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1. Structural variations along the apatite F-OH join: II. The role of hydrogen bonding in fluoridated teeth

   https://doi.org/10.2138/am-2024-9393  

   Hughes, John M; Harlov, Daniel; Rakovan, John F; Ahmadi, Jamshid; Sieber, Melanie J (June 2024, American Mineralogist)

   Abstract Fluoride is one of the most consumed pharmaceuticals in the world, and its facility in preventing dental caries is recognized as one of the top 10 public health achievements of the 20th century. Although hydroxylapatite is often used as an analog of dental enamel, the details of the substitution of F for OH in the apatite anion column are not well known. Using new synthesis techniques, this study extends the structure work on P63/m apatites along the middle portion of the F-OH apatite join to compositions near the composition of fluoridated human teeth. The first F substituent in hydroxylapatite, near fluoridated dental enamel compositions, is dramatically underbonded by the surrounding Ca2 atoms (0.72 vu) in a hydroxylapatite matrix. However, the hydroxyl hydrogen is able to contribute 0.20 or 0.10 vu in hydrogen bonding, depending on whether the substitution creates a reversal site in the anion column; this hydrogen bonding alleviates the bonding requirements of the substituent F. As F concentrations increase along the join, the average hydroxyl contributes increasing amounts of hydrogen bonding to the F column anions; to mitigate the loss of its hydrogen bonding, the hydroxyl oxygen migrates toward the adjacent mirror plane that contains the bonded Ca2 atoms, and the triangle of bonded Ca2 ions concomitantly contracts. These two mechanisms increase bonding to the column hydroxyl oxygen from the adjoining Ca2 atoms to balance the loss of hydrogen bonding that stabilizes the substituent F column anion and the increasing concentration of underbonded F. 

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2. Superlattice ordering transitions driven by short-range structure in barium calcium carbonates

   https://doi.org/10.1039/D1FD00086A  

   Whittaker, Michael. L.; Pri-gal, Efrat; Schmidt, Asher; Joester, Derk (July 2022, Faraday Discussions)

   Balcite (Ba x Ca 1− x CO 3 ) is a synthetic analog of rhombohedral carbonate minerals like calcite and dolomite that is disordered on both the cation and anion sublattices. Here, we show that multiple exotic superlattice structures, including a dolomite analog that we call balcomite, can form from balcite at elevated temperatures. The second-order balcite-to-balcomite conversion at temperatures between 150–600 °C is driven by the preference of barium and calcium for different oxygen coordination numbers and facilitated by local carbonate reorientation. At elevated pressure, further superlattice order arises from cation segregation in all three dimensions, producing a supercell with the same R 3̄ m symmetry as balcite but 6× larger. This highly ordered structure relaxes back to the balcomite structure upon returning to ambient conditions. None of the three naturally occurring polymorphs of Ba 0.5 Ca 0.5 CO 3 (alstonite, paralstonite, barytocalcite) formed from balcite despite being putatively energetically favored. Instead, alstonite transformed to a balcomite-like structure via a first-order process after transiently converting to a paralstonite-like structure via a second-order process. Together, these results show that high temperature transformation pathways between structures in the barium calcium carbonate system can be driven by coarsening and are facilitated by similarity in short-range order, conceptually analogous to previously described low-temperature transformations. Many of the exotic high temperature carbonate structures are unstable, but may participate in transformation pathways between naturally observed metastable mineral phases, suggesting important roles for ephemeral phases in shaping past and current mineral distributions. 

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3. The dense warm ionized medium in the inner Galaxy

   https://doi.org/10.1051/0004-6361/202040223  

   Langer, W. D.; Pineda, J. L.; Goldsmith, P. F.; Chambers, E. T.; Riquelme, D.; Anderson, L. D.; Luisi, M.; Justen, M.; Buchbender, C. (July 2021, Astronomy & Astrophysics)

   null (Ed.)

