Neutron diffraction studies using powder samples have been used to understand the complex sequence of low temperature phase transitions of NaNbO3 in the temperature range from 12 K-350 K. Detailed Rietveld analysis of the diffraction data reveal that the antiferroelectric to ferroelectric phase transition occurs on cooling around 73 K while the reverse ferroelectric to antiferroelectric transition occurs on heating at 245 K. However, the former transformation is not complete till down to 12 K and there is unambiguous evidence for the presence of the ferroelectric R3c phase coexisting with an antiferroelectic phase (Pbcm) over a wide range of temperatures. The coexisting phases and reported anomalous smearing of the dielectric response akin to dipole glasses and relaxors observed in the same temperature range are consistent with competing ferroelectric and antiferroelectric interactions in NaNbO 3 . We have carried out theoretical lattice dynamical calculations which reveal that the free energies of the antiferroelectric Pbcm and ferroelectric R3c phases are nearly identical over a wide range of temperature. The small energy difference between the two phases is of interest as it explains the observed coexistence of these phases over a wide range of temperature. The computed double well depths and energy barriers from paraelectric Pm 3m to antiferroelectric Pbcm and ferroelectric R3c phases in NaNbO 3 are also quite similar, although the ferroelectric R3c phase has a slightly lower energy.
La 2 NiMnO 6 (LNMO) was prepared by a combustion method followed by heating at high temperature. Subsequently, the preformed LNMO was annealed in air, oxygen, or N 2 atmosphere and characterized by powder x-ray diffraction (XRD), neutron diffraction, superconducting quantum interference device magnetometry, and dielectric analysis. Structural studies by XRD and neutron diffraction revealed the coexistence of partially cation disordered monoclinic (31%) and rhombohedral (69%) phases in the sample annealed in air. However, the sample annealed in oxygen shows about 50:50% of monoclinic and rhombohedral phases. Relaxor-like behavior with relative permittivity of the order of 10 4 was observed in the sample annealed in air, while relative permittivity decreases to about 200 in samples annealed in oxygen atmosphere. The magnetic properties indicate a well-defined ferromagnetic phase in the oxygen-annealed sample compared to a feeble ferromagnetic signature in the air-annealed one. The dielectric and ferromagnetism of LNMO samples have been related to formation and annihilation of oxygen vacancies.
Royal Society of Chemistry Achary, S.N.; Errandonea, D.; Muñoz, A.; Rodríguez Hernández, P.;Manjón, F.J.; Kishna, P. S. R.; Patwe, S. J.; Grover, V. ; Tyagi, A. (2013) Lattice-dynamic calculations of the phonon modes were performed at the zone center (Γ point) of the BZ. We used a direct force-constant approach (or supercell method) 43 as it is conceptually simple. These calculations provide information about the symmetry of the modes and their polarization vectors which allowed us to identify the irreducible representations and the character of the phonon modes at the Γ point. However, this method did not include the contribution from dipole-dipole interactions resulting the LO-TO splitting of infrared-active modes, due to the non-analyticity of this term at the Γ point. Results and discussion X-ray and neutron diffraction studies at ambient conditionsThe powder XRD patterns of the three phases of BiPO 4 , namely BiPO4-I, II and III (shown in Fig. 1
In this paper, we report the structural and compostional variation of Ce 2 Zr 2 O 7 pyrochlore under a controlled oxidation/reduction process. On oxidation Ce 2 Zr 2 O 7 transforms to Ce 2 Zr 2 O 8 via an intermediate lattice at Ce 2 Zr 2 O 7.5 . Crystal structures of the Ce 2 Zr 2 O 7 , Ce 2 Zr 2 O 7.5 , and Ce 2 Zr 2 O 8 have been determined accurately from the neutron diffraction data of polycrystalline samples. The retention of cubic pyrochlore-type arrangements is observed even up to the fully oxidized Ce 2 Zr 2 O 8 composition. The unit cell parameters systematically decreased from Ce 2 Zr 2 O 7 (10.6924(3) A ˚) to Ce 2 Zr 2 O 8 ( 10.5443(2) A ˚). All three compositions retain the original pyrochlore-type cation ordering without any intermixing Ce and Zr atoms. All three structures can be explained as cubically coordinated metal atoms, which share the edge of their AO 8 polyhedra similar to that in the fluorite lattice. The transformation from Ce 2 Zr 2 O 7 to Ce 2 Zr 2 O 8 systematically lowers the symmetry from Fd3m to P2 1 3. A complete oxidation of Ce 3þ to Ce 4þ is observed, in contrast to the kinetically and sterically hindered oxidation proposed earlier. The deep crystallographic insights associated with the oxygen intercalation and deintercalation process in Ce 2 O 3 /CeO 2 -ZrO 2 will have immense importance in the development and understanding of new oxygen storage capacitors.
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