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A temperature-induced structural phase transition is reported in layered hybrid perovskite (MA)2PbI2−xBrx(SCN)2 (0 ≤ x ≤ 1.2). The observed transition temperature decreases with Br substitution, suggesting a weakening of bonding interaction between the molecular ions.

Ternary nitride materials hold promise for many optical, electronic, and refractory applications yet their preparation via solid-state synthesis remains challenging. Often, high pressures or reactive gasses are used to manipulate the effective chemical potential of nitrogen, yet these strategies require specialized equipment. Here we report on a simple two-step synthesis using ion-exchange reactions that yield rocksalt-derived MgZrN$_2$ and Mg$_2$NbN$_3$, as well as layered MgMoN$_2$. All three compounds show nearly temperature-independent and weak paramagnetic responses to an applied magnetic field at cryogenic temperatures indicating phase pure products. The key to synthesizing these ternary materials is an initial low-temperature step (300-450 $^{\circ}$C) to promote Mg-M-N bond formation. Then the products are annealed (800-900 $^{\circ}$C) to increase crystalline domains of the ternary product. Calorimetry experiments reveal that initial reaction temperatures are determined by phase transitions of reaction precursors, whereas heating directly to high temperatures results in decomposition. These two-step reactions provide a rational guide to material discovery of other bulk ternary nitrides.

Author(s): Wustrow, A; Huang, G; McDermott, MJ; O'Nolan, D; Liu, CH; Tran, GT; McBride, BC; Dwaraknath, SS; Chapman, KW; Billinge, SJL; Persson, KA; Thornton, K; Neilson, JR | Abstract: Low-temperature synthesis routes are necessary for selectively synthesizing many metastable solid-state materials. Here, we identify a cooperative effect that starting materials have in lowering temperatures in solid-state metathesis reactions by studying the formation of yttrium manganese oxide. Previous studies have shown that YMnO3 can be synthesized by ternary metathesis with an alkali halide being produced as a secondary product. In this contribution, we show that by using alkaline earth metals instead of alkali metals, the polymorph selectivity of the reaction is changed, as orthorhombic YMnO3 forms at lower temperatures than the hexagonal polymorph. Reactions were studied using ex post facto synchrotron X-ray diffraction. These experiments reveal that reactions using alkaline earth manganese oxides as a starting material require high temperatures to progress. Reaction temperatures can be lowered from 700 to 550 °C while maintaining phase selectivity by reacting both MgMn2O4 and CaMn2O4 with YOCl in a cooperative "cometathesis"reaction. The nascent halide salts appear to improve the reaction kinetics. Since the onset temperature for YMnO3 formation falls 50 °C below the MgCl2-CaCl2 liquidus, the enhanced reactivity is consistent with the surface melting of a nasscent salt byproduct at the interfaces. Cometathesis routes have similar phase selectivity and temperature reduction in reactions that form TbMnO3, ErMnO3, and DyMnO3. Cometathesis lowers reaction temperatures while preserving the reaction selectivity of the end members, making it a valuable approach for synthesizing metastable targets.

A cloud-hosted web-based software application, nmfMapping, for carrying out a nonnegative matrix factorization of a set of powder diffraction or atomic pair distribution function datasets is described. This app allows structure scientists to rapidly find trends in sets of related data such as from in situ and operando diffraction experiments. The application is easy to use and does not require any programming expertise. It is available at the pdfitc.org website., Comment: 19 Pages, 7 Figures, submitted to Acta Crystallographica Section A

The layered perovskite (MA)2PbI2(SCN)2 (MA = CH3NH3+) is a member of an emerging series of compounds derived from hybrid organic-inorganic perovskites. Here, we successfully synthesized (MA)2PbI2-xBrx(SCN)2 (0 ≤ x < 1.6) by using a solid-state reaction. Despite smaller bromide substitution for iodine, 1% linear expansion along the a axis was observed at x ∼ 0.4 due to a change of the orientation of the SCN- anions. Diffuse reflectance spectra reveal that the optical band gap increases by the bromide substitution, which is supported by the DFT calculations. Curiously, bromine-rich compounds where x ≥ 0.8 are light sensitive, leading to partial decomposition after ∼24 h. This study demonstrates that the layered perovskite (MA)2PbI2(SCN)2 tolerates a wide range of bromide substitution toward tuning the band gap energy.

