A. Belger

Structure refinement of the superconducting phase YNi2B2C as a function of temperature in the range 25–300 K

Abstract X-ray powder and single crystal diffraction has been used for structure refinement of various YNi 2 B 2 C samples in the temperature range 25–300 and 180–300 K, respectively. Linear thermal expansion is strikingly anisotropic. Coefficients vary between α a =4.1×10 −7 K −1 (25 K)–1.2×10 −5 K −1 (300 K) and α c =−6.1×10 −7 K −1 (25 K)–4.5×10 −7 K −1 (300 K) while the axial ratio c / a of the unit cell decreases with increasing temperature T . Moreover, a small increase of structure parameter z B with T was found. Anisotropic mean square atomic displacements remain almost constant for C and B but increase considerably for both Ni and Y atoms. The influence of the starting composition on the net content and on structure was studied for a series of samples. Lattice parameters as derived from Rietveld refinement deviated significantly among different samples. Relations between structure geometry, superconducting transition temperature, the residual resistance ratios and composition of the samples were discussed.

Crystal structure of K1-xCsxBSi2O6 (x = 0.12, 0.50) boroleucite solid solutions and thermal behaviour of KBSi2O6 and K0.5Cs0.5BSi2O6

Abstract The crystal structures of two K1-xCsxBSi2O6 solid solutions have been refined at room temperature by the Rietveld method: x = 0.12, a = 12.6858(4) Å, Rwp = 7.66%, RF = 5.56% and x = 0.50, a = 12.8480(2) Å, Rwp = 7.64%, RF = 3.10%. They are isostructural to cubic KBSi2O6 with the space group I4̅3d. The structure is built up from (Si,B)O4 tetrahedra linked in four-, six- and eightfold rings which are forming a three-dimensional borosilicate framework. The framework contains large cavities that are placed in continuous channels along the [111] directions. The Cs and K atoms occupy the positions in the channels statistically. Thermal behaviour of KBSi2O6 and K0.5Cs0.5BSi2O6 has been studied by high-temperature powder X-ray diffraction within the temperature range of 293-1073 K. A new tetragonal polymorph of KBSi2O6 has been found in situ under heating. The new polymorphic I4̅3d (cubic) – Ia3̅d (cubic) transition and the new Ia3̅d cubic polymorphic phase has been proposed for K1-xCsxBSi2O6 from our experimental and literature data on crystal structures and thermal expansion of leucites. The structural relaxation under cationic (K, Cs) substitutions and under heating has been investigated.

Nanomechanical characterization of Al–Co–Ni decagonal quasicrystals

This paper addresses some issues related to the nanomechanical responses of decagonal quasicrystals using nanoindentation techniques. A Hysitron Triboscope with a Berkovich indenter was used to carry out the nanoindentation tests on single crystals of Al–Co–Ni decagonal phase. The anisotropy in terms of nanohardness at higher load was observed, whereas it was not possible to establish the same at lower loads. However, the Youngs modulus was found to be indistinguishable in all these cases. Discontinuity in load displacement, which is known as a ‘pop-in’ effect, was observed frequently at various load regime. It was found that this discontinuity was not due to cracking or a phase transformation effect, but to a plastic yielding phenomena and nanodeformation of the material. Significant differences between the nanohardness and microhardness were noticed and are discussed based on various experimental parameters and the consequent mechanical response of materials.

Crystal structure and thermal expansion of β-RbB5O8 from powder diffraction data

Using Rietveld refinement the crystal structure of the β-rubidium pentaborate has been found to be isotypic with β-potassium pentaborate: (61) Pbca - (c)14, oP112, a = 7.550(1) Å, b = 11.842(1) Å, c = 14.805(1) Å, V = 1323.7(2) Å3, Z = 8, Dcalc = 2.68×103 kg/m3. With the aid of high temperature X-ray diffractometry a strongly anisotropic thermal expansion has been observed: αa= 61·10-6 K-1, αb= 23·10-6 K-1 and αc= 4.7·10-6 K-1. This anisotropy may be caused by anisotropic thermal vibrations of heavy atoms as rubidium.

