Syntheses, structures, magnetism, and optical properties of lutetium-based interlanthanide selenides

Ln3LuSe6 (Ln = La, Ce), beta-LnLuSe3 (Ln = Pr, Nd), and LnxLu4-xSe6 (Ln = Sm, Gd; x = 1.82, 1.87) have been synthesized using a Sb2Se3 flux at 1000 degrees C. Ln3LuSe6 (Ln = La, Ce) adopts the U3ScS6-type three-dimensional structure, which is constructed from two-dimensional 2(infinity)[Ln3Se6](3-) slabs with the gaps between these slabs being filled by octahedrally coordinated Lu(3+) ions. The series of beta-LnLuSe3 (Ln = Pr, Nd) are isotypic with UFeS3. Their structures include layers formed from LuSe6 octahedra that are separated by eight-coordinate Ln(3+) (Ln = Pr, Nd) ions in bicapped trigonal prismatic environments. Sm1.82Lu2.18Se6 and Gd1.87Lu2.13Se6 crystallize in the disordered F-Ln2S3 type structure with the eight-coordinate bicapped trigonal prismatic Ln(1) ions residing in the one-dimensional channels formed by three different double chains via edge- and corner-sharing. These double chains are constructed from Ln(2)Se7 monocapped trigonal prisms, Ln(3)Se6 octahedra, and Ln(4)S6 octahedra, respectively. The magnetic susceptibilities of beta-PrLuSe3 and beta-NdLuSe3 follow the Curie-Weiss law. Sm1.82Lu2.18Se6 shows van Vleck paramagnetism. Magnetic susceptibility measurements show that Gd1.87Lu2.13Se6 undergoes an antiferromagnetic transition around 4 K. Ce3LuSe6 exhibits soft ferromagnetism below 5 K. The optical band gaps for La3LuSe6, Ce3LuSe6, beta-PrLuSe3, beta-NdLuSe3, Sm1.82Lu2.18Se6, and Gd1.87Lu2.13Se6 are 1.26, 1.10, 1.56, 1.61, 1.51, and 1.56 eV, respectively.     

Self-consistent electronic band structures of extended polydiacetylene chains with realistic model side groups

Self-consistent semi-empirical band structure calculations for isolated extended polydiacetylene chains with a variety of realistic model side groups have been performed. For side groups with a CH₂ group next to the chain backbone the predicted band gap Δ ≈ 0.5 eV is substantially independent of the detailed structure of the side group and much smaller than the optical absorption thresholds experimentally observed. These results support the hypothesis that the low energy electronic excitations of these systems are better described by an excitonic, rather than an electronic, band model.     

Tuning the structural and physical properties of Cr2Ti3Se8 by lithium intercalation: a study of the magnetic properties, investigation of ion mobility with NMR spectroscopy and electronic band structure calculations

The room temperature intercalation of Cr2Ti3Se8 with butyl lithium yields a phase mixture of the starting material and of the new trigonal phase with composition Li0.4Cr0.5Ti0.75Se2. The phase pure fully intercalated trigonal phase is obtained at elevated temperature (80 degrees C) with the final composition Li0.62Cr0.5Ti0.75Se2. The line profile analysis (LPA) of the powder patterns shows that pronounced strain occurs in the intercalated material. The deintercalation of the material is realized by treatment of the fully intercalated sample with distilled water leading to the composition Li0.15Cr0.5Ti0.75Se2. The intercalation is accompanied by an electron transfer from the guest Li to the host material, and as a consequence significant changes of the interatomic distances are observed. The local environment and the dynamics of the Li+ ions in the fully intercalated sample were studied with 7Li magic angle spinning (MAS) NMR investigations. These reveal different environments of transition metal neighbors for the Li sites and a high mobility of the Li ions. Magnetic measurements show that in the pristine material antiferromagnetic interactions are dominating (theta = -113.5 K) with no long-range order at low temperatures. The magnetic ground state is characterized by a spin-glass behavior. With increasing Li content the antiferromagnetic character vanishes progressively, and the fully intercalated phase exhibits a positive Weiss constant (theta = 12 K) indicating dominating ferromagnetic exchange interactions; i.e., the magnetic properties can be significantly altered by lithiation. The interpretation of our experimental findings is supported by the results of accompanying band structure calculations done within the framework of local spin density functional theory. These demonstrate in particular the role of the charge transfer between the constituents as a function of the Li concentration and its impact on the exchange coupling.     