In the synthesis of complex oxides, solid-state metathesis provides low-temperature reactions where product selectivity can be achieved through simple changes in precursor composition. The influence of precursor structure, however, is less understood in solid-state synthesis. Here we present the ternary metathesis reaction (LiMnO2 + YOCl → YMnO3 + LiCl) to target two yttrium manganese oxide products, hexagonal and orthorhombic YMnO3, when starting from three different LiMnO2 precursors. Using temperature-dependent synchrotron X-ray and neutron diffraction, we identify the relevant intermediates and temperature regimes of reactions along the pathway to YMnO3. Manganese-containing intermediates undergo a charge disproportionation into a reduced Mn(II,III) tetragonal spinel and oxidized Mn(III,IV) cubic spinel, which lead to hexagonal and orthorhombic YMnO3, respectively. Density functional theory calculations confirm that the presence of Mn(IV) caused by a small concentration of cation vacancies (∼2.2%) in YMnO3 stabilizes the orthorhombic polymorph over the hexagonal. Reactions over the course of 2 weeks yield o-YMnO3 as the majority product at temperatures below 600 °C, which supports an equilibration of cation defects over time. Controlling the composition and structure of these defect-accommodating intermediates provides new strategies for selective synthesis of complex oxides at low temperatures.

Although initial studies on hybrid perovskites for photovoltaic applications focused on simple compositions, the most technologically relevant perovskites are heavily substituted. The influence of ...

Olivine Fe2GeS4 has been identified as a promising photovoltaic absorber material introduced as an alternate candidate to iron pyrite, FeS2. The compounds share similar benefits in terms of element...

Functional small molecules afford opportunities to direct solid-state inorganic reactions at low temperatures. Here, we use catalytic amounts of organosilicon molecules to influence the metathesis reaction: FeCl2 + Na2S2 → 2NaCl + FeS2. Specifically, hexaphenyldisiloxane ((C6H5)6Si2O) is shown to increase pyrite yields in metathesis reactions performed at 150 °C. In situ synchrotron X-ray diffraction (SXRD) paired with differential scanning calorimetry (DSC) reveals that diffusion-limited intermediates are circumvented in the presence of (C6H5)6Si2O. Control reactions suggest that the observed change in the reaction pathway is imparted by the Si–O functional group. 1H NMR supports catalytic behavior, as (C6H5)6Si2O is unchanged ex post facto. Taken together, we hypothesize that the polar Si–O functional group coordinates to iron chloride species when NaCl and Na2S4 form, forming an unidentified, transient intermediate. Further exploration of targeted small molecules in these metathesis reaction provides new strategies in controlling inorganic materials synthesis at low-temperatures.

The emission of white light from a single material is atypical and is of interest for solid-state lighting applications. Broadband light emission has been observed in some layered perovskite derivatives, A₂PbBr₄ (A = R-NH₃⁺), and correlates with static structural distortions corresponding to out-of-plane tilting of the lead bromide octahedra. While materials with different organic cations can yield distinct out-of-plane tilts, the underlying origin of the octahedral tilting remains poorly understood. Using high energy resolution (e.g., quasi-elastic) neutron scattering, this contribution details the rotational dynamics of the organic cations in A₂PbBr₄ materials where A = n-butylammonium (nBA), 1,8-diaminooctammonium (ODA), and 4-aminobutyric acid (GABA). The organic cation dynamics differentiate (nBA)₂PbBr₄ from (ODA)PbBr₄ or (GABA)₂PbBr₄ in that the larger spatial extent of dynamics of nBA yields a larger effective cation radius. The larger effective volume of the nBA cation in (nBA)₂PbBr₄ yields a closer to ideal A-site geometry, preventing the out-of-plane tilt and broadband luminescence. In all three compounds, we observe hydrogen dynamics attributed to rotation of the ammonium headgroup and at a time scale faster than the white light photoluminescence studied by time-correlated single photon counting spectroscopy. This supports a previous assignment of the broadband emission as resulting from a single ensemble, such that the emissive excited state experiences many local structures faster than the emissive decay. The findings presented here highlight the role of the organic cation and its dynamics in hybrid organic–inorganic perovskites and white light emission.

Magnetic semiconductors may soon improve the energy efficiency of computers, but materials exhibiting these dual properties remain underexplored. Here, we report the computational prediction and realization of a new magnetic and semiconducting material, MnSnN$_2$, via combinatorial sputtering of thin films. Grazing incidence wide angle X-ray scattering and laboratory X-ray diffraction studies show a wide composition tolerance for this wurtzite-like MnSnN$_2$, ranging from $20\%

