2D hybrid organic-inorganic perovskites (2D HOIPs) are of interest for optoelectronic and phase-change applications. Using ultra-fast (flash) differential scanning calorimetry (FDSC), this study shows the 2D HOIPs (S-Cl-MBA)2PbI4 and (R-Cl-MBA)2PbBr4 (Cl-MBA referring to 4-chloro-α-methylbenzylamine) form a glass on cooling. Both show evidence of a liquid-to-glass transition during quenching from the liquid state; on reheating, a glass-to-liquid transition is followed by crystallization and melting. Using continuous heating in FDSC, the temperature dependence of the liquid viscosity of (S-Cl-MBA)2PbI4 is characterized. The kinetic fragility of the liquid is similar to that of bulk metallic glass-formers and significantly lower than that of organic and phase-change chalcogenide liquids. On cooling the liquid, glass formation is first impeded by thermal degradation, then crystallization. The stages of thermal degradation can be related to known mechanisms. This study highlights the reduced glass-transition temperature and the liquid fragility as key parameters in guiding the optimization of 2D HOIP compositions for targeted applications.

The hexagonal polymorph of Ba3Tb(BO3)3 contains Tb3+ ions on a quasi-2D triangular lattice, resulting in geometric magnetic frustration. Powder samples of Ba3Tb(BO3)3 have been investigated using specific heat, powder neutron diffraction (PND), inelastic neutron scattering (INS), and muon-spin relaxation spectroscopy (μSR). No long-range magnetic ordering is observed down to the lowest measured temperatures of 75 mK in PND and specific heat data and 1.5 K in the μSR data. Modeling the INS spectrum using a point charge model suggests that the ground state is a singlet with a low-lying doublet on each of the two crystallographically independent Tb3+ sites and that both the Tb ions display weak XY single-ion anisotropy. Published by the American Physical Society 2025

The hexagonal polymorph of Ba$_3$Tb(BO$_3$)$_3$ contains Tb$^{3+}$ ions on a quasi-2D triangular lattice, resulting in geometric magnetic frustration. Powder samples of Ba$_3$Tb(BO$_3$)$_3$ have been investigated using specific heat, powder neutron diffraction (PND), inelastic neutron scattering (INS) and muon-spin relaxation spectroscopy ($\mu$SR). No long-range magnetic ordering is observed down to the lowest measured temperatures of 75 mK in PND and specific heat data and 1.5 K in the $\mu$SR data. Modelling the INS spectrum using a point charge model suggests that the ground state is a singlet with a low-lying doublet on each of the two crystallographically independent Tb$^{3+}$ sites and that both the Tb ions display weak XY single-ion anisotropy.

NaNiO2 (NNO) has been investigated as a promising sodium-ion battery cathode material, but it is limited by degradation-induced capacity fade. On desodiation, NNO forms multiple phases with large superstructures due in part to Na+-ion vacancy ordering; however, their structures are unknown. Here, we report a structural solution to the Na2/3NiO2 (P'3) desodiated phase using combined Rietveld refinement of high-resolution synchrotron X-ray (SXRD) and neutron powder diffraction (NPD) data, magnetic susceptibility, and 23Na solid-state nuclear magnetic resonance (ssNMR) spectroscopy. Our experimental results are compared to ab initio molecular dynamics (AIMD) simulations, which indicate multiple low-energy structures that are dynamically populated. We observe a combination of competing effects that contribute to the resultant dynamic nature of the structure, including honeycomb ordering of mixed-valence Ni, orbital ordering of Jahn-Teller (JT) distorted Ni3+, and zigzag Na+/vacancy ordering. Our work provides evidence of multiple contributions to the structures of desodiated Na2/3NiO2, along with a framework for investigating the other unsolved desodiated structures. This work may also inform our understanding of the Jahn-Teller evolution in other nickel-rich lithium- and sodium-ion cathodes, such as LiNiO2.