(EDT-TTF-CONH2)6[Re6Se8(CN)6], a metallic Kagome-type organic-inorganic hybrid compound: electronic instability, molecular motion, and charge localization

(EDT-TTF-CONH2)6[Re6Se8(CN)6], space group R, was prepared by electrocrystallization from the primary amide-functionalized ethylenedithiotetrathiafulvalene, EDT-TTF-CONH2 (E(1/2)1 = 0.49 V vs SCE in CH3CN), and the molecular cluster tetraanion, [Re6Se8(CN)6]4- (E(1/2) = 0.33 V vs SCE in CH3CN), equipped with hydrogen bond donor and hydrogen bond acceptor functionalities, respectively. Its Kagome topology is unprecedented for any TTF-based materials. The metallic state observed at room temperature has a strong two-dimensional character, in coherence with the Kagome lattice symmetry, and the presence of minute amounts of [Re6Se8(CN)6](3-)* identified by electron spin spectroscopy. A structural instability toward a distorted form of the Kagome topology of lesser symmetry is observed at ca. 180 K. The low-temperature structure is associated with a localized, electrically insulating electronic ground state and its magnetic susceptibility accounted for by a model of uniform chains of localized S = 1/2 spins in agreement with the 100 K triclinic crystal structure and band structure calculations. A sliding motion, within one out of the three (EDT-TTF-CONH2)2 dimers coupled to the [Re6Se8(CN6)(3-)*]/[Re6Se8(CN6)4-] proportion at any temperature, and the electronic ground state of the organic-inorganic hybrid material are analyzed on the basis of ESR, dc conductivity, 1H spin-lattice relaxation, and static susceptibility data which qualify a Mott localization in [EDT-TTF-CONH2]6[Re6Se8(CN)6]. The coupling between the metal-insulator transition and a structural transition allows for the lifting of a degeneracy due to the ternary axis in the high temperature, strongly correlated metallic phase which, in turn, leads to Heisenberg chains at low temperature.     

A reduced density-matrix theory of absorption line shape of molecular aggregate

A theory for the absorption line shape of molecular aggregates in condensed phase is formulated based on a reduced density-matrix approach. Intermolecular couplings in the aggregates are assumed to be weak (F?rster type of energy transfer mechanism). The spin-Boson model is employed to include the effect of electron-phonon coupling. Using the projection operator technique, we derive kinetic equations for the reduced electronic density matrix associated with the absorption spectrum. General expressions of time-dependent rate constants in the kinetic equations are derived by using the cumulant expansion technique. The resulting time-dependent kinetic equations are solved numerically. We illustrate the applicability of the present theory by calculating the line shape of a dimer (a pair of donor and acceptor of energy transfer). For a J-aggregate type of molecular pair (with excitonic redshift), a tail appears on the blue side of the absorption spectrum due to the existence of inhomogeneity in electronic state mixing which is originated from the electron-phonon coupling.     

A vibronic approach to the band-filling and temperature-dependent metal-insulator transition

A model of vibronic origin is used to investigate the important issue of metal-insulator transition in low-dimensional materials. For zero temperature, the stability of the single-band model chain is controlled by the competition between the internal electron-phonon coupling and the nearest-neighbor hopping integral. Assuming one particular deformation mode, one can analytically derive an instability criterion in which the band filling is explicitly included. The carrier doping directly controls the stability of a one-dimensional chain. For a half-filled band, the Peierls instability is recovered. For finite temperatures, a similar criterion is derived and can be used to investigate the metal-insulator transition temperatures.     

