A low band gap iron sulfide hybrid semiconductor with unique 2D [Fe(16)S(20)](8-) layer and reduced thermal conductivity

Inducing Ferrimagnetic Exchange in 1D-FeSe2 Chains Using Heteroleptic Amine Complexes: [Fe(en)(tren)][FeSe2]2

[Fe(en)(tren)][FeSe2]2 (en = ethylenediamine, C2H8N2, tren = tris(2-aminoethyl)amine, C6H18N4) has been synthesized by a mixed-ligand solvothermal method. Its crystal structure contains heteroleptic [Fe(en)(tren)]2+ complexes with distorted octahedral coordination, incorporated between 1D-FeSe2 chains composed of edge-sharing FeSe4 tetrahedra. The twisted octahedral coordination environment of the Fe-amine complex leads to partial dimerization of Fe-Fe distances in the FeSe2 chains so that the FeSe4 polyhedra deviate strongly from the regular tetrahedral geometry. 57Fe Mössbauer spectroscopy reveals oxidation states of +3 for the Fechain atoms and +2 for the Fecomplex atoms. The close proximity of Fe atoms in the chains promotes ferromagnetic nearest neighbor interactions, as indicated by a positive Weiss constant, θ = +53.8(6) K, derived from the Curie-Weiss fitting. Magnetometry and heat capacity reveal two consecutive magnetic transitions below 10 K. DFT calculations suggest that the ordering observed at 4 K is due to antiferromagnetic intrachain interactions in the 1D-FeSe2 chains. The combination of two different ligands creates an asymmetric coordination environment that induces changes in the structure of the Fe-Se fragments. This synthetic strategy opens new ways to explore the effects of ligand field strength on the structure of both Fe-amine complexes and surrounding Fe-Se chains.

As-Se Pentagonal Linkers to Induce Chirality and Polarity in Mixed-Valent Fe-Se Tetrahedral Chains Resulting in Hidden Magnetic Ordering

A novel mixed-valent hybrid chiral and polar compound, Fe₇As₃Se₁₂(en)₆(H₂O), has been synthesized by a single-step solvothermal method. The crystal structure consists of 1D [Fe₅Se₉] chains connected via [As₃Se₂]-Se pentagonal linkers and charge-balancing interstitial [Fe(en)₃]²⁺ complexes (en = ethylenediamine). Neutron powder diffraction verified that interstitial water molecules participate in the crystal packing. Magnetic polarizability of the produced compound was confirmed by X-ray magnetic circular dichroism (XMCD) spectroscopy. X-ray absorption spectroscopy (XAS) and ⁵⁷Fe Mössbauer spectroscopy showed the presence of mixed-valent Fe²⁺/Fe³⁺ in the Fe-Se chains. Magnetic susceptibility measurements reveal strong antiferromagnetic nearest neighbor interactions within the chains with no apparent magnetic ordering down to 2 K. Hidden short-range magnetic ordering below 70 K was found by ⁵⁷Fe Mössbauer spectroscopy, showing that a fraction of the Fe³⁺/Fe²⁺ in the chains are magnetically ordered. Nevertheless, complete magnetic ordering is not achieved even at 6 K. Analysis of XAS spectra demonstrates that the fraction of Fe³⁺ in the chain increases with decreasing temperature. Computational analysis points out several competing ferrimagnetic ordered models within a single chain. This competition, together with variation in the Fe oxidation state and additional weak intrachain interactions, is hypothesized to prevent long-range magnetic ordering.

Solvothermal Synthesis of [Cr7 S8 (en)8 Cl2 ]Cl3 ⋅ 2H2 O with Magnetically Frustrated [Cr7 S8 ]5+ Double-Cubes

