High-temperature monoclinic α-SrHfF₆, and isostructural α-SrZrF₆: associating Hf₂F₁₂bipolyhedra and SrF₈snub disphenoids

Abstract

The structure of the high-temperature monoclinic variety α-SrHfF (strontium hafnium hexafluoride) [and of isostructural α-SrZrF (strontium zirconium hexafluoride)] associates HfF bipolyhedra and SrF snub disphenoids, forming zigzag twisted [SrF] layers. The distribution of the Hf and Sr polyhedra forms a three-dimensional framework which can be related to the family of anion-excess ReO-related superstructures. α-SrHfF corresponds to a new ABX type and is compared to the other main families already described. A partial amorphization of this structure is observed in samples quenched from the melt.

Li2PbF6 and SrPbF6 Revisited

“…The Sr cations are coordinated only by eight F2 atoms in a shape best described as a snub disphenoid (Johnson polyhedron J84). The Sr–F distances are 2.335(3) and 2.763(3) Å, and in good agreement to those reported for SrHfF 6 [2.376(3)–2.763(2) Å, P 2 1 / c , 293 K], where are also SrF 8 snub disphenoids are present . Therefore the Sr cations interconnect the [PbF 6 ] 2– anions only via their F2 atoms.…”

Controlling the Phase Behavior of Low and Negative Thermal Expansion ReO3-Type Fluorides using Interstitial Anions: Sc1–xZrxF3+x

“…Upon incorporation of ZrF 4 , the nearest neighbor M−F peak broadens and moves to larger r. As six-coordinate Zr 4+ is smaller than six-coordinate Sc 3+ , the observed increase in the average M−F distance indicates that the solid solution samples must contain cations with a coordination number greater than six. It is notable that in β-BaZr 2 F 11 and α-SrHfF 6 , 28,29 which both contain pairs of edge-sharing Zr/HfF 7 polyehdra, the average M−F distance is 2.05 Å, consistent with the proposal that the observed increase in average M−F distance is associated with the creation of polyhedra with a coordination number greater than six. This is in agreement with the anion interstitial defect mechanism proposed based on the density measurements.…”

“…(b) A cartoon illustrating how corner-sharing polyhedra can be converted to edge-sharing polyhedra by incorporating excess fluoride into the structure. The 3.63 Å M–M distance and ∼2.05 Å M–F distance, marked for the pentagonal bipyramids, are approximately the M–M and average Zr/Hf–F distances found for pairs of edge-sharing MF 7 units in β -BaZr 2 F 11 and α -SrHfF 6 . , However, in reality, the edge-sharing pentagonal bipyramids are expected to be highly distorted due to the single shared edge.…”

“…A similar correlation peak was also observed for YbZrF 7 and the [Mg 1– x Zr x ]­ZrF 6+2 x system and attributed to the metal pairs on the opposite sides of a shared polyhedral edge (see Figure b). It is notable that in β-BaZr 2 F 10 and α-SrHf 6 , which are well ordered and contain edge-sharing ZrF 7 and HfF 7 polyhedra, the Zr–Zr and Hf–Hf separations across a shared edge are 3.61 and 3.63 Å, respectively. The apparent shift in the correlation peak at ∼4 Å to longer distances upon the incorporation of zirconium can be attributed to an increase in the average nearest neighbor M–M distance for metals that share a single fluoride (corner-sharing polyhedra) rather than a pair of fluorides (edge-sharing polyhedral), see Figure b.…”

Controlling the Negative Thermal Expansion and Response to Pressure in ReO3-type Fluorides by the Deliberate Introduction of Excess Fluoride: Mg1–xZr1+xF6+2x, x = 0.15, 0.30, 0.40, and 0.50

“…This is consistent with a move away from corner sharing octahedra, where a single nearest neighbor F–F distance within each octahedron is expected, to more irregular geometry higher coordination number polyhedra, which are partially edge sharing. F–F pairs that lie on a shared edge are likely to be closer together, see for example β-BaZr 2 F 10 and α-SrHfF 6 , where distances of 2.38 and 2.35 Å, respectively, were reported for F–F pairs associated with the shared edges of two ZrF 7 or HfF 7 polyhedra. It is also notable that the Zr–Zr and Hf–Hf separations across this shared edge were 3.61 and 3.63 Å, respectively.…”