Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb 2 Ti 2 O 7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state.

Eron Cemal, Mechthild Enderle, Reinhard K. Kremer, Björn F̊ak, Eric Ressouche, Mariya V. Gvozdikova, Mike E. Zhitomirsky, and Tim Ziman 5 Institut Laue-Langevin, CS 20156, 38042 Grenoble Cedex 9, France Max-Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany Université Grenoble Alpes, CEA, INAC, MEM, F-38000 Grenoble, France CEA, INAC-PHELIQS, F-38000 Grenoble, France LPMMC, UMR-5493, Université Grenoble Alpes and CNRS, 38042 Grenoble, France (Dated: June 23, 2018)

Single crystal neutron diffraction is combined with synchrotron x-ray scattering to identify the different superlattice phases present in Cs0.8Fe1.6Se2. A combination of single crystal refinements and first principles modeling are used to provide structural solutions for the root 5 x root 5 and root 2 x root 2 superlattice phases. The root 5 x root 5 superlattice structure is predominantly composed of ordered Fe vacancies and Fe distortions, whereas the root 2 x root 2 superlattice is composed of ordered Cs vacancies. The Cs vacancies only order within the plane, causing Bragg rods in reciprocal space. By mapping x-ray diffraction measurements with narrow spatial resolution over the surface of the sample, the structural domain pattern was determined, consistent with the notion of a majority antiferromagnetic root 5 x root 5 phase and a superconducting root 2 x root 2 phase.

D.G. Porter, ∗ E. Cemal, 2 D.J. Voneshen, K. Refson, M.J. Gutmann, A. Bombardi, A.T. Boothroyd, A. Krzton-Maziopa, E. Pomjakushina, K. Conder, and J.P. Goff Department of Physics, Royal Holloway, University of London, Egham, TW20 0EX, UK Institut Laue-Langevin, BP156, F-38042 Grenoble Cedex 9, France ISIS, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Didcot OX11 0QX, UK Diamond Light Source Ltd, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, UK Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK Faculty of Chemistry, Warsaw University of Technology, PL-00664 Warsaw, Poland Laboratory for Developments and Methods, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland (Dated: February 18, 2015)

Sodium cobaltate has latterly received attention due to its appealing thermoelectric properties. By combining inelastic X-ray and neutron scattering results with detailed first-principles calculations, it is now shown that low-energy rattling modes of sodium ions within multi-vacancy clusters play a central role in determining the low thermal conductivity of this material.