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All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Thermoelectric properties of Fe and Al double substituted MnSi{sub γ} (γ~1.73)

Two series of Fe and Al double substituted MnSi{sub γ} chimney ladders with a nominal valence electron count, VEC=14 per transition metal were prepared (γ=1.75). Simultaneous replacement of Mn with Fe and Si with Al yielded the Mn{sub 1−x}Fe{sub x}Si{sub 1.75−x}Al{sub x} series while the second Mn{sub 1−x}Fe{sub x}Si{sub 1.75–1.75x}Al{sub 2x} series follows the pseudo-binary between MnSi{sub 1.75} and FeAl{sub 2}. Scanning electron microscopy and elemental mapping revealed that ~60% of the nominal Al content ends up in the product with the remainder lost to sublimation, and that up to 7% Al can be substituted in the main group sublattice. Profile analysis of X-ray powder diffraction data revealed gradual changes in the cell metrics, consistent with the simultaneous substitution of Fe and Al in a fixed ratio. All samples are p-type with VEC≈13.95 from the structural data and ~1×10{sup 21} holes cm{sup −3} from variable temperature Seebeck measurements. The substituted samples have lower electrical resistivities (ρ{sub 300} {sub K}=2–5 mΩ cm) due to an improved microstructure. This leads to increased thermoelectric power factors (largest S{sup 2}/ρ=1.95 mW m{sup −1} K{sup −2}) compared to MnSi{sub γ}. The thermal conductivity for the Mn{sub 0.95}Fe{sub 0.05}Si{sub 1.66}Al{sub 0.1} sample is 2.7 W m{sup −1} K{sup −1} between 300 and 800 K, and is comparable to literature data for the parent material. - Graphical abstract: The crystal structure, microstructure and thermoelectric properties of Fe and Al double substituted MnSi{sub γ} (γ~1.73) have been investigated. - Highlights: • Up to 7% Al can be substituted in MnSi{sub γ} when co-doped with Fe. • Improved microstructure and reduced electrical resistivities for Al substituted samples. • Largest power factor 1.95 mW m{sup −1} K{sup −2} and best estimated ZT=0.5.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Series of coordination polymers based on 4-(5-sulfo-quinolin-8-yloxy) phthalate and bipyridinyl coligands: Structure diversity and properties

Reactions between later metal salts and conjugational N-hetrocyclic sulfonate/ carboxylic acid under the presence of bipyridyl auxiliary ligands afforded a series of manganese, nickel, zinc, silver, cadmium coordination polymers bearing with phenyl pendant arm attached to quinoline skeletons, and they have been characterized by elements analysis, thermogravimetry, infrared spectroscopy and single-crystal X-ray diffraction studying. The series of polymers show interesting structural diversity in coordination environment, dimensions and topologies. They are all built from 2-D networks constructed from metal cluster through sulfonate or carboxylate groups, as the secondary building unit (SBU). The thermalgravimetric analyses show that they display framework stabilities in solid state. Variable-temperature magnetic susceptibility studies reveal the existence of antiferromagnetic interactions between adjacent Mn (II) ions in 1, and ferromagnetic interactions between Ni(II) ions for 2, respectively. The photo-luminescence properties of 3-5 have also been investigated systemically. - Highlights: • A series of coordination polymers based on later transition metal ions have been obtained. • They contain conjugational N-hetrocyclic sulfonate-carboxylic acid and bipyridyl auxiliary ligands. • They have been characterized systemically. • They exhibit structure diversity and interesting properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

A new member of ferrous sulfates, FeSO{sub 4}·2H{sub 2}O with PtS topology showing spin-canted long-range antiferromagnetic ordering

A sanderite ferrous sulfate FeSO{sub 4}·2H{sub 2}O has been synthesized by the hydro/solvothermal method. Its crystal structure (Pccn, a=6.3160 Å, b=7.7550 Å, c=8.9880 Å, V=440.2 Å{sup 3}, Z=4) can be regarded as the condensation of alternately corner-shared FeO{sub 4}(H{sub 2}O){sub 2} octahedra and SO{sub 4} tetrahedra with a similar topology of PtS. By structural comparison with the known hydrated ferrous sulfates, the structural relation among them has been noted and discussed in detail. A variable temperature magnetic study shows a spin-canted long-range antiferromagnetic ordering in the low temperature regime, which might result from a possible phase transition during the cooling from the high temperature. - Graphical abstract: As a new number of ferrous sulfates, sanderite FeSO{sub 4}·2H{sub 2}O has been synthesized under hydro/solvothermal conditions, which exhibits a similar topology of PtS. - Highlights: • Sanderite ferrous sulfate has been synthesized. • The topology of its structure is similar to that of PtS. • A structural relation between these hydrated ferrous sulfates is discovered.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

