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Inorg Chem ; 2021 Dec 10.
Artigo em Inglês | MEDLINE | ID: mdl-34889600


Canfieldite, Ag8SnS6, is a semiconducting mineral notable for its high ionic conductivity, photosensitivity, and low thermal conductivity. We report the solution growth of large single crystals of Ag8SnS6 of mass up to 1 g from a ternary Ag-Sn-S melt. On cooling from high temperature, Ag8SnS6 undergoes a known cubic (F4̅3m) to orthorhombic (Pna21) phase transition at ≈460 K. By studying the magnetization and thermal expansion between 5-300 K, we discover a second structural transition at ≈120 K. Single crystal X-ray diffraction reveals the low-temperature phase adopts a different orthorhombic structure with space group Pmn21 (a = 7.662 9(5) Å, b = 7.539 6(5) Å, c = 10.630 0(5) Å, Z = 2 at 90 K) that is isostructural to the room-temperature forms of the related Se-based compounds Ag8SnSe6 and Ag8GeSe6. The 120 K transition is first-order and has a large thermal hysteresis. On the basis of the magnetization and thermal expansion data, the room-temperature polymorph can be kinetically arrested into a metastable state by rapidly cooling to temperatures below 40 K. We last compare the room- and low-temperature forms of Ag8SnS6 with its argyrodite analogues, Ag8TQ6 (T = Si, Ge, Sn; Q = S, Se), and identify a trend relating the preferred structures to the unit cell volume, suggesting smaller phase volume favors the Pna21 arrangement. We support this picture by showing that the transition to the Pmn21 phase is avoided in Ge alloyed Ag8Sn1-xGexS6 samples as well as in pure Ag8GeS6.

Chem Sci ; 12(44): 14718-14730, 2021 Nov 17.
Artigo em Inglês | MEDLINE | ID: mdl-34820087


An innovative method of synthesis is reported for the large and diverse (RE)6(TM) x (Tt)2S14 (RE = rare-earth, TM = transition metals, Tt = Si, Ge, and Sn) family of compounds (∼1000 members, ∼325 contain Si), crystallizing in the noncentrosymmetric, chiral, and polar P63 space group. Traditional synthesis of such phases involves the annealing of elements or binary sulfides at elevated temperatures. The atomic mixing of refractory components technique, presented here, allows the synthesis of known members and vastly expands the family to nearly the entire transition metal block, including 3d, 4d, and 5d TMs with oxidation states ranging from 1+ to 4+. Arc-melting of the RE, TM, and tetrel elements of choice forms an atomically-mixed precursor, which readily reacts with sulfur providing bulk powders and large single crystals of the target quaternary sulfides. Detailed in situ and ex situ experiments show the mechanism of formation, which involves multiphase binary sulfide intermediates. Crystal structures and metal oxidation states were corroborated by a combination of single crystal X-ray diffraction, elemental analysis, EPR, NMR, and SQUID magnetometry. The potential of La6(TM) x (Tt)2S14 compounds for non-linear optical applications was also demonstrated.

Nat Commun ; 11(1): 3005, 2020 Jun 12.
Artigo em Inglês | MEDLINE | ID: mdl-32532971


Three-dimensional heterostructures are usually created either by assembling two-dimensional building blocks into hierarchical architectures or using stepwise chemical processes that sequentially deposit individual monolayers. Both approaches suffer from a number of issues, including lack of suitable precursors, limited reproducibility, and poor scalability of the preparation protocols. Therefore, development of alternative methods that enable preparation of heterostructured materials is desired. We create heterostructures with incommensurate arrangements of well-defined building blocks using a synthetic approach that comprises mechanical disassembly and simultaneous reordering of layered transition-metal dichalcogenides, MX2, and non-layered monochalcogenides, REX, where M = Ta, Nb, RE = Sm, La, and X = S, Se. We show that the discovered solid-state processes are rooted in stochastic mechanochemical transformations directed by electronic interaction between chemically and structurally dissimilar solids toward atomic-scale ordering, and offer an alternative to conventional heterostructuring. Details of composition-structure-properties relationships in the studied materials are also highlighted.

