The ZrW2O8 family of materials has been shown to exhibit the unusual property of negative thermal expansion over a wide temperature range α-ZrW2O8 itself shows negative thermal expansion from 2 K to 1443 K with a coefficient of thermal expansion α1 of -91×10-6 K-1 between 2 and 300 K This behaviour can be related to the unusual structure of these materials, which possess a family of low energy phonon modes with negative Gruneisen parameters α-ZrW2O8 also shows a phase transition at 448 K to β-ZrW2O8 in which oxygen atoms are dynamically disordered These phenomena have been investigated by a number of techniques Results of powder neutron diffraction at 260 different temperatures are presented

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Negative thermal expansion materials

The updated concepts on the nature of π–π interactions and their use in various fields ranging from crystal engineering to materials science to biochemistry are discussed. This is the opening paper...

The Nature and Applications of π–π Interactions: A Perspective

The founding in 1965 of what is now called the Cambridge Structural Database (CSD) has reaped dividends in numerous and diverse areas of chemical research. Each of the million or so crystal structures in the database was solved for its own particular reason, but collected together, the structures can be reused to address a multitude of new problems. In this Review, which is focused mainly on the last 10 years, we chronicle the contribution of the CSD to research into molecular geometries, molecular interactions, and molecular assemblies and demonstrate its value in the design of biologically active molecules and the solid forms in which they are delivered. Its potential in other commercially relevant areas is described, including gas storage and delivery, thin films, and (opto)electronics. The CSD also aids the solution of new crystal structures. Because no scientific instrument is without shortcomings, the limitations of CSD research are assessed. We emphasize the importance of maintaining database quality: notwithstanding the arrival of big data and machine learning, it remains perilous to ignore the principle of garbage in, garbage out. Finally, we explain why the CSD must evolve with the world around it to ensure it remains fit for purpose in the years ahead.

A Million Crystal Structures: The Whole Is Greater than the Sum of Its Parts

In addition to the underlying basic concepts and early recognition of halogen bonding, this paper reviews the conflicting views that consistently appear in the area of noncovalent interactions and the ability of covalently bonded halogen atoms in molecules to participate in noncovalent interactions that contribute to packing in the solid-state. It may be relatively straightforward to identify Type-II halogen bonding between atoms using the conceptual framework of σ-hole theory, especially when the interaction is linear and is formed between the axial positive region (σ-hole) on the halogen in one monomer and a negative site on a second interacting monomer. A σ-hole is an electron density deficient region on the halogen atom X opposite to the R–X covalent bond, where R is the remainder part of the molecule. However, it is not trivial to do so when secondary interactions are involved as the directionality of the interaction is significantly affected. We show, by providing some specific examples, that halogen bonds do not always follow the strict Type-II topology, and the occurrence of Type-I and -III halogen-centered contacts in crystals is very difficult to predict. In many instances, Type-I halogen-centered contacts appear simultaneously with Type-II halogen bonds. We employed the Independent Gradient Model, a recently proposed electron density approach for probing strong and weak interactions in molecular domains, to show that this is a very useful tool in unraveling the chemistry of halogen-assisted noncovalent interactions, especially in the weak bonding regime. Wherever possible, we have attempted to connect some of these results with those reported previously. Though useful for studying interactions of reasonable strength, IUPAC’s proposed “less than the sum of the van der Waals radii” criterion should not always be assumed as a necessary and sufficient feature to reveal weakly bound interactions, since in many crystals the attractive interaction happens to occur between the midpoint of a bond, or the junction region, and a positive or negative site.

Halogen Bonding: A Halogen-Centered Noncovalent Interaction Yet to Be Understood

Negative thermal expansion in molecular materials.

Two new corrections, namely, area and shape corrections, have been introduced in the statistical analysis of halogen···halogen interactions. Before geometrical corrections, all the halogens show a clear preference for type 1 contact, but after the geometrical corrections the type 2 interaction has taken over type 1 contact for Cl, Br, and I for the Δθ ≠ 0 contacts. In the case of iodine, the population with type 1 contact becomes negligible and the directionality of the contacts increases dramatically after the area along with shape corrections. Without geometrical corrections, F shows a very high preference for lower Δθ, but after the correction it does not show much preference for any angle. Therefore, we anticipate that these corrections would bring a significant change in the concept about the halogen···halogen interactions.

