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Abstract:

We report the magnetic behavior of the hybrid perovskites [Gua]Mn1–xFe2x/3□x/3(HCOO)3 (0 ≤ x ≤ 0.88), showing that vacancy ordering drives bulk ferrimagnetism for x > 0.6. The behavior is rationalized in terms of a simple microscopic model of percolation-induced ferrimagnetism. Monte Carlo simulations driven by this model reproduce the experimental dependence of magnetic susceptibility on x and show that, at intermediate compositions, domains of short-range vacancy order lead to the emergence of local magnetization. Our results open up a new avenue for the design of multiferroic hybrid perovskites.

Abstract:

We study the phenomenology of cooperative off-centering of K+ ions in potassiated Prussian blue analogues (PBAs). The principal distortion mechanism by which this off-centering occurs is termed a “K-ion slide”, and its origin is shown to lie in the interaction between local electrostatic dipoles that couple through a combination of electrostatics and elastic strain. Using synchrotron powder X-ray diffraction measurements, we determine the crystal structures of a range of low-vacancy K2M[Fe(CN)6] PBAs (M = Ni, Co, Fe, Mn, Cd) and establish an empirical link between composition, temperature, and slide-distortion magnitude. Our results reflect the common underlying physics responsible for K-ion slides and their evolution with temperature and composition. Monte Carlo simulations driven by a simple model of dipolar interactions and strain coupling reproduce the general features of the experimental phase behavior. We discuss the implications of our study for optimizing the performance of PBA K-ion battery cathode materials and also its relevance to distortions in other, conceptually related, hybrid perovskites.

Abstract:

We revisit the problem of polytypism in layered MX₂ materials, with a view to reinterpreting the phase space accessible to this family. Our starting point is to develop a simple, constructive, and compact label for the most commonly observed stacking arrangements, similar to the Glazer notation used to label tilt systems in perovskites. The key advantage of this label in the context of MX₂ systems is that it contains sufficient information to generate the corresponding stacking sequences uniquely. Using a related approach, we generate a Cartesian representation of the phase space containing all possible MX₂ polytypes, with the most common structures appearing as limiting cases. We argue that variation in, e.g., composition, temperature, or pressure may allow navigation of this phase space along continuous paths. This interpretation is shown to be consistent with the structural evolution of stacking-faulted MX₂ systems as a function in temperature and pressure. In this way, our study highlights the potential for controlling composition/structure/property relationships among layered MX₂ materials in ways that might not previously have been obvious.

Abstract:

A new reduced phase derived from the excitonic insulator candidate Ta₂NiSe₅ has been synthesized via the intercalation of lithium. LiTa₂NiSe₅ crystallizes in the orthorhombic space group Pmnb (no. 62) with lattice parameters a = 3.50247(3) Å, b = 13.4053(4) Å, c = 15.7396(2) Å, and Z = 4, with an increase of the unit cell volume by 5.44(1)% compared with Ta₂NiSe₅. Significant rearrangement of the Ta-Ni-Se layers is observed, in particular a very significant relative displacement of the layers compared to the parent phase, similar to that which occurs under hydrostatic pressure. Neutron powder diffraction experiments and computational analysis confirm that Li occupies a distorted triangular prismatic site formed by Se atoms of adjacent Ta₂NiSe₅ layers with an average Li–Se bond length of 2.724(2) Å. Li-NMR experiments show a single Li environment at ambient temperature. Intercalation suppresses the distortion to monoclinic symmetry that occurs in Ta₂NiSe₅ at 328 K and that is believed to be driven by the formation of an excitonic insulating state. Magnetometry data show that the reduced phase has a smaller net diamagnetic susceptibility than Ta₂NiSe₅ due to the enhancement of the temperature-independent Pauli paramagnetism caused by the increased density of states at the Fermi level evident also from the calculations, consistent with the injection of electrons during intercalation and formation of a metallic phase.