arXiv subjects

Martin T. Dove

Publications and source records attributed to Martin T. Dove.

24 records · Page 2Linked to original sources

arXiv2010

Structure determination of disordered materials from diffraction data

We show that the information gained in spectroscopic experiments regarding the number and distribution of atomic environments can be used as a valuable constraint in the refinement of the atomic-scale structures of nanostructured or amorphous materials from pair distribution function (PDF) data. We illustrate the effectiveness of this approach for three paradigmatic disordered systems: molecular C60, a-Si, and a-SiO2 . Much improved atomistic models are attained in each case without any a-priori assumptions regarding coordination number or local geometry. We propose that this approach may form the basis for a generalised methodology for structure "solution" from PDF data applicable to network, nanostructured and molecular systems alike.

cond-mat.mtrl-sci↗

arXiv2010

Structure and properties of an amorphous metal--organic framework

We show that ZIF-4, a metal-organic framework (MOF) with a zeolitic structure, undergoes a crystal--amorphous transition on heating to 300 $^\circ$C. The amorphous form, which we term a-ZIF, is recoverable to ambient conditions or may be converted to a dense crystalline phase of the same composition by heating to 400 $^\circ$C. Neutron and X-ray total scattering data collected during the amorphization process are used as a basis for reverse Monte Carlo refinement of an atomistic model of the structure of a-ZIF. We show that the structure is best understood in terms of a continuous random network analogous to that of a-SiO$_2$. Optical microscopy, electron diffraction and nanoindentation measurements reveal a-ZIF to be an isotropic glass-like phase capable of plastic flow on its formation. Our results suggest an avenue for designing broad new families of amorphous and glass-like materials that exploit the chemical and structural diversity of MOFs.

cond-mat.mtrl-sci↗

arXiv2008

Origin of the colossal positive and negative thermal expansion in Ag$_3$[Co(CN)$_6$]: an {\it ab initio} Density Functional Theory study

DFT calculations have been used to provide insights into the origin of the colossal positive and negative thermal expansion in Ag$_3$[Co(CN)$_6]$. The results confirm that the positive expansion within the trigonal basal plane and the negative expansion in the orthogonal direction are coupled due to the existence of a network defined by nearly-rigid bonds within the chains of Co--C--N--Ag--N--C--Co linkages. The origin of the colossal values of the coefficients of thermal expansion arise from an extremely shallow energy surface that allows a flexing of the structure with small energy cost. The thermal expansion can be achieved with a modest value of the overall Grüneisen parameter. The energy surface is so shallow that we need to incorporate a small empirical dispersive interaction to give ground-state lattice parameters that match experimental values at low temperature. We compare the results with DFT calculations on two isostructural systems: H$_3$[Co(CN)$_6$], which is known to have much smaller values of the coefficients of thermal expansion, and Au$_3$[Co(CN)$_6$], which has not yet been synthesised but which is predicted by our calculations to be another candidate material for showing colossal positive and negative thermal expansion.

cond-mat.mtrl-sci↗

arXiv2008

Local structure in Ag$_3$[Co(CN)$_6$]: Colossal thermal expansion, rigid unit modes and argentophilic interactions

Local structure in the colossal thermal expansion material Ag$_3$[Co(CN)$_6$] is studied here using a combination of neutron total scattering and reverse Monte Carlo (RMC) analysis. We show that the large thermal variations in cell dimensions occur with minimal distortion of the [Co(CN)$_6$] coordination polyhedra, but involve significant flexing of the Co--CN--Ag--NC--Co linkages. We find real-space evidence in our RMC configurations for the importance of low-energy rigid unit modes (RUMs), particularly at temperatures below 150 K. Using a reciprocal-space analysis we present the phonon density of states at 300 K and show that the lowest-frequency region is dominated by RUMs and related modes. We also show that thermal variation in the energies of Ag$...$Ag interactions is evident in both the Ag partial pair distribution function and in the Ag partial phonon density of states. These findings are discussed in relation to the thermodynamic properties of the material.

cond-mat.mtrl-sci↗

arXiv2006

Spin-Dynamical Analysis of Supercell Spin Configurations

A model-independent approach capable of extracting spin-wave frequencies and displacement vectors from ensembles of supercell spin configurations is presented. The method is appropriate for those systems whose spin-dynamical motion is well characterised by small-amplitude fluctuations that give harmonic spin waves. First, the the spin-dynamical matrix is described and its eigenvalues shown to correspond to the spin-wave mode frequencies. The generalised spin coordinate matrix - a quantity that may be calculated from the observed spin orientations in an ensemble of spin configurations - is then introduced and its relationship to the spin-dynamical matrix established. Its eigenvalues are subsequently shown to be related to the spin-wave mode frequencies, allowing the extraction of spin-wave dispersion curves from configurational ensembles. Finally, a quantum-mechanical derivation of the same results is given, and the method applied as a case study to spin Monte-Carlo configurations of a 3D Heisenberg ferromagnet.

cond-mat.other↗

arXiv2006

MnO Spin-Wave Dispersion Curves from Powder Diffraction Data

We describe a model-independent approach for the extraction of spin-wave dispersion curves from neutron total scattering data. The method utilises a statistical analysis of real-space spin configurations to calculate spin-dynamical quantities. The RMCProfile implementation of the reverse Monte Carlo refinement process is used to generate a large ensemble of supercell spin configurations from powder diffraction data. Our analysis of these configurations gives spin-wave dispersion curves that agree well with those determined independently using neutron triple-axis spectroscopic techniques.

cond-mat.other↗