Simon J. Hibble

Local distortions in the colossal magnetoresistive manganates La0.70Ca0.30MnO3, La0.80Ca0.20MnO3 and La0.70Sr0.30MnO3 revealed by total neutron diffraction

The manganates La0.80Ca0.20MnO3, La0.70Ca0.30MnO3 and La0.70Sr0.30MnO3 exhibit the colossal magnetoresistive (CMR) effect. Total neutron diffraction was employed to yield information on both the average and local atomic structure of these disordered crystalline materials as a function of temperature. The average structures were determined from Rietveld analysis of the Bragg scattering. Information on the local structures was obtained by Fourier transformation of the total diffraction pattern to yield the total correlation function, T(r). Particular attention is paid to changes in the Mn-O bond distances, which are widely believed to be important in the CMR effect. Jahn-Teller distortions of the MnO6 octahedra are absent at the lowest temperatures in the metallic phase. As the temperature is raised towards the paramagnetic-semiconducting to ferromagnetic-metallic transition at Tc, T(r) exhibits clear increases in the variance of Mn-O bond distances, which greatly exceed those expected from the increase in disorder due to atomic thermal motion. This is confirmed by comparing the behaviour of the three materials, which have different values for Tc. The advantage of studying the local structure directly by determining T(r) from total neutron scattering, rather than extrapolating from the average to local structure from Bragg scattering studies, is demonstrated. Comparisons are made with the results obtained for the ordered compound LaMnO3, which does not exhibit the CMR effect. The three CMR manganates studied here do not show a separation of the Mn-O distances into two well resolved sets above Tc as reported by other workers.

Novel low-temperature route to known (MnS and FeS2) and new (CrS3, MoS4 and WS5) transition-metal sulfides

The reaction of the transition-metal carbonyls, Mn2(CO)10, Fe(CO)5 or M(CO)6(M = Cr, Mo or W) with sulfur in 1, 2-dichlorobenzene yields polycrystalline MnS (alabandite), FeS2(marcarsite), and three new amorphous sulfur-rich sulfides CrS3, MoS4 and WS5.

Relationship between phonons and thermal expansion in Zn(CN)2 and Ni(CN)2 from inelastic neutron scattering and ab initio calculations

Zn(CN)2 and Ni(CN)2 are known for exhibiting anomalous thermal expansion over a wide temperature range. The volume thermal expansion coefficient for the cubic, three dimensionally connected material, Zn(CN)2, is negative (alpha(V) = −51  10(-6) K-1) while for Ni(CN)2, a tetragonal material, the thermal expansion coefficient is negative in the two dimensionally connected sheets (alpha(a) = −7  10(-6) K-1), but the overall thermal expansion coefficient is positive (alpha(V) = 48  10(-6) K-1). We have measured the temperature dependence of phonon spectra in these compounds and analyzed them using ab initio calculations. The spectra of the two compounds show large differences that cannot be explained by simple mass renormalization of the modes involving Zn (65.38 amu) and Ni (58.69 amu) atoms. This reflects the fact that the structure and bonding are quite different in the two compounds. The calculated pressure dependence of the phonon modes and of the thermal expansion coefficient, alpha(V), are used to understand the anomalous behavior in these compounds. Our ab initio calculations indicate that phonon modes of energy approx. 2 meV are major contributors to negative thermal expansion (NTE) in both the compounds. The low-energy modes of approx.8 and 13 meV in Zn(CN)2 also contribute significantly to the NTE in Zn(CN)2 and Ni(CN)2, respectively. The measured temperature dependence of the phonon spectra has been used to estimate the total anharmonicity of both compounds. For Zn(CN)2, the temperature-dependent measurements (total anharmonicity), along with our previously reported pressure dependence of the phonon spectra (quasiharmonic), is used to separate the explicit temperature effect at constant volume (intrinsic anharmonicity).

Helices, chirality and interpenetration: the versatility and remarkable interconversion of silver-copper cyanide frameworks.