Structural and Electronic Peierls Distortions in the Elements (A): The Crystal Structure of Tellurium

The thermodynamically stable tellurium phase (standard conditions) in the trigonal α‐Se structure type may be understood to arise from a three‐dimensional Peierls distortion away from a simple cubic structure. Three‐dimensional electronic structure calculations from first principles reveal that the simple cubic structure exhibits partially filled bands which are folded back when using a hexagonal coordinate system with a tripled volume. The symmetry‐lowering transition from the cubic to the experimentally observed structure is paralled by a removal of band degeneracies and crossings, opening a band gap at the Fermi level.     

Syntheses, crystal structures and characterizations of two new quaternary thioborates: PbMBS4 (M = Sb, Bi)

Two new quaternary thioborates, PbSbBS(4) and PbBiBS(4), have been synthesized from solid-state reaction methods at temperatures from 1073 to 1123 K in evacuated sealed quartz tubes. The crystal structures have been determined by means of single crystal X-ray diffraction and they both crystallize in the P2(1)/m space group of the monoclinic system with a = 5.9532(18) ?, b = 6.2031(13) ?, c = 9.250(3) ?, β = 108.200(16)°, Z = 2 for PbSbBS(4) and a = 5.971(10) ?, b = 6.273(9) ?, c = 9.132(15) ?, β = 107.75(2)°, Z = 2 for PbBiBS(4), respectively. The two compounds are isostructural and both constructed with the infinite one-dimensional [MBS(4)](2-) (M = Sb or Bi) chains as building blocks, which are composed of [BS(3)](3-) trigonal plane units with [MS(3)](3-) (M = Sb or Bi) trigonal pyramids connected alternatively through corner-sharing along the crystallographic b axis. Two adjacent [MBS(4)](2-) chains are further bridged by the intermediate Pb(2+) cations, forming a novel S-shaped Pb-[MBS(4)] dimeric chain structure. In addition, first-principles electronic structure calculations based on the density functional theory (DFT) were performed on compound PbSbBS(4), indicating that the compound belongs to direct semiconductor with a band gap of 1.803 eV, which is in good agreement with the experimental value estimated from the UV-Vis diffuse reflectance spectroscopy.     

Vibronic coupling in molecules and in solids

We utilize the experience gained in our previous studies on the "chemistry of vibronic coupling" in simple homonuclear and heteronuclear molecules to begin assembling theoretical guidelines for the construction of potentially superconducting solids exhibiting large electron-phonon coupling. For this purpose we analyze similarities between vibronic coupling in isolated molecules and in extended solids. In particular, we study vibronic coupling along the antisymmetric stretch coordinate (Q(as)) in linear symmetric AAA molecules, and along the optical phonon "pairing" mode coordinate (Q(opt)) in corresponding one-dimensional [A]( infinity ) chains built of equidistant A atoms. This is done for a broad range of chemical elements (A). The following similarities between vibronic coupling in molecules and phonon coupling in solids emerge from our calculations: 1) The HOMO/LUMO electronic energy gap in an AAA molecule increases along Q(as), and the highest occupied crystal orbital/lowest unoccupied crystal orbital gap in [A]( infinity ) chain increases along Q(opt). 2) The maximum vibronic instability is invariably obtained for a half-filled, singly occupied molecular orbital in AAA molecules, and for a corresponding half-filled band in [A]( infinity ) chains. 3) The vibronic stability of an AAA molecule increases with a decrease of the AA bond length, as does the vibronic stability of [A]( infinity ) chains (external pressure may lead to a reversal of a Peierls distortion). 4) The high degree of s-p mixing and ionic/covalent forbidden curve crossing dramatically enhance the vibronic instability of both AAA molecules and [A]( infinity ) chains. We also introduce one quantitative relationship: The parameter log(R) (where R is molar refractivity, a parameter used by Herzfeld to prescribe the conditions for the metallization of the elements) correlates with a parameter f(AA) (defined as twice the electronegativity of A, divided by the equilibrium AA bond length), used by two of us previously to describe vibronic coupling in AAA molecules for a broad range of elements (A=halogen, H, or an alkali metal). We hope to illustrate that key chemical aspects of vibronic coupling in simple molecules may thus be profitably transferred to corresponding materials in the solid state.     

