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1. Laboratory and Astronomical Discovery of Magnesium Dicarbide, MgC 2

   https://doi.org/10.3847/2041-8213/aca144  

   Changala, P. B. ; Gupta, H. ; Cernicharo, J. ; Pardo, J. R. ; Agúndez, M. ; Cabezas, C. ; Tercero, B. ; Guélin, M. ; McCarthy, M. C. ( November 2022 , The Astrophysical Journal Letters)

   We report the detection of magnesium dicarbide, MgC₂, in the laboratory at centimeter wavelengths and assign²⁴MgC₂,²⁵MgC₂, and²⁶MgC₂to 14 unidentified lines in the radio spectrum of the circumstellar envelope of the evolved carbon star IRC+10216. The structure of MgC₂is found to be T-shaped with a highly ionic bond between the metal atom and the C₂unit, analogous to other dicarbides containing electropositive elements. A two-temperature excitation model of the MgC₂emission lines observed in IRC+10216 yields a very low rotational temperature of 6 ± 1 K, a kinetic temperature of 22 ± 13 K, and a column density of (1.0 ± 0.3) × 10¹²cm⁻². The abundance of MgC₂relative to the magnesium–carbon chains MgCCH, MgC₄H, and MgC₆H is 1:2:22:20 and provides a new constraint on the sequential radiative association–dissociative recombination mechanisms implicated in the production of metal-bearing molecules in circumstellar environments.

    

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2. Elusive Iron: Detection of the FeC Radical (X 3 Δ i ) in the Envelope of IRC+10216

   https://doi.org/10.3847/2041-8213/ad0899  

   Koelemay, L. A. ; Ziurys, L. M. ( November 2023 , The Astrophysical Journal Letters)

   A new interstellar molecule, FeC (X³Δ_i), has been identified in the circumstellar envelope of the carbon-rich asymptotic giant branch star IRC+10216. FeC is the second iron-bearing species conclusively observed in the interstellar medium, in addition to FeCN, also found in IRC+10216. TheJ= 4 → 3, 5 → 4, and 6 → 5 rotational transitions of this free radical near 160, 201, and 241 GHz, respectively, were detected in the lowest spin–orbit ladder, Ω = 3, using the Submillimeter Telescope of the Arizona Radio Observatory (ARO) for the 1 mm lines and the ARO 12 m at 2 mm. Because the ground state of FeC is inverted, these transitions are the lowest energy lines. The detected features exhibit slight U shapes with LSR velocities nearV_LSR≈ −26 km s⁻¹and linewidths of ΔV_1/2≈ 30 km s⁻¹, line parameters characteristic of IRC+10216. Radiative transfer modeling of FeC suggests that the molecule has a shell distribution with peak radius near 300R_*(∼6″) extending out to ∼500R_*(∼10″) and a fractional abundance, relative to H₂, off∼ 6 × 10⁻¹¹. The previous FeCN spectra were also modeled, yielding an abundance off∼ 8 × 10⁻¹¹in a larger shell situated near 800R_*. These distributions suggest that FeC may be the precursor species for FeCN. Unlike cyanides and carbon-chain molecules, diatomic carbides with a metallic element are rare in IRC+10216, with FeC being the first such detection.

    

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3. Laboratory and Astronomical Detection of the SiP Radical (X 2 Π i ): More Circumstellar Phosphorus

   https://doi.org/10.3847/2041-8213/ac9d9b  

   Koelemay, L. A. ; Burton, M. A. ; Singh, A. P. ; Sheridan, P. M. ; Bernal, J. J. ; Ziurys, L. M. ( November 2022 , The Astrophysical Journal Letters)