The hexagonal polymorph of Ba_3Tb(BO_3)_3 contains Tb^3+ ions on a quasi-2D triangular lattice, resulting in geometric magnetic frustration. Powder samples of Ba_3Tb(BO_3)_3 have been investigated using specific heat, powder neutron diffraction (PND), inelastic neutron scattering (INS) and muon-spin relaxation spectroscopy (μSR). No long-range magnetic ordering is observed down to the lowest measured temperatures of 75 mK in PND and specific heat data and 1.5 K in the μSR data. Modelling the INS spectrum using a point charge model suggests that the ground state is a singlet with a low-lying doublet on each of the two crystallographically independent Tb^3+ sites and that both the Tb ions display weak XY single-ion anisotropy.

Lithium nickel oxide, LiNiO2 (LNO), and its doped derivatives are promising battery cathode materials with high gravimetric capacity and operating voltages. They are also of interest to the field of quantum magnetism due to the presumed S = 1/2 triangular lattice and associated geometric frustration. However, the tendency for Li/Ni substitutional defects and off-stoichiometry makes fundamental studies challenging. In particular, there is still a discrepancy between the rhombohedral (R3̅m) bulk structure and the Jahn-Teller (JT) distortions of the NiO6 octahedra inferred on the basis of local structural probes. Karger et al. (Chem. Mater. 2023, 35, 648-657) recently used Na/Li ion exchange to synthesize "defect-free" LNO by exploiting the absence of antisite disorder in NaNiO2 (NNO). Here we characterize the short- and long-range structure of this ion-exchanged material and observe splittings of key Bragg reflections at 100 K in X-ray and neutron diffraction (XRD and NPD), indicative of a monoclinic distortion induced by a cooperative collinear JT distortion, similar to that seen in NNO. Variable temperature XRD reveals a second-order phase transition from the monoclinic (C2/m) low-temperature structure to a rhombohedral (R3̅m) structure above ∼400 K. We propose that this collinear JT ordering is also present in solid-state synthesized LNO with the domain size and extent of monoclinic distortion controlled by defect concentration. This new structural description of LNO will help advance our understanding of its electronic and magnetic properties and the series of phase transformations that this material undergoes upon electrochemical cycling in Li-ion batteries.

The magnetic structure of diamond-like lattice has been studied extensively in terms of the magnetic frustration. Here we report the distortion of stretched diamond lattice of Tb3+ (4f8) in M-TbTaO4 on application of a magnetic field. We have investigated the structural and magnetic properties of M phase terbium tantalate M-TbTaO4 as a function of temperature and magnetic field using magnetometry and powder neutron diffraction. Sharp lambda-shape transitions in d(chi T)/dT, dM/dH and specific heat data confirm the previously reported three-dimensional (3D) antiferromagnetic ordering at TN similar to 2.25 K. On application of a magnetic field the N & eacute;el temperature is found to decrease and variable field neutron diffraction experiments below TN at 1.6 K show an increase in both the bond and angle distortion of the stretched diamond lattice with magnetic field, indicating a potential magneto-elastic coupling effect. By combining our magnetometry, heat capacity and neutron diffraction results we generate a magnetic phase diagram for M-TbTaO4 as a function of temperature and field.

NaNiO2 (NNO) has been investigated as a promising sodium-ion battery (NIB) cathode material, but it is limited by degradation-induced capacity fade. On desodiation, NNO forms multiple phases with large superstructures due in part to Na+-ion vacancy ordering, however, their structures are unknown. Here, we report a structural solution to the Na2/3NiO2 (P/3) de-sodiated phase using combined Rietveld refinement of high-resolution synchrotron X-ray (SXRD) and neutron diffraction (NPD) data, magnetic susceptibility, and 23Na solid-state nuclear magnetic resonance (ssNMR) spectroscopy. Our experimental results are compared to Ab Initio Molecular Dynamics (AIMD) simulations, which indicate multiple low-energy structures that are dynamically populated. We observe a combination of competing effects which contribute to the resultant dynamic nature of the structure, including honeycomb ordering of mixed-valence Ni, orbital-ordering of Jahn-Teller (JT) distorted Ni3+, and zigzag Na+/vacancy ordering. Our work provides evidence of the multiple contributions to the structures of de-sodiated Na2/3NiO2, along with a framework for investigating the other unsolved de-sodiated structures. This work may also inform understanding of the Jahn-Teller evolution in other nickel-rich lithium and sodium ion cathodes, such as LiNiO2.

