Te Bond Lengths Research Articles

Influence of molybdenum doping on the structural, electrical, and optical properties of germanium telluride thin films

This study investigates the influence of molybdenum (Mo) doping on the local atomic structure, morphology, and electrical properties of amorphous germanium telluride (GeTe) thin films. Structural and morphological studies show that Mo-doping inhibits the crystallization of amorphous GeTe thin films. Moreover, the crystallization temperature of the GeTe thin film increases from 197 to 317 °C with increasing Mo dopant concentration. The electrical properties of Mo-doped GeTe thin films are discussed in terms of the capacitance–voltage characteristics in temperatures range 27–357 °C. The optical bandgap values decrease from 0.67 to 0.55 eV when increasing Mo doping in GeTe thin film. Mo integration significantly impacts the Ge–Te bond lengths and structural symmetry, increasing the degree of disorder in amorphous GeTe thin films. This study demonstrates the effectiveness of Mo doping in enhancing the thermal stability of the amorphous phase of GeTe for its envisioned memory and brain-inspired computing applications.

1,4]Ditellurino[2,3-b:5,6-b']di-pyrazine.

[1,4]Ditellurino[2,3-b:5,6-b']di-pyrazine represents the first reported [1,4]chalcogena[2,3-b:5,6-b']di-pyrazine containing a heavy chalcogens The asymmetric unit consists of three mol-ecules. In contrast to its sulfur analog, which is planar [Lynch et al. (1994 ▸) Cryst. Struct. Commun. 50,1470-1472], C8H4N4Te2 is folded along the Te⋯Te axis to accommodate the larger chalcogenide atoms. The dihedral angle between the two Te2C2 rings of the central ring is 57.9° (mean of three). C-Te bond lengths range from 2.1105 (16) Å to 2.1381 (17) Å, in good agreement with those predicted by their covalent radii. All Te atoms are involved in inter-molecular Te⋯N contacts, with distances in the range 2.894 (2) to 2.963 (2) Å. These result in a spiral supra-molecular assembly, forming helical columns.

First-principles study and experimental characterization of metal incorporation in germanium telluride

Germanium telluride is a well-known phase change material (PCM) used in non-volatile memory cells and radio frequency switches. Controlling the properties of GeTe for improved PCM device performance has sometimes been achieved by doping and/or alloying with metals, often at concentrations greater than 10 at. % and using non-equilibrium methods. Since switching PCMs between the low-resistance crystalline and high-resistance amorphous states requires a heating cycle, the stability of metal-incorporated GeTe (Ge0.5−xMxTe0.5) films is also critical to practical implementation of these materials in electronic and optoelectronic devices. In this work, we use both density-functional theory and experimental characterization methods to probe the solubility and critical properties of Ge0.5−xMxTe0.5 films. Using first-principles calculations, we determine the enthalpy of formation for GeTe with 2.08, 4.17, and 6.25 at. % of Cu, Fe, Mn, Mo, and Ti and show trends between the stability of the Ge0.5−xMxTe0.5 systems and the atomic position, composition, and distribution of the metal atoms in the GeTe matrix. Out of all the studied systems, Mo was the only metal to cluster within GeTe. Analysis of the Ge–Te bond lengths and volumes of the Ge0.5−xMxTe0.5 supercells shows that increasing the atomic concentration (2.08, 4.17, 6.25 at. %) of the different metals causes varied distortions of the crystal structure of GeTe that are accompanied by significant changes in the projected density of states. Computational predictions concerning metal solubility and the effect of metal incorporation on critical properties of GeTe are compared to experimental results in the literature (Cu, Mn, Mo, and Ti) and to transmission electron microscopy and transport data from newly characterized co-sputtered Ge0.5−xFexTe0.5 films. The computational predictions of decreasing solubility (Mn > Cu, Fe > Ti, Mo) shows good agreement with experimental observations (Mn, Cu > Fe > Ti, Mo), and Ge0.5−xFexTe0.5 films exhibited increased crystallization temperatures from pure GeTe.

