In this paper, we demonstrate for the first time GaAs-based MOSFETs with excellent performance using of CVD is a very robust, highly manufacturable process.

DC characteristics of a depletion-mode (D-mode) GaAs MOSFET with a thin Ga/sub 2/O/sub 3/(Gd/sub 2/O/sub 3/) gate dielectric layer (74 /spl Aring/) show low gate leakage current, negligible drain current hysteresis and higher than 10 V gale-drain two-terminal breakdown voltage. Compared to MESFET with the same gate length, channel material and fabricated by the same process, the GaAs MOSFET shows higher unity current gain cutoff frequency (Ft). The higher Ft for the MOSFET than that of the MESFET agrees with earlier theoretical predictions.

Thermal degradation of polystyrene and polyvinylchloride was studied by thermomanometric analysis. A design of experiments was used to quantify the influence of 8 physico-chemical parameters on the responses of the analyser and on the different types of degradation compounds.

The replacement of a small amount of chlorine by fluorine has made it possible to synthesize Nb6Cl15-xFx chlorofluoride with the Ta6Cl15 structure-type. The crystal structures of two compositions in the solid solution (x = 2.2 (I) and x = 4.4 (II)) have been determined by single crystal X-ray diffraction and by isotypy with Ta6Cl15 (cubic, Ia-3d n(degrees) 230, a = 20.099(1) Angstrom, V = 8119.2(1) Angstrom(3), Z = 16, R = 0.047, R-w = 0.054 for (I) and a = 19.370(2) Angstrom, V= 7964.3(8) Angstrom(3), Z = 16, R = 0.067, R-w = 0.067 for (II). The structure is based on (Nb6L12L6/2a-a)-L-j (L = ligand) units in which F is randomly distributed on the inner positions for (I) and (II), and additionally on the apical-apical ones for (II). The unit-cell contains 16 units connected in the 3 directions by bent Nb-La-a-Nb bridges. This structure is different from the one encountered for Nb6F15 which exhibits 2 independent interpenetrating networks with linear Nb-Fa-a-Nb bridges. The interatomic distances obtained from structural:data are discussed by comparison with the binary Ta6Cl15 and isotypic NaNb6Cl15 ternary chloride, taking into account electronic effects and the size of the fluorine. The small size of fluorine allows the reduction of the Nb-Nb intracluster distances by steric effects and thus the stabilization of the Ta6Cl15 structure-type with Nb-6 binary halides. (C) 1999 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.

In the literature Mo6Se8−xSx (0≤x≤4) Chevrel phase binaries are stable compounds, synthesized by solid state reaction from stoichiometric mixture of MoS2, MoSe2, and Mo powders at 1200°C. We report the single-crystal structure of these compounds. The structural determinations show as expected a classical Chevrel phase structure with a framework of pseudomolecular Mo6Se8−xSx cubic units but furthermore exhibit molybdenum atoms in the channels of the host structure of these compounds, leading to the new formula Moy[Mo6Se8−xSx] with 0≤x≤5 and y∼0.3. These extra molybdenum atoms stabilize the Chevrel phase host network and seem to be linking with a molybdenum atom of the Mo6 cluster, forming a kind of Mo7 cluster.

A new Mo7S8 binary sulfide structure and its crystal growth are reported. It is the first example of Mo atoms in the channels of the Chevrel phases. The single crystal structure determination indicates extra molybdenum atoms located in the chalcogen site that is centered at the rhombohedral unit-cell origin. The nature and valence state of these extra atoms are discussed through chemical and physical properties.

The title compound synthesized by solid state reaction crystallizes in the cubic system (Pn-3) with a =13.433 Å. The structure has been determined by singlecrystal X-ray diffraction and shows that the compound is based on isolated |(Re 6 S 6 i Br 2 i )Br 6 a | 2− units. The K + cations occupy two different sites. Steric effects lead to the switching from the cubic symmetry present in KRe 6 Se 5 Cl 9 to the monoclinic one in the case of KRe 6 S 5 Br 9 and again the former cubic symmetry in the new compound K 2 Re 6 S 6 Br 8 .

The new series of oxyhalides M2RENb6Cl17O (M = monovalent cation, RE = rare earths) with 16 valency electrons per Nb6 cluster is isotypic with MRENb6Cl18 (trigonal, space group R 3̄). The crystal structure of Cs2LuNb6Cl17O, a = 9.3863(6) Å, c = 25.664(5) Å, Z = 3, refined from single crystal X-ray diffraction data, indicates that it is based on [(Nb6Cl11iOi)Cl6a]5− units in which one oxygen is in statistical occupancy on the twelve inner positions. The observed interatomic distances are correlated with the VEC, the charge of the unit, and the matrix effect owing to both the oxygen and chlorine ligands surrounding the Nb6 cluster.

