Angus P. Wilkinson

Pronounced Negative Thermal Expansion from a Simple Structure: Cubic ScF3

Scandium trifluoride maintains a cubic ReO(3) type structure down to at least 10 K, although the pressure at which its cubic to rhombohedral phase transition occurs drops from >0.5 GPa at ∼300 K to 0.1-0.2 GPa at 50 K. At low temperatures it shows strong negative thermal expansion (NTE) (60-110 K, α(l) ≈ -14 ppm K(-1)). On heating, its coefficient of thermal expansion (CTE) smoothly increases, leading to a room temperature CTE that is similar to that of ZrW(2)O(8) and positive thermal expansion above ∼1100 K. While the cubic ReO(3) structure type is often used as a simple illustration of how negative thermal expansion can arise from the thermally induced rocking of rigid structural units, ScF(3) is the first material with this structure to provide a clear experimental illustration of this mechanism for NTE.

Continuous hydrothermal synthesis and crystallization of magnetic oxide nanoparticles

The continuous hydrothermal synthesis of nanoparticles of two metal oxides (α-Fe 2 O 3 and Co 3 O 4 ) is described. Two variations of the technique were investigated, involving the precipitation reaction between a metal saltsolution and a hydroxide solution at ambient conditions and at elevated temperatures. Elevated temperatures resulted in more uniform particles of α-Fe 2 O 3 and Co 3 O 4 , although the actual sizes of the particles were apparently unaffected by the temperature. This behavior was attributed to the species present in solution and the solubilities of the cation(s), both of which were calculated via a thermodynamic model for the systems under study.

Temperature dependent structural and transport properties of the type II clathrates A8Na16E136 (A=Cs or Rb and E=Ge or Si)

Variable temperature single-crystal structure analyses for Cs8Na16Si136, Rb8Na16Si136, Cs8Na16Ge136, and Rb8Na16Ge136 are reported along with electrical and thermal transport measurements on two polycrystalline specimens. The strong temperature dependence of the atomic displacement parameters for the alkali-metal atoms is indicative of significant disorder associated with the “rattling” alkali-metal atoms inside the two different polyhedra (sixteen dodecahedra and eight hexakaidecahedra per cubic unit cell) that makeup the type II clathrate hydrate framework. This disorder can lead to low lattice thermal conductivities. Transport measurements show these compounds to be metallic. The potential of type II clathrates for thermoelectric applications is discussed.

Preparation of the negative thermal expansion material cubic ZrMo2O8

The negative thermal expansion material cubic ZrMo2O8 can be prepared by the carefully controlled dehydration of ZrMo2O7(OH)2·2H2O. The quality of the end product is dependent upon the route used to prepare the ZrMo2O7(OH)2·2H2O. The influence of both the Zr∶Mo ratio in the solution used to prepare the hydrate and the type of acid used in its preparation (HCl, H2SO4, CH3CO2H, HNO3 and HClO4) are examined. Not all acids provide media suitable for the preparation of ZrMo2O7(OH)2·2H2O. Cubic ZrMo2O8 could only be obtained from precipitates containing the hydrate. Starting solutions with a Zr∶Mo ratio in excess of 1∶2 were necessary to avoid coprecipitation of amorphous MoO3. The nature of the acid used in the hydrate synthesis affects the morphology of both the hydrate precursor and the cubic ZrMo2O8 particles. Many of the syntheses examined led to cubic ZrMo2O8 contaminated with amorphous impurities. Highly crystalline pure material could be obtained by using a perchloric acid medium for the synthesis of the hydrate precursor.

Gallium distribution in the clathrates Sr8Ga16Ge30 and Sr4Eu4Ga16Ge30 by resonant diffraction

The distribution of gallium/germanium in the type-I clathrates Sr8Ga16Ge30 and Sr4Eu4Ga16Ge30 over the three crystallographically distinct framework sites has been determined by resonant diffraction. The analyses indicate a strong preference for the occupation of the 6c site by gallium. This is consistent with theoretical predictions, but contrary to the results of a previous neutron diffraction study. The gallium distribution, and, hence, the thermoelectric properties of these materials, may be amenable to manipulation by heat treatment.