Realization of a particle-in-a-box: electron in an atomic Pd chain

Well-defined Pd chains were assembled from single atoms on a NiAl(110) surface with the tip of a scanning tunneling microscope. The electronic properties of the chains were determined by spatially resolved conductance measurements, revealing a series of quantum well states with parabolic dispersion. The particle-in-a-box states in Pd chains show higher onset energy and larger effective mass than those in Au chains investigated before, reflecting the influence of elemental composition on one-dimensional electronic systems. The intrinsic widths and spectral intensities of Pd induced states provide information on lifetime and spatial localization of states in the atomic chain.     

Chemical bonding variations and electron-phonon interactions

A new functional, Psib(Phi), of an electronic state in solids based on the bonding indicator B(tau,tau') in terms of Mulliken's electron partitioning approach has been introduced. Using Psib(Phi), the bonding variations of an electronic state caused by electron-phonon coupling can be studied. With this proposed approach, the differences between the "flat band" states for Hg in coupling to the phonons and the peaklike structure of electron-phonon coupling constants in the q space are well explained.     

Synthesis, Single-crystal Structure, and Optical and Magnetic Properties of CaEr2S4

Brown needle-like crystals of CaEr2S4 were isolated as the major product from a reaction of elements and binary sulfides by a two-step flux technique. CaEr2S4 crystallizes in the orthorhombic space group Pnma with a = 12.845(4), b = 3.862(4), c = 13.001(2) , V = 645.0(7) 3, Z = 4, F(000) = 880, μ(MoKα) = 27.794 mm-1, the final R = 0.0528 and wR = 0.0562 for 1070 observed reflections with I > 3σ(I). The CaEr2S4 structure forms a three-dimensional framework that consists of interconnected tetra-octahedral Er4S18 fragments. Ca2+ cations, in a monocapped trigonal prism geometry, are stuffed in two parallel rows into the one-dimensional channels along the b direction. CaEr2S4 is an infrared-transparent semiconductor with a band gap of 1.81 eV. Magnetic susceptibility measurements over 6～300 K indicate a Curie-Weiss paramagnetic behavior for the phase, with an effective magnetic moment of 9.64(1) μB per Er3+ ion.     

Ultraviolet-visible-near infrared and mid-Fourier transform infrared spectroscopic studies of intermolecular interaction in cholesteryl oleyl carbonate in mesophases

Ultraviolet-visible-near infrared (UV-Vis-NIR) and Fourier transform infrared (FTIR) spectroscopic studies are presented of molecular association between like molecules of cholesteryl oleyl carbonate, each containing suitable pi-donor (steroid ring C=C) and pi-acceptor (C-O single bonds united with a C=O bond to give a carbonate group) moieties. Frequency shifts and intensity enhancements of donor and acceptor oscillators appear to be governed by reduced mass, vibronic coupling constants, and a few other parameters such as relative change in force constants, etc. Donor-acceptor complex formation is characterized not only by the appearance of new bands in the mid-FTIR spectrum but also by the emergence of a new, intense electronic band centered at approximately 3700 cm(-1), the so-called charge-transfer band, in the UV-Vis-NIR spectrum. This band is strong in the smectic-A and solid phases, but progressively diminishes when temperature is raised to realize the upper end of the cholesteric phase and eventually the isotropic phase. Also, a new, small electronic band at approximately 360 nm, only seen in the entire thermal range of the cholesteric phase, is attributed to the Lifshitz-van der Waals interaction between pretransitional smectic-A domains existing in the cholesteric phase. It is argued that mesophases may owe their thermodynamic stability to both Lifshitz-van der Waals and vibronic coupling (or electron-phonon coupling in extended systems such as smectics and solids) interactions.     

