Hybrid Double Perovskite Containing Helium: [He₂][CaZr]F₆

Abstract

Perovskites are of great technological and geological importance, in large part, due to their considerable compositional and structural flexibility. However, the formation of perovskites with neutral species on their A-sites is very unusual. The formation, phase transitions, and properties of [He 2 ][CaZr]F 6 , which is the first helium-containing perovskite to be made, are reported. It is likely that a large family of related materials can also be prepared. On compression in neon, the negative thermal expansion (NTE) material CaZrF 6 amorphizes at ∼0.5 GPa. However, on compression in helium at room temperature, the gas is inserted into the structure to form a perovskite with helium on the A-site. This suppresses the amorphization until >3 GPa. The volume versus pressure and Raman measurements suggest that filling of the A-site, to give [He 2 ][CaZr]F 6 , is complete at >1 GPa. The presence of helium on the A-site in this perovskite leads to a reduction in the magnitude of NTE when compared to the parent phase CaZrF 6 , likely due to steric impediment of the transverse vibrational motion of fluoride. Helium also leads to considerable stiffening of the structure. At room temperature and ∼2.5 GPa, the helium-containing hybrid perovskite has a bulk modulus of ∼47 GPa, whereas CaZrF 6 has a bulk modulus of ∼40 GPa under ambient conditions. Cubic perovskite [He 2 ][CaZr]F 6 undergoes a structural phase transition at 15 K on compression, which may involve a cooperative tilting of framework octahedra to give a lower-symmetry phase, which is tentatively assigned as tetragonal.

Intercalating Helium into A-Site Vacant Perovskites

“…The recent pressure-induced insertion of helium into the vacant A-site perovskite [CaZr]F 6 has attracted much interest and tantalized with the prospect that other members of this perovskite family could encapsulate this noble gas as well. , Because these experimental studies were unable to fully characterize the crystal structure of the [He] 2 [CaZr]F 6 phase above ∼1 GPa, we employed density functional theory (DFT) calculations to predict the most stable geometry at this pressure. The F m 3̅ m → P 2 1 / c structural phase transition that occurs when [He] 2 [CaZr]F 6 is squeezed was found to be driven by a decrease in the pressure–volume term to the enthalpy.…”

“…57,58 The proclivity of helium incorporation into materials that contain hollow interstices was leveraged in the synthesis of the first heliumcontaining perovskite, [He] 2 [CaZr]F 6 , which was proposed as a gas storage material. 59,60 [CaZr]F 6 is a neutral hybrid double perovskite possessing vacant A-sites, and so, it is naturally porous. Helium could be inserted into the A-site cavities using mild pressures, with a solubility higher than in silica-glass or in cristobalite.…”

“…It is not surprising that pressure causes this porous structure to undergo amorphization at a mere 0.5 GPa when neon is used as the pressure transmitting medium. 60 When helium is used as the pressure transmitting medium instead, the amorphization is delayed until ∼3.5 GPa, aided by an uptake of helium into the pores and accompanied by a phase transition of Fm3̅ m [He] 2 [CaZr]F 6 (Figure 1a) to an unknown space group (suggested to be either I4/m or P4/ mnc) above 1.8 GPa and at low temperature. 60 We found that the most stable phase at 1 GPa possessed the monoclinic P2 1 /c space group instead, and subsequent calculations showed that it was more stable than the proposed I4/m and P4/mnc phases (Figure S1).…”

“…60 When helium is used as the pressure transmitting medium instead, the amorphization is delayed until ∼3.5 GPa, aided by an uptake of helium into the pores and accompanied by a phase transition of Fm3̅ m [He] 2 [CaZr]F 6 (Figure 1a) to an unknown space group (suggested to be either I4/m or P4/ mnc) above 1.8 GPa and at low temperature. 60 We found that the most stable phase at 1 GPa possessed the monoclinic P2 1 /c space group instead, and subsequent calculations showed that it was more stable than the proposed I4/m and P4/mnc phases (Figure S1). In the Fm m P c 3 2 / 1 transformation, the [CaZr]F 6 octahedra rotate in a way that resembles what would be expected for a transition to the P4/mnc spacegroup, with the main difference being an additional tilting along the cdirection that induces a staggered, rather than a linear, orientation of the He atoms within the channels, as shown in Figure 1b.…”

In silico capture of noble gas atoms with a light atom molecule

“…35 No bonding interaction was ever observed involving a He atom, disregarding the heavily debated and still hypothetical He@adamantane case. 36–39 Besides the many attempts to actively bind Ng atoms, they can alternatively be inserted into fullerenes, boron cages and clathrates (confinement strategy), from which they don’t escape due to kinetic stabilization, 40–51 trapped in ionic solid-state-compounds or metals, 52–55 or adsorbed in metal organic frameworks (MOFs). 56,57 Numerous computational studies analyzed the capability of other (mostly hypothetical) cage structures, such as closo -borane derivatives, BN-clusters, etc.…”