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1. 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)

   Abstract 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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2. 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)

   Abstract 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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3. Comparative Thermal Insulation Nature of Ca ₂ FeMnO _6−δ and Sr ₂ FeMnO _6−δ

   https://doi.org/10.1149/2754-2734/ad27dc  

   Schultz, Ebony; Guinn, Mandy; Azure, Alexa D.; Hona, Ram Krishna (February 2024, ECS Advances)

   In this study, we investigate the utility of Ca₂FeMnO_6-δand Sr₂FeMnO_6-δas materials with low thermal conductivity, finding potential applications in thermoelectrics, electronics, solar devices, and gas turbines for land and aerospace use. These compounds, characterized as oxygen-deficient perovskites, feature distinct vacancy arrangements. Ca₂FeMnO_6-δadopts a brownmillerite-type orthorhombic structure with ordered vacancy arrangement, while Sr₂FeMnO_6-δadopts a perovskite cubic structure with disordered vacancy distribution. Notably, both compounds exhibit remarkably low thermal conductivity, measuring below 0.50 Wm⁻¹K⁻¹. This places them among the materials with the lowest thermal conductivity reported for perovskites. The observed low thermal conductivity is attributed to oxygen vacancies and phonon scattering. Interestingly as SEM images show the smaller grain size, our findings suggest that creating vacancies and lowering the grain size or increasing the grain boundaries play a crucial role in achieving such low thermal conductivity values. This characteristic enhances the potential of these materials for applications where efficient heat dissipation, safety, and equipment longevity are paramount. 

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4. Thermoelectric Properties of p‐ and n‐ type Eu ₅ Sn ₂ As ₆

   https://doi.org/10.1002/zaac.202100386  

   Devlin, Kasey P.; Gomez, Braulio; Kauzlarich, Susan M. (February 2022, Zeitschrift für anorganische und allgemeine Chemie)

   Abstract Eu₅Sn₂As₆is a Zintl phase crystalizing in the orthorhombic space groupPbamwith one‐dimensional chains of corner‐shared SnAs₄tetrahedra running in thec‐direction. Eu₅Sn₂As₆has an impressive room temperature Seebeck of >100 μV/K and < – 100 μV/K at 600 K crossing fromp‐ ton‐type at 650 K. The maximum thermoelectric figure of merit,zT, for Eu₅Sn₂As₆is small (0.075), comparable to that of the Zintl phase Ca₅Al₂Sb₆whose thermoelectric performance was improved by doping Na onto the Ca sites. In this study, we show that the thermoelectric properties of Eu₅Sn₂As₆can be improved by substituting with K or La. The series Eu_5‐xK_xSn₂As₆provides an increase in maximumzTof 0.22 forx=0.15 due to a decrease resistivity while the onset of bipolar conduction systematically increases in temperature. Upon La substitution, Eu_5‐xLa_xSn₂As₆results in a newn‐type Zintl phase across the temperature range of 300–800 K. 

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5. Experimental and Theoretical Study on the Substitution Patterns in Lithium Germanides: The Case of Li ₁₅ Ge ₄ vs Li ₁₄ ZnGe ₄

   https://doi.org/10.1002/ejic.202100901  

   Osman, Hussien H.; Bobev, Svilen (December 2021, European Journal of Inorganic Chemistry)

   Abstract A new ternary lithium zinc germanide, Li_13.83Zn_1.17(2)Ge₄, was synthesized by a high‐temperature solid state reaction of the respective elements. The crystal structure was determined by single‐crystal X‐ray diffraction methods. The new phase crystallizes in the body‐centered cubic space groupI3d(no. 220) with unit cell parameter of 10.695(1) Å. The crystal structure refinements show that the parent Li₁₅Ge₄structure is stabilized as Li_15−xZn_xGe₄(x≈1) via random substitution of Li atoms by the one‐electron‐richer atoms of the element Zn, by virtue of which the number of valence electrons increases, leading to a more electronically stable system. The substitution effects in the parent Li₁₅Ge₄structure were investigated through both theory and experiment, which confirm that the Zn atoms in this structure prefer to occupy only one of the two available crystallographic sites for Li. The preferred substitution pattern established from experimental results is supported by DFT electronic structure calculations, which also explore the subtleties of the chemical bonding and the electronic properties of the title compounds. 

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