Samuel G. Duyker

Negative Thermal Expansion in LnCo(CN)6 (Ln=La, Pr, Sm, Ho, Lu, Y): Mechanisms and Compositional Trends†

Negative thermal expansion (NTE) is a comparatively rare phenomenon that is found in a growing number of materials. The discovery of new NTE materials and the elucidation of mechanisms underpinning their behavior is important both in extending the field and enabling tailored thermal expansion properties. NTE has been found throughout a broad family of cyanide coordination frameworks, arising from thermal population of low-energy transverse vibrations of the cyanide bridges, which reduce the average metal–metal distances, and thus the lattice parameters, with increasing temperature. More complex mechanisms have been established in metal– organic framework materials, in which both local and longrange modes contribute to NTE. The low-energy dynamics of metal-based materials are often modeled in terms of rigid unit modes (RUMs), wherein the metal-centered polyhedra are treated as rigid, with only the linkage being flexible. Most NTE cyanide frameworks are members of two cubic structural types: Zn(CN)2 analogues, [2a,b,5] containing tetrahedral metal centers in the diamondoid topology; and Prussian blue analogues, with octahedral metal centers in the a-Po topology. NTE has recently been observed in a framework of a different structural type: ErCo(CN)6, [7] possessing hexagonal symmetry (P63/mmc) owing to the combination of ErN6 trigonal prisms alternating with CoC6 octahedra. ErCo(CN)6 displays near-isotropic NTE with axial coefficients of thermal expansion (CTEs) aa= da/adT= 8 10 6 K , ac= 9 10 6 K 1 and effective linear CTE, al= 1/3 dV/VdT= 9 10 6 K . Herein we probe in detail the novel mechanism for NTE in this structure type through a comprehensive approach combining synthesis, structural and dynamic analysis, and modeling. Substitution of other trivalent lanthanoids for Er yields an extended series, LnCo(CN)6, of which representative members have been selected for characterization (Ln= La, Pr, Sm, Ho, Lu, and Y). Topotactic dehydration of the parent framework hydrates LnCo(CN)6·nH2O (n= 4, 5) yields an extended isostructural series with the trigonal prismatic LnN6 coordination geometry (Figure 1a, inset), which is a rare example of an isostructural

Extreme compressibility in LnFe(CN) 6 coordination framework materials via molecular gears and torsion springs

The mechanical flexibility of coordination frameworks can lead to a range of highly anomalous structural behaviours. Here, we demonstrate the extreme compressibility of the LnFe(CN)6 frameworks (Ln = Ho, Lu or Y), which reversibly compress by 20% in volume under the relatively low pressure of 1 GPa, one of the largest known pressure responses for any crystalline material. We delineate in detail the mechanism for this high compressibility, where the LnN6 units act like torsion springs synchronized by rigid Fe(CN)6 units performing the role of gears. The materials also show significant negative linear compressibility via a cam-like effect. The torsional mechanism is fundamentally distinct from the deformation mechanisms prevalent in other flexible solids and relies on competition between locally unstable metal coordination geometries and the constraints of the framework connectivity, a discovery that has implications for the strategic design of new materials with exceptional mechanical properties.

Topotactic structural conversion and hydration-dependent thermal expansion in robust LnMIII(CN)6·nH2O and flexible ALnFeII(CN)6·nH2O frameworks (A = Li, Na, K; Ln = La–Lu, Y; M = Co, Fe; 0 ≤ n ≤ 5)

The structures of the AxLnM(CN)6·nH2O (A = Li, Na, K; Ln = La–Lu, Y; M = Co, Fe; x = 0, 1; 0 ≤ n ≤ 5) cyanide frameworks, their thermal expansion behaviour, and their transformations upon dehydration are explored using X-ray and neutron single crystal diffraction and X-ray powder diffraction. Modification from positive to negative thermal expansion in the LnCo(CN)6·nH2O phases is achieved by removal of the guest water molecules. Most notable is the unprecedented flexibility demonstrated by the “coiling” of KLnFe(CN)6·nH2O frameworks upon their dehydration, wherein the lanthanoid coordination geometry reversibly converts from a 9-coordinate tri-capped trigonal prism to a 6-coordinate octahedron via a single-crystal-to-single-crystal process, accompanied by a large (14–16%) decrease in unit cell volume.

