Universal geometric frustration in pyrochlores

Abstract

Materials with the pyrochlore/fluorite structure have diverse technological applications, from magnetism to nuclear waste disposal. Here we report the observation of structural instability present in the pyrochlores A2Zr2O6Oʹ (A = Pr, La) and Yb2Ti2O6Oʹ, that exists despite ideal stoichiometry, ideal cation-ordering, the absence of lone pair effects, and a lack of magnetic order. Though these materials appear to have good long-range order, local structure probes find displacements, of the order of 0.01 nm, within the pyrochlore framework. The pattern of displacements of the A2Oʹ sublattice mimics the entropically-driven fluxional motions characteristic of and well-known in the silica mineral β-cristobalite. The universality of such displacements within the pyrochlore structure adds to the known structural diversity and explains the extreme sensitivity to composition found in quantum spin ices and the lack of ferroelectric behavior in pyrochlores.

Magnon interactions in the frustrated pyrochlore ferromagnet Yb2Ti2O7

“…Extrinsic effects, such as structural disorder, can take many distinct forms. Several different types of structural disorder have been discussed for Yb 2 Ti 2 O 7 : these include pointdefects, such as "stuffed" Yb 3+ spins [26,33,83], oxygen vacancies [44,83], as well as internal strains and atomic displacements, such as those that might arise due to line-defects like dislocations [46] or through intrinsic frustration of the structural network [84]. For example, recent experiments have explored the effects of oxygen annealing and stuffed Yb 3+ spins [83], finding that the presence or absence oxygen vacancies has strong effects on the magnetic correlations.…”

Chemically directed structure evolution for crystal structure prediction

“…This is often found to be the case for pyrochlore structures. 28,48,49 The effect of the force-field on this structure is to isotropically expand the unit cell, i.e., the lattice parameter is larger. As a result, the bond valence sum values for Y, O(8b) and O(48f) atoms are all quite far away from the expected values.…”

Kagomé networks of octahedrally coordinated metal atoms in minerals: Relating different mineral structures through octahedral tilting

“…This is an inherent source of strain in pyrochlore structures. The pyrochlore structure is usually described as an aniondeficient fluorite derivative structure with face-centred cubic ( fcc) stacking of alternating A 3 B and B 3 A metal atom layers parallel to the {111} fluorite planes (Subramanian et al, 1983;Fuentes et al, 2018;Trump et al, 2018). Both the A and B atoms form 3D kagomé arrays of anion-centred corner-connected tetrahedra, YA 4 and □B 4 (□ = vacancy) (Pannetier and Lucas, 1970) as shown in Fig.…”

“…The absence of long-range ordering of W displacements in hydrokenoelsmoreite is a specific example of the more general phenomenon of geometric frustration in the pyrochlore structure (McQueen et al , 2008; Trump et al , 2018). As shown in Fig.…”