Crystallisation of caesium borosilicate glasses with approximate boroleucite composition

Abstract Crystallisation and annealing behaviour of two caesium borosilicate glass samples with approximate boroleucite composition were characterised by means of differential scanning calorimetry measurements as well as X-ray powder diffraction investigations with subsequent profile fitting of the observed patterns according to the Rietveld and/or Pawley method. While one sample crystallised primarily in an orthorhombic phase (a = 6.592(3) Å, b = 11.825(5) Å, c = 12.620(6) Å; possible space groups: Pmc21 ((26), setting: P21am), Pma2 (28) and Pmma ((51), setting: Pmam)) which has been unknown until now and transformed irreversibly into the well-known cubic boroleucite phase at T ≈ 850 °C, cubic caesium boroleucite (space group: Ia3̅d (230)) was crystallised directly from the other sample. In this case, the orthorhombic phase was metastable. At the onset temperature of glass crystallisation Tx, on = 770 °C, it existed only for about 75 min. The change of the lattice parameter a of the cubic unit cell of caesium boroleucite in the course of the thermal treatments was explained by the alteration of the size of the TO4 tetrahedra (T = Si, B).

Magnetic ordering and superconductivity in TbxY1−xNi2B2C

Abstract The effect of alloying on magnetic ordering and superconducting properties of pseudoquaternary Tb x Y 1− x Ni 2 B 2 C intermetallic compounds has been investigated by susceptibility and resistivity measurements on polycrystalline samples. The single phase nature of the samples was proven by high-temperature DTA, X-ray and electron microprobe methods. Antiferromagnetic ordering and a depression of the Neel temperature with decreasing Tb content have been revealed for x >0.45. For lower Tb concentrations x T C increases with increasing Y fraction. The magnetic susceptibility χ ( T ) of TbNi 2 B 2 C shows a maximum at T N =15.5 K, passes through a minimum with decreasing temperature, and rises toward a second maximum near 5.5 K. This second maximum was ascribed to weak ferromagnetic ordering which has been confirmed by magnetic hysteresis effects. For decreasing Tb content the ferromagnetic ordering temperature was found to shift to lower temperatures and to disappear for x

On the Impact of Light on Nanoindentations in ZnSe

The (111Zn) surface of single-crystalline ZnSe was subject to nanoindentations in darkness and under illumination using white light. A positive photoplastic effect was observed in the load range from 200 μN to 2 mN: a reduction of the penetration depth by about 5%, resulting in a reversible hardness increase by 10% under illumination. The effect was found to saturate already at 30 mW/ cm 2 , comparable to daylight intensity. There is also a photoplastic after-effect for some seconds, i.e. a reduced hardness increase was found after having turned off the light some seconds before performing the indent.

The defect structure of Y0.915Ni4.12B related to the superstructure of YNi4B

Abstract Using X-ray powder and single crystal diffraction a new phase Y 0.915 Ni 4.12 B has been found ((191)P 6/mmm, hP113; a =14.9085(10) A, c =6.9196(8) A) which may be considered a defect structure of the YNi 4 B structure. X-ray powder patterns measured down to 15 K do not show any structural phase transition. The temperature dependence of the lattice parameters of Y 0.915 Ni 4.12 B was determined by Rietveld refinement. A maximum of the thermal expansion coefficients near 200 K could be observed.

Structural studies with TbNi2B2C single crystals and TbxY(1−x)Ni2B2C polycrystals

Abstract X-ray powder diffraction has been used for structure refinement of Tb x Y (1− x ) Ni 2 B 2 C compounds of different compositions at room temperature. Lattice parameters, structure parameter z B and Tb content x were derived from Rietveld refinements. Lattice parameters a and c of the pseudoquaternary intermetallic compounds Tb x Y (1− x ) Ni 2 B 2 C show a linear behaviour and fulfil Vegards law. For the first time structure data obtained with single crystals are reported for TbNi 2 B 2 C as a function of temperature. Coefficients of linear thermal expansion vary between α a =0.6×10 −5 K −1 (90 K)–1.6×10 −5 K −1 (300 K) and α c =−1.1×10 −6 K −1 (90 K)–0.9×10 −6 K −1 (300 K) while the axial ratio c / a of the unit cell decreases with increasing temperature T .

Photoplastic effects in nanoindentation experiments

Abstract The impact of light on hardness of II-VI semiconductor ZnSe was studied by nanoindentation. A positive photoplastic effect (PPE), that is a hardness increase by 20% from 2.05 GPa in darkness to 2.45 GPa under illumination (16 mN load), was found. The PPE amplitude (hardness change) exhibited saturation for moderate light intensities of about 15mW cm−2. It increased with decreasing loading speed and decreasing effective straining. The spectral sensitivity of the nanoindentation PPE is similar to that found in uniaxial deformation experiments, but displaying somewhat higher effects at shorter wavelengths. The PPE proved to be a dynamic effect; no alteration of penetration depth was found when changing the illumination state in the case of constant load. When varying the illumination state during loading, the resulting penetration depth was governed by the ratio of the volume that was displaced under light to that displaced in darkness.