11 research outputs found
Efficient fitting of single-crystal diffuse scattering in interaction space: a mean-field approach
The diffraction patterns of crystalline materials with strongly-correlated
disorder are characterised by the presence of structured diffuse scattering.
Conventional analysis approaches generally seek to interpret this scattering
either atomistically or in terms of pairwise (Warren--Cowley) correlation
parameters. Here we demonstrate how a mean-field methodology allows efficient
fitting of diffuse scattering directly in terms of a microscopic interaction
model. In this way the approach gives as its output the underlying physics
responsible for correlated disorder. Moreover, the use of a very small number
of parameters during fitting renders the approach surprisingly robust to data
incompleteness, a particular advantage when seeking to interpret single-crystal
diffuse scattering measured in complex sample environments. We use as the basis
of our proof-of-concept study a toy model based on strongly-correlated disorder
in diammine mercury(II) halides
Efficient fitting of single-crystal diffuse scattering in interaction space: a mean-field approach
The diffraction patterns of crystalline materials with strongly-correlated
disorder are characterised by the presence of structured diffuse scattering.
Conventional analysis approaches generally seek to interpret this scattering
either atomistically or in terms of pairwise (Warren--Cowley) correlation
parameters. Here we demonstrate how a mean-field methodology allows efficient
fitting of diffuse scattering directly in terms of a microscopic interaction
model. In this way the approach gives as its output the underlying physics
responsible for correlated disorder. Moreover, the use of a very small number
of parameters during fitting renders the approach surprisingly robust to data
incompleteness, a particular advantage when seeking to interpret single-crystal
diffuse scattering measured in complex sample environments. We use as the basis
of our proof-of-concept study a toy model based on strongly-correlated disorder
in diammine mercury(II) halides
Ferroic multipolar order and disorder in cyanoelpasolite molecular perovskites
We use a combination of variable-temperature high-resolution synchrotron X-ray powder diffraction measurements and Monte Carlo simulations to characterize the evolution of two different types of ferroic multipolar order in a series of cyanoelpasolite molecular perovskites. We show that ferroquadrupolar order in [C3N2H5]2Rb[Co(CN)6] is a first-order process that is well described by a four-state Potts model on the simple cubic lattice. Likewise, ferrooctupolar order in [NMe4]2B[Co(CN)6] (B = K, Rb, Cs) also emerges via a first-order transition that now corresponds to a six-state Potts model. Hence, for these particular cases, the dominant symmetry breaking mechanisms are well understood in terms of simple statistical mechanical models. By varying composition, we find that the effective coupling between multipolar degrees of freedom-and hence the temperature at which ferromultipolar order emerges-can be tuned in a chemically sensible manner. This article is part of the theme issue 'Mineralomimesis: natural and synthetic frameworks in science and technology'
Ferroic multipolar order and disorder in cyanoelpasolite molecular perovskites
We use a combination of variable-temperature high-resolution synchrotron X-ray powder diffraction measurements and Monte Carlo simulations to characterize the evolution of two different types of ferroic multipolar order in a series of cyanoelpasolite molecular perovskites. We show that ferroquadrupolar order in [C3N2H5]2Rb[Co(CN)6] is a first-order process that is well described by a four-state Potts model on the simple cubic lattice. Likewise, ferrooctupolar order in [NMe4]2B[Co(CN)6] (B = K, Rb, Cs) also emerges via a first-order transition that now corresponds to a six-state Potts model. Hence, for these particular cases, the dominant symmetry breaking mechanisms are well understood in terms of simple statistical mechanical models. By varying composition, we find that the effective coupling between multipolar degrees of freedom-and hence the temperature at which ferromultipolar order emerges-can be tuned in a chemically sensible manner. This article is part of the theme issue 'Mineralomimesis: natural and synthetic frameworks in science and technology'
Geometric frustration on the trillium lattice in a magnetic metal-organic framework
In the dense metal-organic framework Na[Mn(HCOO)3], Mn2+ ions (S=5/2) occupy the nodes of a “trillium” net. We show that the system is strongly magnetically frustrated: the Néel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the Néel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at 1/3-saturation magnetization. A simple model of nearest-neighbor Heisenberg antiferromagnetic and dipolar interactions accounts quantitatively for all observations, including an unusual 2-k magnetic ground state. We show that the relative strength of dipolar interactions is crucial to selecting this particular ground state. Geometric frustration within the classical spin liquid regime gives rise to a large magnetocaloric response at low applied fields that is degraded in powder samples as a consequence of the anisotropy of dipolar interactions