Mechanical, electronic and optical properties of K2TeX6 (X=Cl, Br, I) perovskite derivatives using density functional theory

In this study, the elastic, electronic, and optical properties of cubic, tetragonal and monoclinic K2TeX6 (X = Cl, Br, I) perovskite derivatives are investigated using first-principles calculations. The results reveal that different space groups have a minor influence while the atomic number of X has a significant influence on the physical properties of these perovskite derivatives. In their crystal structures, X and K atoms form X–K ionic bond, while X and Te atoms form X–Te ionic bond with more covalent components. For K2TeBr6 perovskite derivatives within the three different space groups, the close Br–Te bond lengths slightly decrease as the symmetry decreases. However, for the monoclinic K2TeX6 (X = Cl, Br, I) perovskite derivatives, as the atomic radius of X increases, the X–Te bond lengths increase with the decrease in covalent bond components. All the K2TeX6 (X = Cl, Br, I) perovskite derivatives exhibit obvious elastic anisotropy. The monoclinic K2Te2I6 perovskite derivative is brittle, but K2TeX6 (X = Cl, Br) perovskite derivatives are ductile. In the electronic structures of the perovskite derivatives, the top valence bands are primarily contributed by X-p (Cl-3p, Br-4p, or I-5p) and Te-5s orbits, and the bottom conduction bands are dominated by Te-6p orbit. The effective masses and carrier mobilities exhibit distinct anisotropy due to the anisotropic crystal structures. As the atomic number of X increases, the effective masses of electrons and holes gradually decrease, while the carrier mobilities increase. Especially, the higher carrier mobilities of monoclinic K2Te2I6 perovskite derivative are caused by its smaller effective masses. The K2TeX6 (X = Cl, Br, I) perovskite derivatives exhibit excellent dielectric functions and absorption coefficients in the visible and ultraviolet regions.

Doping Sumanene with Both Chalcogens and Phosphorus(V): One-Step Synthesis, Coordination, and Selective Response Toward AgI.

Heterasumanenes 4-6 containing chalcogen (S, Se, and Te) and phosphorus atoms have been synthesized in a one-pot reaction from trichalcogenasumanenes 1-3 by replacing one chalcogen atom with a P=S unit. The P=S unit makes 4-6 almost planar and shrinks the HOMO-LUMO gap as compared to 1-3. The bonding between Ag+ and S atom on P=S brings about a distinct change to the optical properties of 4-6; 4 in particular shows a selective fluorescence response toward Ag+ with LOD of 0.21 μm. Compounds 4-6 form complexes with AgNO3 to be (4)2 ⋅AgNO3 , (5)2 ⋅AgNO3 , and (6)2 ⋅(AgNO3 )3 . In complexes, the coordination between Ag+ and P=S is observed, which leads to shrinkage of C-P and C-X (X=S, Se, Te) bond lengths. As a result, 4, 5, and 6 are all bowl-shaped in complexes with bowl-depths reaching to 0.66 Å, 0.42 Å, and 0.40 Å, respectively. There are Ag-Te dative bonds between Ag+ and Te atom on telluorophene in (6)2 ⋅(AgNO3 )3 .

Doping Sumanene with Both Chalcogens and Phosphorus(V): One‐Step Synthesis, Coordination, and Selective Response Toward AgI

AbstractHeterasumanenes 4–6 containing chalcogen (S, Se, and Te) and phosphorus atoms have been synthesized in a one‐pot reaction from trichalcogenasumanenes 1–3 by replacing one chalcogen atom with a P=S unit. The P=S unit makes 4–6 almost planar and shrinks the HOMO–LUMO gap as compared to 1–3. The bonding between Ag+ and S atom on P=S brings about a distinct change to the optical properties of 4–6; 4 in particular shows a selective fluorescence response toward Ag+ with LOD of 0.21 μm. Compounds 4–6 form complexes with AgNO3 to be (4)2⋅AgNO3, (5)2⋅AgNO3, and (6)2⋅(AgNO3)3. In complexes, the coordination between Ag+ and P=S is observed, which leads to shrinkage of C−P and C−X (X=S, Se, Te) bond lengths. As a result, 4, 5, and 6 are all bowl‐shaped in complexes with bowl‐depths reaching to 0.66 Å, 0.42 Å, and 0.40 Å, respectively. There are Ag−Te dative bonds between Ag+ and Te atom on telluorophene in (6)2⋅(AgNO3)3.