N-donor co-ligands driven two new Co(II)- coordination polymers with bi- and trinuclear units: Crystal structures, and magnetic properties

Two new Co(II) coordination polymers(CPs), namely [Co{sub 2}(bpe){sub 2}(Hbppc)]{sub n} (1) and [Co{sub 3}(μ{sub 3}-OH)(bppc)(bpm)(H{sub 2}O)]·3H{sub 2}O (2) (H{sub 5}bppc=biphenyl-2,4,6,3′,5′-pentacarboxylic acid, bpe=1,2-bis(4-pyridyl)ethene, bpm=bis(4-pyridyl)amine), have been obtained and characterized by elemental analysis, single-crystal X-ray diffraction, powder X-ray diffraction (PXRD), IR spectra and thermogravimetric analysis (TGA). 1 shows a binodal (4,6)-connected fsc net with a (4{sup 4}·6{sup 10}·8)(4{sup 4}·6{sup 2}) topology, while 2 shows a binodal (5,7)-connected 3D network based on trinuclear [Co{sub 3}(μ{sub 3}-OH)]{sup 5+} units with unusual (3.4{sup 6}.5{sup 2}.6)(3{sup 2}.4{sup 6}.5{sup 7}.6{sup 5}.7) topology. Variable-temperature magnetic susceptibility measurements reveals that complex 1 shows ferromagnetic interactions between the adjacent Co(II) ions, whereas 2 is a antiferromagnetic system. - Graphical abstract: Two new Co(II) coordination polymers with bi- and trinuclear units have been obtained. 1 shows a binodal (4,6)-connected fsc net with a (4{sup 4}·6{sup 10}·8)(4{sup 4}·6{sup 2}) topology and antiferromagnetic interactions between the adjacent Co(II) ions, while 2 is a binodal (5,7)-connected 3D network with unusual (3.4{sup 6}.5{sup 2}.6)(3{sup 2}.4{sup 6}.5{sup 7}.6{sup 5}.7) topology and a ferromagnetic system. - Highlights: • Two Co(II) coordination polymers with different multimetallic clusters as building units. • A (4,6)-connected fsc net and a (5,7)-connected 3D network. • A antiferromagnetic coupling for 1 and A ferromagnetic coupling for 2.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Magnetic feature and near-infrared absorption of a [Pt(mnt){sub 2}]{sup -}based H-bond supramolecular crystal

A new salt [H{sub 2}DABCO][Pt(mnt){sub 2}]{sub 2} (1) (mnt{sup 2-}=maleonitriledithiolate and H{sub 2}DABCO{sup 2+} is diprotonated 1,4-diazabicyclo[2.2.2]octane) has been synthesized; its crystal structure, magnetic and near-IR absorption properties have been investigated. Two different [Pt(mnt){sub 2}]{sup -} anions form the strong π-dimers, labeled as Pt(1)-dimer and Pt(2)-dimer, with quite shorter Pt…Pt and S…S distances and molecular plane-to-plane distance (<3.5 Å) within a dimer. The [Pt(mnt){sub 2}]{sub 2}{sup 2-} π-dimers are connected through the cations in the strong H-bond manner to form three-dimensional H-bond supramolecular crystal. The salt shows weak paramagnetism in 1.99–300 K and this is due to the existence of strong antiferromagnetic coupling within a π-dimer. In addition, a small thermal hysteresis loop is observed at ca. 120 K, indicating that a phase transition probably occurs that is further confirmed by variable-temperature IR spectra. Another fascinating functionality of 1 is the intense near-IR absorption in the region of 750–2500 nm, and this near-IR absorption feature makes it to be a promising optical material. - Graphical abstract: A H-bond supramolecular crystal of [H{sub 2}DABCO][Pt(mnt){sub 2}]{sub 2} shows a magnetic phase transition at ca. 120 K with sizable thermal hysteresis loop and intense near-IR absorption in the region of 750–2500 nm.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Structural description, luminescent and magnetic properties of novel 2-D coordination polymers containing thiazolo[5,4-d]thiazole rings and trivalent lanthanide ions

