F.J. DiSalvo

Electronic structure and optical index damage of iron‐doped lithium niobate

The properties of Fe2+, Fe3+, and other 3dn ionic impurities in the LiNbO3 structure are investigated with a view to elucidating the mechanism of laser‐induced reversible refractive index damage. Polarized optical absorption spectra of Fe2+ in LiNbO3 and LiTaO3 are reported and interpreted. The 2.66 eV band responsible for laser damage in Fe‐doped LiNbO3 is identified as Fe2+ → Nb5+ intervalence transfer. The Fe d e level lies in the 3.7 eV band gap at about 0.6 eV below the conduction band, and the activation energy for thermal Fe2+ → Nb5+ electron transfer is estimated to be about 0.9 eV, in reasonable agreement with the value of 1.3 ± 0.2 eV observed for thermal bleaching of Fe2+ centers in x‐irradiated LiNbO3. Particular advantages of intervalence transfer as a mechanism for initiating optical index damage are noted. The observed electric field gradients in pure LiNbO3 and LiTaO3 are related to the B20 ligand‐field parameter of a 3dn (n ≠ 5) impurity ion. Comparison with experimental estimates of B20 ...

Valence bands of layer structure transition metal chalcogenides

Abstract The valence band structure of representative MX 2 layer structure compounds has been obtained by X-ray photoemission with monochromatized radiation. Chalcogen s and p and metal d band are identified and their width and position obtained. The results are compared with u.v. and He II photoemission and with recent band structure calculations.

First example of monodispersed (Mo3Se3)1∞ clusters

Abstract The close relationship of the structure of ternary molybdenum chalcogenides to molecular cluster compounds suggests that these chalcogenides might be synthesized from such clusters or be solubilized as clusters. However, in contrast to molybdenum halide clusters which can be obtained in solution from the solid phase, many attempts to solubilize these chalcogenide clusters were unsuccessful. This is the first report that some of the pseudo-one-dimensional compounds ( M 2 Mo 6 X 6 , X = Se, Te; M = Li, Na) can be dissolved when exposed to highly polar solvents such as dimethylsulfoxide or N-methylformamide. We show using optical microscopy, TEM, and light scattering that some of these solutions contain individual (Mo 3 X 3 ) 1 ∞ chains. This is also the first example of a purely inorganic transition-metal polymer solution. The behavior with respect to flocculation is consistent with the double layer theory. We show that the (Mo 3 X 3 ) 1 ∞ chains can be oriented in solution.

Stoichiometry and physical properties of ternary molybdenum chalcogenides MxMo6X8 (X = S, Se; M = Li, Sn, Pb)

Abstract Chemical and electrochemical insertion of Li at room temperature, as well as insertion of lead and tin at moderate temperatures (500°C), into the binary phase Mo 6 X 8 forms ternary molybdenum chalcogenides M x Mo 6 X 8 ( X = S, Se). Crystallographic parameters, superconducting properties, and magnetic susceptibility are reported. The stoichiometry x for lead and tin is shown not to exceed x = 1, while for Li, x can reach approximately 4.0. For the lead and tin sulfide series, the hexagonal lattice parameters and superconducting critical temperatures ( T c ) are invariant to changes in the nominal composition of 0.8 x T c and a small decrease in c h is observed for the selenides; a narrow homogeneity range exists near x = 1 below 500°C for both these sulfides and selenides, the single-phase region being somewhat larger in the selenides. In contrast, several single-phase regions and large unit cell changes are observed in Li x Mo 6 X 8 (0 x x ∼ 3.2 reveals a structural phase transition at 140 and 185 K for the sulfide and selenide, respectively; but neither superconducts down to 1.5 K. At lower lithium concentration near x ∼ 1.0, the T c of the sulfide is raised from that of Mo 6 S 8 (1.8 K) to 5.2 K but the T c of Mo 6 Se 8 (6.5 K) is depressed to 3.9 K.

A systematic study of an isomorphous series of organic solid state conductors based on tetrathiafulvalene

The solid state physical properties of an isomorphous series of TTF salts (TTF11 (SCN)6, TTF11 (SeCN)6, and TTF7I5) were examined. While there was no noticeable trend in the conductivity (as a function of temperature and anion), the effective Fermi energy (?F) and the magnetic susceptibility transition temperature exhibited a definite trend as a function of anion. There is a correlation between the above properties and subtle variations in solid state structure of each of the above salts.

