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1. Structural phase purification of bulk HfO ₂ :Y through pressure cycling

   https://doi.org/10.1073/pnas.2312571121  

   Musfeldt, J. L.; Singh, Sobhit; Fan, Shiyu; Gu, Yanhong; Xu, Xianghan; Cheong, S.-W.; Liu, Z.; Vanderbilt, David; Rabe, Karin M. (January 2024, Proceedings of the National Academy of Sciences)

   We combine synchrotron-based infrared absorption and Raman scattering spectroscopies with diamond anvil cell techniques and first-principles calculations to explore the properties of hafnia under compression. We find that pressure drives HfO ${}_2$:7%Y from the mixed monoclinic ( $P{2}_1/c$) $+$antipolar orthorhombic ( $\mathit{Pbca}$) phase to pure antipolar orthorhombic ( $\mathit{Pbca}$) phase at approximately 6.3 GPa. This transformation is irreversible, meaning that upon release, the material is kinetically trapped in the $\mathit{Pbca}$metastable state at 300 K. Compression also drives polar orthorhombic ( $Pca{2}_1$) hafnia into the tetragonal ( $P{4}_2/nmc$) phase, although the latter is not metastable upon release. These results are unified by an analysis of the energy landscape. The fact that pressure allows us to stabilize targeted metastable structures with less Y stabilizer is important to preserving the flat phonon band physics of pure HfO ${}_2$. 

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2. Stabilizing Ti ₃ C ₂ T _x MXene flakes in air by removing confined water

   https://doi.org/10.1073/pnas.2400084121  

   Fang, Hui; Thakur, Anupma; Zahmatkeshsaredorahi, Amirhossein; Fang, Zhenyao; Rad, Vahid; Shamsabadi, Ahmad A; Pereyra, Claudia; Soroush, Masoud; Rappe, Andrew M; Xu, Xiaoji G; et al (July 2024, Proceedings of the National Academy of Sciences)

   MXenes have demonstrated potential for various applications owing to their tunable surface chemistry and metallic conductivity. However, high temperatures can accelerate MXene film oxidation in air. Understanding the mechanisms of MXene oxidation at elevated temperatures, which is still limited, is critical in improving their thermal stability for high-temperature applications. Here, we demonstrate that Ti ${}_3$C ${}_2$T ${}_x$MXene monoflakes have exceptional thermal stability at temperatures up to 600 ${}^{°}$C in air, while multiflakes readily oxidize in air at 300 ${}^{°}$C. Density functional theory calculations indicate that confined water between Ti ${}_3$C ${}_2$T ${}_x$flakes has higher removal energy than surface water and can thus persist to higher temperatures, leading to oxidation. We demonstrate that the amount of confined water correlates with the degree of oxidation in stacked flakes. Confined water can be fully removed by vacuum annealing Ti ${}_3$C ${}_2$T ${}_x$films at 600 ${}^{°}$C, resulting in substantial stability improvement in multiflake films (can withstand 600 ${}^{°}$C in air). These findings provide fundamental insights into the kinetics of confined water and its role in Ti ${}_3$C ${}_2$T ${}_x$oxidation. This work enables the use of stable monoflake MXenes in high-temperature applications and provides guidelines for proper vacuum annealing of multiflake films to enhance their stability. 

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3. Magnetic order in nanogranular iron germanium (Fe _0.53 Ge _0.47 ) films

   https://doi.org/10.1088/1361-648X/ad8c0a  

   Zielinski, Ruthi; Nguyen, Nhat; Herrington, Bryce; Tarkian, Amir; Taha, Omar; Chin, Wai_Kiat; Mahmood, Ather; Chen, Xiaoqian; Klewe, Christoph; Shafer, Padraic; et al (November 2024, Journal of Physics: Condensed Matter)

   Abstract We study the effect of strain on the magnetic properties and magnetization configurations in nanogranular Fe_xGe ${}_{1-x}$films ( $x=0.53\pm 0.05$) with and without B20 FeGe nanocrystals surrounded by an amorphous structure. Relaxed films on amorphous silicon nitride membranes reveal a disordered skyrmion phase while films near and on top of a rigid substrate favor ferromagnetism and an anisotropic hybridization of Fedlevels and spin-polarized Gespband states. The weakly coupled topological states emerge at room temperature and become more abundant at cryogenic temperatures without showing indications of pinning at defects or confinement to individual grains. These results demonstrate the possibility to control magnetic exchange and topological magnetism by strain and inform magnetoelasticity-mediated voltage control of topological phases in amorphous quantum materials. 

