8 research outputs found
Control of Multipolar and Orbital Order in Perovskite-like [C(NH2)(3)]CuxCd1-x(HCOO)(3) Metal-Organic Frameworks
We study the compositional dependence of molecular orientation (multipolar) and orbital (quadrupolar) order in the family of perovskite-like metalâorganic frameworks [C(NH2)3]CuxCd1âx(HCOO)3. On increasing the fraction x of Jahn-Teller-active Cu2+, we observe first an orbital disorder/order transition and then a multipolar reorientation transition, each occurring at distinct critical compositions xo = 0.45(5) and xm = 0.55(5). We attribute these transitions to a combination of size, charge distribution, and percolation effects. The transitions we observe establish the accessibility in for-mate perovskites of novel structural degrees of freedom beyond the familiar dipolar terms responsible for (an-ti)ferroelectric order. We discuss the symmetry implica-tions of cooperative quadrupolar and multipolar states for the design of relaxor-like hybrid perovskites
Local structure study of the orbital order/disorder transition in LaMnO3
We use a combination of neutron and X-ray total scattering measurements
together with pair distribution function (PDF) analysis to characterise the
variation in local structure across the orbital order--disorder transition in
LaMnO. Our experimental data are inconsistent with a conventional
order--disorder description of the transition, and reflect instead the
existence of a discontinuous change in local structure between ordered and
disordered states. Within the orbital-ordered regime, the neutron and X-ray
PDFs are best described by a local structure model with the same local orbital
arrangements as those observed in the average (long-range) crystal structure.
We show that a variety of meaningfully-different local orbital arrangement
models can give fits of comparable quality to the experimental PDFs collected
within the disordered regime; nevertheless, our data show a subtle but
consistent preference for the anisotropic Potts model proposed in \emph{Phys
Rev.\ B} {\bf 79}, 174106 (2009). The key implications of this model are
electronic and magnetic isotropy together with the loss of local inversion
symmetry at the Mn site. We conclude with a critical assessment of the
interpretation of PDF measurements when characterising local symmetry breaking
in functional materials.Comment: 14 pages, 8 figures, 3 table
Hybrid local-order mechanism for inversion symmetry breaking
Using classical Monte Carlo simulations, we study a simple statistical mechanical model of relevance to the emergence of polarization from local displacements on the square and cubic lattices. Our model contains two key ingredients: a Kitaev-like orientation-dependent interaction between nearest neighbors and a steric term that acts between next-nearest neighbors. Taken by themselves, each of these two ingredients is incapable of driving long-range symmetry breaking, despite the presence of a broad feature in the corresponding heat-capacity functions. Instead, each component results in a âhiddenâ transition on cooling to a manifold of degenerate states; the two manifolds are different in the sense that they reflect distinct types of local order. Remarkably, their intersection, i.e., the ground state when both interaction terms are included in the Hamiltonian, supports a spontaneous polarization. In this way, our study demonstrates how local-order mechanisms might be combined to break global inversion symmetry in a manner conceptually similar to that operating in the âhybridâ improper ferroelectrics. We discuss the relevance of our analysis to the emergence of spontaneous polarization in well-studied ferroelectrics such as BaTiO 3 and KNbO 3
Inorganic co-crystal formation and thermal disproportionation in a dicyanometallate 'superperovskite'
We report the synthesis, crystal structure, and thermally-driven phase transformation of the dicyanometallate superperovskite co-crystal [NBu4]Mn[Au(CN)2]3·[NBu4]ClO4. This phase is understandable in terms of the conventional ABX3 perovskite structure type, but with the NBu4+ A-site cation displaced onto the perovskite cage face and 1-dimensional AX' chains included within framework pores opened up by these displacements. On heating to 380 K, the co-crystal disproportionates into its two inorganic components: a bcs-structured ABX3 phase and [NBu4]ClO4. This system illustrates a new type of structural and phase complexity accessible to dicyanometallate perovskites
Hybrid local-order mechanism for inversion symmetry breaking
Using classical Monte Carlo simulations, we study a simple statistical mechanical model of relevance to the emergence of polarization from local displacements on the square and cubic lattices. Our model contains two key ingredients: a Kitaev-like orientation-dependent interaction between nearest neighbors and a steric term that acts between next-nearest neighbors. Taken by themselves, each of these two ingredients is incapable of driving long-range symmetry breaking, despite the presence of a broad feature in the corresponding heat-capacity functions. Instead, each component results in a âhiddenâ transition on cooling to a manifold of degenerate states; the two manifolds are different in the sense that they reflect distinct types of local order. Remarkably, their intersection, i.e., the ground state when both interaction terms are included in the Hamiltonian, supports a spontaneous polarization. In this way, our study demonstrates how local-order mechanisms might be combined to break global inversion symmetry in a manner conceptually similar to that operating in the âhybridâ improper ferroelectrics. We discuss the relevance of our analysis to the emergence of spontaneous polarization in well-studied ferroelectrics such as
BaTiO
3
and
KNbO
3
Inorganic co-crystal formation and thermal disproportionation in a dicyanometallate 'superperovskite'
We report the synthesis, crystal structure, and thermally-driven phase transformation of the dicyanometallate superperovskite co-crystal [NBu4]Mn[Au(CN)2]3·[NBu4]ClO4. This phase is understandable in terms of the conventional ABX3 perovskite structure type, but with the NBu4+ A-site cation displaced onto the perovskite cage face and 1-dimensional AX' chains included within framework pores opened up by these displacements. On heating to 380 K, the co-crystal disproportionates into its two inorganic components: a bcs-structured ABX3 phase and [NBu4]ClO4. This system illustrates a new type of structural and phase complexity accessible to dicyanometallate perovskites
Orbital dimer model for a spin-glass state in Y2Mo2O7
The formation of a spin glass generally requires that magnetic exchange interactions are both frustrated and disordered. Consequently, the origin of spin-glass behavior in Y2Mo2O7âin which magnetic Mo4ĂŸ ions occupy a frustrated pyrochlore lattice with minimal compositional disorderâhas been a longstanding question. Here, we use neutron and x-ray pair-distribution function (PDF) analysis to develop a disorder model that resolves apparent incompatibilities between previously reported PDF, extended x-rayabsorption fine structure spectroscopy, and NMR studies, and provides a new and physical explanation of the exchange disorder responsible for spin-glass formation. We show that Mo4ĂŸ ions displace according to a local âtwo-inâtwo-outâ rule on each Mo4 tetrahedron, driven by orbital dimerization of Jahn-Teller active Mo4ĂŸ ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative O2â displacements yield a distribution of MoâOâMo angles, which in turn introduces disorder into magnetic interactions. Our study demonstrates experimentally how frustration of atomic displacements can assume the role of compositional disorder in driving a spin-glass transition