Chemical Excision of Tetrahedral FeSe₂ Chains from the Superconductor FeSe: Synthesis, Crystal Structure, and Magnetism of Fe₃Se₄(en)₂

Fragments of the superconducting FeSe layer, FeSe₂ tetrahedral chains, were stabilized in the crystal structure of a new mixed-valent compound Fe₃Se₄(en)₂ (en = ethylenediamine) synthesized from elemental Fe and Se. The FeSe₂ chains are separated from each other by means of Fe­(en)₂ linkers. Mössbauer spectroscopy and magnetometry reveal strong magnetic interactions within the FeSe₂ chains which result in antiferromagnetic ordering below 170 K. According to DFT calculations, anisotropic transport and magnetic properties are expected for Fe₃Se₄(en)₂. This compound offers a unique way to manipulate the properties of the Fe–Se infinite fragments by varying the topology and charge of the Fe-amino linkers

Chemical Excision of Tetrahedral FeSe₂ Chains from the Superconductor FeSe: Synthesis, Crystal Structure, and Magnetism of Fe₃Se₄(en)₂

Fragments of the superconducting FeSe layer, FeSe₂ tetrahedral chains, were stabilized in the crystal structure of a new mixed-valent compound Fe₃Se₄(en)₂ (en = ethylenediamine) synthesized from elemental Fe and Se. The FeSe₂ chains are separated from each other by means of Fe­(en)₂ linkers. Mössbauer spectroscopy and magnetometry reveal strong magnetic interactions within the FeSe₂ chains which result in antiferromagnetic ordering below 170 K. According to DFT calculations, anisotropic transport and magnetic properties are expected for Fe₃Se₄(en)₂. This compound offers a unique way to manipulate the properties of the Fe–Se infinite fragments by varying the topology and charge of the Fe-amino linkers

Thermoelectric Properties of CoAsSb: An Experimental and Theoretical Study

Polycrystalline samples of CoAsSb were prepared by annealing a stoichiometric mixture of the elements at 1073 K for 2 weeks. Synchrotron powder X-ray diffraction refinement indicated that CoAsSb adopts arsenopyrite-type structure with space group <i>P</i>2₁/<i>c</i>. Sb vacancies were observed by both elemental and structural analysis, which indicate CoAsSb_0.883 composition. CoAsSb was thermally stable up to 1073 K without structure change but decomposed at 1168 K. Thermoelectric properties were measured from 300 to 1000 K on a dense pellet. Electrical resistivity measurements revealed that CoAsSb is a narrow-band-gap semiconductor. The negative Seebeck coefficient indicated that CoAsSb is an n-type semiconductor, with the maximum value of −132 μV/K at 450 K. The overall thermal conductivity is between 2.9 and 6.0 W/(m K) in the temperature range 300–1000 K, and the maximum value of figure of merit, zT, reaches 0.13 at 750 K. First-principles calculations of the electrical resistivity and Seebeck coefficient confirmed n-type semiconductivity, with a calculated maximum Seebeck coefficient of −87 μV/K between 900 and 1000 K. The difference between experimental and calculated Seebeck coefficient was attributed to the Sb vacancies in the structure. The calculated electronic thermal conductivity is close to the experimental total thermal conductivity, and the estimated theoretical zT based solely on electronic thermal conductivity agrees with experimental values in the high temperature range, above 800 K. The effects of Sb vacancies on the electronic and transport properties are discussed

Spin Crossover in Fe(II) Complexes with N₄S₂ Coordination

Reactions of Fe­(II) precursors with the tetradentate ligand <i>S,S</i>′-bis­(2-pyridylmethyl)-1,2-thioethane (bpte) and monodentate NCE^– coligands afforded mononuclear complexes [Fe­(bpte)­(NCE)₂] (<b>1</b>, E = S; <b>2</b>, E = Se; <b>3</b>, E = BH₃) that exhibit temperature-induced spin crossover (SCO). As the ligand field strength increases from NCS^– to NCSe^– to NCBH₃^–, the SCO shifts to higher temperatures. Complex <b>1</b> exhibits only a partial (15%) conversion from the high-spin (HS) to the low-spin (LS) state, with an onset around 100 K. Complex <b>3</b> exhibits a complete SCO with <i>T</i>_1/2 = 243 K. While the γ-<b>2</b> polymorph also shows the complete SCO with <i>T</i>_1/2 = 192 K, the α-<b>2</b> polymorph exhibits a two-step SCO with the first step leading to a 50% HS → LS conversion with <i>T</i>_1/2 = 120 K and the second step proceeding incompletely in the 80–50 K range. The amount of residual HS fraction of α-<b>2</b> that remains below 60 K depends on the cooling rate. Fast flash-cooling allows trapping of as much as 45% of the HS fraction, while slow cooling leads to a 14% residual HS fraction. The slowly cooled sample of α-<b>2</b> was subjected to irradiation in the magnetometer cavity resulting in a light-induced excited spin state trapping (LIESST) effect. As demonstrated by Mössbauer spectroscopy, an HS fraction of up to 85% could be achieved by irradiation at 4.2 K