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ABSTRACT Local structure, bonding, and the distribution of vacancies in stoichiometric and highly carbon‐deficient transition metal carbides (TMCs) ZrC, HfC, NbC, and TaC is studied using high‐resolution¹³C magic angle spinning (MAS) NMR spectroscopy. Isotopic enrichment of these TMCs with¹³C allows for remarkable signal enhancement and fast MAS of dielectric‐diluted samples, yielding high‐resolution NMR spectra of these TMCs for the first time. The¹³C MAS NMR spectra display positive correlation between the¹³C isotropic shift of the stoichiometric C site and the metal‐carbon nearest‐neighbor distance in these TMCs. More importantly, these spectra reveal the presence of multiple local carbon environments that can be attributed to either vacancies or oxygen and nitrogen impurities on the carbon sub‐lattice. Comparison between the NMR spectra of stoichiometric and highly carbon deficient TMCs shows that¹³C NMR can be used to differentiate between random distribution and clustering of vacancies and impurities in these lattices. 

ABSTRACT Homogeneous glass formation in binary rare‐earth silicate systems has thus far been precluded due to the presence of extensive liquid–liquid immiscibility and a strong tendency of these liquids toward crystallization. In this study, we demonstrate homogeneous glass formation in the Sc₂O₃–SiO₂binary system within a narrow compositional window (37–39 mol% Sc₂O₃) near a deep eutectic between the compounds Sc₂Si₂O₇and Sc₂SiO₅, using containerless laser melting under aerodynamic levitation. The atomic structure of these unusual glasses is investigated using multinuclear (²⁹Si,⁴⁵Sc,¹⁷O) solid‐state nuclear magnetic resonance (NMR) and Raman spectroscopy. The spectroscopic results, when taken together, provide a comprehensive picture of the structure of these glasses characterized by pyrosilicate [Si₂O₇]⁶⁻anionic units interconnected by Sc cations in ScO₆coordination polyhedra, via Si–O–Sc linkages. A significant fraction (∼6%) of the oxygen atoms in the structure is present as free oxide (FO) ions in Sc–O–Sc linkages, providing connectivity between the ScO₆polyhedra. The formation of the FO species via oxygen disproportionation reaction is promoted by the uniquely high field strength of the Sc³⁺ions, and the resulting structural frustration is hypothesized to suppress crystallization of the stable pyrosilicate phase in these liquids, enabling glass formation in an otherwise non‐glass‐forming binary system. These findings highlight the critical role of rare‐earth cation field strength in controlling oxygen speciation, structure, and glass‐forming ability in this binary silicate system. 

Abstract The environmental conical nozzle levitator (E‐CNL) with dual‐wavelength lasers is an extreme environment materials characterization system that was designed to investigate ultra‐high‐temperature materials: refractory metals, oxides, carbides, and borides above 3000 K in a controlled atmosphere. This article details the characterizations using this system to establish its high‐temperature capabilities and to outline ongoing work on materials under extreme conditions. The system has been used to measure the melting point of several oxide materials (TiO₂, Tm = 2091 ± 3 K; Al₂O₃, Tm = 2310 3 K; ZrO₂, Tm = 2984 31 K; and HfO₂, Tm = 3199 ± 45 K) and several air‐sensitive refractory metals (Ni, Tm = 1740 K; Ti, Tm = 1983 K; Nb, Tm = 2701 K; and Ta, Tm = 3368 K—note: mean ± standard deviation) during levitation which matched literature values within 0.17–2.43 % demonstrating high accuracy and precision. This containerless measurement approach is critical for probing properties without container‐derived contamination, and dual‐wavelength laser heating is essential to heat both relatively poor electrical conductors (some refractory metals and carbides) and insulators (oxides). The highest temperature achieved utilizing both lasers in these experiments was ∼4250 ± 34 K on a 76.6 mg, molten HfO₂sample using a normal spectral emissivity of 0.91. Stable levitation was demonstrated on spherical samples (yttria‐stabilized zirconia) while adjusting levitation gas composition from pure oxygen to pure argon, verifying atmospheric control up to 3173 K on solid or molten samples. These successes demonstrate the viability of in situ high‐temperature environmentally controlled studies potentially up to 4000 K on all classes of ultra‐high‐temperature materials in one system. These measurements highlight the E‐CNL system will be essential for the development of next‐generation ultra‐high‐temperature materials for hypersonic platforms, nuclear fission and fusion, and space exploration. 