High Thermoelectric Performance in the Wide Band‐Gap AgGa1‐xTe2 Compounds: Directional Negative Thermal Expansion and Intrinsically Low Thermal Conductivity

AbstractA deficiency of Ga in wide band‐gap AgGa1‐xTe2 semiconductors (1.2 eV) can be used to optimize the electrical transport properties and reduce the thermal conductivity to achieve ZT > 1 at 873 K. First‐principles density functional theory calculations and a Boson peak observed in the low temperature heat capacity data indicate the presence of strong coupling between optical phonons with low frequency and heat carrying acoustical phonons, resulting in a depressed maximum of Debye frequency in the first Brillouin zone and low phonon velocities. Moreover, the AgTe bond lengths and TeAgTe bond angles increase with rising temperature, leading to a significant distortion of the [AgTe4]7− tetrahedra, but an almost unmodified [GaTe4]5− tetrahedra. This behavior results in lattice expansion in the ab‐plane and contraction along the c‐axis, corresponding to the positive and negative Gruneisen parameters in the phonon spectral calculations. This effect gives rise to the large anharmonic behavior of the lattice. These factors together with the low frequency vibrations of Ag and Te atoms in the structure lead to an ultralow thermal conductivity of 0.18 W m−1 K−1 at 873 K.

Conformational trimorphism of bis(2,6-dimesitylphenyl)ditelluride

AbstractBesides the previously known α-form (monoclinic,P21/c, Z=4) of bis(2,6-dimesitylphenyl)ditelluride, two new polymorphic modifications, namely the β-form (monoclinic,P21/c, Z=8) and the γ-form (triclinic,P1̅, Z=2), were obtained serendipitously during chemical reactions. In all three modifications, the individual molecules possess significantly different conformations and bond parameters, such as Te–Te bond lengths, C–Te–Te bond angles, C–Te–Te–C torsion angles and intramolecular Menshutkin interactions, which is also reflected in their non-covalent interactions with adjacent molecules in the crystal lattice via London dispersion and electrostatic forces. The interplay between intermolecular and intramolecular forces in these conformational polymorphs was investigated using quantum chemical calculations, which reveal that the β-form should be thermodynamically stable at absolute zero. In contrast, crystallization experiments and thermoanalytical investigations indicate that the α-form is stable at high temperatures and therefore, both forms might be related by enantiotropism.

The first crystal structures of six- and seven-membered tellurium- and nitrogen-containing (Te—N) heterocycles: 2H-1,4-benzotellurazin-3(4H)-one and 2,3-dihydro-1,5-benzotellurazepin-4(5H)-one

There is a paucity of data concerning the structures of six- and seven-membered tellurium- and nitrogen-containing (Te-N) heterocycles. The title compounds, C8H7NOTe, (I), and C9H9NOTe, (II), represent the first structurally characterized members of their respective classes. Both crystallize with two independent molecules in the asymmetric unit. When compared to their sulfur analogs, they exhibit slightly greater deviations from planarity to accommodate the larger chalcogenide atom, with (II) adopting a pronounced twist-boat conformation. The C-Te-C angles of 85.49 (15) and 85.89 (15)° for the two independent molecules of (I) were found to be somewhat smaller than those of 97.4 (2) and 97.77 (19)° for the two independent molecules of (II). The C-Te bond lengths [2.109 (4)-2.158 (5) Å] are in good agreement with those predicted by the covalent radii. Intermolecular N-H...O hydrogen bonding in (I) forms centrosymmetric R2(2)(8) dimers, while that in (II) forms chains. In addition, intermolecular Te...O contacts [3.159 (3)-3.200 (3) Å] exist in (I).

Electronic aspects of formation and properties of local structures around Mn in Cd1−xMnxTe1−ySey

Local electronic and structural features around Mn in Cd1−xMnxTe0.97Se0.03 (x = 0.02; 0.05; 0.1; y = 0.03) were studied by means of X-ray Absorption Fine Structure (XAFS) techniques. Manganese ions with an average valence 2+, are found to be well incorporated into the host CdTe lattice, with clear preference for Te atoms as the first neighbors. However, Mn and Te are found to form two essentially different types of bonds, one short, strong and directional (cubic MnTe-alike bond), and three much longer, predominantly ionic in nature (hexagonal MnTe-alike bonds), thereby distorting the tetrahedral coordination around Mn. The origin of peculiar Mn–Te bonds distribution and details of their nature and strength are further elaborated by employing the first principle electronic structure calculations. That way a thorough insight in impact of the Mn–Te bond length variation on the electronic structure of the compound is obtained. The relations established between the local structures and electronic properties offer a reliable procedure for detailed analysis of the structural and electronic consequences of the 3d-transition metals (TM) incorporation in II–VI semiconductor host. Clear distinction between various influences makes the procedure easily adoptable also to the studies of TM impurities in other semiconductors.

