Andy N. Fitch

Phase coexistence in the charge ordering transition in CaMn7O12

The structural phase transition in CaMn7O12 has been investigated by using high-resolution synchrotron and neutron powder diffraction. Both measurements show a phase coexistence phenomenon: between 409 and 448 K two different crystallographic phases coexist in the material. The first one is trigonal and it has a charge ordering (CO) of the Mn3+ and Mn4+ ions, while the second one is cubic and charge delocalized (CD). The volume fraction of the CD phase increases with temperature from zero at 400 K up to 100% about 460 K. Both phases have domains of at least 150 nm at each temperature in the PS region. A percolation scenario assuming a growth of the volume of the highly conducting CD regions at the expense of the volume of the insulating CO matrix is discussed and it is found to be in agreement with literature data of the CaMn7O12 resistivity.

Variable temperature study of the crystal and magnetic structures of the giant magnetoresistant materials LMnAsO (L =La, Nd)

A variable temperature neutron and synchrotron diffraction study have been performed on the giant magnetoresistant oxypnictides LnMnAsO (Ln = La, Nd). The low temperature magnetic structures have been studied and results show a spin reorientation of the Mn2+ spins below TN (Nd) for NdMnAsO. The Mn2+ spins rotate from alignment along c to alignment into the basal plane and the Mn2+ and Nd3+ moments refine to 3.54(4) mu B and 1.93(4) mu B respectively at 2 K. In contrast there is no change in magnetic structure with temperature for LaMnAsO. There is no evidence of a structural transition down to 2 K, however discontinuities in the cell volume, Ln-O and Mn-As bond lengths are detected at sim 150 K for both materials. This temperature coincides with the electronic transition previously reported and suggests a coupling between electronic and lattice degrees of freedom.

Negative (and very low) thermal expansion in ReO3 from 5 to 300 K

This paper reports the accurate measurement of the ReO3 cell parameter as a function of temperature. The thermal expansion is confirmed to be negative over most of the temperature range from 5 to 300u2005K. The main problems with the measurements are the very small variations (in the range of 10−5u2005A) in the cell parameter at each temperature, requiring tight control of the stability and reliability of instrumental effects. In particular, achieving monochromator stability over time might be challenging with the high energy and high beam current variations of a third-generation synchrotron facility. On the other hand, such effects are usually checked by the addition of silicon as an internal standard, but the accuracy (and precision) of the published thermal expansion (which is not certified) might not be sufficient for its use when dealing with very small cell parameter variations.

Controlling mesoscopic phase separation near electronic topological transitions via quenched disorder in ternary diborides

A phase separation driven by the negative compressibility of the electron gas, near electronic topological transitions (ETT), could drive the system at the verge of a catastrophe. We show here that the metastable phases very close to the ETT transition are observed in a mesoscopic phase separation (MePhS) driven by the quenched lattice disorder. By using high resolution synchrotron radiation x-ray powder diffraction we have identified the MePhS for the intermetallic ternary Mg1−x Alx B2 in the proximity of two ETTs: the first at x1 = 0. 1a nd the second at x2 = 0.3. We have identified the competition between a first ‘relaxed’ (R) hole poor and a second ‘tense’ (T) hole rich phase, and by micro-Raman we observe the splitting of the in-plane phonon E2g mode in the proximity of the first ETT at x = 0.1. The anisotropic quenched disorder due to a random distribution of Al 3+ and Mg 2+ ions both in the axial (c axis) direction and planar (ab plane) direction, probed by x-ray diffraction and Raman data, is proposed to be the physical variable that allows the formation of metastable phases near the critical points of electronic topological transitions, where Feshbach shape resonances in interband pairing amplifies the superconducting critical temperature. (Some figures in this article are in colour only in the electronic version) Q.1

A determination of the structure of liquid Ga2Te3 using combined X-ray diffraction and neutron diffraction with isotopic substitution

The partial structure factors of liquid Ga2Te3 have been determined using a combination of neutron diffraction with isotopic substitution (NDIS) and anomalous X-ray diffraction around the Te K edge. The inclusion of X-ray diffraction data significantly improves the statistical accuracy of the partial structure factors obtained compared with those obtained by the NDIS method alone. The results show that the liquid is hetero-coordinated in a way similar to that of typical two-component glasses or ionic liquids. There is no evidence, within the accuracy of this experiment, for substantial numbers of homopolar bonds as suggested by recent ab initio molecular dynamics simulations. The bond distances and coordination numbers show strong similarities to the tetrahedrally bonded defective zinc blende structure of the crystalline solid before melting.

Unraveling the nature of electric field- and stress- induced structural transformations in soft PZT by a new powder poling technique

A powder-poling technique was developed to study electric field induced structural transformations in ferroelectrics exhibiting a morphotropic phase boundary (MPB). The technique was employed on soft PZT exhibiting a large longitudinal piezoelectric response (d(33) ∼ 650 pC N(-1)). It was found that electric poling brings about a considerable degree of irreversible tetragonal to monoclinic transformation. The same transformation was achieved after subjecting the specimen to mechanical stress, which suggests an equivalence of stress and electric field with regard to the structural mechanism in MPB compositions. The electric field induced structural transformation was also found to be accompanied by a decrease in the spatial coherence of polarization.