In sharp contrast to molecular synthesis, materials synthesis is generally presumed to lack selectivity. The few known methods of designing selectivity in solid-state reactions have limited scope, such as topotactic reactions or strain stabilization. This contribution describes a general approach for searching large chemical spaces to identify selective reactions. This novel approach explains the ability of a nominally "innocent" Na2CO3 precursor to enable the metathesis synthesis of single-phase Y2Mn2O7: an outcome that was previously only accomplished at extreme pressures and which cannot be achieved with closely related precursors of Li2CO3 and K2CO3 under identical conditions. By calculating the required change in chemical potential across all possible reactant-product interfaces in an expanded chemical space including Y, Mn, O, alkali metals, and halogens, using thermodynamic parameters obtained from density functional theory calculations, we identify reactions that minimize the thermodynamic competition from intermediates. In this manner, only the Na-based intermediates minimize the distance in the hyperdimensional chemical potential space to Y2Mn2O7, thus providing selective access to a phase which was previously thought to be metastable. Experimental evidence validating this mechanism for pathway-dependent selectivity is provided by intermediates identified from in situ synchrotron-based crystallographic analysis. This approach of calculating chemical potential distances in hyperdimensional compositional spaces provides a general method for designing selective solid-state syntheses that will be useful for gaining access to metastable phases and for identifying reaction pathways that can reduce the synthesis temperature, and cost, of technological materials.

In solid-state chemistry, stable phases are often missed if their synthesis is impractical, such as when decomposition or a polymorphic transition occurs at relatively low temperature. In the preparation of complex oxides, reaction temperatures commonly exceed 1000 °C with little to no control of the reaction pathway. Thus, a prerequisite for exploring the synthesis of complex oxides is to identify reactions with intermediates that are kinetically competent at low temperatures, as provided by assisted metathesis reactions. Here, we study the assisted metathesis reaction Mn2O3 + 2.2YCl3·6H2O + 3Li2CO3 → 2YMnO3 + 5.8LiCl + 0.2LiYCl4 + 3CO2 using in situ synchrotron X-ray diffraction. By changing the atmosphere, oxygen vs inert gas, the reaction product changes from the overoxidized perovskite YMnO3+δ to the hexagonal YMnO3 polymorph at the reaction temperature of 850 °C, respectively. Analysis of the reaction pathways reveals two parallel reaction pathways in forming YMnO3 phases: (1) the slow reaction of metal oxides in a LiCl flux (Y2O3 + Mn2O3 [Formula: see text] 2YMnO3) and (2) the fast reaction from ternary intermediates (YOCl + LiMnO2 → LiCl + YMnO3). Control reactions reveal that both proposed pathways in isolation result in product formation, but the direct preparation of ternary intermediates (YOCl + LiMnO2 → LiCl + YMnO3) occurs at lower temperatures (500 °C) and shorter times (

The synthesis of inorganic metal nitrides poses a challenge due to the low reactivity of N2 gas at low temperatures, yet entropy driven formation of N2 gas at high temperatures. In contrast, synthe...

The choice of organic cation in hybrid perovskites has large implications for optoelectronic properties, material stability, and crystal structure. In particular, formamidinium ((CH(NH2)2+) perovsk...

Two-dimensional (2D) organic−inorganic hybrid perovskites (OIHPs) have showed impressive stability, compared to their three-dimensional (3D) counterparts. However, tuning the chemical structure of the organic cations to simultaneously improve the device performance and stability of 2D OIHP solar cells is rarely reported. Here, we demonstrate that by introducing a classic noncovalent aryl-perfluoroaryl interaction, 2D OIHP solar cells with 1:1 mixed phenethylammonium (PEA) and perfluorophenethylammonium (F5-PEA) can achieve an efficiency of >10% with much enhanced stability using a simple deposition at low temperature without using any additives. The competing effects of surface morphology and crystal orientation with an increased amount of F5-PEA result in the highest efficiency at a 1:1 ratio, while single-crystal studies reveal the expected aryl-perfluoroaryl interaction, accounting for the highest device stability of 2D OIHP solar cell at 1:1 ratio as well. This work provides an example where tuning the interactions of organic cations via molecular engineering can have a profound effect on device performance and stability of 2D OIHP solar cells.

Mixed valence perovskite materials present an opportunity to understand how structural motifs influence electronic properties in semiconducting materials. Here, we report the preparation of high-qu...

Hybrid metal halides yield highly desirable optoelectronic properties and offer significant opportunity due to their solution processability. This contribution reports a new series of hybrid semiconductors, (C7H7)MX4 (M = Bi3+, Sb3+; X = Cl-, Br-, I-), that are composed of edge-sharing MX6 chains separated in space by π-stacked tropylium (C7H7+) cations; the inorganic chains resemble the connectivity of BiI3. The Bi3+ compounds have blue-shifted optical absorptions relative to the Sb3+ compounds that span the visible and near-IR region. Consistent with observations, DFT calculations reveal that the conduction band is composed of the tropylium cation and valence band primarily the inorganic chain: a charge-transfer semiconductor. The band gaps for both Bi3+ and Sb3+ compounds decrease systematically as a function of increasing halide size. These compounds are a rare example of charge-transfer semiconductors that also exhibit efficient crystal packing of the organic cations, thus providing an opportunity to study how structural packing affects optoelectronic properties.

