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 (LiMnO 2 + YOCl → YMnO 3 + LiCl) to target two yttrium manganese oxide products, hexagonal and orthorhombic YMnO 3 , when starting from three different LiMnO 2 precursors. Using temperature-dependent synchrotron X-ray and neutron diffraction, we identify the relevant intermediates and temperature regimes of reactions along the pathway to YMnO 3. Manganesecontaining 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 YMnO 3 , respectively. Density functional theory calculations confirm that the presence of Mn(IV) caused by a small concentration of cation vacancies (∼2.2%) in YMnO 3 stabilizes the orthorhombic polymorph over the hexagonal. Reactions over the course of 2 weeks yield o-YMnO 3 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.

Magnesium-ion batteries are a promising energy storage technology because of their higher theoretical energy density and lower cost of raw materials. Among the major challenges has been the identification of cathode materials that demonstrate capacities and voltages similar to lithium-ion systems. Thiospinels represent an attractive choice for new Mg-ion cathode materials owing to their interconnected diffusion pathways and demonstrated high cation mobility in numerous systems. Reported magnesium thiospinels, however, contain redox inactive metals such as scandium or indium, or have low voltages, such as MgTiS. This article describes the direct synthesis and structural and electrochemical characterization of MgCrS, a new thiospinel containing the redox active metal chromium and discusses its physical properties and potential as a magnesium battery cathode. However, as chromium(III) is quite stable against oxidation in sulfides, removing magnesium from the material remains a significant challenge. Early attempts at both chemical and electrochemical demagnesiation are discussed.

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 YMnO 3 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 YMnO 3 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 °C to 550 °C while maintaining phase selectivity by reacting both MgMn 2 O 4 and CaMn 2 O 4 with YOCl in a cooperative "cometathesis" reaction. The nascent halide salts appear to improve the reaction kinetics. Since the onset temperature for YMnO 3 formation falls 50 °C below the MgCl 2 -CaCl 2 liquidus, the enhanced reactivity is consistent with surface melting of a nasscent salt biproduct at the interfaces. Cometathesis routes have similar phase selectivity and temperature reduction in reactions that form TbMnO 3 , ErMnO 3 , and DyMnO 3 .Cometathesis lowers reaction temperatures while preserving reaction selectivity of the end members, making it a valuable approach for synthesizing metastable targets.

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 temperaturedependent 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 3Dprinted 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 temperaturedependent phase evolution in a complex solid-state reaction are demonstrated.

Ternary nitride phase space holds great potential for new functional materials, as suggested by computational predictions of yet-to-be discovered stable phases. Here, we report a metathesis route to bulk powders of MgZrN2 and the solid solutions Mg x Zr2–x N2 (0 < x < 1). These ternary phases only result when lower temperature reactions are used, in contrast to previous work using the similar Mg-based metathesis reactions that resulted in the formation of exclusively ZrN. Thermochemical calculations illustrate why lower temperature metathesis reactions yield the incorporation of Mg, while higher temperature ceramic reactions yield exclusively ZrN. Experimental in situ X-ray diffraction of metathesis reactions during heating reveals two stages in the reaction pathway: initial consumption of the precursors to make an amorphous product (T rxn > 350 °C) followed by crystallization at higher temperatures (T rxn > 500 °C). Changing the ratio of the metathesis precursors (Mg2NCl and ZrCl4) controllably varies the composition of Mg x Zr2–x N2, which crystallizes as a cation-disordered rock salt, as evidenced by high-resolution synchrotron X-ray diffraction, electron microscopy, and bulk compositional analysis. Variation in composition leads to a gradual metal-to-insulator transition with increasing x, similar to other reports of analogous thin film specimens produced by combinatorial sputtering. Meanwhile, the optical behavior of these powders suggests nanoscale compositional inhomogeneity, as signatures of ZrN-like absorption are detectable even in Mg-rich samples. This metathesis approach appears to be generalizable to the synthesis of bulk ternary nitride materials.