Friedrich Frey

Powder diffraction investigations of plasma sprayed zirconia

The crystallographic and microstructural parameters of plasma sprayed pure zirconia powders were studied by X-ray and neutron powder diffraction. A significant influence of the flow rate of the quenching gas on the phase composition (the tetragonal to monoclinic ratio varied between 0.6 and 3.5) and on the micro-structure was observed, while structural parameters of both phases of all investigated samples remained essentially unchanged and were close to those reported in the literature. These results do not support the concept of a critical particle size effect as a stabilizing factor for the tetragonal phase. A transition from a tetragonal to a monoclinic phase was observed without any measurable change in the crystallite sizes by heating at 845‡C. A very high background on the neutron powder patterns may have been caused by the presence of pores in the samples.

Cation substitution models of congruent LiNbO3 investigated by X-ray and neutron powder diffraction

Abstract The average structure of congruent lithium niobate has been reexamined by X-ray and neutron powder diffraction. The Rietveld refinements show that the Li-site vacancy model [Li 1 − 5 x Nb x □ 4 x ]NbO 3 in which the Nb site is fully occupied describes best the average defect structure. The Nb-site vacancy model [Li 1 − 5 x Nb 5 x ][Nb 1 − 4 x □ 4 x ]O 3 and the Li split model [Li 1 − 5 x − y Nb x □ 4 x + y ][Li y ]NbO 3 can be definitely rejected. Several Nb split models in which the excess Nb atoms are distributed among the Li and the vacant cation sites, however, are also possible. The necessity of combined X-ray and neutron refinements is pointed out.

Neutron powder investigation of tetragonal and cubic stabilized zirconia, TZP and CSZ, at temperatures up to 1400 K

A comparative study of tetragonal (Y-TZP, 3 mol% Y 2 O 3 ) and cubic (CSZ, 15 mol% CaO) zirconia powder samples was carried out to learn about a possible structural origin of the superior ionic conductivity of TZP at low temperatures. Structure refinements including anharmonic temperature factors and oxygen occupancies as well as a qualitative analysis of the diffuse background revealed large differences especially of the anisotropy of thermal motion and the underlying disorder in both samples

High temperature thermal expansion of mullite: an in situ neutron diffraction study up to 1600°C

Abstract Structural thermal expansions of undoped and Cr-doped 3/2 mullites (10.8 wt.% Cr2O3) were measured with in situ neutron (25–1600°C) and X-ray synchrotron (25–1000°C) diffraction techniques. Heat treatments between 25 and 300°C cause little and non-linear structural expansions. Above 300°C and up to 1000°C the mullites display linear and low increase of the lattice constants. The mean values of undoped mullite are slightly higher ( α av ≈ 5.45×10−6) than those of Cr-doped mullite ( α av ≈ 4.95×10−6), thus confirming earlier in situ high-temperature diffraction data. Above about 1000°C the expansion behaves discontinuously. Structural expansions of undoped and Cr-doped mullite strongly increase yielding similar mean expansion coefficients ( α av ≈ 7.50×10−6) for both materials. The highest increase of lattice constants has been determined in b axis direction throughout 1600°C in undoped and Cr-doped mullite. High temperature diffraction studies under heating-up and cooling-down conditions yielded reversible expansion effects, including the anomalous behaviour at about 1000°C.

Single-crystal neutron diffraction study of metamict zircon up to 2000 K

High-temperature single-crystal measurements of metamict zircon were performed up to 2000 K and the recrystallization was monitored using the width and intensity of selected peaks. Full data sets were collected at room temperature, 1573 and 1823 K. The changes in the probability density function (p.d.f.) of the O atoms revealed important features of the recrystallization. The measurements were done in air using a mirror furnace and performed on the D19 four-circle diffractometer at the ILL equipped with a vertically curved two-dimensional position-sensitive detector.