A novel transition metal chalcohalide [Cr7 S8 (en)8 Cl2 ]Cl3  ⋅ 2H2 O, with [Cr7 S8 ]5+ dicubane cationic clusters, has been synthesized by a low temperature solvothermal method, using dimethyl sulfoxide (DMSO) and ethylenediamine (en) solvents. Ethylenediamine ligand exhibits bi- and monodentate coordination modes; in the latter case ethylenediamine coordinates to Cr atoms of adjacent clusters, giving rise to a 2D polymeric structure. Although magnetic susceptibility shows no magnetic ordering down to 1.8 K, a highly negative Weiss constant, θ=-224(2) K, obtained from Curie-Weiss fit of inverse susceptibility, suggests strong antiferromagnetic (AFM) interactions between S=3/2 Cr(III) centers. Due to the complexity of the system with (2S+1)7 =16384 microstates from seven Cr3+ centers, a simplified model with only two exchange constants was used for simulations. Density-functional theory (DFT) calculations yielded the two exchange constants to be J1 =-21.4 cm-1 and J2 =-30.2 cm-1 , confirming competing AFM coupling between the shared Cr3+ center and the peripheral Cr3+ ions of the dicubane cluster. The best simulation of the experimental data was obtained with J1 =-20.0 cm-1 and J2 =-21.0 cm-1 , in agreement with the slightly stronger AFM exchange within the triangles of the peripheral Cr3+ ions as compared to the AFM exchange between the central and peripheral Cr3+ ions. This compound is proposed as a synthon towards magnetically frustrated systems assembled by linking dicubane transition metal-chalcogenide clusters into polymeric networks.

Tuning Fe-Se Tetrahedral Frameworks by a Combination of [Fe(en)3]2+ Cations and Cl- Anions

A one-dimensional (1D) chain compound [Fe(en)3]3(FeSe2)4Cl2 (en = ethylenediamine), featuring tetrahedral FeSe₂ chains separated by [Fe(en)₃]²⁺ cations and Cl^- anions, has been synthesized by a low temperature solvothermal method using simple starting materials. The degree of distortion in the Fe-Se backbone is similar to previously reported compounds with isolated 1D FeSe₂ chains. ⁵⁷Fe Mössbauer spectroscopy reveals the mixed-valent nature of [Fe(en)₃]₃(FeSe₂)₄Cl₂ with Fe³⁺ centers in the [FeSe₂]^- chains and Fe²⁺ centers in the [Fe(en)₃]²⁺ complexes. SQUID magnetometry indicates that [Fe(en)3]3(FeSe2)4Cl2 is paramagnetic with a reduced average effective magnetic moment, μ_eff = 9.51 μ_B per formula unit, and a negative Weiss constant, θ = -10.9(4) K, indicating antiferromagnetic (AFM) nearest neighbor interactions within the [FeSe₂]^- chains. Weak antiferromagnetic coupling between chains, combined with rather strong intrachain AFM coupling, leads to spin-glass behavior at low temperatures, as indicated by a frequency shift of the peak observed at 3 K in AC magnetic measurements. A combination of [Fe(en)₃]²⁺ and Cl^- ions is also capable of stabilizing mixed-valent 2D Fe-Se puckered layers in the crystal structure of [Fe(en)₃]₄(Fe₁₄Se₂₁)Cl₂, where Fe₁₄Se₂₁ layers have a unique topology with large open pores. Property measurements of [Fe(en)₃]₄(Fe₁₄Se₂₁)Cl₂ could not be performed due to the inability to either grow large crystals or synthesize this material in single-phase form.

Spin Crossover in a Hexaamineiron(II) Complex: Experimental Confirmation of a Computational Prediction

Single crystal structural analysis of [FeII (tame)2 ]Cl2 ⋅MeOH (tame=1,1,1-tris(aminomethyl)ethane) as a function of temperature reveals a smooth crossover between a high temperature high-spin octahedral d6 state and a low temperature low-spin ground state without change of the symmetry of the crystal structure. The temperature at which the high and low spin states are present in equal proportions is T1/2 =140 K. Single crystal, variable-temperature optical spectroscopy of [FeII (tame)2 ]Cl2 ⋅MeOH is consistent with this change in electronic ground state. These experimental results confirm the spin activity predicted for [FeII (tame)2 ]2+ during its de novo artificial evolution design as a spin-crossover complex [Chem. Inf.

MODEL

2015, 55, 1844], offering the first experimental validation of a functional transition-metal complex predicted by such in silico molecular design methods. Additional quantum chemical calculations offer, together with the crystal structure analysis, insight into the role of spin-passive structural components. A thermodynamic analysis based on an Ising-like mean field model (Slichter-Drickammer approximation) provides estimates of the enthalpy, entropy and cooperativity of the crossover between the high and low spin states.