Highlights: • Reaction between thiazolo[5,4-d]thiazole-2,5-dicarboxylate and lanthanide ions yielded six novel coordination polymers. • Crystallization in gel improved yields and quality of products. • Dc magnetic susceptibilities (from 5 to 300 K) and luminescence spectra are reported. • Eu(III) luminescence indicated a single emitting center, in disagreement with the existence of the two crystallographically distinct ion sites. - Abstract: In this work, we have synthesized six coordination polymers [Ln{sub 2}(Thz){sub 3}·8H{sub 2}O)]·4H{sub 2}O (Ln = La{sup 3+}, Ce{sup 3+}, Nd{sup 3+}, Sm{sup 3+}, Eu{sup 3+}, Gd{sup 3+}; Thz = thiazolo[5,4-d]thiazole-2,5-dicarboxylate). These are the first structures combining this ligand with lanthanides. All compounds crystallize in the P-1 triclinic system, forming a 2-D coordination network. Luminescence spectra for the Nd{sup 3+}, Sm{sup 3+}, and Eu{sup 3+} structures show the typical transitions of these ions, through the antenna effect promoted by the ligand. The magnetic properties of Ce{sup 3+}, Nd{sup 3+}, Sm{sup 3+}, Eu{sup 3+}, Gd{sup 3+} compounds were analyzed in terms of the free ion approximation. The variable-temperature susceptibility fitted with a Curie-Weiss law shows the existence of weak magnetic interactions at low temperatures for the Ce{sup 3+}, Nd{sup 3+} and Sm{sup 3+} compounds, and paramagnetic behavior for the Gd{sup 3+} compound.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Effect of stored strain energy heterogeneity on microstructure evolution of 90% cold rolled AISI 304L stainless steel during interrupted annealing treatment

Highlights: • Study of usefulness of stored strain energy distribution map of the material for variable temperature range. • Advantage of using stored strain energy distribution map • Variation observed in stored energy between isothermal annealing treatment and interrupted annealing treatment. - Abstract: This research work is an attempt to understand the effect of stored strain energy heterogeneity over macro-scale, induced during interrupted annealing treatment, on the micro structural evolution in a heavily cold rolled AISI 304L austenitic stainless steel. The cold rolled AISI 304L stainless steel (SS) samples were subjected to different annealing treatments. The microstructures obtained after each annealing treatment were characterized using optical and electron microscopy techniques. Further, an attempt was made to estimate the stored strain energy by a novel approach of micro hardness measurement both over small scale (area 3600 μm{sup 2}) and large scale (area ~0.25 mm{sup 2}). The stored strain energy was determined using hardness values. Using these data, spatial stored strain energy distribution contour maps were created. They were, in turn, used to characterize the local softening behavior both at micro and macro-scale levels after each interrupted annealing treatment. The results showed that, interrupted annealing treatment induces increased stored strain energy heterogeneity after each annealing interruptions. Also, it was possible to determine recrystallization kinetics of the material using stored strain energy map, which helps in designing annealing cycles for achieving ultrafine grained microstructure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The structural and dynamical role of water in natural organic matter: A 2H NMR and XRD study

