CaZrF6 and CaHfF6 display much stronger negative thermal expansion (NTE) (α L100 K ∼ −18 and −22 ppm·K–1, respectively) than ZrW2O8 and other corner-shared framework structures. Their NTE is comparable to that reported for framework solids containing multiatom bridges, such as metal cyanides and metal–organic frameworks. However, they are formable as ceramics, transparent over a wide wavelength range and can be handled in air; these characteristics can be beneficial for applications. The NTE of CaZrF6 is strongly temperature-dependent, and first-principles calculations show that it is largely driven by vibrational modes below ∼150 cm–1. CaZrF6 is elastically soft with a bulk modulus (K 300K) of 37 GPa and, upon compression, starts to disorder at ∼400 MPa. The strong NTE of CaZrF6, which remains cubic to <10 K, contrasts with cubic CoZrF6, which only displays modest NTE above its rhombohedral to cubic phase transition at ∼270 K. CaZrF6 and CaHfF6 belong to a large and compositionally diverse family of materials, AIIBIVF6, providing for a detailed exploration of the chemical and structural factors controlling NTE and many opportunities for the design of controlled thermal expansion materials.

Varying amounts of Co and Ni were substituted into the metal−organic framework Mg-MOF-74 via a one-pot solvothermal reaction, and the effects of these substitutions on CO 2 adsorption and kinetic water stability properties were examined. Based on elemental analyses, Co and Ni are more favorably incorporated into the MOF-74 framework from solution than Mg. In addition, reaction temperature more strongly impacts the final metal composition in these mixed-metal (MM) MOF-74 structures than does the reaction solvent composition. Single-component CO 2 adsorption isotherms were measured for the MM-MOF-74 systems at 5, 25, and 45 °C, and isosteric heats of adsorption were calculated. These results suggest that CO 2 adsorption properties can be adjusted by partial metal substitution. Water adsorption isotherms were also measured for the MM-MOF-74 samples, with powder X-ray diffraction patterns and Brunauer−Emmett−Teller surface areas measured both before and after water exposure. Results show that Mg-MOF-74 can gain partial kinetic water stability by the incorporation of Ni 2+ or Co 2+ metal ions that are less vulnerable to hydrolysis than Mg 2+ . Of particular note, Mg−Ni-MM-MOF-74 shows a significant increase in water stability when incorporating as little as 16 mol % Ni into the Mg-MOF-74 structure.

The cubic ReO 3 -type material ScF 3 exhibits strong isotropic negative thermal expansion (NTE) over a wide temperature range while remaining cubic. Control of its thermal expansion was investigated by forming Sc 1-x Ti x F 3 solid solutions, which were characterized by synchrotron powder diffraction at ambient pressure from 100 to 500 K. TiF 3 is fully soluble in ScF 3 at a synthesis temperature of 1338 K. The temperature for the cubic-torhombohedral phase transition in Sc 1-x Ti x F 3 varies linearly with composition (above 100 K), and, at large x, the transition is clearly first-order. The rhombohedral phase for each composition examined exhibits strongly positive thermal expansion, while the expansion of the cubic phase (between 420 and 500 K) is negative for all Sc 1-x Ti x F 3 .

Metal−organic frameworks (MOFs), such as MOF-74, can have open metal sites to which adsorbates such as CO 2 preferentially bind. 13 C NMR of 13 CO 2 is highly informative about the binding sites present in Mg-MOF-74. We used this technique to investigate loadings between ∼0.88 and 1.15 molecules of CO 2 per metal in Mg-MOF-74 at 295 K. 13 C lineshapes recorded as a function of loading can be understood in terms of the dependence of the CO 2 NMR frequency on the angle (θ) with respect to the CO 2 axis and the channel of the MOF, reflected in the Legendre polynomial, P 2 . In the fast motion limit, the NMR spectra reveal the time-averaged value of P 2 , where θ is the angle between the instantaneous CO 2 axis and the channel axis. DFT calculations were used to determine a weighted average of P 2 in this regime and are in good agreement with experimental data. Static variable temperature 13 C NMR from cryogenic temperatures to room temperature was used to investigate 13 CO 2 binding in Mg-MOF-74 loaded at two levels (∼0.88 and 1.08 molecules of CO 2 per metal), revealing temperature-dependent lineshapes. We have investigated the effect of partial substitution of Cd for Mg in Mg-MOF-74 on the 13 CO 2 variable temperature NMR spectra. The chemical shift anisotropy (CSA) that leads to characteristic lineshapes of 13 C indicates that incorporation of Cd leads to weaker binding energies for adsorbed CO 2 .

Scandium fluoride displays isotropic negative thermal expansion (NTE) from at least 10 to 1100 K and retains a cubic ReO3-type structure over this range; the NTE is most pronounced at low temperatures. Control of thermal expansion was explored by forming Sc1–xYxF3 (x≤0.25), which were characterized with synchrotron powder diffraction at ambient pressure from 100 to 800 K. The behavior of the solid solutions under pressure (≤0.276 GPa) was also examined while heating from 298 to 523 K. Insertion of the relatively large Y3+ ion into ScF3 results in a cubic-to-rhombohedral phase transition upon cooling from ambient temperature to 100 K, even at low substitution levels (5%). The coefficient of thermal expansion (CTE) of the solid solutions in the rhombohedral phase is strongly dependent on both composition and temperature; however, above 400 K, where all samples are cubic, the CTE appears to be largely independent of composition. The isothermal bulk modulus and CTE of ScF3, but not those of the solid solutions, are independent of temperature and pressure, respectively. Yttrium substitution lowers the bulk modulus, even at temperatures where the samples are cubic. Finally, the solid solutions stiffen upon heating.