Browsing by Author "Bobev, Svilen"
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Item Calcium Substitution in Rare-earth Metal Germanides with the Gd5Si4 Type Structure(Journal of Inorganic and General Chemistry, 2022-02-23) Suen, Nian-Tzu; Bobev, SvilenAn extended series of rare-earth metal calcium germanides have been synthesized and structurally characterized. The compounds have the general formula RE5−xCaxGe4 (1.5Item Crystal and electronic structure of the ternary Zintl bismuthide BaLiBi(Zeitschrift für anorganische und allgemeine Chemie | Journal of Inorganic and General Chemistry, 2023-10-02) Ovchinnikov, Alexander; Bobev, SvilenReported is the accurate refinement of the structure of the ternary bismuthide BaLiBi, based on single-crystal X-ray diffraction data. This compound crystallizes with the ZrBeSi structure type with the space group P63/mmc (no. 194), a=4.9917(6) Å, c=9.079(2) Å, V=195.92(7) Å3 with two formula units per unit cell. In addition to being a colored ternary variant of the AlB2 type, the crystal structure of BaLiBi can be also viewed as a “stuffed” variant of the NiAs structure, where the Bi atoms form a hexagonal close packing, the Ba atoms occupy the octahedral voids in this packing, and the Li atoms are located between adjacent tetrahedral voids on their common triangular faces. In the absence of direct Bi–Bi interactions, the BaLiBi crystal structure rationalized according to the notation (Ba2+)(Li+)(Bi3−), suggesting an electron-balanced composition, i. e., a Zintl phase. In line with this notation, scalar-relativistic first-principle calculations with the LMTO code reveal a semiconducting ground state, with a bandgap of about 0.6 eV. Fully relativistic electronic structure calculations predict a semimetallic ground state.Item Experimental and Theoretical Study on the Substitution Patterns in Lithium Germanides: The Case of Li15Ge4 vs Li14ZnGe4(European Journal of Inorganic Chemistry, 2021-12-23) Osman, Hussien H.; Bobev, SvilenA new ternary lithium zinc germanide, Li13.83Zn1.17(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 group /43d 3d (no. 220) with unit cell parameter of 10.695(1) Å. The crystal structure refinements show that the parent Li15Ge4 structure is stabilized as Li15−xZnxGe4 (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 Li15Ge4 structure 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.Item Rare-earth Metal Substitution in Calcium Germanides with the Tetragonal Cr5B3 Type Structure(Journal of Inorganic and General Chemistry, 2022-07-20) Suen, Nian-Tzu; Bobev, SvilenCalcium germanides with two mid-late rare-earth metals, Ca5−xGdxGe3 and Ca5−xTbxGe3 (x≈0.1−0.2), have been synthesized and structurally characterized. Additionally, a lanthanum-rich germanide with calcium substitutions, La5−xCaxGe3 (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 group I4/mcm (no. 140; Z=4; Pearson symbol tI32), 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 Ca5−xGdxGe3 and 2.75 Å in La5−xCaxGe3. Such an elongation of the Ge−Ge bond is consistent with the notion that the parent Ca5Ge3 Zintl 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 Cr5B3 structure type to accommodate substitutions and wider valence electron count while maintaining their global structural integrity.Item Results from Exploratory Work in Li-Rich Regions of the AE-Li-Ge Systems (AE = Ca, Sr, Ba)(Crystals, 2023-12-31) Zhang, Jiliang; Bobev, SvilenThe compounds AELi2Ge (AE = Ca, Sr and Ba) were synthesized, and their structures were determined as a part of the exploratory work in the Li-rich regions of the respective ternary systems. The three compounds are isostructural, and their crystal structure is analogous with the orthorhombic structure of BaLi2Si and KLi2As (space group Pmmn). The atomic arrangement can be viewed as an intergrowth of corrugated AEGe layers, alternated with slabs of Li atoms, suggestive of the possible application of these phases as electrode materials for lithium-ion batteries. Both experimental electronic density and calculated electronic structure suggest the existence of Li–Li and Li–Ge interactions with largely covalent character. Despite that, the valence electrons can be partitioned as (AE2+)(Li+)2(Ge4–), i.e., the title compounds can be viewed as valence-precise Zintl phases. The band structure calculations for BaLi2Ge show that a bona fide energy gap in the band structure does not exist and that the expected poor metallic behavior is originated from the AEGe sub-lattice and related to hybridization of Ba5d and Ge3p states in the valence band in proximity of the Fermi level. In addition, electrochemical measurements indicate that Li atoms can be intercalated into CaGe with a maximum capacity of 446 mAh/g, close to the theoretical value of 480 mAh/g of CaLi2Ge, which reveals the possibility of this Li-rich compound to be used as an electrode in Li-ion batteries.Item Synthesis and Crystal Structure of the Zintl Phases Na2CaCdSb2, Na2SrCdSb2 and Na2EuCdSb2(Inorganics, 2022-12-18) Saparov, Bayram; Bobev, SvilenThis work details the synthesis and the crystal structures of the quaternary Zintl phases Na2CaCdSb2, Na2SrCdSb2 and Na2EuCdSb2. They are isostructural and their noncentrosymmetric structure is with the space group Pmc21 (Pearson code oP12). All structural work is carried out via single-crystal X-ray diffraction methods. The