Halide perovskite semiconductors such as methylammonium lead iodide (CH3NH3PbI3) have achieved great success in photovoltaic devices; however, concerns surrounding toxicity of lead and material stability have motivated the field to pursue alternative perovskite compositions and structures. Vacancy-ordered double perovskites are a defect-ordered variant of the perovskite structure characterized by an antifluorite arrangement of isolated octahedral units bridged by A-site cations. In this Review, we focus upon the structure–dynamics–property relationships in vacancy-ordered double perovskite semiconductors as they pertain to applications in photovoltaics, and we propose avenues of future study within the context of the broader perovskite halide literature. We describe the compositional and structural motifs that dictate the optical gaps and charge transport behavior and discuss the implications of charge ordering, lattice dynamics, and organic–inorganic coupling upon the properties of these materials. The d...

Many functional materials have relatively low decomposition temperatures (T≤ 400 °C), which makes their synthesis challenging using conventional high-temperature solid-state chemistry. Therefore, non-conventional techniques such as metathesis, hydrothermal, and solution chemistry are often employed to access low-temperature phases; the discovery of new chemistries is needed to expand access to these phases. This contribution discusses the use of triphenylphosphine (PPh3) as a molten flux to synthesize superconducting iron selenide (Fe1+δSe) at low temperature (T = 325 °C). Powder X-ray diffraction and magnetism measurements confirm the successful formation of superconducting iron selenide while nuclear magnetic resonance spectroscopy and in situ X-ray diffraction show that the formation of superconducting FeSe at low temperatures is enabled by an adduct between the triphenylphosphine and selenium. Exploration of the Fe-Se-PPh3 phase space indicates that the PPh3-Se adduct effectively reduces the chemical potential of the selenium at high concentrations of triphenylphosphine. This contribution demonstrates that the use of a poorly-solvating yet reactive flux has the potential to enable the synthesis of new low-temperature phases of solid materials.

Hybrid halide perovskite semiconductors exhibit complex, dynamical disorder while also harboring properties ideal for optoelectronic applications that include photovoltaics. However, these materials are structurally and compositionally distinct from "traditional" compound semiconductors composed of tetrahedrally-coordinated elements with an average valence electron count of silicon. As discussed here, the additional dynamic degrees of freedom of hybrid halide perovskites underlie many of their potentially transformative physical properties. Neutron scattering and spectroscopy studies of the atomic dynamics of these materials have yielded significant insights to the functional properties. Specifically, inelastic neutron scattering has been used to elucidate the phonon band structure, and quasi-elastic neutron scattering (QENS) has revealed the nature of the uncorrelated dynamics pertaining to molecular reorientations. Understanding the dynamics of these complex semiconductors has elucidated the temperature-dependent phase stability and origins of the defect-tolerant electronic transport from the highly polarizable dielectric response. Furthermore, the dynamic degrees of freedom of the hybrid perovskites provides additional opportunities for application engineering and innovation., 25 pages. 8 Figures

Interest in inorganic ternary nitride materials has grown rapidly over the past few decades, as their diverse chemistries and structures make them appealing for a variety of applications. Due to synthetic challenges posed by the stability of N2, the number of predicted nitride compounds dwarfs the number that has been synthesized, offering a breadth of opportunity for exploration. This review summarizes the fundamental properties and structural chemistry of ternary nitrides, leveraging metastability and the impact of nitrogen chemical potential. A discussion of prevalent defects, both detrimental and beneficial, is followed by a survey of synthesis techniques and their interplay with metastability. Throughout the review, we highlight applications (such as solid-state lighting, electrochemical energy storage, and electronic devices) in which ternary nitrides show particular promise.

Hybrid perovskites are a technologically relevant family of materials, with potential applications in photovoltaics, solid-state lighting, and radiation detection. Interactions between the inorganic octahedral framework and the organic sublattice have been implicated in the structure and optoelectronic properties, but characterization of these interactions has been challenging, because of competition between organic-inorganic coupling and intraoctahedral interactions. Owing to their decreased octahedral connectivity, vacancy-ordered double perovskites present an ideal case study to examine organic-inorganic coupling in hybrid perovskites and their derivatives. Here, we describe the low-temperature, hysteretic phase transition of formamidinium tin(IV) iodide from the high-symmetry cubic phase to a lower-symmetry monoclinic phase. We propose that the hysteresis stems from organic-inorganic coupling mediated by local and spontaneous strain from the orientations of the formamidinium cations, which result in a ferroelastic phase transition.

