Ion bombardment-induced charge state effects CaO single crystals: F+ and F centers

The effects of heavy-and light-ion bombardment on defect formation in CaO have been investigated by UV-absorption spectroscopy and volume measurements. While 500 keV Ar or Ca implantation produces only F⁺ centers, 240 keVH produces both F⁺ and F centers at a F⁺ to F ratio of 5.6 to 1. On the other hand, when an argon implanted sample is subsequently bombarded with hydrogen, about 30% of the F⁺ centers anneal during 1 ×10¹⁴ H/cm²; at higher H fluences, new F⁺ and F centers are produced. An effect of energy partition between ionization and nuclear/atomic collision processes for the incident ions on the charge state of the resulting defect is thus clearly demonstrated.

The formation and annealing of these defects are accompanied by volume changes in the ion implanted surface layer which can be monitored in sltu with a cantilever beam technique. The measurements show volume expansion of the order of 1.5% following 10¹⁶ 500 keV Ar implantation; subsequent implantation of 10¹⁸ 240 keV H compacts the previously expanded material by 25 %. These results are in qualitative agreement with the optical data and seem to indicate that volume changes are associated with the formation and annealing of F⁺ centers.     

Synthesis and crystal structure of RMgSi2 compounds (R=La, Ce, Pr, Nd), a particular example of linear intergrowth

A new series of rare earth compounds with stoichiometry RMgSi₂ (R=La, Ce, Pr, Nd) is reported. The single crystal X-ray diffraction showed that CeMgSi₂, which melts congruently at 1200 °C, crystallizes in a new tetragonal structure type (I4₁/amd, tI32, a=4.2652(4) Å, c=36.830(4) Å, Z=8; wR₂=0.042 (19 parameters, 393F₀²), R₁=0.018 (297F₀>4σF₀). The crystal structure of CeMgSi₂ can be formally built up by alternating along the z direction four CeMg₂Si₂-type CeMg₂Si₂ slabs with four AlB₂-type CeSi₂ slabs, one after the other. The structural model obtained from a CeMgSi₂ single crystal has been confirmed for the La, Pr and Nd homologous compounds by means of Rietveld refinement. The trend of the unit-cell parameters, plotted versus the R³⁺ ionic radius, shows a linear behaviour, which strongly suggests a trivalent state for the Ce atoms. An analysis of the features of this new structure is reported, in comparison with the other known CeMg₂Si₂/AlB₂-type linear intergrowth compounds.     

A double-temperature-gradient technique for the growth of single-crystal fullerites from the vapor phase

Fullerite single crystals were prepared by a sublimation-condensation method in a closed evacuated glass tube situated in a double-temperature-gradient furnace. Crystals of various size and up to 9 mg weight with well expressed smooth and shiny faces were obtained. X-ray analysis, interfacial angle measurements and observed morphological habits of selected crystals of C₆₀ confirm the theoretically predicted and experimentally well established fcc structure at room temperature with two types of morphological faces, namely {111} and {100}. A strong tendency to twinning was observed. In the case of C₇₀ crystals, a pure fcc structure was observed. Information on growth kinetics and on instability versus exposure to air and light were obtained from surface studies. Characteristic changes in a thin surface layer were observed when crystals were exposed to air and light. A new phase of C₆₀ stabilized by oxygen was characterized.     

Determination of mercury in biological materials by CV-AAS after wet digestion

The paper describes the determination of ultratrace concentrations of mercury in biological materials. Samples were digested in nitric acid under pressure in a polytetrafluoroethylene bomb. Mercury was determined in the digest solution by cold vapour atomic absorption spectrometry using stannous chloride reduction and two-stage gold amalgamation. The detection limit is 0.12 ng. Analysis of several certified reference materials yielded results in excellent agreement with the certified values. The results for blood serum of several subjects ranged from 0.2 to 0.8g 1^–1.Of the National Fund for Scientific Research (Belgium), to whom correspondence should be addressed     

Heavy fermion superconductivity and magnetic order in noncentrosymmetric CePt3Si

CePt3Si is a novel heavy fermion superconductor, crystallizing in the CePt3B structure as a tetragonally distorted low symmetry variant of the AuCu3 structure type. CePt3Si exhibits antiferromagnetic order at T(N) approximately 2.2 K and enters into a heavy fermion superconducting state at T(c) approximately 0.75 K. Large values of H(')(c2) approximately -8.5 T/K and H(c2)(0) approximately 5 T refer to heavy quasiparticles forming Cooper pairs. Hitherto, CePt3Si is the first heavy fermion superconductor without a center of symmetry.     