Natural organic matter (NOM) is an important component in many near-surface geochemical environments, and its properties are greatly affected by the incorporation of water. Because of its importance, the macroscopic behavior and effects of water in NOM and soil organic matter (SOM) have been extensively studied using a wide range of experimental and computational methods. The molecular scale structural and dynamical behavior of water in these materials, however, is less well understood. This paper presents a variable temperature 2 H NMR and XRD study of water in Suwannee River NOM and its fulvic acid (FA) and humic acid (HA) fractions that provides new insight into the dynamical behavior of structurally different types of water and exchangeable hydrogen environments in NOM. The results provide a basis for future studies of more complex natural organic materials and the interaction of organic materials with mineral surfaces. Room temperature 2 H NMR spectra of samples hydrated in 2 H 2 O and then dehydrated, distinguish 2 H 2 O molecules that are in rapid reorientational motion (correlation times, ν c, > 10 5 Hz), 2 H exchanged onto carboxylic sites of the NOM that do not undergo rapid reorientation at frequencies >~10 3 Hz, and 2 H exchanged onto phenolic and possibly other alcohol sites of the NOM that undergoes rapid, but anisotropic, dynamical reorientation. For samples exposed to water and not dried, the XRD results collected at temperatures from 173 to 298 K show the formation of ice-1h in samples exposed to 100% relative humidity (R.H.) but not in samples exposed to 43% R.H. 2 H NMR of those samples collected at temperatures from 313 K to 173 K show the presence of multiple sites. Near room temperature, the spectra contain a narrow resonance for mobile water undergoing rapid isotropic motion, and a broader symmetrical resonance probably due to a combination of more dynamically restricted water molecules and 2 H exchanged onto phenolic and alcohol functional groups undergoing rapid anisotropic motion. The 43% R.H. samples also yield a broader, quadrupole-dominated, resonance for 2 H exchanged onto functional groups of the NOM. With decreasing temperature the resonances for dynamically restricted water molecules and 2 H exchanged onto phenolic and alcohol functional groups become broader, reflecting a decreasing rate of exchange between the water molecules and functional groups and a decreasing rate of reorientation of the 2 H 2 O molecules. The formation of ice-1h is directly reflected in the 2 H spectra of the 100% R.H. samples as a resonance with a quadrupole coupling constant (QCC) of ~180 kHz. For the 43% R.H. samples, there is also a broad, poorly resolved resonance with typical QCCs of ~180 kHz for which the relative signal intensity increases with decreasing temperature. This signal represents 2 H 2 O molecules that are not crystallized in ice-1h but have greatly reduced reorientation frequencies at low temperature and a hydrogen bonding network with hydrogen bond strengths similar to, but somewhat weaker than, ice-1h. Such molecules are also likely to be present in the 100% R.H. samples. At both R.H.s, some of the 2 H 2 O molecules do not freeze and retain their isotropic motion down to 173 K, the lowest temperature investigated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Cyanate groups in higher oxidation state metal cluster chemistry: Mixed-valence (II/III) Mn 16 and Mn 18 clusters

Here, the employment of cyanato (OCN - ) group in high oxidation state manganese cluster chemistry, in conjunction with carboxylate ions and the organic chelating/bridging ligand 2-(hydroxymethyl)pyridine (hmpH), is reported. The syntheses, crystal structures, and magnetochemical characterization are described for [Mn 16 O 8 (OR) 4 (OCN) 4 (O 2 CMe) 12 (hmp) 6 (ROH) 2 ] (R = Me (1), Et (2)) and [Mn 18 O 14 (O 2 CR) 18 (hmp) 4 (hmpH) 2 (H 2 O) 2 ] (R = Me (3), Et (4)). The 2:1:1:1 reactions of Mn(O 2 CMe) 2 ·4H 2 O, hmpH, NaOCN and NEt 3 in solvent MeOH or EtOH afford the isostructural complexes [Mn 16 O 8 (OR) 4 (OCN) 4 (O 2 CMe) 12 (hmp) 6 (ROH) 2 ] (R = Me (1), Et (2)). The [Mn 16 (μ 4 -O) 4 (μ 3 -O) 4 (μ-OMe) 4 (μ 3 -OR) 6 (μ-OR) 6 ] 10+ core of representative complex 1 comprises a Mn II 4 Mn III 4 double-cubane subunit attached on either side to two symmetry-related Mn II Mn III 3 defective dicubanes. A similar reaction of Mn(O 2 CR) 2 ·4H 2 O, hmpH, NaOCN and NEt 3 , but in solvent MeCN, led instead to the formation of [Mn 18 O 14 (O 2 CR) 18 (hmp) 4 (hmpH) 2 (H 2 O) 2 ] (R = Me (3), Et (4)). Compounds 3 and 4 are very similar to each other and can be described as a central [Mn III 4 (μ-O) 6 ] rodlike subunit attached on either side to two symmetry-related [Mn 7 O 9 ] subunits. Variable-temperature, solid-state dc and ac magnetic susceptibility studies revealed the presence of predominant antiferromagnetic exchange interactions in all compounds, and possible S = 2 or 1 (for 1 and 2) and S = 0 (for 3 and 4) ground state spin values. The combined results demonstrate the ability of cyanato groups to facilitate the formation of new polynuclear Mn II/III complexes with structures different than these obtained from the use of the related azides.