Shape and Geometry Corrected Statistical Analysis on Halogen···Halogen Interactions

Here for the first time thermal expansion study has been explored to understand the bending mechanism in crystals. Dimorphic 4-chlorobenzonitrile has been chosen to demonstrate it. We have postulated that some of the structural features of the concave and convex sides of the bent crystal would resemble thermally compressed and expanded crystal structures respectively in the expansion–contraction bending mechanism. In this mechanism it has been shown that the strain is proportional to the thickness of the crystal and inversely proportional to the radius of the curvature of the bent crystal. On the other hand, in the case of slip plane mechanism, the amount of sliding of a layer with respect to its neighbor at the two terminals of the crystal is proportional to the arc angle of the bent crystal and the distance between the two consecutive slip planes.

Thermal Expansion Study as a Tool to Understand the Bending Mechanism in a Crystal

Three different approaches of statistical analyses, after surface area corrections, on the C–X···X–C (X = halogen) and C–H···X–C interactions have been performed. The analyses suggest that with respect to the accessible surface area on H and X, the C–H···X–C interactions are preferable over C–X···X–C interactions for all the halogens in the organic crystals. This is in contrast to the previous statistical analysis which stated that lighter halogens prefer C–H···X–C interactions whereas heavier halogens prefer C–X···X–C interactions. We have shown here the origin of the difference from the previous analysis. We also have shown here why it gives the impression that lighter halogens generally do not go for C–X···X–C interactions, but when they are replaced with the heavier halogens in the same molecule, the probability of formation of the C–X···X–C contacts increases. The preferred geometry of the C–X···X–C interactions with respect to the two ∠CXX angles and X···X distance have been shown. This study sugges...

C–X···X–C vs C–H···X–C, Which One Is the More Dominant Interaction in Crystal Packing (X = Halogen)?

Geometrical correction, used in statistical analysis on interhalogen interactions, provides a new insight into the nature of halogens. Statistical analysis without geometrical correction shows preference for Type 1 interactions over Type 2 interactions, but the trend is reversed after geometrical correction. It is known that polarizability in F is very small and hence should not show much preference for any particular angle in C–F···F–C interactions. Statistical analysis without geometrical correction could not prove it, but it is demonstrated here after geometrical correction. It is shown here that population after geometrical corrections vs θ1 and θ2 plots provides more meaningful information than the usually practiced population vs Δθ plot. The geometrical corrections proposed here are more general and can be used in several other interactions to eliminate geometrical bias from the statistical data in the process of extracting true chemical information from statistical analysis.

Interhalogen Interactions in the Light of Geometrical Correction

Establishment of structure-property relationship is one of the utmost important parts in structural chemistry and materials science. Thermal expansion of the strong bonds is known to be smaller than the weaker bonds. With that condition, can the dimensionality of coordination polymers be correlated to its thermal expansion? In this study, we found that the two 1D polymers exhibit much higher thermal expansion than the 2D polymer of the polymorphic coordination complex of ZnCl2 and 4,4'-bipyridine. It suggests that higher dimensionality can induce lower thermal expansion in coordination compounds. The thermal expansion property has also been used here in explaining the solid state phase transformation between the 1D and 2D structures. We anticipate that the dimensionality would be considered as an effective tool in designing new coordination polymer with desired thermal expansion properties and thermal expansion study would be used in explaining solid state phase transformation in many cases.

Sumair A. Rather

Papers

Shape and Geometry Corrected Statistical Analysis on Halogen···Halogen Interactions

Thermal Expansion Study as a Tool to Understand the Bending Mechanism in a Crystal

C–X···X–C vs C–H···X–C, Which One Is the More Dominant Interaction in Crystal Packing (X = Halogen)?

Interhalogen Interactions in the Light of Geometrical Correction

Dimensionality of a Coordination Polymer as a Tool To Control Thermal Expansion in a Polymorphic Coordination Compound