Determining metal ion distributions using resonant scattering at very high-energy K-edges:Bi/Pb in Pb5Bi6Se14

Powder diffraction data collected at ∼86 keV, and just below both the Pb and the Bi K-edges, on an imaging plate detector using synchrotron radiation from the Advanced Photon Source have been used to examine the Pb/Bi distribution over the 11 crystallographically distinct sites in Pb 5 Bi 6 Se 14 [space group P2 1 /m, a = 16.0096 (2) A, b = 4.20148 (4) A, c= 21.5689 (3) A and β = 97.537 (1)°]. The scattering factors needed for the analyses were determined both by Kramers-Kronig transformation of absorption spectra and by analyses of diffraction patterns from reference compounds. Even with the relatively low scattering contrast that is available at the K-edges, it was possible to determine the Pb/Bi distribution and probe the presence of cation site vacancies in the material. The current results indicate that resonant scattering measurements at high-energy K-edges are a viable, and perhaps preferable, route to site occupancies when absorption from the sample or sample environment/container is a major barrier to the acquisition of high-quality resonant scattering data at lower-energy edges.

Negative thermal expansion and compressibility of Sc1–xYxF3 (x≤0.25)

Scandium fluoride displays isotropic negative thermal expansion (NTE) from at least 10 to 1100 K and retains a cubic ReO3-type structure over this range; the NTE is most pronounced at low temperatures. Control of thermal expansion was explored by forming Sc1–xYxF3 (x≤0.25), which were characterized with synchrotron powder diffraction at ambient pressure from 100 to 800 K. The behavior of the solid solutions under pressure (≤0.276 GPa) was also examined while heating from 298 to 523 K. Insertion of the relatively large Y3+ ion into ScF3 results in a cubic-to-rhombohedral phase transition upon cooling from ambient temperature to 100 K, even at low substitution levels (5%). The coefficient of thermal expansion (CTE) of the solid solutions in the rhombohedral phase is strongly dependent on both composition and temperature; however, above 400 K, where all samples are cubic, the CTE appears to be largely independent of composition. The isothermal bulk modulus and CTE of ScF3, but not those of the solid solution...

High pressure synchrotron x-ray powder diffraction study of Sc2Mo3O12 and Al2W3O12

Synchrotron x-ray powder diffraction was used to study Sc2Mo3O12 and Al2W3O12 at high pressure in a DAC. Both compounds adopt the orthorhombic Sc2W3O12 structure under ambient conditions and exhibit anisotropic negative thermal expansion. A phase transition from orthorhombic (Pnca) to monoclinic (P 21/a) symmetry was observed at ~0.25 GPa for Sc2Mo3O12 and at ~0.1 GPa for Al2W3O12 associated with a volume reduction of ~1.5–2%. A second crystalline to crystalline phase transition was clearly seen only for Sc2Mo3O12 (2.5–3.0 GPa). Peak broadening and almost complete amorphization were observed for Sc2Mo3O12 at ~8 GPa, and this was not fully reversible on decompression. At 7 GPa, the amorphization of Al2W3O12 was not as advanced as for the molybdate and on decompression crystalline material was recovered. The compressibility of orthorhombic Sc2Mo3O12 is highly anisotropic, but it is almost isotropic for both monoclinic Sc2Mo3O12 and Al2W3O12. Both compounds show a reduction in their bulk moduli (K0) at the orthorhombic to monoclinic transition: 32(2) GPa for orthorhombic and 16(1) GPa for monoclinic Sc2Mo3O12, and 48 GPa for orthorhombic and 28(1) GPa for monoclinic Al2W3O12. Sc2Mo3O12 displays very similar high pressure behaviour to the previously studied Sc2W3O12.

Structural analysis of Sr8Ga16Ge30 clathrate compound

Crystal structural characterization of the Sr8Ga16Ge30 compound was carried out with the Rietveld refinement method using x-ray powder diffraction data. The structure corresponds to that of the type-I clathrate hydrate, with the Sr atoms filling the “cages” formed by the Ga and Ge atoms. Anisotropic displacement parameters were refined. The limiting figure for rms values of the Sr(1) atoms enclosed in the dodecahedral cages is a sphere, while that for the Sr(2) atoms in the tetrakaidecahedral cages move is an oblate ellipsoid. A comparison of the sizes of the cages in this structure with the ones in other M8GaxGe46−x (M=K, Ba, or void) structures shows that once the structure is filled (i) there is very little variation in size of the tetrakaidecahedral cages and (ii) the dodecahedral cages expand only slightly (measurably) as a function of the filler size. This is an important result from the thermoelectric point of view because it means that once the structure is full, the cages will not “close over” th...

Fluorinert as a pressure-transmitting medium for high-pressure diffraction studies

Fluorinert is a liquid pressure-transmitting medium that is widely used in high-pressure diffraction work. A systematic study of five different fluorinerts was carried out using single-crystal x-ray diffraction in a diamond-anvil cell in order to determine the pressure range over which they provide a hydrostatic stress state to the sample. It was found that none of the fluorinerts studied can be considered hydrostatic above 1.2 GPa, a lower pressure than reported previously.