Phase transitions and ordering in urea inclusion compounds with n-paraffins

Abstract Urea inclusion compounds consist of a pseudo-hexagonal framework of urea with “infinite” channels parallel to c , into which paraffins are embedded. The compound with hexadecane exhibits phase transitions at ≅ 365, ≅ 148 and ≅ 120 K (phases I–IV), which have been investigated by X-ray and neutron scattering. Phase I is hexagonal with a (longitudinal and orientational) random distribution of the included molecules. In phase II, which uniquely occurs in the adduct with C 16 H 34 , a doubling of the lattice constant c 0 of the host structure indicates partial longitudinal ordering. This is assigned to the approximately equal length of the hexadecane molecule and 2 c o leading to mutual deformations of both constituents. The transition II→ III is connected with a lateral orientational ordering of the guest molecules in adjacent channels and a simultaneous orthorhombic deformation of the host. Correlated rotational motions of the included molecules around their long axes are deduced from the increasing critical scattering above T c In phase IV a crystal-like ordering tendency of the paraffins plays the dominant role, while the host breaks into a heavily disordered domain structure. Experimental results indicate a transformation process as no thermal equilibrium was reached during the measurements. The different ordering mechanisms are discussed in terms of interactions between host and guest, guest and guest along each channel, and guest and guest in adjacent channels.

The HCP-FCC transition in pure Co investigated by neutron scattering

The martensitic HCP-FCC transition in pure Co at TM approximately 700K exhibits premonitory effects found by purely elastic and by inelastic neutron scattering. Above T/TM approximately 0.95 preformed cubic nuclei start to grow considerably. At about the same temperature the elastic shear constant c44 decreases sharply. Approaching TM the entire TA branch corresponding to c44 goes down in energy. No pronounced soft mode behaviour, however, could be found.

Disorder diffuse scattering from decagonal phase alloys

Abstract The isotypic decagonal phases Al70Ni15Co15 (ANC) and Al65Cu20Co15 (ACC) show remarkable diffuse scattering of various kinds and of different origins. Most prominent are ‘modulated’ diffuse layers which are due to columns with doubled period along the unique ‘periodic’ axis. There are long-range correlations along this direction, but only restricted correlations in the lateral, i.e. quasiperiodic arrangement of these columns. This ordering vanishes gradually upon heating to 800°C, but recovers fully after cooling down again. An explanation is given by a reversible chemical order/disorder process of the Ni/Co atoms which might be facilitated by diffusion within the columns. Diffuse scattering contributions within the Bragg layers are traced back to orientational disorder of the columns and to positional disorder as consequence of the ‘longitudinal’ ordering process. In contrast to ANC and ACC, decagonal Al70.5Mn16.5Pd13 (AMP) reveals disorder phenomena only related to the quasiperiodic ordering. Disordering in decagonal phases seems to be partly specific for the thermodynamically stable state, and partly related to the growth conditions.

Dynamic structure determination for two interacting modes at the M-point in α- and β-quartz by inelastic neutron scattering

The eigenvectors of a temperature-independent and a strongly temperature-dependent M-point mode have been determined. Near the alpha - beta transition they exchange their eigenvectors. In both modes the SiO4 tetrahedra stay rigid. In addition, in the temperature-dependent mode, all Si-Si distances (Si-O-Si bond angle) are unaffected, similar to the well known soft mode. In the beta phase a flat low-frequency dispersion branch in the Gamma -M direction connects the temperature-dependent mode at M with the soft mode of the Gamma point. This connection is permitted by group theory for the Gamma -M direction but not for the Gamma -K-M direction. The results are consistent with a domain model of the beta phase.

Neutron scattering investigation of the high temperature phase transition in NiTiO3

Nickel titanate, NiTiO3, undergoes a second order phase transition at Tc = 1570(10) K from the ilmenite structure with space group R3 to the corundum structure with space group R3c. It is predominantly due to a Ni/Ti order-disorder transition. The temperature dependence of certain superstructure reflexions, investigated by powder neutron diffraction suggests, however, a coupling to other displacive degrees of freedom of the order parameter. The behaviour of the essential part of the order parameter below the critical region may be described by the law of mass action applied to the exchange of Ni and Ti on nonequivalent sites.