Anisotropic, lightweight, strong, and super thermally insulating nanowood with naturally aligned nanocellulose

There has been a growing interest in thermal management materials due to the prevailing energy challenges and unfulfilled needs for thermal insulation applications. We demonstrate the exceptional thermal management capabilities of a large-scale, hierarchal alignment of cellulose nanofibrils directly fabricated from wood, hereafter referred to as nanowood. Nanowood exhibits anisotropic thermal properties with an extremely low thermal conductivity of 0.03 W/m·K in the transverse direction (perpendicular to the nanofibrils) and approximately two times higher thermal conductivity of 0.06 W/m·K in the axial direction due to the hierarchically aligned nanofibrils within the highly porous backbone. The anisotropy of the thermal conductivity enables efficient thermal dissipation along the axial direction, thereby preventing local overheating on the illuminated side while yielding improved thermal insulation along the backside that cannot be obtained with isotropic thermal insulators. The nanowood also shows a low emissivity of <5% over the solar spectrum with the ability to effectively reflect solar thermal energy. Moreover, the nanowood is lightweight yet strong, owing to the effective bonding between the aligned cellulose nanofibrils with a high compressive strength of 13 MPa in the axial direction and 20 MPa in the transverse direction at 75% strain, which exceeds other thermal insulation materials, such as silica and polymer aerogels, Styrofoam, and wool. The excellent thermal management, abundance, biodegradability, high mechanical strength, low mass density, and manufacturing scalability of the nanowood make this material highly attractive for practical thermal insulation applications.

Ba3(Cr0.97(1)Te0.03(1))2TeO9: in Search of Jahn-Teller Distorted Cr(II) Oxide

A novel 6H-type hexagonal perovskite Ba₃(Cr_0.97(1)Te_0.03(1))₂TeO₉ was prepared at high pressure (6 GPa) and temperature (1773 K). Both transmission electron microscopy and synchrotron powder X-ray diffraction data demonstrate that Ba₃(Cr_0.97(1)Te_0.03(1))₂TeO₉ crystallizes in P6₃/mmc with face-shared (Cr_0.97(1)Te_0.03(1))O₆ octahedral pairs interconnected with TeO₆ octahedra via corner-sharing. Structure analysis shows a mixed Cr²⁺/Cr³⁺ valence state with ∼10% Cr²⁺. The existence of Cr²⁺ in Ba₃(Cr²⁺_0.10(1)Cr³⁺_0.87(1)Te⁶⁺_0.03)₂TeO₉ is further evidenced by X-ray absorption near-edge spectroscopy. Magnetic properties measurements show a paramagnetic response down to 4 K and a small glassy-state curvature at low temperature. In this work, the octahedral Cr²⁺O₆ component is stabilized in an oxide material for the first time; the expected Jahn-Teller distortion of high-spin (d⁴) Cr²⁺ is not found, which is attributed to the small proportion of Cr²⁺ (∼10%) and the face-sharing arrangement of CrO₆ octahedral pairs, which structurally disfavor axial distortion.

Control over connectivity and magnetism of tetrahedral FeSe2 chains through coordination Fe–amine complexes

Magnetic interactions in compounds containing tetrahedral _∞¹(FeSe₂) chains separated by Fe–amine complexes are controlled by the denticity of the amine.

Photo-electroactive ternary chalcogenido-indate-stannates with a unique 2-D porous structure

A series of photo-electroactive 2-D In–Sn–Q ternary compounds are prepared using metal–phenanthroline complex templates. The 2-D network joint of In–Sn–Q is a (In/Sn)₃Q₃ six-membered ring, which is different from the Sn₃S₄ pseudosemicube of most 2-D Sn–Q binary compounds.

Ultralow thermal conductivity of atomic/molecular layer-deposited hybrid organic-inorganic zincone thin films

Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques with atomic level control enable a new class of hybrid organic-inorganic materials with improved functionality. In this work, the cross-plane thermal conductivity and volumetric heat capacity of three types of hybrid organic-inorganic zincone thin films enabled by MLD processes and alternate ALD-MLD processes were measured using the frequency-dependent time-domain thermoreflectance method. We revealed the critical role of backbone flexibility in the structural morphology and thermal conductivity of MLD zincone thin films by comparing the thermal conductivity of MLD zincone films with an aliphatic backbone to that with aromatic backbone. Much lower thermal conductivity values were obtained in ALD/MLD-enabled hybrid organic-inorganic zincone thin films compared to that of the ALD-enabled W/Al2O3 nanolaminates reported by Costescu et al. [Science 2004, 303, 989-990], which suggests that the dramatic material difference between organic and inorganic materials may provide a route for producing materials with ultralow thermal conductivity.