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.

CaZrF6 has recently been shown to combine strong negative thermal expansion (NTE) over a very wide temperature range (at least 10–1000 K) with optical transparency from mid-IR into the UV range. Variable-temperature and high-pressure diffraction has been used to determine how the replacement of calcium by magnesium and zirconium by niobium­(IV) modifies the phase behavior and physical properties of the compound. Similar to CaZrF6, CaNbF6 retains a cubic ReO3-type structure down to 10 K and displays NTE up until at least 900 K. It undergoes a reconstructive phase transition upon compression to ∼400 MPa at room temperature and pressure-induced amorphization above ∼4 GPa. Prior to the first transition, it displays very strong pressure-induced softening. MgZrF6 adopts a cubic (Fm3̅m) structure at 300 K and undergoes a symmetry-lowering phase transition involving octahedral tilts at ∼100 K. Immediately above this transition, it shows modest NTE. Its’ thermal expansion increases upon heating, crossing through zero at ∼500 K. Unlike CaZrF6 and CaNbF6, it undergoes an octahedral tilting transition upon compression (∼370 MPa) prior to a reconstructive transition at ∼1 GPa. Cubic MgZrF6 displays both pressure-induced softening and stiffening upon heating. MgNbF6 is cubic (Fm3̅m) at room temperature, but it undergoes a symmetry-lowering octahedral tilting transition at ∼280 K. It does not display NTE within the investigated temperature range (100–950 K). Although the replacement of Zr­(IV) by Nb­(IV) leads to minor changes in phase behavior and properties, the replacement of the calcium by the smaller and more polarizing magnesium leads to large changes in both phase behavior and thermal expansion.

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. The insertion of helium gas into CaZrF both elastically stiffens the material and reduces the magnitude of its negative thermal expansion. It also suppresses the onset of structural disorder, which is seen on compression in other media. Measurements of the gas released on warming to room temperature and Rietveld analyses of neutron diffraction data at low temperature indicate that exposure to helium gas at 500 MPa leads to a stoichiometry close to (He□)(CaZr)F. Helium has a much higher solubility in CaZrF than silica glass or crystobalite. An analogue with composition (H)(CaZr)F would have a volumetric hydrogen storage capacity greater than current US DOE targets. We anticipate that other hybrid perovskites with small neutral molecules on the A-site can also be prepared and that they will display a rich structural chemistry.

Although compounds of the formula AMoO2F3 (A = K, Rb, Cs, NH4, Tl) have been known for decades, crystal structures have only been reported for CsMoO2F3 and NH4MoO2F3. The three compounds (Rb/NH4/Tl)­MoO2F3 are isostructural and crystallize in the centrosymmetric space group C2/c (No. 15). The compounds contain the MoO2F3 – anionic chain, composed of corner-sharing MoO2F4 octahedra, with Mo6+ coordinated by two cis bridging fluoride anions that are trans to terminal oxide anions. The MoO2F3 – chain has a very unusual and complex chain structure; a single chain contains alternating zigzag and helical sections. These helical regions alternate in chirality along the chain, and thus the chains exhibit periodic tendril perversion. To the best of the authors’ knowledge, no other materials with a similar chain structure have been reported. On the other hand, KMoO2F3 is noncentrosymmetric and chiral, crystallizing in the enantiomorphic space group P212121 (No. 19). KMoO2F3 also contains the MoO2F3 – anionic chain. However, the chain is helical, with only one enantiomer present, resulting in a chiral, noncentrosymmetric structure.

Oxides with the CaFe 2 O 4 -type structure have been predicted as being suitable hosts for reactions of intercalation of light cations such as Li and Mg because of their favorable cationic diffusion. Although Li has been shown to intercalate into the Mn 2 O 4 variant, the key structure property correlations determining function are not fully ascertained. This basic information is needed before attempting the intercalation of divalent cations, which face comparably higher migration barriers. For this purpose, the electrode function of CaFe 2 O 4 -type Li 0.8 Mn 2 O 4 was compared for materials made by a direct high-pressure route or through cation exchange from NaMn 2 O 4 . X-ray diffraction and absorption spectroscopy revealed that, despite having largely the same bulk structure, the presence of surface defects blocked Li + transfer in Li 0.8 Mn 2 O 4 made at high pressure. These defects were not present in the cation-exchanged material, which resulted in highly reversible Li intercalation with very fast kinetics in micrometric crystals. Delithiated electrodes from the cation-exchange synthesis were subsequently reduced in an ionic liquid electrolyte containing Mg 2+ . The process induced topotactic changes in the bulk, strongly suggesting the existence of intercalation, but it is accompanied by severe reactivity with the electrolyte that impedes reversibility. This study uncovers that defects affect the fundamentals of cation intercalation in this novel class of materials. The ability of the cation-exchanged material to conduct fast reactions with Li is consistent with calculated activation energy barriers and creates promise for their use as Mg hosts, provided that novel electrolytes enhanced stability at high potential can be realized.