The structural transformations between cesium silver-copper cyanides under modest conditions, both in solution and in the solid state, are described. Three new cesium silver(I) copper(I) cyanides with three-dimensional (3-D) framework structures were prepared as single crystals from a one-pot reaction initially heated under hydrothermal conditions. The first product to appear, Cs(3)Ag(2)Cu(3)(CN)(8) (I), when left in contact with the supernatant produced CsAgCu(CN)(3) (II) and CsAgCu(CN)(3).1/3H(2)O (III) over a few months via a series of thermodynamically controlled cascade reactions. Crystals of the hydrate (III) can be dehydrated to polycrystalline CsAgCu(CN)(3) (II) on heating at 100 degrees C in a remarkable solid-state transformation involving substantial breaking and reconnection of metal-cyanide linkages. Astonishingly, the conversion between the two known polymorphs of CsAg(2)Cu(CN)(4), which also involves a major change in connectivity and topology, occurs at 180 degrees C as a single-crystal to single-crystal transformation. Structural features of note in these materials include the presence of helical copper-cyanide chains in (I) and (II), which in the latter compound produce a chiral material. In (II) and (III), the silver-copper cyanide networks are both self- and interpenetrating, features also seen in the known polymorphs of CsAg(2)Cu(CN)(4).

Total neutron diffraction: the correct way to determine the true structure of crystalline materials?

Crystallography, using conventional Bragg diffraction, and the study of atomic correlation functions, using total diffraction, have historically been carried out separately. There exist two different scientific communities, which in the case of neutron diffraction normally use different instruments. However, modem time-of-flight neutron diffractometers allow data to be collected to high maximum momentum transfer, Q max , and with good reciprocal-space resolution, Δd/d. The high Q max yields correlation functions with good real-space resolution, whilst the good reciprocal-space resolution yields data well suited to conventional crystallographic analysis. We show how the Liquids and Amorphous Diffractometer, LAD, at the ISIS spallation neutron source at the Rutherford Appleton Laboratory has been used to obtain new information on a number of disordered crystalline molybdates, Li 2 MoO 3 , LiMoO 2 and D 2 MoO 3 . The average crystal structures are determined using Rietveld refinement of the Bragg diffraction data, whilst the local structures are determined by modelling the correlation functions, T(r), obtained from total neutron diffraction data. Reconciling the information from the two techniques provides a deeper understanding of structures than is possible using either technique in isolation. Finally, we discuss how the next generation of instruments will allow the development of this technique with specific reference to the new General Materials Diffractometer, GEM, at ISIS.

Structures and negative thermal expansion properties of the one-dimensional cyanides, CuCN, AgCN and AuCN

Abstract The behaviour of the lattice parameters of HT-CuCN (high-temperature form), AgCN and AuCN have been investigated as a function of temperature over the temperature range 90–490 K. All materials show one-dimensional negative thermal expansion (NTE) along the —(M—C≡N)— chain direction c(αc(HT-CuCN) = –32.1 × 10–6 K–1, αc(AgCN) = –23.9 × 10–6 K–1 and αc(AuCN) = –9.3 × 10–6 K–1 over the temperature range 90–490 K). The origin of this behaviour has been studied using RMC modelling of Bragg and total neutron diffraction data from AgCN and AuCN at 10 and 300 K. These analyses yield details of the local motions within the chains responsible for NTE. The low-temperature form of CuCN, LT-CuCN, has been studied using single-crystal X-ray diffraction. In this form of CuCN, wavelike distortions of the —(Cu—C≡N)— chains occur in the static structure, which are reminiscent of the motions seen in the RMC modelling of AgCN and AuCN responsible for the NTE behaviour.

