P. Rogl

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.

Structural chemistry and magnetic behavior of binary uranium silicides

Binary uranium silicides have been thoroughly reinvestigated with respect to crystal chemistry and magnetic properties; for most compounds, magnetic studies have been carried out at low temperatures for the first time (2 to 300 K, 0 to 5 T). Formation of all known binary silicides has been confirmed: U3Si, U3Si2 (U3Si2-type), USi (USi-type), U3Si5 (defect AlB2-type), USi1.88 (defect ThSi2-type) and USi3 (Cu3Au-type). At the composition U3Si∼5 three different phases have been observed: the defect AlB2-type as well as a phase separation into two orthorhombically distorted Alb2-type related phases. USi2−x with the tetragonal defect ThSi2-type structure at its silicon poor phase boundary was found to be in equilibrium with USi2−x of the orthorhombic defect GdSi2 type. Precise U-U distances have been derived from X-ray single crystal counter data for U3Si2, USi, for the AlB2-type subcell of U3Si5, and for USi1.84 (defect ThSi2-type). From susceptibility measurements, a band type paramagnetism was observed for U3Si2, whereas a temperature independent paramagnetism was confirmed for USi3. Curie-Weiss paramagnetism was encountered within the investigated temperature range for U3Si5 and USi1.88. A modified Curie-Weiss law was revealed for USi with the USi-type, whereas ferromagnetic ordering at Tc = 125 K was observed for the oxygen stabilized “USi” with the FeB-type. Magnetism of the uranium silicides is discussed as a function of the distant dependent 5f-5f electron overlap and d-f electron hybridization. No superconductivity was observed above 2 K.

Evidence for a novel state of superconductivity in noncentrosymmetric CePt3Si: a 195Pt-NMR study.

We report on novel antiferromagnetic (AFM) and superconducting (SC) properties of noncentrosymmetric CePt3Si through measurements of the 195Pt nuclear spin-lattice relaxation rate 1/T(1). In the normal state, the temperature (T) dependence of 1/T(1) unraveled the existence of low-lying levels in crystal-electric-field multiplets and the formation of a heavy-fermion (HF) state. The coexistence of AFM and SC phases that emerge at T(N)=2.2 K and T(c)=0.75 K, respectively, takes place on a microscopic level. CePt3Si is the first HF superconductor that reveals a peak in 1/T(1) just below T(c) and, additionally, does not follow the T3 law that used to be reported for most unconventional HF superconductors. We remark that this unexpected SC characteristic may be related to the lack of an inversion center in its crystal structure.

Magnetic properties and structural chemistry of ternary silicides (RE, Th, U) Ru2Si2 (RE=rare earth)

Abstract Ternary silicides (RE, Th, U)Ru 2 Si 2 have been synthesized from the elements. All the compounds (RE = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were found to be isotypic and to crystallize with the structure type of ThCr 2 Si 2 (ordered derivative of the BaAl 4 -type). The magnetic behavior of these alloys was studied in the temperature range 1.5 K T T > 300 K closely follow a typical Van Vleck paramagnetism of free RE 3+ -ions. In the case of CeRu 2 Si 2 susceptibilities are well described for 20 K T eff = 2.12 BM indicates a high percentage (85%) of Ce 3+ . SmRu 2 Si 2 yields an effective moment μ eff = 0.54 BM, which compares reasonably well with the Hunds rule J = 5/2 ground level for free Sm + and a low-lying excited level with J = 7/2. For temperatures T > 15 K the magnetic susceptibility as a function of temperature follows the “Van Vleck behavior” for free Sm 3+ . At low temperatures ferromagnetic ordering was encountered for (Pr, Nd, Ho, Er, Tm)Ru 2 Si 2 , whereas antiferromagnetic ordering was observed for (Sm, Gd, Tb, Dy)Ru 2 Si 2 . The ordering temperatures are generally below 55 K. No superconductivity was found for temperatures as low as 1.8 K.

Thermoelectricity of clathrate I Si and Ge phases

We report on investigations of type I clathrate Si and Ge compounds with Ba partially substituted by rare earth atoms. Novel compounds from framework-deficient solid solutions Ba8 Alx Si42−3/4x 4−1/4x and Ba8 Gax Si42−3/4x 4−1/4x (x = 8, 12, 16; , open square... lattice defect) have been prepared and characterized. All x-ray intensity data are consistent with the standardized clathrate I-Ba8Al16Ge30 type structure (space group Pmn). In rare earth substituted clathrates, Eu2Ba6MxSi46−x (M = Cu, Al, Ga), rare earth atoms completely occupy the 2a position and thus form a new quaternary ordered version of the Ba8Al16Ge30 structure type. From a geometrical analysis of clathrate crystal structures, a systematic scheme for all known clathrate compounds is proposed. All clathrates studied are metals with low electrical conductivity. The highest Seebeck coefficient in the present series is deduced for Ba8In16Ge30, S = −75μV K−1, indicating transport processes dominated by electrons as carriers. The Eu-based clathrates investigated exhibit long-range magnetic order as high as 32 K for Eu2Ba6Al8Si36 of presumably ferromagnetic type. Magnetic susceptibility indicates in all cases a 2+ ground state for the Eu ions, in fine agreement with LIII absorption edge spectra.

On the crystal structure and magnetic properties of the ternary rare earth compounds RETSb2 with RE≡rare earth and T≡Ni, Pd, Cu and Au

Abstract Novel compounds RETSb 2 have been prepared and characterized for T  Cu (RE  rare earth from La to Lu), Ni (RE  La to Ho), Pd (RE  La to Tb) and Au (RE  La to Sm). From X-ray powder diffraction analyses all compounds were found to crystallize as the ZrCuSi 2 type. Magnetic susceptibilities were generally measured in the temperature range from 4 to 100 K. YCuSb 2 and LaTSb z are temperature-independent paramagnets. RETSb 2 compounds are found to order antiferromagnetically below T  20 K. PrPdSb 2 and TbPdSb 2 undergo metamagnetic transitions, whereas PrCuSb 2 and ErCuSb 2 are simple ferromagnets. The Sm-containing compounds are typical Van Vleck paramagnets owing to the closely spaced multiplets.

Heavy-fermion behavior, crystalline electric field effects, and weak ferromagnetism in Sm Os 4 Sb 12

The filled skutterudite compound

Al, Ga substitution in RE2Fe17 (RE=Ce,Pr,Nd): magnetic behavior of RE2Fe15(Al,Ga)2 alloys

\mathrm{Sm}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}

Ternary clathrates Ba–Zn–Ge: phase equilibria, crystal chemistry and physical properties

was prepared in single-crystal form and characterized using x-ray diffraction, specific-heat, electrical resistivity, and magnetization measurements. The

High thermoelectric performance of triple-filled n-type skutterudites (Sr,Ba,Yb)yCo4Sb12

\mathrm{Sm}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}