Synthesis and Properties of the Helium Clathrate and Defect Perovskite [He □ ][CaNb]F.

The defect double perovskite [He □ ][CaNb]F, with helium on its A-site, can be prepared by the insertion of helium into ReO-type CaNbF at high pressure. Upon cooling from 300 to 100 K under 0.4 GPa helium, ∼60% of the A-sites become occupied.

Equation of state predictions for ScF3 and CaZrF6 with neural network-driven molecular dynamics.

In silico property prediction based on density functional theory (DFT) is increasingly performed for crystalline materials. Whether quantitative agreement with experiment can be achieved with current methods is often an unresolved question, and may require detailed examination of physical effects such as electron correlation, reciprocal space sampling, phonon anharmonicity, and nuclear quantum effects (NQE), among others. In this work, we attempt first-principles equation of state prediction for the crystalline materials ScF3 and CaZrF6, which are known to exhibit negative thermal expansion (NTE) over a broad temperature range.

Controlling the Phase Behavior of Low and Negative Thermal Expansion ReO-Type Fluorides using Interstitial Anions: ScZrF.

Density measurements suggest that the incorporation of ZrF into the cubic ReO-type structure of ScZrF is associated with the creation of anion interstitials. X-ray total scattering measurements are consistent with the conversion of corner-sharing octahedra to edge-sharing polyhedra as the solid solutions become richer in ZrF. The cubic (3̅) to rhombohedral (3) cooperative octahedral tilting transition seen for ScF moves to a higher pressure as increasing amounts of zirconium are added, and it is eventually suppressed completely ( = 0.

Tuning Thermal Expansion in Metal-Organic Frameworks Using a Mixed Linker Solid Solution Approach.

Several metal-organic frameworks are known to display negative thermal expansion (NTE). However, unlike traditional NTE material classes, there have been no reports where the thermal expansion of a MOF has been tuned continuously from negative to positive through the formation of single-phase solid solutions. In the system Zn-DMOF-TM, Zn[(bdc)(TM-bdabco)][dabco], the introduction of increasing amounts of TM-bdc, with four methyl groups decorating the benzene dicarboxylate linker, leads to a smooth transition from negative to positive thermal expansion in the - plane of this tetragonal material.

Negative Thermal Expansion, Response to Pressure and Phase Transitions in CaTiF.

Strong volume negative thermal expansion over a wide temperature range typically only occurs in ReO-type fluorides that retain an ideal cubic structure to very low temperatures, such as ScF, CaZrF, CaHfF, and CaNbF. CaTiF was examined in an effort to expand this small family of materials. However, it undergoes a cubic ( Fm3̅ m) to rhombohedral ( R3̅) transition on cooling to ∼120 K, with a minimum volume coefficient of thermal expansion (CTE) close to -42 ppm K at 180 K and a CTE of about -32 ppm K at room temperature.

Synthesis of Defect Perovskites (He□)(CaZr)F by Inserting Helium into the Negative Thermal Expansion Material CaZrF.

Defect perovskites (He□)(CaZr)F can be prepared by inserting helium into CaZrF at high pressure. They can be recovered to ambient pressure at low temperature. There are no prior examples of perovskites with noble gases on the A-sites.

Orientational order-dependent thermal expansion and compressibility of ZrW2O8 and ZrMo2O8.

The role of MO4 (M = W, Mo) orientational disorder in the thermal expansion and compressibility of ZrW2O8 and ZrMo2O8 was investigated via in situ powder X-ray diffraction at elevated temperature and pressure. A dramatic reduction in the bulk modulus of α-ZrW2O8, which has ordered WO4 tetrahedra at room temperature, from 65 GPa at room temperature to 47 GPa at 386 K was observed to be concomitant with the onset of a reversible WO4 orientational disordering upon compression. Additionally, the coefficient of thermal expansion (CTE) of the α phase became more negative upon compression within the temperature range in which pressure-dependent disorder was observed; αl, over the range 298 to 386 K, was ~-11 ppm K(-1) at 35 MPa but ~-16 ppm K(-1) at 276 MPa.

Pronounced negative thermal expansion from a simple structure: cubic ScF(3).

Scandium trifluoride maintains a cubic ReO(3) type structure down to at least 10 K, although the pressure at which its cubic to rhombohedral phase transition occurs drops from >0.5 GPa at ∼300 K to 0.1-0.