A combined in situ X-ray absorption spectroscopy and X-ray diffraction study of the thermal decomposition of ammonium tetrathiotungstate

The structural changes occurring during the thermal decomposition of ammonium tetrathiotungstate to form amorphous WS 3 and poorly crystalline WS 2 have been studied in situ using combined extended X-ray absorption fine structure (EXAFS) spectroscopy at the tungsten L III -edge and X-ray diffraction. Data collected during isothermal decomposition at both 135 and 150 °C show that ammonium tetrathiotungstate decomposes to produce amorphous WS 3 without the formation of any intermediate phases. Decay curves of starting material and growth curves of product derived from both X-ray absorption data and diffraction data show that the two-phase approach to data analysis is appropriate. The fit of various simple kinetic models to the thermal decay curves is tested and it is found that the Prout-Tompkins expression describes the process very well. Heating ammonium tetrathiotungstate from 100 to 350 °C produces first amorphous WS 3 and finally disordered WS 2 , a material of catalytic interest. The EXAFS data of this freshly prepared WS 2 are compared to those of crystalline 2H-WS 2 . Debye-Waller factors are increased and occupation numbers are significantly reduced from those in the crystalline material for all atomic shells. This behaviour is compared to previous results obtained from poorly crystalline molybdenum disulfide and possible structural models suggested to account for the results of the first EXAFS study of the disordered WS 2 .

Structure of two disordered molybdates, Li2MoIVO3 and Li4Mo3IVO8, from total neutron scattering

The structures of the disordered lithium molybdates Li2MoO3 and Li4Mo3O8 have been investigated using total neutron scattering from polycrystalline powders. Rietveld analysis of the Bragg scattering is used to determine the average structures. Shortcomings in this method of analysis are demonstrated by comparing the total correlation function, T(r), determined from total neutron scattering, with those calculated from the structures determined from Rietveld analysis. Much more satisfactory models for these materials are derived from the structurally related ordered material LiZn2Mo3O8, using information from Mo K-edge extended X-ray absorption fine-structure spectroscopy (EXAFS). These models include metal–metal-bonded Mo3O13 clusters [d(Mo—Mo) = 2.58 A in Li2MoO3 and 2.56 A in Li4Mo3O8] not present in the average structure determined from Rietveld analysis [d(Mo—Mo) = 2.88 A in Li2MoO3]. In contrast to EXAFS studies neutron diffraction yields information on all the pair correlations in the material, not merely those involving molybdenum, and allows, for example, the location of lithium. Remaining discrepancies between our models and the experimental T(r)s give an insight into the disorder in the two materials.

Preparation of new selenium-rich selenides, CrSe3, MoSe≈5, WSe≈6–7, and ReSe≈6–7 and known selenides, by the reaction of metal carbonyls with selenium

The reaction of the transition-metal carbonyls, Cr(CO)6, Mn2(CO)10, Fe(CO)5, Co2(CO)8, Mo(CO)6, W(CO)6 and Re2(CO)10, with selenium in 1,2-dichlorobenzene heated under reflux produced a number of known transition-metal Selenides, MnSe, MnSe2, FeSe2, CoSe2, MoSe2 and WSe2 and four new selenium-rich compounds, CrSe3, MoSe≈5, WSe≈6–7 and ReSe≈6–7. The new selenides, with high selenium-to-metal ratios, were obtained as amorphous solids, while the known compounds, MnSe, MnSe2, FeSe2, CoSe2 were formed as polycrystalline powders and MoSe2 and WSe2 as poorly ordered fibres. The products were characterised by analytical electron microscopy and powder X-ray diffraction and MoSe≈5 was further characterised using thermal methods.

An in situ structural study of the thermal decomposition reactions of the ammonium thiomolybdates, (NH4)2Mo2S12·2H2O and (NH4)2Mo3S13·2H2O

A combined in situ Mo K-edge extended X-ray absorption fine structure (EXAFS) and X-ray powder diffraction study has been carried out to follow the evolution of structure in the thermal decomposition of the ammonium thiomolybdates, (NH4)2Mo3S13·2H2O and (NH4)2Mo2S12·2H2O, under nitrogen. Additional information on the course of the decomposition reactions has been obtained from thermogravimetric and differential thermal analysis, and from IR spectroscopy. Molybdenum–molybdenum bonded triangular units are found to persist in all the amorphous intermediate decomposition products of (NH4)2Mo3S13·2H2O, and are also formed during the decomposition of (NH4)2Mo2S12·2H2O. Structural models for the intermediates are presented. The final decomposition product in both cases is poorly crystalline MoS2.