A facile synthesis of pseudo one-monodimensional ternary molybdenum chalcogenides M2Mo6X6 (X = Se,Te; M = Li,Na..Cs)

Abstract Compounds of this family are of great interest because of their pseudo one-dimensional structure of linear chains of (Mo3)1∞ clusters. High temperature solid state reactions (1100°C) are generally used to prepare members of this class, such as In2Mo6Se6. This paper describes ion exchange reactions of In2Mo6X6 with alkali iodide and chloride salts at 550°C to give M2Mo6X6 and mixed solid solutions In2−xMxMo6X6 (M=Li, Na, K, Rb, Cs; X=Se, Te). Furthermore, we show that ion exchange provides a low temperature route to prepare phases which are not stable at high temperature, such as In2−xMxMo6Se6 0 ≤ x ≤ 2 and M = alkali. When M = Li, the lithium can be partially removed by oxidation with iodine producing non stoichiometric ternary molybdenum chalcogenides In2−xMo6Se6 (0 ≤ x ≤ 1.8). Superconducting measurements indicate that only Tl2Mo6Se6 is superconducting above 1K.

Softening of charge density wave excitations at the superstructure transition in 2H-TaSe2

Abstract Raman scattering measurements performed between 5 K and 300 K on 2H-TaSe2 reveal new modes which are assigned to the modes of the charge density wave, observed in light scattering due to the Fermi surface induced distortion. The mode at 49 cm−1 of E2g symmetry softens (with concurrent line-width broadening) towards 122 K, the transition temperature from the incommensurate distorted to the undistorted phase. The mode at 82 cm−1 of A1g symmetry appears to be connected with the transition at 90 K from the commensurate to the incommensurate superstructure. The mode at 24.5 cm−1 of E2g shows no temperature dependence and is clearly due to the rigid-layer vibration.

Synthesis and physical properties of new superconducting Chevrel phases HgxMo6S8

New ternary molybdenum chalcogenides HgxMo6S8 (0 < x < 1) have been synthesized and investigated for their structural, magnetic, and superconducting behavior. These new phases prepared at low temperature by reaction of mercury with Mo6S8 (obtained from oxidation of Cu2Mo6S8 by iodine) have been characterized by x-ray diffraction, static Faraday susceptibility, and superconducting transition temperature studies. HgMo6S8 crystallizes in the rhombohedral space group R3 (ar = 6.51 A, αr = 92.53°), has a paramagnetic, temperature dependent susceptibility, and superconducts at 8.1K. As the mercury content decreases, a continuous decrease in Tc from 8.1K to 1.7K for x=0 (Mo6S8) is observed.

Structure and properties of La2Mo2O7: A quasi-two-dimensional metallic oxide with strong MoMo bonds

Single crystals of La/sub 2/Mo/sub 2/O/sub 7/, prepared by fused salt electrolysis, were used for structural and electronic characterization. La/sub 2/Mo/sub 2/O/sub 7/ is orthorhombic with a = 6.034 A, b = 12.236 A, and c = 3.888 A. The dominant feature of the structure, which was refined in space group Pnnm, is Mo/sub 2/O/sub 10/ units formed by edge-sharing MoO/sub 6/ octahedra which contain Mo-Mo distances of only 2.478 A. These groups then share corners in two dimensions to give rise to Mo-O layers which are held together by the lanthanum ions. The relationship of the La/sub 2/Mo/sub 2/O/sub 7/ structure to those of other reduced oxides is discussed. La/sub 2/Mo/sub 2/O/sub 7/ is a metallic conductor down to 125 K where a phase transition takes place. A similar transition is seen in the magnetic susceptibility. The anomalous electric and magnetic behavior of this compound may be associated with a charge density wave instability of the type often found in quasi-two-dimensional materials.

Structure and properties of Y5Mo2O12 and Gd5Mo2O12: Mixed valence oxides with structurally equivalent molybdenum atoms

Abstract Crystals of Ln 5 Mo 2 O 12 ( Ln = Y, Gd) were grown by electrochemical reduction of alkali-molybdate/rare-earth oxide melts at 1075–1100°C. A single crystal of Y 5 Mo 2 O 12 , used for structure determination, was found to be monoclinic with a = 12.2376(7) A, b = 5.7177(8) A, c = 7.4835(5) A, β = 108.034(5)°, and Z = 2. Although the structure was refined in space group C2 m , the true space group appears to be P2 1 m . In Y 5 Mo 2 O 12 , rutile-like sheets of edge-shared MoO 6 chains linked by YO 6 octahedra are interconnected with YO 7 monocapped trigonal prisms. The Mo atoms in the chains have alternating distances of 2.496 and 3.221 A and in that respect are similar to MoO 2 . However, in contrast to metallic MoO 2 both the Y and Gd compounds are n -type semiconductors with room temperature resistivities of the order of 10 3 ohm-cm. Magnetic susceptibility measurements confirm the presence of one unpaired electron per Mo 2 unit. The semiconducting behavior can be explained in terms of an unfavorable bridging oxygen coordination which prevents electron delocalization through metal-oxygen pi bonding as in MoO 2 .