We report the discovery of a trigonal polymorph of the prospective Li-ion battery material ${\mathrm{Li}}_{2}\mathrm{Mn}{\mathrm{O}}_{3}$ and its synthesis in bulk, single-crystal form. Crystal growth of trigonal ${\mathrm{Li}}_{2}\mathrm{Mn}{\mathrm{O}}_{3}$ is strongly dependent upon the quality of a polycrystalline $\mathrm{Li}\mathrm{Mn}{\mathrm{O}}_{2}$ precursor consumed in the synthesis process. The crystal structure of the trigonal phase is composed of ordered honeycomb layers of $\mathrm{Li}{\mathrm{O}}_{6}$ and $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra segregated by layers of $\mathrm{Li}{\mathrm{O}}_{6}$ octahedra and represents an ordered stacking variant of the known monoclinic polymorph. Diffuse reflectance spectroscopy reveals a direct optical gap of $2.47\ifmmode\pm\else\textpm\fi{}0.11$ eV and a series of charge excitations that are well explained by the expected ${\mathrm{Mn}}^{4+}\phantom{\rule{4pt}{0ex}}3{d}^{3}$ valence. Density functional theory calculations are in excellent agreement with the spectroscopic measurements and find a near degeneracy in the formation energies of the two polymorphs. Our results suggest that the trigonal structure resolves the compositional and structural disorder often manifested in the monoclinic phase.

Functional small molecules afford opportunities to direct solid-state inorganic reactions at low temperatures. Here, we use catalytic amounts of organosilicon molecules to influence the metathesis reaction: FeCl2 + Na2S2 → 2NaCl + FeS2. Specifically, hexaphenyldisiloxane ((C6H5)6Si2O) is shown to increase pyrite yields in metathesis reactions performed at 150 °C. In situ synchrotron X-ray diffraction (SXRD) paired with differential scanning calorimetry (DSC) reveals that diffusion-limited intermediates are circumvented in the presence of (C6H5)6Si2O. Control reactions suggest that the observed change in the reaction pathway is imparted by the Si-O functional group. 1H NMR supports catalytic behavior, as (C6H5)6Si2O is unchanged ex post facto. Taken together, we hypothesize that the polar Si-O functional group coordinates to iron chloride species when NaCl and Na2S4 form, forming an unidentified, transient intermediate. Further exploration of targeted small molecules in these metathesis reaction provides new strategies in controlling inorganic materials synthesis at low-temperatures.

Magnetic frustration in ${\mathrm{Fe}}_{3}{\mathrm{PO}}_{4}{\mathrm{O}}_{3}$ produces an unusual magnetic state below ${\mathrm{T}}_{N}=163\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where incommensurate antiferromagnetic order is restricted to nanosized needle-like domains, as inferred from neutron powder diffraction. Here we show using single-crystal neutron diffraction that ${\mathrm{Fe}}_{3}{\mathrm{PO}}_{4}{\mathrm{O}}_{3}$ does not exhibit a preferred ordering wave vector direction in the $ab$ plane despite having a well-defined ordering wave vector length. This results in the observation of continuous rings of scattering rather than satellite Bragg peaks. The lack of a preferred incommensurate ordering wave vector direction can be understood in terms of an antiferromagnetic Heisenberg model with nearest-neighbor (${J}_{1}$) and second-neighbor (${J}_{2}$) interactions, which produce a quasidegenerate manifold of ordering wave vectors. This state appears to be similar to the partially ordered phase of MnSi, but in ${\mathrm{Fe}}_{3}{\mathrm{PO}}_{4}{\mathrm{O}}_{3}$ arises in a frustrated antiferromagnet rather than a chiral ferromagnet.

