Dr. Götz Schuck

Mechanism of Proton Conduction in Solid-State Protonic Conductors: Method and Results from Investigations by QENS Techniques

schuck — Wed, 04 Aug 2010 11:36:24 +0000

The proton conduction mechanism in M3H(XO4)2 crystals has been employed as an example for demonstrating, how such a mechanism can be broken up into a number of different types of proton motion starting with internal and lattice vibrations at short times of the order of 10–13 s and ultimately leading to H+-diffusion on the 10–9 s to 10–8 s time-scale as required for proton conductivity.

Neutron Scattering on KOs2O6 powder

schuck — Thu, 20 Mar 2008 18:54:29 +0000

The HRPT data, measured between 2 K and 290 K, was refined with the Rietveld program FullProf, based on detailed crystallographic data at room temperature. As a result of the refinement, we received lattice constants, structural parameters and fraction values for the present phases (β-pyrochlore KOs2O6 and precipitation phases).

The HRPT measurement was carried out on powder that beside β-pyrochlore KOs2O6 (fraction 33.08 %) also contain three parasitic phases (KOs2O4 fraction 26.14 %; OsO2 fraction 37.14 % and Os fraction 3.64 %).

Do to the chosen synthesis path it is not possible to produce polycrystalline β-pyrochlore KOs2O6 without precipitations because washing and etching is not possible. In contrast to β-pyrochlore RbOs2O6 were it is possible to remove precipitations. This is probably the reason why we are the first group that measured polycrystalline β-pyrochlore KOs2O6 with neutron diffraction.

Crystal Structures of Co1-xFexTiSb compounds

schuck — Fri, 29 Feb 2008 15:41:54 +0000

CoTiSb belongs to the family of half-Heusler compounds [1] which have the general form XYZ, with X and Y transition metals and Z an sp electron metal. These materials are thought to have the MaAgAs type structure, derived from the general Heusler structure (X₂Y Z) by leaving every other X site vacant. The latter is thought to lead to unusual electronic properties, including the so-called half-metallic ferromagnetism with a 100% polarized electronic band responsible for the metallic conduction [2]. In addition, the ferromagnetism in this series has been claimed to be associated with a metal to semiconductor transition [3]. All this makes the half metallic ferromagnets of this family good candidates for potential applications involving spin-polarized transport (spintronics) [4]. The Co_xFe_1-xTiSb samples for this investigation (x = 0, x = 0.015 and x = 0.05) have very different transport properties what is shown in Figure 1 were the normalized dc-resistances R(T)/R(300 K) of Ref. [3] is displayed.

Figure 1: Normalized dc-resistances R(T)/R(300 K) of Ref. [3]

According to electronic band structure calculations [4], should be a paramagnetic semiconductor with an energy gap of the order of 1 eV. However, the situation is complicated because the results are very sensitive to which elements occupy the X, Y and Z positions (Fig. 2) in the structure [5]. For instance, CoTiSb ought to be a nonmagnetic semimetal, and TiCoSb a ferromagnetic metal. Although this claim obtained some support from measurements on a polycrystalline sample of CoTiSb, it was seriously questioned by recent work on single crystals of CoTiSb [6,7]. In fact none of the above possibilities matched the experimental results. In addition it was found that replacing a small amount (5 %) of Co by Fe strongly reduces the low temperature electrical conductivity, by nearly 2 orders of magnitude. This clearly shows that the sample is close to a metal-semiconductor instability. In addition the properties of CoTiSb are sensitive to details of the annealing of the sample.

Figure 2

For all three samples (x = 0, x = 0.015 and x = 0.05) neutron single crystal measurements on TriCS (PSI) and X-ray single crystal measurements on BM1A beamline (SNBL/ESRF) each at 100 K has been carried out. Presented are preliminary refinement results on the joint refinement on the neutron- and the X-ray datasets, with a new beta-version of the JANA2006 software [8].

References

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R. A. De Groot, F. M. Muller, P.G. Van Engen, and K.H. Buschow, Phys. Rev. Lett., 50, 2024 (1983)
M.A. Kouacou, J. Pierre, and R.V. Skolozdra, J. Phys.: Condens. Matter 7, 7373 (1995).
B.R.K. Nanda and I. Dasgupta, J. Phys. Condens. Matter 17, 5037 (2005).
S. Ishida, T. Masaki, S. Fujii, and S. Asano, Physica B Vol. 239, 163 (1997).
L. Degiorgi, A. V. Sologubenko, H. R. Ott, F. Drymiotis and Z. Fisk, Phys. Rev. B, 65, 41101 (2001)
H.R. Ott, Physica B, 318, 77 (2002).
V. Petricek et al., JANA, Inst. of Physics, AVCR, Praha, Czech. Rep. (2000).