Inorg Chem ; 56(18): 11104-11112, 2017 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-28862443


Two borophosphates, (NH4)1-2xM1+x(H2O)2(BP2O8)·yH2O with M = Mn (I) and Co (II), synthesized hydrothermally crystallize in enantiomorphous space groups P6522 and P6122 with a = 9.6559(3) and 9.501(3) Å, c = 15.7939(6) and 15.582(4) Å, and V = 1275.3(1) and 1218.2(8) Å3 for I and II, respectively. Both compounds feature helical chains composed of vertex-sharing tetrahedral PO4 and BO4 groups that are connected through O atoms to transition-metal cations, Mn2+ and Co2+, respectively. For the two crystallographically distinct transition-metal cation sites present in the structure, this results in octahedral coordination with different degrees of distortion from the ideal symmetry. The crystal-field parameters, calculated from the corresponding absorption spectra, indicate that Mn2+ and Co2+ ions are located in a weak octahedral-like crystal field and suggest that the Co-ligand interactions are more covalent than the Mn-ligand ones. Luminescence measurements at room temperature reveal an orange emission that red-shifts upon lowering of the temperature to 77 K for I, while II is not luminescent. The luminescence lifetimes of I are 33.4 µs at room temperature and 1.87 ms at 77 K. Both compounds are Curie-Weiss paramagnets with negative Weiss constants and effective magnetic moments expected for noninteracting Mn2+ and Co2+ cations but no clear long-range magnetic order above 2 K.

Nat Commun ; 8: 15565, 2017 06 01.
Artigo em Inglês | MEDLINE | ID: mdl-28569753


How photoexcitations evolve into Coulomb-bound electron and hole pairs, called excitons, and unbound charge carriers is a key cross-cutting issue in photovoltaics and optoelectronics. Until now, the initial quantum dynamics following photoexcitation remains elusive in the hybrid perovskite system. Here we reveal excitonic Rydberg states with distinct formation pathways by observing the multiple resonant, internal quantum transitions using ultrafast terahertz quasi-particle transport. Nonequilibrium emergent states evolve with a complex co-existence of excitons, carriers and phonons, where a delayed buildup of excitons under on- and off-resonant pumping conditions allows us to distinguish between the loss of electronic coherence and hot state cooling processes. The nearly ∼1 ps dephasing time, efficient electron scattering with discrete terahertz phonons and intermediate binding energy of ∼13.5 meV in perovskites are distinct from conventional photovoltaic semiconductors. In addition to providing implications for coherent energy conversion, these are potentially relevant to the development of light-harvesting and electron-transport devices.

Inorg Chem ; 54(21): 10296-308, 2015 Nov 02.
Artigo em Inglês | MEDLINE | ID: mdl-26479308


Four complex intermetallic compounds BaAu(6±x)Ga(6±y) (x = 1, y = 0.9) (I), BaAu(6±x)Al(6±y) (x = 0.9, y = 0.6) (II), EuAu6.2Ga5.8 (III), and EuAu6.1Al5.9 (IV) have been synthesized, and their structures and homogeneity ranges have been determined by single crystal and powder X-ray diffraction. Whereas I and II originate from the NaZn13-type structure (cF104-112, Fm3̅c), III (tP52, P4/nbm) is derived from the tetragonal Ce2Ni17Si9-type, and IV (oP104, Pbcm) crystallizes in a new orthorhombic structure type. Both I and II feature formally anionic networks with completely mixed site occupation by Au and triel (Tr = Al, Ga) atoms, while a successive decrease of local symmetry from the parental structures of I and II to III and, ultimately, to IV correlates with increasing separation of Au and Tr on individual crystallographic sites. Density functional theory-based calculations were employed to determine the crystallographic site preferences of Au and the respective triel element to elucidate reasons for the atom distribution ("coloring scheme"). Chemical bonding analyses for two different "EuAu6Tr6" models reveal maximization of the number of heteroatomic Au-Tr bonds as the driving force for atom organization. The Fermi levels fall in broad pseudogaps for both models allowing some electronic flexibility. Spin-polarized band structure calculations on the "EuAu6Tr6" models hint to singlet ground states for europium and long-range magnetic coupling for both EuAu6.2Ga5.8 (III) and EuAu6.1Al5.9 (IV). This is substantiated by experimental evidence because both compounds show nearly identical magnetic behavior with ferromagnetic transitions at TC = 6 K and net magnetic moments of 7.35 µB/f.u. at 2 K. The effective moments of 8.3 µB/f.u., determined from Curie-Weiss fits, point to divalent oxidation states for europium in both III and IV.