Hybrid organic-inorganic perovskites (HOIPs) have garnered significant attention for their crystalline properties, yet recent findings reveal that they can also form liquid and glassy phases, offering an alternative platform for understanding non-crystalline materials. In this study, we present a detailed investigation into the structural dynamics of the melting and glass formation process of a two-dimensional (2D) HOIP, (S-(-)-1-(1-naphthyl)ethylammonium)2PbBr4. Compared to its crystalline counterpart, the glass exhibits superior mechanical properties, including higher Young's modulus and hardness. Our structural studies reveal that the liquid and glass formed from the 2D HOIP exhibit network-forming behaviour, featuring limited short-range order within individual octahedra, partial retention of metal-halide-metal connectivity between neighbouring octahedra, and residual structural correlations mediated by organic cations. We then combine in situ variable-temperature X-ray total scattering experiments, terahertz far-infrared absorption spectroscopy and solid-state nuclear magnetic resonance techniques to study the melting mechanism and the nature of the HOIP liquid obtained. Our results deepen the understanding of the structural evolution and property relationships in HOIP glasses, providing a foundation for their potential applications in advanced phase-change material technologies.

The rapid rise in battery demand, coupled with growing lithium prices and resource concerns, is driving significant research into alternative rechargeable battery technologies, with growing interest in multivalent ion batteries that promise high cost-effectiveness and sustainability [1] . Rechargeable Magnesium-ion Batteries (RMBs), use the divalent Mg 2+ as the charge storing ion. Although promising prototypes have been presented in the last decades [2] , RMBs must achieve competitive energy densities to be viable alternatives to lithium-ion batteries in commercial applications. The enhancement of RMB energy densities can be achieved by developing novel cathode materials with higher gravimetric capacities and/or operating voltages [3] . Recent literature has identified borates as potential novel cathode materials due to their ability to deliver high operating voltages, attributed to the polyanion inductive effect [4] . Evaluating the electrochemical performance of cathodes for Mg-ion batteries is challenging with many reports relying on an observed electrochemical capacity rather than demonstrating Mg-ion (de)-intercalation [5] . We present the first systematic study of the viability of borate polyanions as intercalation cathode materials for RMBs, focusing on three chemically distinct classes of borate polyanions: orthoborates M 3 (BO 3 ) 2 , ludwigites M 3 BO 5 , and pyroborates M 2 B 2 O 5 (Fig 1). We select five representative materials and use a suite of experimental techniques to investigate Mg 2+ ion mobility and demagnesiation after cycling vs Li metal with a Li electrolyte. For all the borates in this work ball-milling combined with high-temperature cycling (55⁰C) produced significant first-charge capacities up to 200 mAhg -1 in a voltage range of 3-4.2 V (vs Li metal). This promising capacity, alongside the extended voltage plateaus, suggested the potential viability of these candidate cathode materials for RMBs. This electrochemical performance was investigated using a wide range of in-depth post-cycling analyses, such as cutting-edge operando X-ray absorption near-edge structure (XANES) analysis, Mössbauer spectroscopy and scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDS) spectroscopy. Comprehensive analysis revealed that demagnesiation from the cathode structure accompanied by transition metal oxidation was not the source of the observed first charge capacity in the borates investigated. A further effort was put into identifying the origin of the promising first charge capacities common to the borate polyanion systems studied in this work. Through high-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy we propose a mechanism associated with a non-faradaic and irreversible reaction of an amorphous surface of the cathode particles. Such a surface composition could arise from the synthesis of these materials. The effect of ball-milling activates and increases the surface area and amorphous nature of these surface coatings, which when exposed to the electrolyte at high temperatures yield the significant capacities observed in this work. We believe this study will be of interest to the wider battery community, in demonstrating that an overreliance on measured capacities and structural changes can lead to ambiguous results when studying novel battery systems. We show that similar phenomena can arise from competing reactions of amorphous surface coatings, of trace impurities [6] or within the electrolyte rather than from Mg (de)intercalation. This highlights the complexity of cathode material behaviour in RMBs and emphasizes the necessity of thorough post-cycling characterization to fully understand these complexities. References [1] Y. Liang, H. Dong, D. Aurbach, and Y. Yao, Nat. Energy 5 , 646–656 (2020). [2] L. F. Wan, B. R. Perdue, C. A. Apblett, and D. Prendergast, Chem. Mater. 27 , 5932–5940 (2015). [3] J. Muldoon, C. B. Bucur, and T. Gregory, Angewandte Chemie Int. Ed. 56 , 12064–12084 (2017). [4] G. D. D. Sanglay, J. S. Garcia, M. S. Palaganas, M. Sorolla, S. See, L. A. Limjuco, and J. D. Ocon, Molecules 27 (2022). [5] I. D. Johnson, B. J. Ingram, and J. Cabana, ACS Energy Lett. 6 , 1892–1900 (2021). [6] C. Tacconis , S. Dey, C. D. McLaughlin, M. T. Sougrati, C. A. O’Keefe, I. Mikulska, C. P. Grey and S. E. Dutton, Chem. Mater, Accepted for publication , (2024). DOI: 10.1021/acs.chemmater.4c02855 Figure 1