Temperature is a ubiquitous environmental variable used to explore materials structure, properties and reactivity. This article reports a new paradigm for variable-temperature measurements that varies the temperature continuously across a sample such that temperature is measured as a function of sample position and not time. The gradient approach offers advantages over conventional variable-temperature studies, in which temperature is scanned during a series measurement, in that it improves the efficiency with which a series of temperatures can be probed and it allows the sample evolution at multiple temperatures to be measured in parallel to resolve kinetic and thermodynamic effects. Applied to treat samples at a continuum of temperatures prior to measurements at ambient temperature, the gradient approach enables parametric studies of recovered systems, eliminating temperature-dependent structural and chemical variations to simplify interpretation of the data. The implementation of spatially resolved variable-temperature measurements presented here is based on a gradient-heater design that uses a 3D-printed ceramic template to guide the variable pitch of the wire in a resistively heated wire-wound heater element. The configuration of the gradient heater was refined on the basis of thermal modelling. Applications of the gradient heater to quantify thermal-expansion behaviour, to map metastable polymorphs recovered to ambient temperature, and to monitor the time- and temperature-dependent phase evolution in a complex solid-state reaction are demonstrated.

The quasi-one-dimensional structure of ${\mathrm{Li}}_{2}{\mathrm{Mn}}_{2}{({\mathrm{MoO}}_{4})}_{3}$ consists of three mutually distinct chains of ${\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}$-centered polyhedra in which Mn ostensibly adopts a $J=5/2\phantom{\rule{4pt}{0ex}}{\mathrm{Mn}}^{2+}$ configuration. In situ x-ray scattering experiments carried out as crystallites emerge from a molten oxide solution facilitate the synthesis of large single crystals. Ex situ x-ray diffraction finds no evidence of long-range Li/Mn occupancy ordering, suggesting that the structure is effectively composed of finite chains of Mn moments of statistically varying lengths. UV/visible diffuse reflectance spectroscopy measurements establish a wide 3.43(12)-eV direct charge gap consistent with the local polyhedral coordination of the nominally ${\mathrm{Mn}}^{2+}$ species. The temperature $T$ dependence of the DC magnetic susceptibility $\ensuremath{\chi}$ reveals a fluctuating moment of only $2.74{\ensuremath{\mu}}_{B}\ifmmode\pm\else\textpm\fi{}0.01{\ensuremath{\mu}}_{B}$/Mn, dramatically reduced from the $5.9{\ensuremath{\mu}}_{B}$/Mn expected for ${\mathrm{Mn}}^{2+}$. Meanwhile, the Weiss temperature ${\mathrm{\ensuremath{\Theta}}}_{W}=\ensuremath{-}89\ifmmode\pm\else\textpm\fi{}1$ K reveals antiferromagnetic fluctuations that are stymied from reaching an ordered state apparently by the chemical disorder intrinsic to the polyhedral chains. Measurements of magnetization vs field $H$ at $T\ensuremath{\le}10$ K are far from saturation even at $H=5$ T and are strongly non-Brillouin-like, instead scaling as $H/{T}^{0.24(3)}$ and suggesting the presence of quantum fluctuations associated with an eventual quasi-one-dimensional, disordered magnetic phase.

Two-dimensional (2D) Ruddlesden–Popper hybrid perovskites are a homologous series of compounds with the formula A′2An–1BnX3n+1 (A′ = bulky organic cation; A = small organic cation; B = Pb2+ or Sn2+, X = Cl–, Br–, I–; n is an integer) composed of inorganic octahedral layers separated by organic spacer cations. The octahedral layer thickness is modified by the stoichiometry of the A-site cation, but limited methods exist for controlled and discriminating synthesis for all compositions. We report a general synthesis method and its principles that yield phase-pure 2D hybrid perovskites; the chemistry operates within a dynamic equilibrium established by the molar solubility of the compounds within the homologous series. A solvent–antisolvent (HI–acetic acid) pair and the common-ion effect provide selective control over the molar solubility to precipitate phase-pure compounds, as is supported by simple and predictive calculations. Here, this approach is demonstrated in detail with A′ = n-butylammonium, A = meth...

Ternary intermetallic compounds with the ThCr2Si2 crystal structure have recently been shown to exhibit pseudo-elastic responses to hydrostatic and c-axis uniaxial compression. While the sensitivity of their electronic properties to the presence of point defects has been previously studied, there is a dearth of investigations into their effects on the mechanical properties of these materials. In this paper, we present the experimental results of the uniaxial compression of a defected LaRu2P2 micropillar. With a combination of materials theory and density functional theory (DFT), we examine the role defects have on the mechanical response and phase stability of LaRu2P2. First, we explore the nature of isolated point defects in these materials. Then, we investigate the stability of high densities of these defects. We further investigate the mechanical response of these defected structures using both DFT as well as composite models, which allow us to account for the coexistence of pristine and defected structures. These results are then compared against recent experiments, explaining the processing-microstructure-property relations of differently-fabricated samples.