La dechloration des PCB est realisee avec des rendements eleves, la reaction faisant appel au systeme catalytique palladium-zinc en milieu acide. L'optimisation du rendement resulte de l'etude de differents parametres par plans d'experiences les resultats observes sont commentes.

Determination of factors influencing the dechlorination of PCB in the presence of palladium. PCB dechlorination is obtained with high yields; the reaction process involves the acidic palladium-zinc catalytic system. Rate optimization results from the study of several parameters; the system's behaviour is discussed according to their variations.

A new ternary oxobromide series, M2RE Ta6Br15O3 (M = monovalent cation, RE = rare earth) based on (Ta6Br15O3)5- units, for which the valency electron count (VEC) is 14, has been isolated by solid state chemistry. The crystal structure of Cs2LaTa6Br15O3, which crystallizes in the P3̄1C space group (a = 9.512(2) Å and c = 17.935(3) Å), has been refined from single crystal X-ray diffraction data. This structure is strongly related to the structure of CsErTa6Br18 with a shift of the units along the c axis, which allows the building of new cationic sites. The developed formula of this compound is M2RE [(Ta6Bri9Oi3)Bra6]. The lanthanum is located in an original environment formed by three Oi and six Bra belonging to three adjacent units, and the cesium is 12-coordinated in a complex site formed by six Bri and six Bra belonging to six adjacent units. Short distances between rare earths and Ta6 clusters are encountered.

Sintered samples of EuBa2Cu3O6 have been halogenated at low temperature (t < 300-degrees-C) by treatments under NF3 or CCl4 flow diluted in nitrogen, or by reaction with iodine in sealed tubes. Such mild conditions of synthesis allowed to avoid the decomposition of the material during the reactions. The incorporation of the halogen in the sample has been evidenced by the weight gain, by the evolution of the unit-cell parameters and by SEM and EDS analyses. After fluorination and chlorination, the samples become superconducting, but no superconducting behaviour is observed after iodination. These results are compared to the ones previously obtained during the halogenation of YBa2Cu3O6.

Des echantillons frittes de EuBa2Cu3O6 ont ete halogenes a basse temperature ($t < 300~^\circ$C) par traitement sous courant de NF3 ou de CCl4 dilue dans de l'azote, ou par reaction avec de l'iode en tube scelle. De telles conditions de synthese ont permis de limiter la decomposition du materiau au cours de la reaction. L'incorporation de l'halogene dans l'echantillon est mise en evidence par variation de masse, par l'evolution des parametres de maille, par observations au MEB et analyses EDS. Apres fluoration et chloration l'echantillon devient supraconducteur, tandis qu'aucun comportement supraconducteur n'est observe apres iodation. Ces resultats sont compares avec ceux qui avaient ete obtenus prealablement lors de l'halogenation de YBa2Cu3O6.

GaMo4Se8 (I) and AlMo4S8 (II) transform at T1 = 45(2) K and 24(2) K, respectively, from a cubic high-temperature modification with ordered defect spinel structure (space group F43mBAR, a = 10.1747(2) angstrom (I), 9.7289(2) angstrom (II) at 300 K) to a rhombohedrally distorted low-temperature modification (R3m, a(rh) = 7.1537(3) angstrom, 6.8500(3) angstrom (II), alpha(rh) = 60.572(5)-degrees, 60.374(5)-degrees (II) at 10 K). Structure parameters for both modifications are refined by the Rietveld method from X-ray powder data recorded at T = 300 K (I, II), 60 K (I), 50 K (II) and 10 K (I, II) on a Guinier diffractometer. In the cubic modification the tetrahedral Mo4 clusters have 43mBAR symmetry and intracluster bond lengths of [Mo-Mo] = 2.881(2) angstrom (I), 2.829(2) angstrom (II). During the phase transition their symmetry is reduced to 3m as they expand each by about 0.05 angstrom at one of their triangular bases.

GaMo4Se8 (I) and AlMo4S8 (II) transform at T1 = 45(2) K and 24(2) K, respectively, from a cubic high-temperature modification with ordered defect spinel structure (space group F[unk]3m, a = 10.1747(2) Å (I), 9.7289(2) Å (II) at 300 K) to a rhombohedrally distorted low-temperature modification (R3m, arh = 7.1537(3) Å, 6.8500(3) Å (II), αrh = 60.572(5)°, 60.374(5)° (II) at 10 K). Structure parameters for both modifications are refined by the Rietveld method from X-ray powder data recorded at T = 300 K (I, II), 60 K (I), 50 K (II) and 10 K (I, II) on a Guinier diffractometer. In the cubic modification the tetrahedral Mo4 clusters have [unk]3m symmetry and intracluster bond lengths of [Mo – Mo] = 2.881(2) Å (I), 2.829(2) Å (II). During the phase transition their symmetry is reduced to 3m as they expand each by about 0.05 Å at one of their triangular bases.