Atomic displacements in the charge ice pyrochloreBi2Ti2O6O′studied by neutron total scattering

Abstract

The oxide pyrochlore Bi 2 Ti 2 O 6 OЈ is known to be associated with large displacements of Bi and OЈ atoms from their ideal crystallographic positions. Neutron total scattering, analyzed in both reciprocal and real space, is employed here to understand the nature of these displacements. Rietveld analysis and maximum entropy methods are used to produce an average picture of the structural nonideality. Local structure is modeled via large-box reverse Monte Carlo simulations constrained simultaneously by the Bragg profile and real-space pair distribution function. Direct visualization and statistical analyses of these models show the precise nature of the static Bi and OЈ displacements. Correlations between neighboring Bi displacements are analyzed using coordinates from the large-box simulations. The framework of continuous symmetry measures has been applied to distributions of OЈBi 4 tetrahedra to examine deviations from ideality. Bi displacements from ideal positions appear correlated over local length scales. The results are consistent with the idea that these nonmagnetic lone-pair containing pyrochlore compounds can be regarded as highly structurally frustrated systems.

First-principles calculation of the structure and dielectric properties ofBi2Ti2O7

“…The structure of the insulating pyrochlore Bi 2 Ti 2 O 6 OЈ ͑BTi͒ has been studied by x-ray and neutron-scattering [1][2][3] and density-functional theory ͑DFT͒ methods. [4][5][6][7] The ideal pyrochlore structure belongs to the Fd3m space group and consists of interpenetrating Bi 2 OЈ and Ti 2 O 6 polyhedral networks.…”

Orbital Dimer Model for the Spin-Glass State in Y2Mo2O7Thygesen1, Paddison2, Zhang3 et al. 2017Phys. Rev. Lett.411510View full textAdd to dashboardCite

“…3(d) shows a disordered example that would more closely approximate the true structure. This is the same degeneracy responsible for the unusual physics of static and dynamic spin-ice materials [39,40], cubic water ice [37], and charge-ice materials [41,42].…”

Neutron scattering studies of spin ices and spin liquids

“…Consequently, both the ice and spin ice have statistically identical ground state manifolds and entropy, as shown by integration of specific heat measurements [44][45][46]. Because mappings can be made between "two-in-two-out" Ising spins with ferromagnetic coupling, and "two-up-two-down" Ising spins with antiferromagnetic coupling 4 [42,43,[47][48][49] the form of spin correlations obeying the ice rules becomes an important result in both pyrochlore ferromagnets and antiferromagnets [48][49][50][51][52].…”

“…Frustration is most commonly associated with spin systems [1], where its consequences can be particularly well identified, but is by no means limited to magnetism. Frustrated interactions are also relevant in certain structural problems [2][3][4][5][6] Interest in frustrated spin systems stems from the idea that conventional order will be suppressed by the frustration, and an unconventional state will appear in its place. For example, Anderson proposed that in high temperature superconductors, antiferromagnetic Néel ordering would be replaced by a resonating valence bond state (RVB) [17], which would host the superconductivity on doping.…”

“…Frustration is most commonly associated with spin systems [1], where its consequences can be particularly well identified, but is by no means limited to magnetism. Frustrated interactions are also relevant in certain structural problems [2][3][4][5][6], colloids and liquid crystals [7], spin glasses [8], stripe phases [9,10], Josephson junction arrays [11], stellar nuclear matter [12,13], social dynamics [14], origami [15], and protein folding [16], to name a few.…”