Crystals of the ThCr2Si2-type structure comprise a large class of known compounds, and observations of superconductivity in some of the compounds generated significant interest in these materials. Recently, nano-indentation experiments have shown that at room temperature, small-scale crystals of CaFe2As2 exhibit pseudo-elastic behavior with recoverable strains of over 10%. These experiments also demonstrate the potential for shape memory effects at cryogenic temperatures, behavior which has previously been related to its magnetic phase transitions. In this work, the phase transitions of CaFe2As2 are investigated using density functional theory (DFT) in conjunction with analytical models. The models demonstrate that both uniaxial and hydrostatic loading can give rise to pseudo-elastic behavior. These models are then applied to LaRu2P2, which does not exhibit a magnetic phase change, but is still found to have a similar pseudo-elastic response. A suite of parameters useful in quantifying the complex responses of these compounds is presented and it is demonstrated that c-axis strain is the critical loading parameter in predicting the pseudo-elastic behavior. These results provide a method of connecting local chemical tuning to macroscopic behavior.

Anharmonic lattice dynamics are intimately linked with optical and electronic properties in perovskite halide semiconductors. Vacancy-ordered double perovskites are a subset of the perovskite halide family containing isolated octahedral units. The absence of polyhedral connectivity engenders the vacancy-ordered double perovskites with additional degrees of dynamic freedom, which presents an ideal structural framework to study dynamic–property relationships in perovskite halide semiconductors. In the present study, we examine the structure and bonding origins of anharmonicity in the vacancy-ordered double perovskites Cs2Sn1−xTexI6. While X-ray diffraction indicates that all members adopt the cubic vacancy-ordered double perovskite structure, the local coordination environment probed by X-ray pair distribution function (XPDF) analysis reveals asymmetry of the Cs–I/I–I pair correlation that smoothly decreases with increasing tellurium content. Temperature-dependent neutron total scattering suggests that this asymmetry in the PDF occurs due to anharmonic lattice dynamics arising from octahedral tilting and Cs+ displacements, as supported by Reverse Monte Carlo simulations of the Cs2SnI6 and Cs2TeI6 end members. We further correlate the trends in asymmetry and anharmonicity with the bond valence sum of the Cs+ ion, and find that the anharmonicity vanishes when the bonding preferences of the Cs+ are satisfied by the size of the cuboctahedral void. This study presents a simple and effective approach for understanding the origin of anharmonicity in vacancy-ordered double perovskite materials.

Herein we report the first examples of single-molecule magnet (SMM) behaviour in S = 1/2 Ni(III) complexes. We find that low-spin 3d7trans-[NiIII(cyclam)(X)2]Y complexes (cyclam = 1,4,8,11-tetraazacyclotetradecane; X and Y are singly charged anions) exhibit field-induced slow relaxation of magnetization for O-donor axial ligands (nitrate) but not for N-donor variants (isothiocyanate). Experimental and electronic structure computational investigations indicate that intrinsic spin polarisation of low-spin Ni(III) is modulated significantly by local coordination geometry and supramolecular interactions. Solid state dilution of Ni(III) with diamagnetic Co(III) ions forms a related complex salt, [NixCo1−x(cyclam)(NO3)2](NO3)·2HNO3 (0.1 < x < 1), which preserves slow magnetic dynamics, thus supporting a molecular component to slow relaxation. An initial analysis of magnetic relaxation lifetime fits best to a combination of Raman and direct relaxation processes.

Porous materials have widespread industrial applications in adsorption and catalysis, but experimental studies providing atomistic data regarding gas–sorbent interactions under application-relevant conditions are limited. Current analytical methods give information about either the crystal structure or the macroscopic kinetics involved in these processes, but not their interplay. The development of a new combination of stroboscopic, isotope-contrasted neutron total scattering and steady-state isotopic transient kinetic analysis provides insight into both areas. An advanced data reduction procedure was developed to isolate the differential isotope signal from the stroboscopic neutron diffraction. These data, combined with measurement of adsorption isotherms and theoretical considerations from Grand Canonical Monte Carlo simulation, enabled the location of a heterogeneous distribution of nitrogen adsorption sites in calcium exchanged zeolite X under operational conditions (1 atm, 300 K). The nitrogen is fou...

The advantageous performance of hybrid organic–inorganic perovskite halide semiconductors in optoelectronic applications motivates studies of their fundamental crystal chemistry. In particular, recent studies have sought to understand how dipolar, dynamic, and organic cations such as methylammonium (CH3NH3+) and formamidinium (CH(NH2)2+) affect physical properties such as light absorption and charge transport. To probe the influence of organic–inorganic coupling on charge transport, we prepared the series of vacancy-ordered double perovskite derivatives A2SnI6, where A = Cs+, CH3NH3+, and CH(NH2)2+. Despite nearly identical cubic structures by powder X-ray diffraction, replacement of Cs+ with CH3NH3+ or CH(NH2)2+ reduces conductivity through a reduction in both carrier concentration and carrier mobility. We attribute the trends in electronic behavior to anharmonic lattice dynamics from the formation of hydrogen bonds that yield coupled organic–inorganic dynamics. This anharmonicity manifests as asymmetry ...