Synthesis, crystal and band structures, and optical properties of a new quaternary metal pnictidehalide: (Hg2Cd2As2Br)Br

A new quaternary cadmium and mercury pnictidehalide semiconductor (Hg2Cd2As2Br)Br (1) has been prepared by the solid-state reaction of HgBr2 with elemental Cd and As at 420 degrees C. Compound 1 crystallizes in the space group Pmma of the orthorhombic system with two formula units in a cell: a = 8.791(4) A, b = 4.701(2) A, c = 9.779(6) A, V = 404.2(3) A(3). The structure of 1 is composed of parallel slabs bridged by linearly coordinated Hg atoms to form a 3D cationic network with the channels occupied by discrete Br anions, in which the layer consists of interlinks of linear (HgAs2Br2) tetrahedral chains and (CdAs2Br) trigonal chains. The optical properties were investigated in terms of the diffuse reflectance and Fourier transform infrared spectra. The electronic band structure along with the density of states (DOS) calculated by the DFT method indicate that compound 1 is a semiconductor with an indirect band gap and that the optical absorption mainly originates from the charge transitions from the Br2-4p and As-4p to Cd-5s and Hg-6s states.     

Closed-pore crystal capable of adsorbing CO2 onto isolated cavities generated by disorderly mixing of substituents on host skeleton

A single crystal adsorbent, [Rh(II)(2)(bza)(4)(2,3-empyz)](n) (2,3-empyz = 2-ethyl-3-methylpyrazine) (1), was synthesized by self-assembly reaction of a Rh(2) benzoate complex and substituted pyrazine linker. The compound consists of one-dimensional zigzag chains, which generated a closed-pore structure without channels. The cavities were statistically generated by the static disorder of substituents on pyrazine and are separated by long intervals within the crystal. The property of CO(2) absorption was characterized in this closed-pore system. The CO(2) inclusion structure was determined by single-crystal X-ray diffraction measurements. These studies suggest that CO(2) molecules were adsorbed and diffused in the nonporous crystal with the isolated cavities.     

Helical polymer 1/infinity[P2Se6(2-)]: strong second harmonic generation response and phase-change properties of its K and Rb salts

The selenophosphates A2P2Se6 (A = K, Rb) crystallize in the chiral trigonal space group P3121, with a = 7.2728(9) A, c = 18.872(4) A, and Z = 3 at 298(2) K and a = 14.4916(7) A, c = 18.7999(17) A, and Z = 12 at 173(2) K for K+ salt and a = 7.2982(5) A, c = 19.0019(16) A, and Z = 3 at 100(2) K for Rb+ salt. The A2P2Se6 feature parallel one-dimensional helical chains of 1/infinity[P2Se62-] which depict an oxidative polymerization of the ethane-like [P2Se6]4- anion. On cooling well below room temperature K2P2Se6 exhibits a displacive phase transition to a crystallographic subgroup and forms a superstructure with a cell doubling along the a- and b-axes. The Rb analogue does not exhibit the phase transition. The compounds are air stable and show reversible glass-crystal phase-change behavior with a band gap red shift of 0.11 and 0.22 eV for K+ and Rb+ salts, respectively. Raman spectroscopy, 31P magic angle spinning solid-state NMR, and pair distribution function (PDF) analysis for crystalline and glassy K2P2Se6 give further understanding of the phase transition and the local structure of the amorphous state. K2P2Se6 exhibits excellent mid-IR transparency and a strong second harmonic generation (SHG) response. The SHG response is type-I phase-matchable and in the wavelength range of 1000-2000 nm was measured to be 50 times larger than that of the commercially used material AgGaSe2. Glassy K2P2Se6 also exhibits an SHG response without the application of electric field poling. In connection with the NLO properties the thermal expansion coefficients for K2P2Se6 are reported.     

A series of novel rare-earth bismuth tungstate compounds LnBiW2O9 (Ln=Ce, Sm, Eu, Er): synthesis, crystal structure, optical and electronic properties

