Effect of Extra-Framework Cations on Negative Linear Compressibility and High-Pressure Phase Transitions: A Study of KCd[Ag(CN)₂]₃

Abstract

The negative thermal expansion material potassium cadmium dicyanoargentate, KCd­[Ag­(CN)2]3, is studied at high pressure using a combination of X-ray single-crystal diffraction, X-ray powder diffraction, infrared and Raman spectroscopy, and density functional theory calculations. In common with the isostructural manganese analogue, KMn­[Ag­(CN)2]3, this material is shown to exhibit very strong negative linear compressibility (NLC) in the crystallographic c direction due to structure hinging. We find increased structural flexibility results in enhanced NLC and NTE properties, but this also leads to two pressure-induced phase transitionsto very large unit cells involving octahedral tilting and shearing of the structurebelow 2 GPa. The presence of potassium cations has an important effect on the mechanical and thermodynamic properties of this family, while the chemical versatility demonstrated here is of considerable interest to tune unusual mechanical properties for application.

Probing the Influence of Defects, Hydration, and Composition on Prussian Blue Analogues with Pressure

“…This is similar to B 0 of MnPt, suggesting that the incorporation of extra-framework cations does not significantly affect the compliance of the cubic phase—in contrast to some other framework materials. 65 − 67 In the region 1.6–2.0 GPa, CsMnCo appears to undergo pressure-induced softening and gradually transitions into a tetragonal phase around 2 GPa. Due to the gradual nature of the transition, it is difficult to pinpoint the exact transition pressure given the resolution of the data.…”

Spin crossover in the Prussian blue analogue FePt(CN)6induced by pressure or X-ray irradiation

“…55 Another key difference between the two PBAs is the presence of Cs ions in CsFeCr(CN) 6 , in contrast to the open nature of FePt(CN) 6 . Extra-framework cations can exert a strong influence on the dynamics of the framework, 56 which may also affect the electronic structure. Thus, local-structure and computational studies would help rationalise the effects of varying chemistry (e.g.…”

Reentrant Negative Linear Compressibility in MIL-53(Al) over an Ultrawide Pressure Range

“…The broad bands centered at 25,200 and 17,714 cm −1 can be ascribed to the transitions of 4 A 2 → 4 T 1 and 4 A 2 → 4 T 2 , respectively. In contrast to the location of emission peaks, the small absorption peak at the low frequency region was separated into two peaks centered at 14,460 and 14,220 cm −1 , assigned to the transitions of 4 A 2 → 2 T 1 and 4 A 2 → 2 E, respectively. The value of the 2 T 2 energy level was calculated by using the spectral data according to the ligand-field theory and put at 23,700 cm −1 .…”

“…Negative linear compressibility (NLC) is an anomalous mechanical behavior describing that a material can expand along a specific direction under uniform compression without any phase transition. NLC is thermodynamically allowed for it couples to the volume reduction but goes against the intuition that the compression is bound to generate the contraction. − NLC materials spur great interests in the exploration of inherent mechanisms for their unusual elasticity, which may also exploit a range of promising application fields such as interferometric pressure sensors, artificial muscles, and shock-resisted optical fibers. − Up to now, the NLC phenomenon has been observed in a diversity of materials, involving inorganic compounds such as BPO 4 , BAsO 4 , and α-BiB 3 O 6 , and metal–organic frameworks (MOFs) such as Ag 3 [Co­(CN) 6 ], Zn­[Au­(CN) 2 ] 2 , and KMn­[Ag­(CN) 2 ] 3 . − Several kinds of mechanisms for the generation of NLC have been proposed for specific materials, such as wine-rack and honeycomb networks, and the Lifshitz model. − …”