Structural and Superconducting Properties of RbOs2O6 Single Crystals

schuck — Thu, 10 Jan 2008 08:40:04 +0000

Single crystals of RbOs2O6 have been grown from Rb2O and Os in sealed quartz ampoules. The crystal structure has been identified at room temperature as cubic with the non-centrosymmetric space group F43m and lattice constant a = 10.1242(12) A. The anisotropy of the tetrahedral and octahedral networks is lower and the displacement parameters of alkali metal atoms are smaller than for KOs2O6, so the “rattling” of the alkali atoms in RbOs2O6 is less pronounced. Superconducting properties of RbOs2O6 in the mixed state have been well described within the London approach and the Ginzburg-Landau parameter kappa(0) = 31 has been derived from the reversible magnetization. This parameter is field dependent and changes at low temperatures from kappa = 22 (low fields) to kappa = 31 at H_{c2}. The thermodynamic critical field H_{c}(0) = 1.3 kOe and the superconducting gap 2delta/k_{B}T_{c} = 3.2 have been estimated. These results together with slightly different H_{c2}(T) dependence obtained for crystals and polycrystalline RbOs2O6 proof evidently that this compound is a weak-coupling BCS-type superconductor close to the dirty limit.

Crystal structures of two rubrene derivatives

schuck — Mon, 20 Aug 2007 12:34:32 +0000

5,11-bis-(4-tert-butyl-phenyl)-6,12-diphenyl-naphthacene (5,11-BTBR) is a derivate of rubrene with two polymorphs (A and B) where t-butyl sidegroups are added.

5,11-BTBR Polymorph A has a structure drastically different from the packing of unsubstituted rubrene with enhanced backbone-backbone spacing between two molecules [2]. Remarkably, the naphthacene backbone of the molecules is significantly twisted in polymorph A, with a twist angle of 43° between the two opposite C-C bonds at both ends of the backbone. The resulting backbone-backbone distances are not shorter than 4.36 Å, which is commonly expected to drastically reduce the pi - pi * overlap.

In the case of 5,12-bis-(4-tert-butyl-phenyl)-6,11-diphenyl-naphthacene (5,12-BTBR) [3], the in-plane arrangement of the molecules is very similar to that of rubrene [4], with even shorter distances between the naphthacene backbones (3.62 Å compared to 3.75 Å). However, the addition of the t-butyl groups increases the inter-layer spacing by 31%. Interestingly, it leaves the backbone almost perfectly planar.

The structure of polymorph 5,11-BTBR (B) could not be solved so far, as the material seems to exclusively grow as ultrathin platelets. From measurements of the d-spacing perpendicular to the extended crystal surface [1], a structure closely related to the one found for a constitutional isomer, 5,12-bis-(4-tert-butyl-phenyl)-6,11-diphenyl-naphthacene was assumed for polymorph B.

The electric transport properties of the two polymorphs are completely different: in polymorph B the in-plane hole mobility of 12 cm2/Vs measured on single crystal FETs is just as high as in rubrene crystals, whereas polymorph A does not show any measurable field-effect [1].

Reference: [1] Haas, S., Stassen, A. F., Schuck, G., Pernstich, K. P., Gundlach, D. J., Batlogg, B., Berens, U. & Kirner, H. J. (2007). Phys. Rev. B. Accepted. (cond-mat, arXiv:0707.0461); [2] G. Schuck, S. Haas, A. F. Stassen, U. Berens and B. Batlogg, Acta Cryst. (2007). E63, o2894; [3] G. Schuck, S. Haas, A. F. Stassen, H.-J. Kirner and B. Batlogg, Acta Cryst. (2007). E63, o2893; [4] O. D. Jurchescu, A. Meetsma and T. T. M. Palstra, Acta Cryst. (2006). B62, 330-334

