Richard M. Ibberson

Structural Phase Transitions in the Fullerene C60

High-resolution powder neutron diffraction has been used to study the crystal structure of the fullerence C60 in the temperature range 5 K to 320 K. Solid C60 adopts a cubic structure at all temperatures. The experimental data provide clear evidence of a continuous phase transition at ca. 90 K and confirm the existence of a first-order phase transition at 260 K. In the high-temperature face-centred-cubic phase (T > 260 K), the C60 molecules are completely orientationally disordered, undergoing continuous reorientation. Below 260 K, interpretation of the diffraction data is consistent with uniaxial jump reorientation principally about a single 111 direction. In the lowest-temperature phase (T < 90 K), rotational motion is frozen although a small amount of static disorder still persists.

Formation of isomorphic Ir3+ and Ir4+ octamers and spin dimerization in the spinel CuIr2S4.

Inorganic compounds with the AB2X4 spinel structure have been studied for many years, because of their unusual physical properties. The spinel crystallographic structure, first solved by Bragg in 1915, has cations occupying both tetrahedral (A) and octahedral (B) sites. Interesting physics arises when the B-site cations become mixed in valence. Magnetite (Fe3O4) is a classic and still unresolved example, where the tendency to form ordered arrays of Fe2+ and Fe3+ ions competes with the topological frustration of the B-site network. The CuIr2S4 thiospinel is another example, well known for the presence of a metal–insulator transition at 230 K with an abrupt decrease of the electrical conductivity on cooling accompanied by the loss of localized magnetic moments. Here, we report the determination of the crystallographic structure of CuIr2S4 below the metal–insulator transition. Our results indicate that CuIr2S4 undergoes a simultaneous charge-ordering and spin-dimerization transition—a rare phenomenon in three-dimensional compounds. Remarkably, the charge-ordering pattern consists of isomorphic octamers of Ir83+S24 and Ir84+S24 (as isovalent bi-capped hexagonal rings). This extraordinary arrangement leads to an elegant description of the spinel structure, but represents an increase in complexity with respect to all the known charge-ordered structures, which are typically based on stripes, slabs or chequerboard patterns.

Investigation of the unusual magnetic spiral arrangement in BiFeO3

Abstract In the ferroelectric-antiferromagnet, BiFeO3, the cycloidal spiral arrangement of the magnetic moments of the Fe3+ ions whose length is 620 A was confirmed. This magnetic ordering remains up to the Neel temperature (643K).

NITROGEN ATOM LOCATION IN RHOMBOHEDRAL AND HEXAGONAL RE2FE17NX COMPOUNDS

High resolution neutron powder diffraction studies were performed on Nd2Fe17Nx and Y2Fe17Nx. A strong preferential site occupation of the N atoms on the 9e position of the rhombohedral compound Nd2Fe17Nx was observed. In the hexagonal compound Y2Fe17Nx the N atoms occupy primarily the 6h sites (82%) and only a small fraction goes into the 12i site (12%). The refined nitrogen occupation numbers correspond to the formula compositions Nd2Fe17N2.6 and Y2Fe17N2.9.

Lithium location in NASICON-type Li+ conductors by neutron diffraction. I. Triclinic α′-LiZr2(PO4)3

Abstract High-resolution powder neutron diffraction data have been collected at room temperature on the triclinic α′ phase of LiZr2(PO4)3 (HRPD, ISIS Facility, U.K.). Rietveld refinement analysis (space group C1; Z=4; a=15.0718(2), b=8.8556(1), c=9.1234(1) A, α=89.660(1), β=123.912(1), γ=90.429(1)°) and Fourier difference maps have revealed the presence of monoclinic pseudo-symmetry −x, y, 1/2−z for all atoms but lithium, which is disordered over two sites Li1 and Li2 with occupancies 0.71(3) and 0.29(3), respectively. The final agreement indexes are Rp=0.0672, wRp=0.0919, R(F2)=0.0603 for 92 refined parameters. Both Li1 and Li2 are in distorted tetrahedral coordinations, with 〈Li–O〉=2.09 and 2.18 A, respectively. Pairs of centrosymmetrical Li1 sites are located within half of the available M′ holes of the NASICON structure, which are split into two independent sets by triclinic breaking of symmetry. This accounts for the lower conductivity of the triclinic phase.

Temperature- and Pressure-induced Proton Transfer in the 1:1 Adduct Formed between Squaric Acid and 4,4 '-Bipyridine

We have applied a combination of spectroscopic and diffraction methods to study the adduct formed between squaric acid and bypridine, which has been postulated to exhibit proton transfer associated with a single-crystal to single-crystal phase transition at ca. 450 K. A combination of X-ray single-crystal and very-high flux powder neutron diffraction data confirmed that a proton does transfer from the acid to the base in the high-temperature form. Powder X-ray diffraction measurements demonstrated that the transition was reversible but that a significant kinetic energy barrier must be overcome to revert to the original structure. Computational modeling is consistent with these results. Modeling also revealed that, while the proton transfer event would be strongly discouraged in the gas phase, it occurs in the solid state due to the increase in charge state of the molecular ions and their arrangement inside the lattice. The color change is attributed to a narrowing of the squaric acid to bipyridine charge-transfer energy gap. Finally, evidence for the possible existence of two further phases at high pressure is also presented.

Ba8ZnTa6O24: a high-Q microwave dielectric from a potentially diverse homologous series

The ceramic Ba8ZnTa6O24 has been synthesized in isolation and its dielectric and crystallographic properties characterized. The material affords excellent dielectric properties, with a high unloaded quality factor Qu=20 800 at 3.28 GHz, high relative permittivity er=29 and a temperature coefficient of resonant frequency τf=29.4 ppm/°C. The crystal structure adopted is complex, comprising mixed cubic and hexagonal perovskite subunits, and contains cation vacancies on the octahedral sites. A second phase with a closely related structure is identified, demonstrating the existence of a family of materials. This structural complexity offers diverse opportunities for substitutions calculated to enhance the figures of merit reported.

Structural origins of the differing grain conductivity values in BaZr0.9Y0.1O2.95 and indication of novel approach to counter defect association

Proton conducting oxides such as BaCe0.9Y0.1O3−δ have considerable promise for intermediate temperature fuel cells. Unfortunately these tend to be unstable, e.g. to attack by carbonation. Previous work has highlighted the possibility of utilising barium zirconate to provide a chemically stable electrolyte; however such materials are difficult to sinter yielding very high overall resistances. Whilst this sintering problem is soluble, there are still very significant questions about the intrinsic grain conductivity, which varies by orders of magnitude for different reports. Here we demonstrate that there are two variants of BaZr0.9Y0.1O2.95, both with the cubic perovskite structure. The α-form exhibits a slightly smaller unit cell and much lower protonic conductivity than the β-form. The α-form is observed in better equilibrated samples and neutron diffraction demonstrates that this difference originates in a small degree of cross substitution of the Y atom onto the A-sites for the β-form, suggesting a novel approach to enhance ionic conductivity by reducing defect association through A-site substitution.

Does the modulated magnetic structure of BiFeO3 change at low temperatures

Figure 2 and its caption were incorrect, along with a line of the text. Full details can be found in the PDF; these corrections do not change the conclusions of the paper.

On the preferential site occupation of Fe in RFe4Al8 and related compounds

High-resolution neutron powder diffraction studies were performed on several compounds of the type YFe12-xAlx (x=8, 7, 6). The authors observed a strong preferential site occupation of Fe on one of the three non-equivalent transition metal sites of the hMn12-type structure. In particular the 8f site is found to be markedly populated by Fe atoms, while there is a strong preference of Al for the 8i site.