109 research outputs found
Neutron scattering study of PbMgTaO and BaMgTaO complex perovskites
Neutron scattering investigations were carried out in
PbMgTaO and BaMgTaO complex
perovskites. The crystal structure of both compounds does not show any phase
transition in the temperature range 1.5 -- 730 K. Whereas the temperature
dependence of the lattice parameter of BaMgTaO follows the
classical expectations, the lattice parameter of relaxor ferroelectric
PbMgTaO exhibits anomalies. One of these anomalies is
observed in the same temperature range as the peak in the dielectric
susceptibility. We find that in PbMgTaO, lead ions are
displaced from the ideal positions in the perovskite structure at all
temperatures. Consequently short-range order is present. This induces strong
diffuse scattering with an anisotropic shape in wavevector space. The
temperature dependences of the diffuse neutron scattering intensity and of the
amplitude of the lead displacements are similar
Anomalous pressure dependence of the atomic displacements in the relaxor ferroelectric PbMgTaO
The crystal structure of the PbMgTaO (PMT) relaxor
ferroelectric was studied under hydrostatic pressure up to GPa by
means of powder neutron diffraction. We find a drastic pressure-induced
decrease of the lead displacement from the inversion centre which correlates
with an increase by 50 % of the anisotropy of the oxygen temperature
factor. The vibrations of the Mg/Ta are, in contrast, rather pressure
insensitive. We attribute these changes being responsible for the previously
reported pressure-induced suppression of the anomalous dielectric permittivity
and diffuse scattering in relaxor ferroelectrics
Phase Diagram of the Dzyaloshinskii-Moriya Helimagnet Ba2CuGe2O7 in Canted Magnetic Fields
The evolution of different magnetic structures of non-centrosymmetric
Ba2CuGe2O7 is systematically studied as function of the orientation of the
magnetic field H. Neutron diffraction in combination with measurements of
magnetization and specific heat show a virtually identical behaviour of the
phase diagram of Ba2CuGe2O7 for H confined in both the (1,0,0) and (1,1,0)
plane. The existence of a recently proposed incommensurate double-k AF-cone
phase is confirmed in a narrow range for H close to the tetragonal c-axis. For
large angles enclosed by H and the c-axis a complexely distorted non-sinusoidal
magnetic structure has recently been observed. We show that its critical field
Hc systematically increases for larger canting. Measurements of magnetic
susceptibility and specific heat finally indicate the existence of an
incommensurate/commensurate transition for H /sim 9 T applied in the basal
(a,b)-plane and agree with a non-planar, distorted cycloidal magnetic
structure.Comment: 14 pages, 13 figure
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