2-(Hydroxymethyl)pyridine↗
DOE OSTI2019

Molecular spin frustration in mixed-chelate Fe5 and Fe6 oxo clusters with high ground state spin values

The synthesis, structures, and magnetic properties are reported of three new polynuclear Fe III complexes containing the anions of picolinic acid (picH) and triethanolamine (teaH 3 ) as chelates. The complexes [Fe 6 O 2 (OH) 2 (O 2 CR) 4 (pic) 4 (teaH) 2 ] (R = Me (1), Ph (2)) and [Fe 5 O 2 (O 2 CBu t ) 4 (pic)3(teaH) 2 ] (3) were obtained from the reaction of [Fe 3 O(O 2 CR)6(H 2 O) 3 ](NO 3 ) (R = Me, Ph, But) with picH and teaH3 in a 1:2:1 ratio in MeCN. The core of 1 and 2 consists of an [Fe 4 (µ 3 -O) 2 ] 8+ ‘planar-butterfly’ unit to which is attached an Fe atom on either side by bridging O atoms. The core of 3 consists of an [Fe 5 (µ 3 -O) 2 ] 11+ unit comprising two near-perpendicular vertex-sharing [Fe 3 (µ 3 -O)] 7+ triangular units. Variable-temperature (T) and -field (H) solid-state dc and ac magnetization (M) studies in the 5.0–300 K temperature range revealed that 1 and 2 have an S = 5 ground state spin whereas 3 has an S = 5/2 ground state. J ij exchange couplings were calculated by DFT and a magnetostructural correlation (MSC) for polynuclear Fe III /O complexes. This allowed rationalization of the observed ground states from the analysis of the spin frustration effects operative, and provided good input values for fits of the experimental χMT vs T data to obtain the J ij values.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Analysis of spin frustration in an Fe III 7 cluster using a combination of computational, experimental, and magnetostructural correlation methods

The synthesis, structure, and magnetic properties are reported for [Fe 7 O 3 (O 2 C t Bu) 9 (mda) 3 (H 2 O) 3 ] ( 1 ), where mdaH 2 is N -methyldiethanolamine. 1 was prepared from the reaction of [Fe 3 O(O 2 C t Bu) 6 (H 2 O) 3 ](NO 3 ) with mdaH 2 in a 1:~3 ratio in MeCN. The core of 1 consists of a central octahedral Fe III ion held within a non-planar Fe 6 loop by three μ 3 -O 2- and three μ 2 -RO - arms from the three mda 2- chelates. Variable-temperature dc and ac magnetic susceptibility studies revealed dominant antiferromagnetic coupling, leading to a ground state spin of S = 5 / 2 . The ground state was confirmed by a fit of magnetization data collected in the 0.1–7.0 T and 1.8–10.0 K ranges. The four Fe 2 pairwise exchange parameters ( J 1 - J 4 ) were estimated by independent methods: theoretical calculations using either broken symmetry energy differences (-46.3, -16.2, -3.9, and - 28.1 cm -1 , respectively) or Green’s function approximation methods (-41.4, -14.8, -13.2, and - 24.7 cm -1 ), and a magnetostructural correlation (MSC) previously developed for high nuclearity Fe III /O complexes (-39.5, -13.8, -6.7, and - 23.5 cm -1 ). Additionally, the J 1 - J 4 obtained from the MSC and theoretical methods were used with the program PHI to both simulate χ M T vs T as well as to serve as reasonable input values to fit the experimental data (-41.0, -11.4, -5.0, and - 27.3 cm -1 ). Analysis of the J ij led to identification of the spin frustration effects operative and the resultant spin vector alignments at each Fe III ion, thus allowing for the rationalization of the experimental ground state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Gamma Irradiation and Oxidative Degradation of a Silica-Filled Silicone Elastomer

The radiation oxidative degradation of a commonly used silica-filled silicone elastomer DC745 was investigated by a series of experimental techniques. This elastomer is known to be chemically and thermally stable, but insufficient data exist on the radiation resistance. In the present work, gamma doses up to 200 kGy were applied under air at RT and 1Gy/s. Radiation chemical changes were investigated by NMR, FT-IR, Raman, and mass spectroscopy. DSC and TGA experiments probed thermal transitions and thermal stability changes with exposure dose. SEM probed variations on the surface of the elastomer, and solvent swelling methods were used to investigate changes in the polymer network properties. Electron paramagnetic resonance was employed to detect and identify free radicals. Uniaxial compression load tests at variable temperatures were performed to assess changes in the material’s mechanical response as a function of radiation dose. Results demonstrate that, with increasing exposure, DC745 undergoes changes in chemistry that lead to an increase in thermal stability and cross-link density, formation of free radical species, decrease in heat of fusion and increase in stiffness at low temperatures. Taken together, these results indicate that oxidative cross-linking is the dominant radiolysis mechanism that occurs when this material is exposed to gamma irradiation in air at high dose rates.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2020

