrogen bonds, which act as highly compressible springs at low pressures but robust struts at high pressures. Hence, the crystal prefers to compress the hydrogen bonds coupled with PLC at first and flatten the molecules, coupled with later NLC to resist the increasing external pressure. The compressive behavior of BATZM provides a strategy to design more hidden NLC materials via the rational use of the hydrogen bonds.

Negative linear compressibility exhibited by the hybrid perovskite [(NH2)3C]Er(HCO2)2(C2O4)

Extended framework materials with specific topologies can exhibit unusual mechanical behaviour, such as expanding in one direction under hydrostatic (uniform) pressure, known as negative linear compressibility (NLC). Here, two hybrid perovskite frameworks with winerack structures, a known NLC topology, are investigated under pressure. [C(NH2)3]Er(HCO2)2(C2O4) exhibits NLC from ambient pressure to 2.63(10) GPa and is the first reported NLC hybrid perovskite from ambient pressure. However, isostructural [(CH3)2NH2]Er(HCO2)2(C2O4) instead compresses relatively moderately along all axes before it undergoes a phase transition above 0.37(10) GPa. The differences in the mechanical properties can be interpreted from differences in host-guest interactions within these frameworks, primarily their hydrogen bond networks.

Structural flexibility in crystalline coordination polymers: a journey along the underlying free energy landscape

In this perspective, we discuss structural flexibility in crystalline coordination polymers. We identify that the underlying free energy landscape unites scientific disciplines, and discuss key areas to advanced the field.

Pressure-and temperature induced phase transitions, piezochromism, NLC behaviour and pressure controlled Jahn-Teller switching in a Cu-based framework

In situ single-crystal diffraction and spectroscopic techniques have been used to study a previously unreported Cu-framework bis[1-(4-pyridyl)butane-1,3-dione]copper(ii) (CuPyr-I). CuPyr-I was found to exhibit high-pressure and low-temperature phase transitions, piezochromism, negative linear compressibility, and a pressure induced Jahn–Teller switch, where the switching pressure was hydrostatic media dependent.

In situ high-pressure single-crystal diffraction and spectroscopic techniques have been used to study a previously unreported Cu-framework bis[1-(4-pyridyl)butane-1,3-dione]copper(ii) (CuPyr-I).

Colossal Negative Linear Compressibility in Porous Organic Salts

Negative linear compressibility (NLC) is a common sense violation (that is, crystal phases expand in one or more directions under hydrostatic compression). The excellent NLC performance of crystal materials is intrinsically related to the geometric structure of its skeleton. Here, we discovered a crystalline porous organic salt (CPOS-1); high-pressure X-ray diffraction experiments reveal that the CPOS-1 shows colossal NLC (K_c = -90.7 T Pa^-1) behavior along the c axis. This incredible performance arises from the flexible "supramolecular spring" formed by the charge-enhanced N-H⁺···^-O-S hydrogen bond interaction between the anionic sulfonate and the cationic ammonium ion. Furthermore, we reveal the relationship between this rare NLC behavior and single crystal proton conductivity using high-pressure electrochemical impedance spectroscopy (EIS) method. We believe that NLC behavior research on such inexpensive and readily available porous organic materials is of great significance for accelerating the research and application of NLC materials, especially in organic system.

Anomalous Lattice Dynamics in AgC4N3: Insights From Inelastic Neutron Scattering and Density Functional Calculations

We have performed temperature dependent inelastic neutron scattering measurements to study the anharmonicity of phonon spectra of AgC4N3. The analysis and interpretation of the experimental spectra is done using ab-initio lattice dynamics calculations. The calculated phonon spectrum over the entire Brillouin zone is used to derive linear thermal expansion coefficients. The effect of van der Waals interaction on structure stability has been investigated using advanced density functional methods. The calculated isothermal equation of states implies a negative linear compressibility along the c-axis of the crystal, which also leads to a negative thermal expansion along this direction. The role of elastic properties inducing the observed anomalous lattice behavior is discussed.

Unusual and Tunable Negative Linear Compressibility in the Metal-Organic Framework MFM-133(M) (M = Zr, Hf)

High-pressure single-crystal X-ray structural analyses of isostructural MFM-133(M) (M = Zr, Hf) of flu topology and incorporating the tetracarboxylate ligand TCHB^4- [H₄TCHB = 3,3',5,5'-tetrakis(4-carboxyphenyl)-2,2',4,4',6,6'-hexamethyl-1,1'-biphenyl] and {M₆(μ₃-OH)₈(OH)₈(COO)₈} clusters confirm negative linear compressibility (NLC) behavior along the c axis. This occurs via a three-dimensional wine-rack NLC mechanism leading to distortion of the octahedral cage toward a more elongated polyhedron under static compression. Despite the isomorphous nature of these two structures, MFM-133(Hf) shows a higher degree of NLC than the Zr(IV) analogue. Thus, for the first time, we demonstrate here that the NLC property can be effectively tuned in a framework material by simply varying the inorganic component of the frameworks without changing the network topology and structure.