Melt alloying, the process of melting a physical powder blend to create a homogeneous alloy, is widely used in materials processing. By carefully selecting the materials and their proportions, the physical properties of the resulting alloy can be precisely controlled. In this study, we investigate the possibility of utilizing melt alloying principles for meltable two-dimensional hybrid organic-inorganic perovskites (2D-HOIPs). We blend and melt mixtures of two selected 2D-HOIPs: the glass-forming (S-NEA)2PbBr4 (S-NEA = (S)-(-)-1-(1-naphthyl)ethylammonium) and the liquid-forming (1-MHA)2PbI4 (1-MHA = 1-methylhexylammonium). Upon melting and cooling, 1-MHA-poor blends (X1-MHA ≤ 50% mol, where X1-MHA corresponds to the relative molar concentration of (1-MHA)2PbI4 in the blend) form a hybrid glass, while 1-MHA-rich blends (X1-MHA ≥ 70% mol) crystallize. The melting temperature of all blends, as well as the glass transition temperature of the glass-forming blends, change according to blend composition. In all cases, melting produces a homogeneous structure, either glassy or crystalline, which remains such after the glassy samples are recrystallized upon a second heat treatment. This method enables band gap tuning of the blends, given that it varies with composition and crystallinity. Overall, this work demonstrates the applicability of classical melt processing to binary-component functional hybrid systems, and paves the way to solvent-free perovskite-based device fabrication.

The atomistic structure of lithium nickelate (LiNiO2), the parent compound of Ni-rich layered oxide cathodes for Li-ion batteries, continues to elude a comprehensive understanding. The common consensus is that the material exhibits local Jahn-Teller distortions that dynamically reorient, resulting in a time-averaged undistorted R3 ̅m structure. Through a combination of ab initio molecular dynamics (AIMD) simulations and variable-temperature X-ray diffraction (VT-XRD), we explore Jahn-Teller distortions in LiNiO2 as a function of temperature. Static Jahn-Teller distortions are observed at low temperatures (T < 250 K), followed by a broad phase transition occurring between 250 K and 350 K, leading to a highly dynamic, displacive phase at high temperatures (T > 350 K), which does not show the four short and two long bonds characteristic of local Jahn-Teller distortions. This transition is seen in AIMD simulations via abrupt changes in the calculated pair distribution function and the bond-length distortion index, and in X-ray diffraction via the monoclinic lattice parameter ratio amon/bmon and angle, the fit quality of an R3 ̅m-based structural refinement, and a peak-sharpening of the diffraction peaks on heating consistent with the loss of distorted domains. Between 250 K and 350 K, a mixed-phase regime is found via the AIMD simulations where distorted and undistorted domains coexist. The repeated change between the distorted and undistorted states in this mixed phase regime allows the Jahn-Teller long axes to change direction, these pseudorotations of the Ni-O long axes being a side effect of the onset of the displacive phase transition. Antisite defects, involving Li ions in the Ni layer and Ni ions in the Li layer, are found to pin the undistorted domains at low temperatures, impeding cooperative ordering at a longer length scale.