A square-planar tellurium(II) complex with Te,Te'-chelating ligands.

While exploring the chemistry of tellurium-containing dichalcogenidoimidodiphosphinate ligands, the first all-tellurium member of a series of related square-planar E(II)(E')4 complexes (E and E' are group 16 elements), namely bis(P,P,P',P'-tetraphenylditelluridoimidodiphosphinato-κ(2)Te,Te')tellurium(II) (systematic name: 2,2,4,4,8,8,10,10-octaphenyl-1λ(3),5,6λ(4),7λ(3),11-pentatellura-3,9-diaza-2λ(5),4λ(5),8λ(5),10λ(5)-tetraphosphaspiro[5.5]undeca-1,3,7,9-tetraene), C48H40N2P4Te5, was obtained unexpectedly. The formally Te(II) centre is situated on a crystallographic inversion centre and is Te,Te'-chelated to two anionic [(TePPh2)2N](-) ligands in an anti conformation. The central Te(II)(Te)4 unit is approximately square planar [Te-Te-Te = 93.51 (3) and 86.49 (3)°], with Te-Te bond lengths of 2.9806 (6) and 2.9978 (9) Å.

Synthesis, Spectral Characterization of Four Symmetrical and Unsymmetrical Organotellurium(II) Compounds: O−H…N, CH…π, and CH…O Secondary Interactions in X‐Ray Crystal Structures of 4‐MeOC6H4TeCH2CH2CH2N═C(CH3)C6H4‐2‐OH (1) and Te[CH2CH2CH2N═C(CH3)C6H4‐2‐OH]2 (2)

ABSTRACTThe condensation of 3‐(4‐methoxyphenyltelluro)propylamine and bis(3‐aminopropyl) telluride with 2‐hydroxyacetophenone formed two Schiff's bases 2‐[1‐(3‐(4‐methoxyphenyltellanyl)propylimino)ethyl]phenol (1) and Bis 2‐[1‐(3‐iminopropyltellanyl)ethyl]phenol (2), respectively. The reduction of 1 and 2 gave compounds 2‐[1‐(3‐(4‐methoxyphenyltellanyl)propylamino)ethyl]phenol (3) and Bis 2‐[1‐(3‐aminopropyltellanyl)ethyl]phenol (4), respectively. These four new organotellurium compounds (1–4) were characterized by 1H and 13C NMR and FTIR spectroscopies and atomic absorption spectrophotometer (AAS) (Te). The conductance and molecular weights and composition by elemental analysis (C, H, and N) of 1 and 2 were determined. The compound 4 was also characterized by 13C DEPT‐135 NMR spectrum. The compounds 1 and 3 are unsymmetrical, and 2 and 4 are symmetrical type of telluroethers. The single crystals of 1 and 2 were grown, and their molecular structures were determined by single crystal X‐Ray diffraction. The ArC–Te bond length in 1 was found 2.117(7) Å. The RC–Te bond length in 1 is 2.141(5) Å and in 2 is 2.146(5) and 2.150(6) Å. The >C═N bond lengths in 1 and 2 were between 1.278(6) and 1.295(6) Å. There exists intramolecular O−H…N hydrogen bonding [between 1.758(4) and 1.792(4) Å] in 1 and 2. In compound 2, these nonbonded interactions result in the formation of a (34) dimeric ring. CH…O [2.505(4)–2.516(3) Å] in 1 and 2 and CH…π [2.61(5) and 2.89(4) Å] secondary interactions are also present in 2.

A Silver(I) Iodide Complex of a Tellurophosphorane

AbstractThe formation and structural characterization of the first coordination complex of the tellurophosphorane [tBu(H)NP(μ‐NtBu)2P(Te)N(H)tBu] are presented. The silver(I) iodide complex {(I)Ag[tBu(H)NP(μ‐NtBu)2P(Te)N(H)tBu]3} (2) was isolated from the reaction of [Li(TMEDA)]2[tBuN(Te)P(μ‐NtBu)2P(Te)NtBu)] (1) with AgI in toluene and characterized by X‐ray analysis and multinuclear NMR spectroscopy. Complex 2 is comprised of three tellurophosphorane ligands coordinated to AgI in a distorted tetrahedral arrangement [102.90(3)–114.51(5)o]. The P=Te bond lengths of 2.3883(13)–2.4043(17) Å in 2 are slightly elongated compared to those in the free ligand.

