Evolution of Negative Thermal Expansion and Phase Transitions in Sc_1‑xTi_<i>x</i>F₃

The cubic ReO₃-type material ScF₃ 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‑<i>x</i>Ti_<i>x</i>F₃ solid solutions, which were characterized by synchrotron powder diffraction at ambient pressure from 100 to 500 K. TiF₃ is fully soluble in ScF₃ at a synthesis temperature of 1338 K. The temperature for the cubic-to-rhombohedral phase transition in Sc_1‑<i>x</i>Ti_<i>x</i>F₃ varies linearly with composition (above 100 K), and, at large <i>x</i>, 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‑<i>x</i>Ti_<i>x</i>F₃

Composition, Response to Pressure, and Negative Thermal Expansion in M^IIB^IVF₆ (M = Ca, Mg; B = Zr, Nb)

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 (<i>Fm</i>3̅<i>m</i>) 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 (<i>Fm</i>3̅<i>m</i>) 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

Synthesis of Defect Perovskites (He_2–<i>x</i>□_<i>x</i>)(CaZr)F₆ by Inserting Helium into the Negative Thermal Expansion Material CaZrF₆

Defect perovskites (He_2–<i>x</i>□_<i>x</i>)­(CaZr)­F₆ can be prepared by inserting helium into CaZrF₆ 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 CaZrF₆ 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 (He₁□₁)­(CaZr)­F₆. Helium has a much higher solubility in CaZrF₆ than silica glass or crystobalite. An analogue with composition (H₂)₂(CaZr)­F₆ 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

Role of Anion Site Disorder in the Near Zero Thermal Expansion of Tantalum Oxyfluoride

Materials with the cubic ReO₃-type structure are, in principle, excellent candidates for negative thermal expansion (NTE). However, many such materials, including TaO₂F, do not display NTE. It is proposed that local distortions away from the ideal structure, associated with the need to accommodate the different bonding requirements of the disordered O/F, contribute to the occurrence of near zero thermal expansion rather than NTE. The local structure of TaO₂F is poorly described by an ideal cubic ReO₃-type model with O and F randomly distributed over the available anion sites. A supercell model featuring −Ta–O–Ta–O–Ta–F– chains along ⟨1 0 0⟩, with different Ta–O and Ta–F distances and O/F off-axis displacements, gives much better agreement with pair distribution functions (PDFs) derived from total X-ray scattering data for small separations (<8 Å). Analyses of PDFs derived from variable temperature measurements (80 to 487 K), over different length scales, indicate an average linear expansion coefficient of close to zero with similar contributions from the geometrically distinct Ta–OTa and Ta–FTa links in TaO₂F

Zero Thermal Expansion and Abrupt Amorphization on Compression in Anion Excess ReO₃‑Type Cubic YbZrF₇

Heat treatment of cubic YbZrF₇, after quenching from 1000 °C, leads to a material displaying precisely zero thermal expansion at ∼300 K and negative thermal expansion at lower temperatures. The zero thermal expansion is associated with a minimum in the lattice constant at ∼300 K. X-ray total scattering measurements are consistent with a previously proposed model in which the incorporation of interstitial fluoride into the ReO₃-related structure leads to both edge and corner sharing coordination polyhedra. The temperature dependence of the experimental pair correlation functions suggests that the expansions of edge and corner sharing links partly compensate for one another, supporting the hypothesis that the deliberate incorporation of excess fluoride into ReO₃ structure materials can be used as a design strategy for controlling thermal expansion. Cubic YbZrF₇ has a bulk modulus, <i>K</i>₀, of 55.4(7) GPa and displays pronounced pressure-induced softening [<i>K</i>₀′ = −27.7(6)] prior to an abrupt amorphization on compression above 0.95 GPa. The resulting glass shows a single sharp scattering maximum at <i>Q</i> ∼ 1.6 Å^–1

Large Negative Thermal Expansion and Anomalous Behavior on Compression in Cubic ReO₃‑Type A^IIB^IVF₆: CaZrF₆ and CaHfF₆

CaZrF₆ and CaHfF₆ display much stronger negative thermal expansion (NTE) (<i>α</i>_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 (<i>K</i>_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, A^IIB^IVF₆, providing for a detailed exploration of the chemical and structural factors controlling NTE and many opportunities for the design of controlled thermal expansion materials