X-ray and neutron scattering

Abstract:

The sodium reordering in Nax CoO2 in the vicinity of room temperature is rationalized at high x in terms of phase transitions between square and striped phases. A striking hexagon-of-hexagons diffraction pattern observed for x=0.78 can be reproduced using coexisting square and striped phases that are related by simple shear deformations. All compositions exhibit a partial melting transition to a disordered stripe phase just below room temperature, which alters the topology of the electrical conduction pathways. © 2009 The American Physical Society.

Abstract:

We have investigated orbital ordering in the bilayer manganite, Pr (Sr0.1 Ca0.9) 2 Mn2 O7, using synchrotron x-ray diffraction techniques, in particular resonant soft x-ray diffraction. Diffraction signals are observed at (0, 1 2, 0) above 300 K and (1 2, 0,0) below 300 K, corresponding to orbital stripes along the a and b axes, respectively. The previously observed transition at 300 K is evidence of a rotation of the orientation of the 3d orbital stripes. Large resonances of the orbital signals are observed at the MnL edges, with a complex dependence on incident photon energy indicative of a weakly Jahn-Teller distorted system. A structural transition observed at 92 K is found not to involve a further change in the orbital order. © 2009 The American Physical Society.

Abstract:

The structure of the A-site substituted perovskite K x Na1 - x NbO3, x = 0.24-0.36, where a phase boundary was previously reported, has been determined by high-resolution X-ray powder and neutron powder diffraction studies. The structure of the x = 0.3 compound was refined in the monoclinic space group Pm at 293 K and in P4mm at 523 K. The Glazer tilt system of the room-temperature monoclinic phase is a 0 b + c 0, which has implications for the nature of the next symmetry change with composition towards pure potassium niobate. A phase-coexistence region at the transition between monoclinic and tetragonal phases was also identified, consistent with a first-order phase boundary. There is also evidence for an intermediate oxygen-octahedra tilted tetragonal phase. © 2009 International Union of Crystallography Printed in Singapore - all rights reserved.