   The millimeter-wave spectrum of the SiP radical (X²Π_i) has been measured in the laboratory for the first time using direct-absorption methods. SiP was created by the reaction of phosphorus vapor and SiH₄in argon in an AC discharge. Fifteen rotational transitions (J+ 1 ←J) were measured for SiP in the Ω = 3/2 ladder in the frequency range 151–533 GHz, and rotational, lambda doubling, and phosphorus hyperfine constants determined. Based on the laboratory measurements, SiP was detected in the circumstellar shell of IRC+10216, using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory at 1 mm and 2 mm, respectively. Eight transitions of SiP were searched: four were completely obscured by stronger features, two were uncontaminated (J= 13.5 → 12.5 and 16.5 → 15.5), and two were partially blended with other lines (J= 8.5 → 7.5 and 17.5 → 16.5). The SiP line profiles were broader than expected for IRC+10216, consistent with the hyperfine splitting. From non-LTE radiative transfer modeling, SiP was found to have a shell distribution with a radius ∼300R_*, and an abundance, relative to H₂, off∼ 2 × 10⁻⁹. From additional modeling, abundances of 7 × 10⁻⁹and 9 × 10⁻¹⁰were determined for CP and PN, respectively, both located in shells at 550–650R_*. SiP may be formed from grain destruction, which liberates both phosphorus and silicon into the gas phase, and then is channeled into other P-bearing molecules such as PN and CP.

    

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4. Calcium Substitution in Rare‐earth Metal Germanides with the Gd 5 Si 4 Type Structure

   https://doi.org/10.1002/zaac.202200016  

   Suen, Nian‐Tzu ; Bobev, Svilen ( February 2022 , Zeitschrift für anorganische und allgemeine Chemie)

   An extended series of rare‐earth metal calcium germanides have been synthesized and structurally characterized. The compounds have the general formulaRE_5−xCa_xGe₄(1.5<x<3.6;RE=rare‐earth metal; Ce, Nd, Sm, Tb−Lu) and their structures have been established from single‐crystal X‐ray diffraction methods. They crystallize with the Gd₅Si₄‐type in the orthorhombic space groupPnma(No. 62;Z=4; Pearson symboloP36), where the germanium atoms are interconnected into two kinds of Ge₂‐dimers, formally [Ge₂]⁶⁻. These studies show that Ca can be successfully incorporated into the hostRE₅Ge₄structure, whereby trivalent rare‐earth metal atoms can be substituted by divalent calcium atoms. Rare‐earth metal and calcium atoms are arranged in distorted trigonal prisms and cubes, centered by either Ge or Ca atoms. On one of the metal sites, the substitution is preferential and in 9 out of the 10 refined structures, the Wyckoff site 4cis found almost exclusively occupied by Ca. On the other two metal sites the substitution patterns appear to be governed by the mismatch between the size of theRE³⁺and Ca²⁺ions. This work further demonstrates the ability for the Gd₅Si₄structure type to accommodate the substitution of a non‐magnetic element while maintaining the global structural integrity.

    

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5. Rare‐earth Metal Substitution in Calcium Germanides with the Tetragonal Cr 5 B 3 Type Structure

   https://doi.org/10.1002/zaac.202200183  

   Suen, Nian‐Tzu ; Bobev, Svilen ( July 2022 , Zeitschrift für anorganische und allgemeine Chemie)

   Calcium germanides with two mid‐late rare‐earth metals, Ca_5−xGd_xGe₃and Ca_5−xTb_xGe₃(x≈0.1−0.2), have been synthesized and structurally characterized. Additionally, a lanthanum‐rich germanide with calcium substitutions, La_5−xCa_xGe₃(x≈0.5) has also been identified. The three structures have been established from single‐crystal X‐ray diffraction methods and confirmed to crystallize with the Cr₅B₃‐type in the tetragonal space groupI4/mcm(no. 140;Z=4; Pearson symboltI32), where part of the germanium atoms are interconnected into Ge₂‐dimers, formally [Ge₂]⁶⁻. Rare‐earth metal and calcium atoms are arranged in distorted trigonal prisms, square‐antiprisms and cubes, centered by Ge or rare‐earth/calcium metal atoms. These studies show that the amount of trivalent rare‐earth metal atoms substituting divalent calcium atoms is in direct correlation with the lengths of the Ge−Ge bond within the Ge₂‐dimers, with distance varying between 2.58 Å in Ca_5−xGd_xGe₃and 2.75 Å in La_5−xCa_xGe₃. Such an elongation of the Ge−Ge bond is consistent with the notion that the parent Ca₅Ge₃Zintl phase (e. g. (Ca²⁺)₅[Ge₂]⁶⁻[Ge⁴⁻]) is being driven out of the ideal valence electron count and further reduced. In this context, this work demonstrates the ability of the germanides with the Cr₅B₃structure type to accommodate substitutions and wider valence electron count while maintaining their global structural integrity.

    

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