Tuning magnetic properties in layered van der Waals (vdW) materials has captured significant attention due to the efficient control of ground states by heterostructuring and external stimuli. Electron doping by electrostatic gating, interfacial charge transfer, and intercalation is particularly effective in manipulating the exchange and spin-orbit properties, resulting in a control of Curie temperature (TC) and magnetic anisotropy. Here, an uncharted role of intercalation is discovered to generate magnetic frustration. As a model study, Na atoms are intercalated into the vdW gaps of pristine Cr2Ge2Te6 (CGT) where generated magnetic frustration leads to emerging spin-glass states coexisting with a ferromagnetic order. A series of dynamic magnetic susceptibility measurements/analysis confirms the formation of magnetic clusters representing slow dynamics with a distribution of relaxation times. The intercalation also modifies other macroscopic physical parameters including the significant enhancement of TC from 66 to 240 K and the switching of magnetic easy-hard axis direction. This study identifies intercalation as a unique route to generate emerging frustrated spin states in simple vdW crystals.

High Ni-rich layered cathodes, such as; LiNi0.8Co0.15Al0.05O2 (NCA) and LiNi0.8Mn0.1Co0.1O2 (NMC811) materials, offer very high capacity and excellent rate capability fulfilling the pressing priorities. However, the cyclic performance is poor compared to other cathodes with lower Ni content. Due to high surface reactivity, first-cycle irreversibility is quite high in all these Ni-rich cathodes. Such surface reactions lead to different electrochemically inactive phases and reduce cyclic performance. Herein, we report a comprehensive analysis of plane selective cathode and electrolyte interface reactions and link the relationship between the formation of different cathode-electrolyte interphase (CEI) by-products with surface reconstruction mechanisms. Moreover, we established a guiding principle of coating strategy to enhance plane selective surface properties. Both bulk and thin-film Ni-rich NMC811 electrodes were used to investigate the surface properties. We fabricated epitaxial thin films of Ni-rich NMC811 cathodes of different orientations, such as; (104), (018), and (003). Al2O3 was chosen as a model coating material to understand how it modulates surface properties. The surface by-products depend highly on the surface charge and subsequently change with Al2O3 coating. Likewise, the surface reconstruction depends on these crystallographic planes and the coating layer exposed to the electrolyte. Keywords: NMC811; Thin film; Epitaxial; CEI; Surface reconstruction

This work uses a combination of neutron diffraction and bulk property measurements to establish the low temperature magnetic states in the dense metal-organic frameworks Ho(HCO2)3 and Er(HCO2)3; whose structures feature chains of face-sharing LnO9 polyhedra packed into a triangular lattice. Below 0.7 K Ho(HCO2)3 is found to adopt a state in which the magnetic moment on its ferromagnetic chains vary from neighbouring chains but the sum around each triangle is constant. Er(HCO2)3 is found to be the first lanthanide formate to adopt an ordered magnetic state with antiferromagnetic coupling within its chains near 50 mK. The potential to combine the ferromagnetic and antiferromagnetic coupling within chains associated with Ho and Er cations, respectively, in the same compound has also been explored via the series Ho1-xErx(HCO2)3. Ho0.5Er0.5(HCO2)3 remains paramagnetic to 0.4 K, suggesting it may be a starting point to search for a random spin chain paramagnet.

