Uncovering the Interplay of Competing Distortions in the Prussian Blue Analogue K₂Cu[Fe(CN)₆]

Abstract

We report the synthesis, crystal structure, thermal response, and electrochemical behavior of the Prussian blue analogue (PBA) K 2 Cu[Fe(CN) 6 ]. From a structural perspective, this is the most complex PBA yet characterized: its triclinic crystal structure results from an interplay of cooperative Jahn–Teller order, octahedral tilts, and a collective “slide” distortion involving K-ion displacements. These different distortions give rise to two crystallographically distinct K-ion channels with different mobilities. Variable-temperature X-ray powder diffraction measurements show that K-ion slides are the lowest-energy distortion mechanism at play, as they are the only distortion to be switched off with increasing temperature. Electrochemically, the material operates as a K-ion cathode with a high operating voltage and an improved initial capacity relative to higher-vacancy PBA alternatives. On charging, K + ions are selectively removed from a single K-ion channel type, and the slide distortions are again switched on and off accordingly. We discuss the functional importance of various aspects of structural complexity in this system, placing our discussion in the context of other related PBAs.

The uniaxial zero thermal expansion and zero linear compressibility in distorted Prussian blue analogue RbCuCo(CN)6

“…As a typical distorted PBA, RbCuCo(CN) 6 has an orthorhombic structure (Cccm symmetry), which is considered to originate from the interaction of the collective Jahn-Teller order with either the tilt distortion or ''rodlike'' Rb cation order. 25,[37][38][39] Based on group theory analysis, any two of the above three distortions necessarily generate the third, so any two distortions can cause the symmetry lowering of RbCuCo(CN) 6 . As seen from Fig.…”

Computational and Experimental Determination of the Electronic Structure and Optical Properties of Three-Dimensional Zn[M(CN)4] Tetracyanates (M = Ni, Pd, and Pt)

“…The electronic behavior and derived potential technological applications of cyanide-based coordination polymers are currently subjects of great interest and development. − The electronic structure of an isolated square-planar [Ni­(CN) 4 ] unit has been broadly studied and represents a particular case study within crystal field theory. − The molecular orbitals and the energy structure of this moiety are determined by the central nickel through its 3d (d xy , d yz , d xz , d x 2 – y 2 , and d z 2 ), 4s, and 4p (p x , p y , and p z ) orbitals overlapping with the σ, π, and π* orbitals from the cyanide (CN) ligand. Similar to other cyanide-based coordination polymers, the effect of charge donation through back-bonding between d metal orbitals and π ligand orbitals plays an important role in the electronic behavior .…”

Predicting Distortion Magnitudes in Prussian Blue Analogues

“…23 The reduced A-site occupancy of the oxidized state means there is an insufficient electrostatic driving force to drive distortion, but on reduction the increased A-site occupancy induces long-range symmetry breaking. 24 Our derivation is intentionally simplistic and involves a number of crude approximations. We have not taken into account electronic distortion mechanisms, such as the Jahn− Teller effect, which needs to be treated separately.…”

“…Since one expects the thermal volume of A-and X-site ions to grow more quickly than that of the B-site, α should increase with temperature, rationalizing the thermal quenching of distortions observed experimentally. 21,24 So the structural chemistry of PBAs differs conceptually from that of other hybrid perovskites as a consequence of the linear ground-state geometry of the B−X−B linkages. In conventional perovskites, for example, the tilt modes are usually mechanically unstable in the absence of sufficiently large A-site cations.…”

“…Immediately it rationalizes why low-vacancy PBA cathode materials switch between distorted and undistorted phases as A-site cations are shuttled in and out during electrochemical cycling, for example . The reduced A-site occupancy of the oxidized state means there is an insufficient electrostatic driving force to drive distortion, but on reduction the increased A-site occupancy induces long-range symmetry breaking …”