Predicting Distortion Magnitudes in Prussian Blue Analogues

Based on simple electrostatic and harmonic potential considerations, we derive a straightforward expression linking the composition of a Prussian blue analogue (PBA) to its propensity to undergo collective structural distortions. We demonstrate the existence of a threshold value, below which PBAs are undistorted and above which PBAs distort by a degree that is controlled by a geometric tolerance factor. Our analysis rationalizes the presence, absence, and magnitude of distortions in a wide range of PBAs and distinguishes their structural chemistry from that of other hybrid perovskites

Dicyanometallates as Model Extended Frameworks

We report the structures of eight new dicyanometallate frameworks containing molecular extra-framework cations. These systems include a number of hybrid inorganic–organic analogues of conventional ceramics, such as Ruddlesden–Popper phases and perovskites. The structure types adopted are rationalized in the broader context of all known dicyanometallate framework structures. We show that the structural diversity of this family can be understood in terms of (i) the charge and coordination preferences of the particular metal cation acting as framework node, and (ii) the size, shape, and extent of incorporation of extra-framework cations. In this way, we suggest that dicyanometallates form a particularly attractive model family of extended frameworks in which to explore the interplay between molecular degrees of freedom, framework topology, and supramolecular interactions

Ordered B‑Site Vacancies in an ABX₃ Formate Perovskite

We report the synthesis and structural characterization of the ABX3 perovskite frameworks [C­(NH2)3]­Mn1–x2+(Fe2x/33+,□x/3)­(HCOO)3 (□ = B-site vacancy). For large x, the vacancies order, lowering the crystal symmetry. This system establishes B-site vacancies as a new type of defect in formate perovskites, with important chemical, structural, and functional implications. Monte Carlo simulations driven by nearest-neighbor vacancy repulsions show checkerboard vacancy order to emerge for x > 0.6, in accord with experiment

Dicyanometallates as Model Extended Frameworks

We report the structures of eight new dicyanometallate frameworks containing molecular extra-framework cations. These systems include a number of hybrid inorganic–organic analogues of conventional ceramics, such as Ruddlesden–Popper phases and perovskites. The structure types adopted are rationalized in the broader context of all known dicyanometallate framework structures. We show that the structural diversity of this family can be understood in terms of (i) the charge and coordination preferences of the particular metal cation acting as framework node, and (ii) the size, shape, and extent of incorporation of extra-framework cations. In this way, we suggest that dicyanometallates form a particularly attractive model family of extended frameworks in which to explore the interplay between molecular degrees of freedom, framework topology, and supramolecular interactions

Guest-Dependent Negative Thermal Expansion in Nanoporous Prussian Blue Analogues M^IIPt^IV(CN)₆·<i>x</i>{H₂O} (0 ≤ <i>x</i> ≤ 2; M = Zn, Cd)

The guest-dependent thermal expansion behavior of the nanoporous Prussian Blue analogues MIIPtIV(CN)6·x{H2O} (0 ≤ x ≤ 2; M = Zn, Cd) has been investigated using variable temperature single-crystal X-ray diffraction. The dehydrated phases MIIPtIV(CN)6 were found to exhibit negative thermal expansion, attributed to thermal population of low energy transverse vibrations of the bridging cyanide ligands. The presence of guest molecules within the framework pore system was found capable of dampening the effect of these transverse vibrational modes. The guest-loaded ZnII phase, in which the available pore volume is commensurate with the volume occupied by individual water molecules, possesses a considerably higher coefficient of thermal expansion, with the material switching from positive to negative thermal expansion behavior upon guest removal

Guest-Dependent Negative Thermal Expansion in Nanoporous Prussian Blue Analogues M^IIPt^IV(CN)₆·<i>x</i>{H₂O} (0 ≤ <i>x</i> ≤ 2; M = Zn, Cd)

The guest-dependent thermal expansion behavior of the nanoporous Prussian Blue analogues MIIPtIV(CN)6·x{H2O} (0 ≤ x ≤ 2; M = Zn, Cd) has been investigated using variable temperature single-crystal X-ray diffraction. The dehydrated phases MIIPtIV(CN)6 were found to exhibit negative thermal expansion, attributed to thermal population of low energy transverse vibrations of the bridging cyanide ligands. The presence of guest molecules within the framework pore system was found capable of dampening the effect of these transverse vibrational modes. The guest-loaded ZnII phase, in which the available pore volume is commensurate with the volume occupied by individual water molecules, possesses a considerably higher coefficient of thermal expansion, with the material switching from positive to negative thermal expansion behavior upon guest removal

Dicyanometallates as Model Extended Frameworks

We report the structures of eight new dicyanometallate frameworks containing molecular extra-framework cations. These systems include a number of hybrid inorganic–organic analogues of conventional ceramics, such as Ruddlesden–Popper phases and perovskites. The structure types adopted are rationalized in the broader context of all known dicyanometallate framework structures. We show that the structural diversity of this family can be understood in terms of (i) the charge and coordination preferences of the particular metal cation acting as framework node, and (ii) the size, shape, and extent of incorporation of extra-framework cations. In this way, we suggest that dicyanometallates form a particularly attractive model family of extended frameworks in which to explore the interplay between molecular degrees of freedom, framework topology, and supramolecular interactions

Thermal Expansion Matching via Framework Flexibility in Zinc Dicyanometallates

Thermal Expansion Matching via Framework Flexibility in Zinc Dicyanometallate

Rational Design of Materials with Extreme Negative Compressibility: Selective Soft-Mode Frustration in KMn[Ag(CN)₂]₃

We show that KMn[Ag(CN)₂]₃ exhibits the strongest negative linear compressibility (NLC) effect over the largest pressure range yet observed. Variable pressure neutron powder diffraction measurements reveal that its crystal lattice expands along the <i>c</i> axis of its trigonal cell under increasing hydrostatic pressure, while contracting along the <i>a</i> axis. This corresponds to a “wine-rack”-like mechanism for NLC that we find also results in anisotropic negative thermal expansion (NTE) in the same material. Inclusion of extra-framework K⁺ counterions has minimal effect on framework flexibility (and hence the magnitude of NTE/NLC) but selectively frustrates the soft phonon modes responsible for destroying NLC in the related material Ag₃[Co(CN)₆]

Guest-Activated Forbidden Tilts in a Molecular Perovskite Analogue

The manipulation of distortions in perovskite structures is critical to tailoring the properties of these materials for a variety of applications. Here we demonstrate a violation of established octahedral tilt rules in the double perovskite analogue (NH₄)₂SrFe­(CN)₆·2H₂O. The forbidden tilt pattern we observe arises through coupling to hydration-driven Jahn–Teller-like distortions of the Sr coordination environment. Access to novel distortion mechanisms and the ability to switch these distortions on and off through chemical modification fundamentally expands the toolbox of techniques available for engineering symmetry-breaking processes in solid materials