Structural chemistry,electrical resistivity,magnetization and specific heat in La1.85Sr0.15Cu1−xZn x O4

Samples from the oxide series of solid solutions La_1.85Sr_0.15Cu₁–xZn _x O₄, quenched from 1100°C, have been characterized by X-ray powder analysis. We report on the structural chemistry and on measurements of the electrical resistivity, magnetization, ac susceptibility and specific heat of these samples. Zinc was found to statistically replace the copper atoms in the body centered tetragonal K₂NiF₄-type of structure up to values ofx smaller than 0.15. At higher zinc-concentrations (0.15<x<0.45) the crystal structure transformes into the orthorhombically distorted K₂NiF₄-type, known for the low-temperature form of Sr-and Zn-free La₂CuO₄. Limited solid solutions are encountered when Cu is substituted by Cd and Hg. Whilst the Zn-free sample La_1.85Sr_0.15CuO₄ was superconducting with a sharp onset at 35 K, substitution of Cu by Zn by as little as 6 mole percent was sufficient to suppress superconductivity belowT _c<4.5 K. Isoelectronic substitution of Cu by (Ni_0.5Zn_0.5) was found to cause an even larger degradation ofT _c than equivalent amounts of Zn. At the superconducting transition the discontinuity of the specific heat C _p is observed in agreement with recent publications, but is found to decrease with Zn substitution. A linear term inC _p(T) is resolved at low temperatures which significantly rises upon Cu/Zn substitution.     

Neue τ-Boride

The following -borides have been synthesized: Cr₁₃Ir₁₀B₆, Mn₁₆Ir₇B₆, Fe_10–15.4Ir_13–7.6B₆, Co₁₅Ir₈B₆.

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Herrn Prof. Dr.A. Knappwost in Freundschaft gewidmet.     

Structure and Properties of Yb3Ge5

Single crystals of Yb₃Ge₅, grown from pure gallium or indium metal flux and investigated by means of CCD four-circle diffractometry, were consistent with the Th₃Pd₅-type of structure, displaying full order of vacancies and germanium atoms. From magnetic susceptibility and electrical resistivity measurements, performed on single-crystalline specimens, Yb₃Ge₅ was found to be an intermediate valent systems.     

A constitutional diagram of the system VC0.88−HfC0.98−WC

The system VC_0.88–HfC_0.98–WC was investigated by means of melting point, differential-thermoanalytical, X-ray diffraction and metallographic techniques on hot pressed and heat treated as well as melted alloy specimens and a complete constitutional diagram from 1500°C through the melting range established.The phase behaviour within VC_0.88–HfC_0.98–WC is characterized by the presence of a large (binary) miscibility gap within VC–HfC (T _crit.,hypo.=3000°C). Additions of WC decrease the miscibility gap in the ternary. Interaction of the solvus (boundary of the cubic-B1 monocarbide solution) and the ternary miscibility gap was established at 2075°C and (VC)_0.17(HfC)_0.37(WC)_0.46: On cooling below 2075°C alloys of this composition enter a decomposition reaction into two isotypic cubic B1 phases and hexagonal WC.Solid state and melting behaviour was established within the isopleths VC_0.88–WC and VC_0.88–HfC_0.98 as well as within (V_0.8W_0.2)C–(Hf_0.8W_0.2)C and WC–(V_0.38Hf_0.62)C. The isopleth VC_0.88–HfC_0.98 intersects the four phase plane of the ternary V–Hf–C euctetic [2580±10°C, (VC)_0.78(HfC)_0.22]. Originating at the VC–HfC binary this eutetic trough proceeds into the VC–HfC–WC ternary with raising temperatures connecting the maximum critical point of the disappearing miscibility gap [(VC)_0.35(HfC)_0.51(WC)_0.14] by a limiting tie line (2830±20°C).Isothermal sections have been calculated assuming regular solutions.With 9 Figures