Crystal and magnetic structure of cesium superoxide

CsO2 is a member of the family of alkali superoxides formula AO2 with A Na, K, Rb, and Cs that exhibit magnetic behavior arising from open p shell electrons residing on O amp; 8722;2 molecules. We use neutron diffraction to solve the crystal and magnetic structures of CsO2 and observe a complex series of structures on cooling from room temperature to 1.6 K. These include an incommensurate modulation along the a axis of the structure at intermediate temperatures, which then locks into a commensurate modulation that doubles the unit cell compared to the previously supposed orthorhombic unit cell. In both incommensurate and commensurate phases, our structural solution involves a staggering of the cesium ion positions along the b axis, in contrast to studies of other alkali superoxides in which staggered tilts of the O amp; 8722;2 dimers relative to the c axis are seen. Below T 10 K, we observe magnetic Bragg reflections arising from an antiferromagnetically ordered structure with a wave vector of k 0, 0, 0 relative to the doubled crystallographic unit cell , with moments that point predominantly along the b axis with a small component along the a axis that hints at possible anisotropic exchange coupling consistent with the crystal structure . Measurements of the magnetic Bragg reflections in an applied magnetic field suggest a spin flop transition takes place between 2 T and 4 T in which moments likely flop to point along the crystallographic a axis. Our measurements indicate that CsO2 is an interesting example of magnetic properties being inherently linked to the crystal structure, in that the staggered displacement of the cesium ions activates antisymmetric exchange, which then permits the observed spin cantin

High order Ho multipoles in HoB2C2 observed with soft resonant x ray diffraction

Soft resonant x ray Bragg diffraction SRXD at the Ho M 4,M 5 edges has been used to study Ho 4f multipoles in the combined magnetic and orbitally ordered phase of HoB2C2. A full description of the energy dependence for both sigma and pi incident x rays at two different azimuthal angles, as well as the ratio I sigma I pi as a function of azimuthal angle for a selection of energies, allows a determination of the higher order multipole moments of rank 1 dipole to 6 hexacontatetrapole . The Ho 4f multipole moments have been estimated, indicating a dominant hexadecapole rank 4 order with an almost negligible influence from either the dipole or the octupole magnetic terms. The analysis incorporates both the intra atomic magnetic and quadrupolar interactions between the 3d core and 4f valence shells as well as the interference of contributions to the scattering that behave differently under time reversal. Comparison of SRXD, neutron diffraction and non resonant x ray diffraction shows that the magnetic and quadrupolar order parameters are distinct. The 001 2 component of the magnetic order exhibits a Brillouin type increase below the orbital ordering temperature T Q, while the quadrupolar order increases more sharply. We conclude that the quadrupolar interaction is strong, but quadrupolar order only occurs when the magnetic order gives rise to a quasi doublet ground state, which results in a lock in of the orbitals at T

REXS contribution to electronic ordering investigation in solids

Resonant Elastic X-Ray Scattering (REXS) has played a fundamental role in understanding electronic properties and in revealing hidden order, local symmetries and exotic states realized in correlated solids. This article reports on some of the relevant scientific contributions and technical advances over the last 20 years, by providing a list of related publications produced by various groups all around the world. The given perspective is that of a group of young scientists involved at various times in the investigation of the beauty of electronic ordering by the REXS technique. © 2012 EDP Sciences, Springer-Verlag