Theoretical and experimental investigations of the properties of Ge2Sb2Te5 and indium-doped Ge2Sb2Te5 phase change material

We have carried out comprehensive computational and experimental study on the face-centered cubic Ge2Sb2Te5 (GST) and indium (In)-doped GST phase change materials. Structural calculations, total density of states and crystal orbital Hamilton population have been calculated using first-principle calculation. 5 at.% doping of In weakens the Ge–Te, Sb–Te and Te–Te bond lengths. In element substitutes Sb to form In–Te-like structure in the GST system. In–Te has a weaker bond strength compared with the Sb–Te bond. However, both GST and doped alloy remain in rock salt structure. It is more favorable to replace Sb with In than with any other atomic position. X-ray diffraction (XRD) analysis has been carried out on thin film of In-doped GST phase change materials. XRD graph reveals that In-doped phase change materials have rock salt structure with the formation of In2Te3 crystallites in the material. Temperature dependence of impedance spectra has been calculated for thin films of GST and doped material. Thickness of the as-deposited films is calculated from Swanepoel method. Absorption coefficient (α) has been calculated for amorphous and crystalline thin films of the alloys. The optical gap (indirect band gap) energy of the amorphous and crystalline thin films has also been calculated by the equation \( \alpha h\nu = \beta (h\nu - E_{\text{g }} )^{2} \). Optical contrast (C) of pure and doped phase change materials have also been calculated. Sufficient optical contrast has been found for pure and doped phase change materials.

Directional Compton profiles of CdTe using a low intensity 241Am source

We report the first-ever experimental Compton profiles (CPs) of CdTe measured along [100] and [111] directions using 100mCi 241Am Compton spectrometer. The experimental Compton data have been interpreted in terms of theoretical anisotropy in the momentum densities and also energy bands computed using density functional theory with local density approximation, generalized gradient approximation (GGA) and second order GGA. It is found that DFT–GGA based CPs are in marginally better agreement with the experimental data in comparison to other DFT based profiles. The anisotropy in CPs is found to be in tune with the energy bands. The bond length (5.23±0.11a.u.) deduced from the oscillation in the experimental anisotropy reconciles well with the theoretical Cd–Te bond length (5.30a.u.) which establishes the role of γ-ray Compton measurements in determination of structural parameters.

Transfer Hydrogenation of Ketones and Catalytic Oxidation of Alcohols with Half-Sandwich Complexes of Ruthenium(II) Designed Using Benzene and Tridentate (S, N, E) Type Ligands (E = S, Se, Te)

The complexes of composition fac-[(η6-C6H6)Ru(L)][PF6][X] (1−6; X = PF6 or Cl), formed by reacting 2-MeSC6H4CH═NCH2CH2E-C6H4-4−R (L1−L3) and 2-MeSC6H4CH2-NHCH2CH2E-C6H4-4−R (L4−L6) (where E = S or Se, R = H; E = Te, R = OMe) with [{(η6-C6H6)RuCl(μ-Cl)}2] and NH4PF6, have been characterized by 1H, 13C{1H}, 77Se{1H}, and 125Te{1H} NMR spectroscopy and X-ray crystallography. The Ru−Se and Ru−Te bond lengths are in the ranges 2.4837(14)−2.4848(14) and 2.6234(6)−2.6333(7) A, respectively. Complexes 1−6 have been found to be efficient catalysts for catalytic oxidation of alcohols with N-methylmorpholine-N-oxide, tBuOOH, NaOCl, and NaIO4 and transfer hydrogenation reaction of ketones with 2-propanol. The TON values are up to 9.9 × 104 and 9.8 × 104 for two catalytic processes, respectively. The oxidation probably involves the formation of intermediate species having Ru(IV)═O. Complexes 1−3 are as efficient as 4−6 for transfer hydrogenation of ketones. In transfer hydrogenation, the mechanism does not appear to b...