Below its Jahn-Teller transition temperature, TJT, NaNiO2 has a monoclinic layered structure consisting of alternating layers of edge-sharing NaO6 and Jahn-Teller-distorted NiO6 octahedra. Above TJT where NaNiO2 is rhombohedral, diffraction measurements show the absence of a cooperative Jahn-Teller distortion, accompanied by an increase in the unit cell volume. Using neutron total scattering, solid-state Nuclear Magnetic Resonance (NMR), and extended X-ray absorption fine structure (EXAFS) experiments as local probes of the structure we find direct evidence for a displacive, as opposed to order-disorder, Jahn-Teller transition at TJT. This is supported by ab initio molecular dynamics (AIMD) simulations. To our knowledge this study is the first to show a displacive Jahn-Teller transition in any material using direct observations with local probe techniques.

ABX3-type hybrid organic-inorganic structures have recently emerged as a new class of meltable materials. Here, by the use of phenylphosphonium derivatives as A cation, we study liquid- and glass-forming behavior of a new family of hybrid structures, (RPh3P)[Mn(dca)3] (R = Me, Et, Ph; dca = dicyanamide). These new compounds melt at 196-237 degrees C (T m) and then vitrify upon cooling to room temperature, forming glasses. In situ glass formation of this new family of materials was probed on a large scale using a variable-temperature PXRD experiment. Structure analyses of the crystalline and the glasses were carried out by solid-state nuclear magnetic resonance spectroscopy and synchrotron X-ray total scattering techniques for using the pair distribution function. The mechanical properties of the glasses produced were evaluated showing promising durability. Thermal and electrical conductivities showed low thermal conductivities (kappa similar to 0.07-0.09 W m-1 K-1) and moderate electrical conductivities (sigma similar to 10-4-10-6 S m-1) at room temperature, suggesting that by the precise control of the A cation, we can tune meltable hybrid structures from moderate conductors to efficient thermal insulators. Our results raise attention on the practical use of this new hybrid material in applications including, e.g., photovoltaic devices to prevent light-deposited heat (owing to low kappa RT), energy harvesting thermoelectric, etc., and advance the structure-property understanding.

We present the crystal-glass transformation of two-dimensional hybrid organic–inorganic perovskites (HOIPs) via ball-milling.

We investigate magnesium-iron pyroborate MgFeB2O5 as a potential cathode material for rechargeable magnesium-ion batteries. Synchrotron powder X-ray diffraction and Mossbauer spectroscopy confirm its successful synthesis and iron stabilization in the high-spin Fe(II) state. Initial electrochemical testing against a lithium metal anode yields a first charge capacity near the theoretical value (147.45 mAhg-1), suggesting MgFeB2O5 as a promising cathode candidate. However, multimodal analyses, including scanning electron microscopy energy-dispersive X-ray (SEM-EDS) analysis, operando X-ray absorption near edge spectroscopy (XANES), and Mossbauer spectroscopy, reveal the absence of any Fe redox reactions. Instead, we propose that the source of the observed capacity involves the irreversible reaction of a small (4-7 wt%) Fe metal impurity. These findings highlight the need for diverse characterization techniques in evaluating the performance of new Mg cathode materials, since promising initial cycling may be caused by competing side reactions rather than Mg (de)intercalation.

Van Vleck modes describe all possible displacements of octahedrally coordinated ligands about a core atom. They are a useful analytical tool for analysing the distortion of octahedra, particularly for first-order Jahn–Teller distortions, but determination of the Van Vleck modes of an octahedron is complicated by the presence of angular distortion of the octahedron. This problem is most commonly resolved by calculating the bond distortion modes ( Q 2 , Q 3 ) along the bond axes of the octahedron, disregarding the angular distortion and losing information on the octahedral shear modes ( Q 4 , Q 5 and Q 6 ) in the process. In this paper, the validity of assuming bond lengths to be orthogonal in order to calculate the Van Vleck modes is discussed, and a method is described for calculating Van Vleck modes without disregarding the angular distortion. A Python package for doing this, VanVleckCalculator , is introduced and some examples of its use are given. Finally, it is shown that octahedral shear and angular distortion are often, but not always, correlated, and a parameter η is proposed as the shear fraction. It is demonstrated that η can be used to predict whether the values will be correlated when varying a tuning parameter such as temperature or pressure.