Hybrid halide perovskites have radically transformed photovoltaics research. While these materials, exemplified by (CH3NH3)PbI3, are chemically complex relative to traditional elemental (Si) and co...

Hybrid organic–inorganic perovskites have gained notoriety in the photovoltaic community for their composition-tunable band gaps and long-lived electronic excited states, which are known to be related to the crystalline phase. While it is known that the inorganic and organic components are coupled through structural phase transitions, it remains unclear as to what role each plays in directing the structure of hybrid perovskites such as methylammonium lead halides (CH3NH3PbX3). Here, we present crystallographic and spectroscopic data for the series (CH3NH3)1–xCsxPbBr3. CH3NH3PbBr3 behaves as a plastic crystal in the high temperature cubic phase, and substitution of CH3NH3+ with Cs+ leads to the formation of an orientational glass. While the organic molecule exhibits slow, glassy reorientational dynamics, the inorganic framework continues to undergo crystallographic phase transitions. These crystallographic transitions occur in the absence of thermodynamic signatures in the specific heat from molecular orie...

In situ total scattering measurements are increasingly utilized to follow atomic and nanoscale structural details of phase transitions and other transient processes in materials. This contribution presents an automated method and associated tool set to analyze series of diffraction and pair distribution function data with a linear combination of end-member states. It is demonstrated that the combinatorial appraisal of transition states (CATS) software tracks phase changes, relative phase fractions and length scales of interest in experimental data series. It is further demonstrated, using a series of local structure data simulations, that the misfit of such a model can reveal details of phase aggregation and growth related to the pair distribution function's sensitivity to interphase correlations. CATS may be applied to quantitative evaluation of many transient processes, including amorphous-to-crystalline phase transitions, the evolution of solid-solution behaviors, the precipitation and growth of aggregates, and other atomic to nanoscale details of crystallization and phase transformation phenomena.

The control of solid state reaction pathways will enable the design and discovery of new functional inorganic materials. A range of synthetic approaches have been used to shift solid state chemistry away from thermodynamic control, in which the most energetically favorable product forms, toward a regime of kinetic control, so that metastable materials can be controllably produced. In this Perspective, we focus on the kinetic control of solid state metathesis reactions to alter solid state reaction pathways and products. We provide insight into the necessary components of a kinetically controlled solid state reaction and illustrate the utility of studying reactions in situ in order to observe the various intermediates and kinetic pathways that may extend synthetic solid state chemistry toward a paradigm of reaction-by-design.

Controlling the connectivity and topology of solids is a versatile way to target desired physical properties. This is especially relevant in the realm of hybrid halide semiconductors, where the long-range connectivity of the inorganic substructural unit can lead to significant changes in optoelectronic properties such as photoluminescence, charge transport, and absorption. We present a new series of hybrid metal-halide semiconductors, (phenH2)BiI5·H2O, (2,2-bpyH2)BiI5, (BrbpyH)BiI4·H2O, (phenH2)2Pb3I10·2H2O, and (2,2-bpyH2)2Pb3I10 where (phenH2)2+ = 1,10-phenanthroline-1,10-diium, (2,2-bpyH2)2+ = 2,2'-bipyridine-1,1'-diium and (BrbpyH)+ = 6,6'-dibromo-2,2'-bipyridium. These compounds allow us to observe how the planarity of the cation, induced either through structural modification in the case of (phenH2)2+ or through non-covalent interactions in (BrbpyH)+, both relative to (2,2-bpyH2)2+, modifies the inorganic substructural unit. While the Pb2+ series of compounds show minimal changes in inorganic connectivity, we observe large differences in the Bi3+ series, ranging from 0-D dimers to corner- and edge-sharing 1-D chains of octahedra. We find that compounds containing (phenH2)2+ and (BrbpyH)+ pack more efficiently than those with (2,2-bpyH2)2+ due to their retention of planarity leading to greater inorganic connectivity. Electronic structure calculations and optical diffuse reflectance reveal that the band gaps of these compounds are influenced by the degree of inorganic connectivity and the inorganic substructural unit distances. These results show that the structure and planarity of organic cations can directly influence both the inorganic connectivity and the optical properties that could be tuned for certain optoelectronic applications.