The electrical properties of the entropy stabilized oxides: Zr₆Nb₂O₁₇, Zr₆Ta₂O₁₇, Hf₆Nb₂O₁₇and Hf₆Ta₂O₁₇were characterized. The results and the electrical properties of the products (i.e. ZrO₂, HfO₂, Nb₂O₅and Ta₂O₅) led us to hypothesize the A₆B₂O₁₇family is a series of mixed ionic-electronic conductors. Conductivity measurements in varying oxygen partial pressure were performed on A₆Nb₂O₁₇and A₆Ta₂O_17.The results indicate that electrons are involved in conduction in A₆Nb₂O₁₇while holes play a role in conduction of A₆Ta₂O₁₇. Between 900 °C–950 °C, the charge transport in the A₆B₂O₁₇system increases in Ar atmosphere. A combination of DTA/DSC and in situ high temperature X-ray diffraction was performed to identify a potential mechanism for this increase. In-situ high temperature X-ray diffraction in Ar does not show any phase transformation. Based on this, it is hypothesized that a change in the oxygen sub-lattice is the cause for the shift in high temperature conduction above 900 °C–950 °C. This could be:(i)Nb(Ta)⁴⁺- oxygen vacancy associate formation/dissociation,(ii)formation of oxygen/oxygen vacancy complexes(iii)ordering/disordering of oxygen vacancies and/or(iv)oxygen-based superstructure commensurate or incommensurate transitions. In-situ high temperature neutron diffraction up to 1050 °C is required to help elucidate the origins of this large increase in conductivity. 

Details of the carbothermic reduction/nitridation to synthesize hafnium nitride (HfN) and hafnium carbide (HfC) are scarce in the literature. Therefore, this current study was carried out to evaluate two pathways for synthesizing these two refractory materials: direct nitridation and carbothermic reduction/nitridation. Two mixtures of hafnium dioxide and carbon with C/ HfO2 molar ratios of 2.15 and 3.1 were nitridized directly using flowing nitrogen gas at elevated temperatures (1300−1700 °C). The 3.1 C/HfO2 molar ratio mixture was also carbothermically reduced under flowing argon gas to synthesize HfC, which was converted into HfN by introducing a nitridation step under both N2(g) and N2(g)-10% H2(g). X-ray diffraction results showed the formation of HfN at 1300 and 1400 °C and HfC1−yNy at ≥1400 °C under direct nitridation of samples using a C/HfO2 molar ratio of 2.15. These phase analysis data together with lower lattice strain and greater crystallite sizes of HfC1−yNy that formed at higher temperatures suggested that the HfC1−yNy phase is preferred over HfN at those temperatures. Carbothermic reduction of 3.1 C/HfO2 molar ratio samples under an inert atmosphere produced single-phased HfC with no significant levels of dissolved oxygen. Carbothermic reduction nitridation made two phases of different carbon levels (HfC1−yNy and HfC1−y′Ny′, where y′ < y), while direct nitridation produced a single HfC1−yNy phase under both N2 and N2-10% H2 cover gas environments. 

Abstract There is an ever-increasing need for material systems to operate in the most extreme environments encountered in space exploration, energy production, and propulsion systems. To effectively design materials to reliably operate in extreme environments, we need an array of tools to both sustain lab-scale extreme conditions and then probe the materials properties across a variety of length and time scales. Within this article, we examine the state-of-the-art experimental systems for testing materials under extreme environments and highlight the limitations of these approaches. We focus on three areas: (1) extreme temperatures, (2) extreme mechanical testing, and (3) chemically hostile environments. Within these areas, we identify six opportunities for instrument and technique development that are poised to dramatically impact the further understanding and development of next-generation materials for extreme environments. Graphical abstract