Structure of compounds E(SnMe3)4 (E = Si, Ge) as seen by high-resolution X-ray powder diffraction and solid-state NMR

The compounds tetrakis(trimethylstannyl)germane, Ge(SnMe3)4 (1), and tetrakis(trimethylstannyl)silane, Si(SnMe3)4 (2), have crystal structures with the quasispherical molecules in a closed-packed stacking. At room temperature both structures have the space group P1 (Z = 2) with a = 9.94457 (5), b = 14.52927 (8), c = 9.16021 (5) A, alpha = 90.53390 (30), beta = 111.73080 (30), gamma = 90.0049 (4) degrees, and V = 1229.414 (12) A3 for (1) and a = 9.92009 (7), b = 14.51029 (11), c = 9.13585 (7) A, alpha = 90.4769 (4), beta = 111.6724 (4), gamma = 89.9877 (6) degrees, and V = 1222.037 (16) A3 for (2). The molecules are found to be ordered as a result of steric interactions between neighboring molecules, as shown by analyzing the distances between the atoms. Upon heating, both compounds undergo a first-order phase transition at temperatures T(c) = 348 +/- 5 K, as characterized by a relative jump of the lattice parameter of approximately 16%. At 353 K, both structures have the space group P1 (Z = 4), with a = 14.2037 (2) A, and V = 2865.52 (7) A3 for (1) and a = 14.1346 (2) A, and V = 2823.90 (7) A3 for (2). Rietveld refinements were performed for the low-temperature phases measured at T = 295 K [R(wp) = 0.0844 for (1), R(wp) = 0.0940 for (2)] and for the high-temperature phases measured at T = 353 K [R(wp) = 0.0891 for (1), R(wp) = 0.0542 for (2)]. The combination of high-resolution X-ray powder diffraction measurements and variable-temperature magic-angle-spinning 13C, 29Si and 119Sn NMR experiments demonstrates low crystallographic and molecular (C1) symmetries for the low-temperature phases of (1) and (2) at temperatures T < 348 +/- 5 K and high crystallographic symmetry due to rotational disorder for the high-temperature phases at temperatures T > 348 +/- 5 K.

A determination of the partial structure factors of liquid TlSe using combined x-ray and neutron diffraction

The partial structure factors of liquid thallium selenide, TlSe, have been determined using a combination of two neutron diffraction measurements using isotopic substitution and one x-ray diffraction measurement. We show that the statistical accuracy obtained using this method is considerably higher than that achievable using neutron diffraction and isotopic substitution alone. The results unambiguously show the formation of Se polyanionic species in the liquid.

Structural analysis of metal-doped calcium oxalate

Calcium oxalate crystals are the most common biominerals found in plants. They also make their presence known as painful kidney stones in humans and animals. Their function in plants is extraordinarily versatile and encompasses calcium storage and defense mechanisms against herbivores and detoxification processes. Since plants containing calcium oxalate crystals are often exposed to metallic environments, we studied the interaction of such environmental metals with calcium oxalate in vitro. We showed that selected metals are indeed incorporated into synthetic calcium oxalate, and in a manner that depends on their ionic radius when precipitated in solution. One such mechanism of incorporation is based on the replacement of calcium ions by other metal cations within the host unit cell. The unit cell of calcium oxalate expands when incorporating elements with larger atomic radii and shrinks when doped with elements possessing ionic radii smaller than that of calcium. In this systematic study, metal-doped calcium oxalate crystals were characterized by means of high-resolution synchrotron X-ray powder diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic emission spectroscopy, and transmission electron microscopy. Better understanding of metal incorporation into mineral hosts might lead to ways of developing new and more efficient sorbent materials for detoxification processes.

Charge ordering in CaCuxMn7−xO12 (x = 0.0 and 0.1) compounds

The crystal structure of CaMn7O12 and CaCu0.1Mn6.9O12 has been studied by synchrotron radiation (SR) based powder x-ray diffraction and neutron powder diffraction in the temperature range from 10 K up to 290 K. The lattice parameter a exhibits a minimum at 45 K in CaMn7O12. The c lattice parameter in CaMn7O12 and CaCu0.1Mn6.9O12 has a maximum at the same temperature. Additional Bragg peaks have been found in the SR diffraction patterns in CaMn7O12 and CaCu0.1Mn6.9O12 below 250 K and 220 K, respectively. All diffraction peaks have been indexed as (h,k,l ± κ), where κ was equal to 0.079(15) for CaMn7O12 and 0.093(15) for CaCu0.1Mn6.9O12. The incommensurate low-temperature diffraction peaks are not observed in neutron diffraction patterns. This leads to the conclusion that the phase transition to the incommensurate structure is due to charge ordering rather than atomic position modulation. The charge ordering temperature coincides with dielectric constant changes of four orders of magnitude for CaMn7O12.