   Context. Ionized interstellar gas is an important component of the interstellar medium and its lifecycle. The recent evidence for a widely distributed highly ionized warm interstellar gas with a density intermediate between the warm ionized medium (WIM) and compact H  II regions suggests that there is a major gap in our understanding of the interstellar gas. Aims. Our goal is to investigate the properties of the dense WIM in the Milky Way using spectrally resolved SOFIA GREAT [N  II ] 205 μm fine-structure lines and Green Bank Telescope hydrogen radio recombination lines (RRL) data, supplemented by spectrally unresolved Herschel PACS [N  II ] 122μm data, and spectrally resolved 12 CO. Methods. We observed eight lines of sight (LOS) in the 20° < l < 30° region in the Galactic plane. We analyzed spectrally resolved lines of [N  II ] at 205 μm and RRL observations, along with the spectrally unresolved Herschel PACS 122 μm emission, using excitation and radiative transfer models to determine the physical parameters of the dense WIM. We derived the kinetic temperature, as well as the thermal and turbulent velocity dispersions from the [N  II ] and RRL linewidths. Results. The regions with [N  II ] 205 μm emission are characterized by electron densities, n ( e ) ~ 10−35 cm −3 , temperatures range from 3400 to 8500 K, and nitrogen column densities N(N + ) ~ 7 × 10 16 to 3 × 10 17 cm −2 . The ionized hydrogen column densities range from 6 × 10 20 to 1.7 × 10 21 cm −2 and the fractional nitrogen ion abundance x (N + ) ~ 1.1 × 10 −4 to 3.0 × 10 −4 , implying an enhanced nitrogen abundance at a distance ~4.3 kpc from the Galactic Center. The [N  II ] 205 μm emission lines coincide with CO emission, although often with an offset in velocity, which suggests that the dense warm ionized gas is located in, or near, star-forming regions, which themselves are associated with molecular gas. Conclusions. These dense ionized regions are found to contribute ≳50% of the observed [C  II ] intensity along these LOS. The kinetic temperatures we derive are too low to explain the presence of N + resulting from electron collisional ionization and/or proton charge transfer of atomic nitrogen. Rather, these regions most likely are ionized by extreme ultraviolet (EUV) radiation from nearby star-forming regions or as a result of EUV leakage through a clumpy and porous interstellar medium. 

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4. On stars and spikes: Resolving the skeletal morphology of planktonic Acantharia using synchrotron X-ray nanotomography and deep learning image segmentation

   https://doi.org/10.1016/j.actbio.2023.01.037  

   Raja Somu, Dawn; Cracchiolo, Timothy; Longo, Elena; Greving, Imke; Merk, Vivian (March 2023, Acta Biomaterialia)

   Acantharia ( Acantharea ) are wide-spread marine protozoa, presenting one of the rare examples of stron- tium sulfate mineralization in the biosphere. Their endoskeletons consist of 20 spicules arranged accord- ing to a unique geometric pattern named Müller’s principle. Given the diverse mineral architecture of the Acantharia class, we set out to examine the complex three-dimensional skeletal morphology at the nanometer scale using synchrotron X-ray nanotomography, followed by image segmentation based on deep learning methods. The present study focuses on how the spicules emanate from the robust central junction in the orders Symphyacanthida and Arthracanthida , the geometry of lateral spicule wings as well as pockets of interspicular space, which may be involved in cell compartmentalization. Through these morphometric studies, we observed subtle deviations from the previously described spatial arrangement of the spicules. According to our data, spicule shapes are adjusted in opposite spicules as to accommodate the overall spicule arrangement. In all types examined, previously unknown interspicular interstices were found in areas where radial spicules meet, which could have implications for the crystal growth mecha- nism and overall endoskeletal integrity. A deeper understanding of the spiculogenesis in Acantharia can provide biomimetic routes towards complex inorganic shapes. 

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5. Anion order in oxysulfide perovskites: origins and implications

   https://doi.org/10.1038/s41524-020-0338-1  

   Pilania, Ghanshyam; Ghosh, Ayana; Hartman, Steven T.; Mishra, Rohan; Stanek, Christopher R.; Uberuaga, Blas P. (June 2020, npj Computational Materials)

   Abstract Heteroanionic oxysulfide perovskite compounds represent an emerging class of new materials allowing for a wide range of tunability in the electronic structure that could lead to a diverse spectrum of novel and improved functionalities. Unlike cation ordered double perovskites—where the origins and design rules of various experimentally observed cation orderings are well known and understood—anion ordering in heteroanionic perovskites remains a largely uncharted territory. In this contribution, we present and discuss insights that have emerged from our first-principles-based electronic structure analysis of a prototypical anion-ordered SrHf(O_0.5S_0.5)₃oxysulfide chemistry, studied in all possible anion configurations allowed within a finite size supercell. We demonstrate that the preferred anion ordering is always an all-cisarrangement of anions around an HfO₃S₃octahedron. As a general finding beyond the specific chemistry, the origins of this ordering tendency are traced back to a combined stabilization effect stemming from electronic, elastic, and electrostatic contributions. These qualitative notions are also quantified using state-of-the-art machine learning models. We further study the relative stability of the identified ordering as a function of A (Ca, Sr, Ba) and B (Ti, Zr, Hf) site chemistries and probe chemistry-dependent trends in the electronic structure and functionality of the material. Most remarkably, we find that the identified ground-state anion ordering breaks the inversion symmetry to create a family of oxysulfide ferroelectrics with a macroscopic polarization >30 μC/cm², exhibiting a significant promise for electronic materials applications. 

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