The structural, optical, and electronic properties of four rare-earth bismuth tungstate compounds, LnBiW(2)O(9) (Ln = Ce, Sm, Eu, Er), have been investigated by means of single-crystal X-ray diffraction, elemental analyses, and spectral measurements. For some of the compounds, the calculations of energy band structures and density of states have also been made by the density functional theory. The structure of CeBiW(2)O(9) features a three-dimensional (BiW(2)O(9))(3-) anionic framework with interesting channels where Ce atoms are located. The framework is constructed by one-dimensional BiO(9) polyhedra chains and one-dimensional zigzag W(2)O(9) chains via edge- and face-sharing. LnBiW(2)O(9) (Ln = Sm, Eu, Er) are isostructural and their structures feature a three-dimensional network based on alternating (BiO(2))(-) layers and (Ln(2)W(2)O(12))(6-) layers connected by corner-linked chains of WO(6) octahedra. Results of spectral measurements indicate that EuBiW(2)O(9) exhibit the characteristic yellow-red light emission under excitation at 395 nm, and it will be a red phosphor in designing white light-emitting diode device. The calculated results of band structures by using the density functional theory (DFT) show that the solid-state compound CeBiW(2)O(9) and SmBiW(2)O(9) are indirect band gap materials.     

The role of the π-bonding network for trigonal level splittings of tris-bidentate Cr(acac)3 and Cr(ox) 3 3−

The varying -bonding contributions in the title compounds caused by the different electronic and molecular structure of the chelate rings are used for explaining the large band splittings in the absorption spectra by trigonal symmetry. It is shown that usual ligand field theory and the angular overlap model are not able to account for the trigonal level splitting of Cr(acac)₃ for which the coordination sphere of oxygen atoms is nearly octahedrally arranged. The experimental finding can, however, be rationalized by an extended angular overlap model which considers the phase coupling of -orbitals in the ligands leading to non-additive contributions to the metal-ligand bond energy.On leave of absence from the Bulgarian Academy of Sciences, Sofia, Bulgaria     

Carbothermal chemical vapor deposition route to Se one-dimensional nanostructures and their optical properties

A simple and practical carbothermal chemical vapor deposition route has been developed for the growth of trigonal phase selenium nanowires and nanoribbons. In detail, the mixture of active carbon and selenium was heated for the chemical reaction to occur, followed by thermal evaporation and decomposition into elemental selenium. The as-prepared sample was characterized by X-ray diffraction, transmission electron microscopy, high-resolution electron microscopy, UV-vis absorption, and photoluminescence. The results show that trigonal Se nanowires have uniform diameters ranging from 20 to 60 nm and grow along the [001] direction, with the same growth direction found for nanoribbons. Spectral measurements suggest a large blue shift and two types of electron transition activity. The influences of experimental conditions on morphologies and growth processes are also discussed. This synthetic method should be able to be extended to grow other one-dimensional chalcogens and chalcogenides nanostructures.     

Comparative electronic band structure study of the intrachain ferromagnetic versus antiferromagnetic coupling in the magnetic oxides Ca3Co2O6 and Ca3FeRhO6

In the (MM'O6)infinity chains of the transition-metal magnetic oxides Ca3MM'O6 the MO6 trigonal prisms alternate with the M'O6 octahedra by sharing their triangular faces. In the (Co(2O6)infinity chains of Ca3Co2O6 (M = M' = Co) the spins are coupled ferromagnetically, but in the (FeRhO6)infinity chains of Ca3FeRhO6 (M = Fe, M' = Rh) they are coupled antiferromagnetically. The origin of this difference was probed by carrying out spin-polarized density functional theory electronic band structure calculations for ordered spin states of Ca3Co2O6 and Ca3FeRhO6. The spin state of a (MM'O6)infinity chain determines the occurrence of direct metal-metal bonding between the adjacent trigonal prism and octahedral site transition-metal atoms. The extent of direct metal-metal bonding in the (Co2O6)infinity chains of Ca3Co2O6 is stronger in the intrachain ferromagnetic state than in the intrachain antiferromagnetic state, so that the intrachain ferromagnetic state becomes more stable than the intrachain antiferromagnetic state. Such a metal-metal-bonding-induced ferromagnetism is expected to occur in magnetic insulators and magnetic metals of transition-metal elements in which direct metal-metal bonding can be enhanced by ferromagnetic ordering. In the (FeRhO6)infinity chains of Ca3FeRhO6 the ferromagnetic coupling does not lead to a strong metal-metal bonding and the adjacent spins interact by the Fe-O...O-Fe super-superexchange, hence leading to an antiferromagnetic coupling.