structure features [CdSb2]4– layers of corner-shared CdSb4 tetrahedra, which are stacked along the b-crystallographic axis and are separated by cations. The results from the structure refinements suggest that in addition to full cation ordering, which is typical for this structure, there also exists a possibility for an accommodation of a small degree of cation disorder.Item Synthesis and Crystal Structure of the Zintl Phases NaSrSb, NaBaSb and NaEuSb(Materials, 2023-02-08) Wang, Yi; Bobev, SvilenThis work details the synthesis and the crystal structures of the ternary compounds NaSrSb, NaBaSb and NaEuSb. They are isostructural and adopt the hexagonal ZrNiAl-type structure (space group P6¯2m; Pearson code hP9). The structure determination in all three cases was performed using single-crystal X-ray diffraction methods. The structure features isolated Sb3– anions arranged in layers stacked along the crystallographic c-axis. In the interstices, alkali and alkaline-earth metal cations are found in tetrahedral and square pyramidal coordination environments, respectively. The formal partitioning of the valence electrons adheres to the valence rules, i.e., Na+Sr2+Sb3–, Na+Ba2+Sb3– and Na+Eu2+Sb3– can be considered as Zintl phases with intrinsic semiconductor behavior. Electronic band structure calculations conducted for NaBaSb are consistent with this notion and show a direct gap of approx. 0.9 eV. Additionally, the calculations hint at possible inverted Dirac cones, a feature that is reminiscent of topological quantum materials.Item Synthesis, crystal and electronic structure of the Zintl phase Ba16Sb11. A case study uncovering greater structural complexity via monoclinic distortion of the tetragonal Ca16Sb11 structure type(Zeitschrift für anorganische und allgemeine Chemie | Journal of Inorganic and General Chemistry, 2023-09-14) Baranets, Sviatoslav; Ovchinnikov, Alexander; Samarakoon, S. M. Gayomi K.; Bobev, SvilenThe binary Zintl phase Ba16Sb11 has been synthesized and structurally characterized. Detailed studies via single-crystal X-ray diffraction methods indicate that although Ba16Sb11 appears to crystallize in the tetragonal Ca16Sb11 structure type (space group Purn:x-wiley:00442313:media:zaac202300148:zaac202300148-math-0001 21m with a=13.5647(9) Å, c=12.4124(12) Å, Z=2, R1=3.14 %; wR2=4.77 %), there exists an extensive structural disorder. Some Ba16Sb11 crystals were found to be monoclinic and the structure was solved and refined in space group P21 (a=18.3929(12) Å, b=13.5233(8) Å, c=18.3978(12) Å, β=94.6600(10)°; Z=4, R1=5.84 %; wR2=9.58 %). The latter corresponds to a 2-fold superstructure of the tetragonal one, which provides a disorder-free structural model. In both descriptions, the disordered tetragonal and the ordered monoclinic superstructure, the basic building units that make up the structure of this Ba-rich compound are pairs of face-shared square antiprisms of Ba atoms, which are centered by Sb atoms. The dimerized antiprisms are linked into parallel chains via square prisms of Ba atoms, which are also centered by Sb atoms. The Zintl concept can be applied in a straightforward manner and as result, the structure of Ba32Sb22 (=2×Ba16Sb11) can be rationalized as (Ba2+)32(Sb3−)20[Sb2]4−. The partitioning of the valence electrons is done taking into an account the homoatomic Sb−Sb contacts (d=3.01 Å), which can be clearly distinguished in the lower symmetry space group. Electronic structure calculations of Ba16Sb11 are in good accordance with the Zintl rationalization and predict a semiconductor with a band gap of 0.77 eV.Item Yet Another Case of Lithium Metal Atoms and Germanium Atoms Sharing Chemistry in the Solid State: Synthesis and Structural Characterization of Ba2LiGe3(Chemistry - A European Journal, 2023-12-06) Ghosh, Kowsik; Bobev, SvilenSeveral Ba−Li−Ge ternary phases are known and structurally characterized, including the title compound Ba2LiGe3. Its structure is reported to contain [Ge6]10− anions that exhibit delocalized bonding with a Hückel-like aromatic character. The Ge atoms are in the same plane with the Li atoms, and if both types of atoms are considered as covalently bonded, [LiGe3]4− honeycomb-like layers will result. The latter are separated by slabs of Ba2+ cations. However, based on the systematic work detailed herein, it is necessary to re-evaluate the phase as Ba2Li1−xGe3+x (x<0.05). Although small, the homogeneity range is clearly demonstrated in the gradual change of the unit cell for four independent samples. Subsequent characterization by single-crystal X-ray diffraction methods shows that the Ba2Li1−xGe3+x structure, responds to the varied number of valence electrons and the changes are most pronounced for the refined lengths of the Li−Ge and Ge−Ge bonds. Indirectly, the changes in the Ge−Li/Ge distances within layers affect the stacking too, and these changes can be correlated to the variation of the c-cell parameter. Chemical bonding analysis based on TB-LMTO-ASA level calculations affirms the notion for covalent character of the Ge−Ge bonds; the Ba−Ge and Li−Ge interactions also show some degree of covalency. Graphical Abstract available at: https://doi.org/10.1002/chem.202302385 The structure of Ba2Li1−xGe3+x, emphasizing the honeycomb-like [Li1−xGe3+x] layers with Li and Ge atoms (denoted in blue and gold) in a partially disordered state.