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4. Development of broad modulus profile upon polymer–polymer interface formation between immiscible glassy–rubbery domains

   https://doi.org/10.1073/pnas.2312533120  

   Gagnon, Yannic J; Burton, Justin C; Roth, Connie B (January 2024, Proceedings of the National Academy of Sciences)

   Interfaces of glassy materials such as thin films, blends, and composites create strong unidirectional gradients to the local heterogeneous dynamics that can be used to elucidate the length scales and mechanisms associated with the dynamic heterogeneity of glasses. We focus on bilayer films of two different polymers with very different glass transition temperatures ( $T_{\text{g}}$) where previous work has demonstrated a long-range (∼200 nm) profile in local $T_{\text{g}}\left(z\right)$is established between immiscible glassy and rubbery polymer domains when the polymer–polymer interface is formed to equilibrium. Here, we demonstrate that an equally long-ranged gradient in local modulus $\tilde{G}\left(z\right)$is established when the polymer–polymer interface ( $\approx$5 nm) is formed between domains of glassy polystyrene (PS) and rubbery poly(butadiene) (PB), consistent with previous reports of a broad $T_{\text{g}}\left(z\right)$profile in this system. A continuum physics model for the shear wave propagation caused by a quartz crystal microbalance across a PB/PS bilayer film is used to measure the viscoelastic properties of the bilayer during the evolution of the PB/PS interface showing the development of a broad gradient in local modulus $\tilde{G}\left(z\right)$spanning $\approx$180 nm between the glassy and rubbery domains of PS and PB. We suggest these broad profiles in $T_{\text{g}}\left(z\right)$and $\tilde{G}\left(z\right)$arise from a coupling of the spectrum of vibrational modes across the polymer–polymer interface as a result of acoustic impedance matching of sound waves with $\lambda \sim 5$nm during interface broadening that can then trigger density fluctuations in the neighboring domain. 

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5. Designing multicomponent hydrides with potential high T _c superconductivity

   https://doi.org/10.1073/pnas.2413096121  

   Denchfield, Adam; Park, Hyowon; Hemley, Russell J (November 2024, Proceedings of the National Academy of Sciences)

   While hydrogen-rich materials have been demonstrated to exhibit high T_csuperconductivity at high pressures, there is an ongoing search for ternary, quaternary, and more chemically complex hydrides that achieve such high critical temperatures at much lower pressures. First-principles searches are impeded by the computational complexity of solving the Eliashberg equations for large, complex crystal structures. Here, we adopt a simplified approach using electronic indicators previously established to be correlated with superconductivity in hydrides. This is used to study complex hydride structures, which are predicted to exhibit promisingly high critical temperatures for superconductivity. In particular, we propose three classes of hydrides inspired by the Fm $\overline{3}$m RH ${}_3$structures that exhibit strong hydrogen network connectivity, as defined through the electron localization function. The first class [RH ${}_{11}$X ${}_3$Y] is based on a Pm $\overline{3}$m structure showing moderately high T_c, where the T_cestimate from electronic properties is compared with direct Eliashberg calculations and found to be surprisingly accurate. The second class of structures [(RH ${}_{11}$) ${}_2$X ${}_6$YZ] improves on this with promisingly high density of states with dominant hydrogen character at the Fermi energy, typically enhancing T_c. The third class [(R ${}^1$H ${}_{11}$)(R ${}^2$H ${}_{11}$)X ${}_6$YZ] improves the strong hydrogen network connectivity by introducing anisotropy in the hydrogen network through a specific doping pattern. These design principles and associated model structures provide flexibility to optimize both T_cand the structural stability of complex hydrides. 

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