Thermodynamic and Kinetic Transitions of Liquids in Nanoconfinement

Core principles of chemistry are ubiquitously invoked to shed light on the nature of molecular level interactions in nanoconfined fluids, which play a pivotal role in a wide range of processes in geochemistry, biology, and engineering. A detailed understanding of the physicochemical processes involved in the flow, structural transitions, and freezing or melting behavior of fluids confined within nanometer-sized pores of solid materials is thus of enormous importance for both basic research and technological applications. This Account provides a perspective on new insights into the thermodynamic and kinetic transitions of nanoconfined fluids in their stable and metastable forms. After briefly introducing the unique properties of mesoporous silicas from the SBA, MCM, and FDU families that serve as the confinement matrices, combining highly ordered single and bimodal mesopore architectures with tunable pore sizes in the ~2-15 nm range and narrow size distributions, recent studies on melting/freezing behavior of water confined in these host matrices are reviewed. While differential scanning calorimetry (DSC) reveals a linear relationship between melting point depression and pore size (independent of the pore shape), as predicted by the Gibbs-Thomson relation, variable temperature 2 H wide-line nuclear magnetic resonance (NMR) spectroscopy studies confirm the core-shell model of water and give evidence for a layer-by-layer freezing mechanism, which gives rise to an apparent fragile-to-strong transition in the solidification dynamics. In contrast to the freezing/melting behavior of water, the effect of nanoconfinement on the glass transition of supercooled liquids is nonuniversal and the glass transition temperature T g can either increase or decrease with the dimensionality and extent of confinement. This nonuniversal behavior is exemplified by the two glass-forming molecular liquids, glycerol and ortho-terphenyl (OTP). While glycerol shows an increase in T g and a pronounced slowdown of the rotational dynamics of the constituent molecules due to a change in the molecular packing between the bulk and the confined liquid, OTP displays a linear and confining-media-dependent depression of T g with increased confinement that is strongly influenced by the pore-liquid interface characteristics. This Account concludes with a focus on recent experimental evidence of extreme spatial and dynamical heterogeneity in both freezing and glass transition processes. This discovery was enabled by the unique mesoporous structures of SBA-16 and FDU-5, possessing bimodal architectures with two interconnected pore types of different size and shape (spherical and cylindrical). For the very first time, two melting points for water and two glass transitions for supercooled OTP, corresponding to a specific pore type, were observed. Collectively, these observations strongly suggest a close mechanistic connection between the local fluctuations in the structure and dynamics of nanoconfined liquids. Finally, while the findings reviewed in this Account provide new insights into thermodynamic and kinetic transitions of fluids, there remain many unanswered questions regarding the effects of nanoconfinement on the fundamental properties of fluids, which offer exciting future opportunities in chemical research.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Polymorphism and Negative Linear Compressibility in Pyrazine-d4

Pyrazine (1,4-diazine) is a widely used linker in coordination polymers, molecular magnets, and metal–organic frameworks, yet its solid-state polymorphism is controlled by weak C–H···N interactions. Using variable-temperature and high-pressure neutron diffraction, a previously unrecognized low-temperature transition to phase IV is identified, showing that the form stabilized under pressure can also be accessed below ∼90 K at ambient pressure in the perdeuterated compound. The transition from phase III → IV involves a symmetry-breaking lattice distortion driven by the reorganization of C–H···N hydrogen bonds, accompanied by negative linear compressibility, which is primarily accommodated through the reduction in void-space within the structure. Structural analysis reveals that while the overall monoclinic framework of phase IV is preserved under both temperature and pressure routes, the evolution of hydrogen-bonded network is distinct depending on the pathway taken. These results establish pyrazine as a model molecular solid for studying pressure–temperature polymorphism and demonstrate how weak directional interactions can produce anomalous mechanical responses in organic crystals.