Guest-Activated Forbidden Tilts in a Molecular Perovskite Analogue

The manipulation of distortions in perovskite structures is critical to tailoring the properties of these materials for a variety of applications. Here we demonstrate a violation of established octahedral tilt rules in the double perovskite analogue (NH4)2SrFe(CN)6·2H2O. The forbidden tilt pattern we observe arises through coupling to hydration-driven Jahn-Teller-like distortions of the Sr coordination environment. Access to novel distortion mechanisms and the ability to switch these distortions on and off through chemical modification fundamentally expands the toolbox of techniques available for engineering symmetry-breaking processes in solid materials.

Powder sample-positioning system for neutron scattering allowing gas delivery in top-loading cryofurnaces

A system for positioning powder samples in top-loading cryofurnaces during neutron scattering experiments, while facilitating the successive delivery of gas doses at set temperatures to the sample, has been designed and tested. The positioning system is compatible with a Hiden Isochema IMI instrument as a gas-dosing platform, enabling gases to be delivered to the sample through a centrally located and thermally stabilized capillary line and valve. The positioning system separates into an upper and a lower section, with the lower section enabling the sample to be isolated and inserted into a glove box. This work describes the system using example neutron powder diffraction results obtained with this system in closed-cycle cryofurnaces.

Using neutron powder diffraction and first-principles calculations to understand the working mechanisms of porous coordination polymer sorbents

Metal-organic frameworks (MOFs) are promising solid sorbents, showing gas selectivity and uptake capacities relevant to many important applications, notably in the energy sector. To improve and tailor the sorption properties of these materials for such applications, it is necessary to gain an understanding of their working mechanisms at the atomic and molecular scale. Specifically, it is important to understand how features such as framework porosity, topology, chemical functionality and flexibility underpin sorbent behaviour and performance. Such information is obtained through interrogation of structure-function relationships, with neutron powder diffraction (NPD) being a particularly powerful characterization tool. The combination of NPD with first-principles density functional theory (DFT) calculations enables a deep understanding of the sorption mechanisms, and the resulting insights can direct the future development of MOF sorbents. In this paper, experimental approaches and investigations of two example MOFs are summarized, which demonstrate the type of information and the understanding into their functional mechanisms that can be gained. Such information is critical to the strategic design of new materials with targeted gas-sorption properties.

Ammonia-storage in lithium intercalated fullerides

Ammonia has been proposed as an indirect hydrogen carrier, as solid-state ammonia-storage could be easier than directly absorbing hydrogen in materials. Here we investigate the structural evolution of hyper-ammoniated lithium fullerides (ND3)yLi6C60 during ammonia desorption, using in-situ high intensity neutron powder diffraction. In (ND3)yLi6C60, ammonia molecules are stored in their neutral state inside the inter-fullerene interstices and are coordinated to the intercalated Li ions, forming Li–ND3 clusters. Li6C60 is found to absorb up to 36.8 wt% ND3, which corresponds to approximately 14 ammonia molecules per C60. The ammonia release, studied either in-situ or ex-situ by means of manometric analyses and differential scanning calorimetry, takes place in two main steps, at 350–410 K and 500–540 K, respectively. This corresponds to two clear 1st order structural phase transitions and the absorption process is partially reversible. These findings suggest that the system could be a good candidate for ammonia-storage applications.

Putting pressure on WOMBAT – outcomes and unique capabilities

Helen Elizabeth Maynard-Casely1, Stanley Lee2, Norman Booth2, Andrew Studer2, Vanessa Peterson2, Samuel Duyker3, Kazuki Komatsu4, Ryo Yamane4, Gabriel Murphy3, Thomas Vogt5 1Australian Nuclear Science And Technology Organisation, Kirrawee Dc, Australia, 2Australian Nuclear Science And Technology Organisation, Kirrawee, Australia, 3Department of Chemistry, University of Sydney, Sydney, Australia, 4Geochemical Research Centre, University of Tokyo, Toyko, Japan, 5University of South Carolina, Colombia, United States E-mail: helenmc@ansto.gov.au