High charge-carrier mobility and low trap density in a rubrene derivative

schuck — Thu, 09 Aug 2007 08:24:27 +0000

We have synthesized, crystallized and studied the structural and electric transport properties of organic molecular crystals based on a rubrene derivative with {\em t}-butyl sidegroups at the 5,11 positions. Two crystalline modifications are observed: one (A) distinct from that of rubrene with larger spacings between the naphtacene backbones, the other (B) with a in-plane structure presumably very similar compared to rubrene. The electric transport properties reflect the different structures: in the latter phase (B) the in-plane hole mobility of 12 cm$^2$/Vs measured on single crystal FETs is just as high as in rubrene crystals, while in the A phase no field-effect could be measured. The high crystal quality, studied in detail for B, reflects itself in the density of gap states: The deep-level trap density as low as $10^{15}$ cm$^{-3}$ eV$^{-1}$ has been measured, and an exponential band tail with a characteristic energy of 22 meV is observed. The bulk mobility perpendicular to the molecular planes is estimated to be of order of $10^{-3}$ — $10^{-1}$ cm$^2$/Vs.

5,11-Bis(4-tert-butylphenyl)-6,12-diphenylnaphthacene (form A)

schuck — Thu, 10 May 2007 14:02:16 +0000

The title compound, C₅₀H₄₄, is a derivative of rubrene where tert-butyl side groups are added to two of the pendant aromatic rings. Two polymorphs of this derivative, the title compound (form A) and form B, have been identified. The molecule of form A displays a strongly twisted naphthacene backbone. The in-plane arrangement differs from the classical herringbone structure, resembling a slip–stack structure type with the backbones separated by a minimum 7.0 Å in the direction of possible π-stacking.

5,12-Bis(4-tert-butylphenyl)-6,11-diphenylnaphthacene

schuck — Thu, 10 May 2007 13:53:13 +0000

The title compound, C₅₀H₄₄, is a derivative of rubrene in which tert-butyl side groups are added to two of the pendant aromatic rings. The complete molecule is generated by a mirror plane, and the unsubstituted and substituted pendant aromatic rings are almost perpendicular to the main backbone of the molecule, which is essentially planar.

Temperature dependent structural studies of beta-pyrochlore KOs2O6 single crystals

schuck — Fri, 02 Mar 2007 18:36:15 +0000

The discovery of superconductivity in ß-pyrochlore oxides AOs₂O₆ (A = Cs, Rb and K) has recently attracted considerable interest. In particular profound influence of broken spatial inversion symmetry on the nature of pair-breaking in KOs₂O₆ was discussed [1]. We have studied the temperature dependent structural properties (100K - 400K) of the superconducting compound ß-pyrochlore KOs₂O₆ using synchrotron radiation at the BM1A beamline at ESRF. The KOs₂O₆ crystals have metallic conductivity and show a sharp transition to the superconducting state at T_c = 9.65 K. So far crystallographic data is available only at room temperature. The structure at room temperature is cubic with non-centrosymmetric space group F-43m [2]. A strong anisotropic character of the potassium channels was observed. Compared to the hypothetical ideal ß-pyrochlore lattice (Fd-3m), both osmium tetrahedral and oxygen octahedral network exhibit strong breathing mode like volume changes. We have shown that reflections that are forbidden in the ideal pyrochlore space group symmetry Fd-3m must be taken into consideration for the ß-pyrochlore compounds [2]. One main task was to study the evolution with temperature of these “forbidden” Bragg intensities in order to elucidate the influence of the observed Bragg peak intensity changes on the anisotropic character of the crystal structure of ß-pyrochlore KOs₂O₆.

Structural studies of beta-pyrochlore RbOs2O6

schuck — Wed, 21 Feb 2007 16:23:20 +0000

The discovery of superconductivity in beta-pyrochlore oxides AOs₂O₆ (A = Cs, Rb and K, [1]) has recently attracted considerable interest. We have studied the structural properties of the new superconducting beta-pyrochlore RbOs₂O₆, at room temperature, by single crystal X-ray diffraction to elucidate the effect of geometrical frustration in this compound. The RbOs₂O₆crystals have metallic conductivity and show a sharp transition to the superconducting state at T_C = 6.3 K. We show that reflections witch are forbidden in the ideal pyrochlore space group symmetry Fdm must be taken into consideration for the RbOs₂O₆ compound. We also present details of the crystallographic analysis of RbOs₂O₆in order to understand the structural relationships of this compound to superconducting beta-pyrochlore KOs₂O₆ and alpha-pyrochlore Cd₂Re₂O₇ [2,3].