Ridley, Christopher J. [Oak Ridge National Laborat↗
DOE OSTI2020

Mixed-Valent Copper Chalcogenides: Tuning Structures and Electronic Properties Using Multiple Anions

A series of mixed-anion copper chalcogenides have been prepared using solid-state methods including ACu 4.2 TeS 2 (A = K, Rb, Cs), which adopt the KCu 4 S 3 structure type. The mixedanion motif has an expanded sublattice, relative to KCu 4 S 3 , that can accommodate additional Cu atoms at its interstitial sites that are unoccupied in the parent structure. The variable temperature transport shows that the materials are p-type metals with carrier densities on the order of 10 21 cm -3 and room-temperature electrical conductivity as high as 4000 S cm -1 . Band structures calculated using density functional theory corroborate the experimental data and indicate that the interstitial Cu atoms lower the carrier concentration and increase the Fermi level of the materials. The layered structure has Te and S atoms occupying unique sites within the ACu 4.2 TeS 2 structure, where relatively hard S 2- anions prefer Wyckoff positions where they can form energetically favorable acid-base interactions with hard alkali cations. The phenomenon is observed in the related K 3 Cu 8 Te 2 S 4 system, which also has a fully ordered layered structure. Here, we believe that the report provides new chemical guidelines for targeting ordered multianion structures, as well as a unique method for tuning the electronic properties of metallic chalcogenides.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

ACuZrQ 3 (A = Rb, Cs; Q = S, Se, Te): Direct Bandgap Semiconductors and Metals with Ultralow Thermal Conductivity

ACuZrQ 3 (A = Rb, Cs; Q = S, Se, Te) were synthesized as black platelet crystals. RbCuZrS 3 , RbCuZrSe 3 , and CsCuZrS 3 crystallize in the KCuZrSe 3 structure type with space group Cmcm, and RbCuZrTe 3 and CsCuZrTe 3 crystallize in the lower symmetry space group Pnma. The tellurides exhibit a second order Jahn-Teller distortion with off-centering of Zr in its octahedral environment. The magnitude of the distortion is larger in RbCuZrTe 3 than in CsCuZrTe 3 . The structures of beta-CsCuS 4 and Rb 2 Cu 5 Te 5 were also determined. CsCuZrS 3 melts at 910 C-circle and exhibits partial decomposition upon heating at 275 C-circle, while CsCuZrTe 3 melts incongruently. Our DFT calculations of RbCuZrQ 3 (Q = S, Se) and CsCuZrS 3 indicate direct gap semiconductors in agreement with experiments. ACuZrTe 3 (A = Rb, Cs) were calculated to be metals which was confirmed for RbCuZrTe 3 with variable temperature conductivity measurements and consistent with heat capacity measurements. Spectroscopic measurements found a bandgap and work function of 1.44(5) eV and 4.89(5) eV for RbCuZrS 3 and 0.95(5) eV and 4.67(5) eV for RbCuZrSe 3 , respectively. Finally, RbCuZrTe 3 did not exhibit an optical bandgap and has a work function of 4.64(5) eV. RbCuZrTe 3 exhibits a low thermal conductivity under 0.5 W m -1 K -1 at room temperature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Polymorphism and Phase Control in Dion–Jacobson 2D 3-(Aminomethyl)piperidinium-Based Metal Iodide Perovskites

Two-dimensional halide perovskites exhibit rich structural diversity and tunable optoelectronic properties, making them promising materials for energy, sensing, and photonic applications. In this work, we report the structural mapping of four distinct polymorphs, γ (P2 1 /c), β (P4/mbm), α (P4/mmm), and δ (Cmcm) in the two-dimensional iodoplumbate, iodostannate, and iodogermanate perovskite so-called Dion-Jacobson series (3AMP)MI4 (M = Sn, Pb, Ge), where 3AMP is 3-(aminomethyl)piperidinium. The phases exhibit systematic evolution in octahedral distortion, lattice symmetry, and metal–halide geometry, enabling structural control over optoelectronic properties. Notably, the α-phase of (3AMP)SnI 4 represents a rare, ambient-stable, high-symmetry structure for Sn-based perovskites, without a phase transition down to 100 K. Variable-temperature single-crystal diffraction, powder x-ray diffraction (PXRD), and calorimetry reveal metal- and temperature-dependent polymorph interconversions, including the emergence of long range supercell reflections in Pb-rich compositions at low temperature. Optical spectroscopy and photoelectron yield spectroscopy confirm band gap tunability and band alignment trends, highlighting symmetry-dependent shifts and anomalous band gap bowing in mixed-metal systems, verified by electronic structure calculations. Calculations additionally indicate that the higher symmetry phases have reduced electron and hole effective masses compared to the lower symmetry phases.

36 MATERIALS SCIENCE↗