Critical Temperature Enhancement by Biaxial Compressive Strain in FeSe0.5Te0.5 Thin Films

High-purity epitaxial FeSe0.5Te 0.5 thin films with different thicknesses were grown by pulsed laser ablation on different substrates. By varying the film thickness, T c values of up to 21 K were observed, significantly larger than the bulk value. Structural analyses indicated that the a axis changes significantly with the film thickness and is linearly related to T c. The latter result indicates the important role of the compressive strain in enhancing T c: the compressive strain derives from the Volmer–Weber growth of the films. The critical temperature is also related to both the Fe–(Se,Te) bond length and angle, suggesting the possibility of further enhancement.

Structure of Te-rich Te–Ge–X (X = I, Se, Ga) glasses

The structure of glassy Te78Ge11Ga11, Te79Ge16Ga5,Te70Ge20Se10 andTe73Ge20I7—promising materials for far infrared applications—was investigated by means ofx-ray and neutron diffraction as well as extended x-ray absorption fine structuremeasurements at various edges. Experimental data sets were fitted simultaneously in theframework of the reverse Monte Carlo simulation technique. Short range order inTe85Ge15 was reinvestigated by fitting a new x-ray diffraction measurement together with availableneutron diffraction and extended x-ray absorption fine structure data. It was found thatTe85Ge15 consistsmostly of GeTe4 structural units linked together directly or via bridging Te atoms. Te is predominantly twofold coordinatedin Te85Ge15,Te70Ge20Se10 andTe73Ge20I7 whilein Te78Ge11Ga11 and Te79Ge16Ga5 the Te coordination number is significantly higher than 2. The Te–Te bond length is2.80 ± 0.02 Å inTe78Ge11Ga11 while it isas short as 2.70 ± 0.02 Å and 2.73 ± 0.02 Å in Te73Ge20I7 and Te70Ge20Se10, respectively. Our results show that the strengths ofGeTe4 (GeTe3I,GeTe3Se) ‘units’ are very similar in all glasses investigated but the connection between these unitsdepends on the third component. Differences in the Te coordination number suggest thatunlike Se or I, Ga does not build into the Ge–Te covalent network. Instead, it forms acovalent bond with the non-bonding p electrons of Te, which results in an increase in theaverage Te coordination number.

“Piano‐Stool” Complexes of Ruthenium(II) Designed with Arenes and N‐[2‐(Arylchalcogeno)ethyl]morpholines: Highly Active Catalysts for the Oxidation of Alcohols with N‐Methylmorpholine N‐Oxide, tert‐Butyl Hydroperoxide and Sodium Periodate and Oxychloride

AbstractThe reactions of [{(η6‐C6H6)RuCl(μ‐Cl)}2] and [{(η6‐p‐cymene)RuCl(μ‐Cl)}2] with N‐[2‐(arylchalcogeno)ethyl]morpholines (L) (aryl = Ph/2‐pyridyl for S, Ph for Se, 4‐MeOC6H4 for Te) and NH4PF6 result in “piano‐stool” complexes of RuII of composition [RuCl(η6‐C6H6)(L)][PF6]/[RuCl(η6‐p‐cymene)(L)][PF6], which give characteristic 1H, 13C{1H}, 77Se{1H}, and 125Te{1H} NMR spectra. Some of them have also been characterized by X‐ray crystallography [Ru–S, Ru–Se, and Ru–Te bond lengths: 2.3815(12)/2.3742(14), 2.4837(14), and 2.6143(7) Å, respectively]. The cyclic voltammograms show that all the complexes undergo irreversible oxidation (E${1 \over 2}$ = 0.290–0.586 V). All the ruthenium complexes have been explored for their catalytic activity in the oxidation of primary and secondary alcohols with N‐methylmorpholine N‐oxide (NMO), tBuOOH, NaOCl, and NaIO4 (TON values upto 9.8 × 104). The efficiency of the catalytic oxidation reaction decreases in the order Te > Se > S. The intermediate species involved in the oxidation reactions appear to incorporate the RuIV=O group.

ChemInform Abstract: Ternary and Quaternary Uranium and Thorium Chalcogenides

A survey of the structural chemistry of ternary and quaternary uranium (U) and thorium (Th) chalcogenides is presented. Specific attention is paid to the coordination environments of U and Th in a variety of classical and nonclassical structure types. The problems of defining the oxidation state of U from U-Q (Q = S, Se, Te) bond lengths and magnetic susceptibility data are also discussed.