244 research outputs found
Electron density distribution in paramagnetic chromium A gamma ray diffraction study
High accuracy single crystal structure factors, complete up to sin amp; 61553; amp; 61548; amp; 61472; amp; 61501; amp; 61472; amp; 61489; amp; 61486; amp; 61495; amp; 61496; amp; 61472; amp; 61485; amp; 61489; amp; 61484; amp; 61472; have been measured from paramagnetic chromium at 333 K using 316.5 keV gamma radiation. A detailed description of the electron density distribution is derived in terms of a multipolar atomic deformation model. There is pronounced charge asphericity in the valence region arising from preferential occupancy of the t2g subshell. The 3d charge distribution is contracted by 12.6 relative to the free atom, in accordance with magnetic synchrotron x ray and neutron measurements. By contrast, the atomic crystal scattering factor deduced from experiment is found to be in contradiction with earlier experimental and theoretical work. Achievement of a reliable Debye Waller factor is of vital importance in this context. There is no evidence for an anharmonic term in the atomic potential. Real space and energetic features of the charge density topology are used to characterize the directed metallic bonds. Special attention is paid to the form factor approximation in diffraction data analysis
Incommensurate spin density modulation in a copper-oxide chain compound with commensurate charge order
Neutron diffraction has been used to determine the magnetic structure of
NaCuO, a stoichiometric compound containing chains based on
edge-sharing CuO plaquettes. The chains are doped with 2/5 hole per Cu site
and exhibit long-range commensurate charge order with an onset well above room
temperature. Below K, the neutron data indicate long-range collinear
magnetic order with a spin density modulation whose propagation vector is
commensurate along and incommensurate perpendicular to the chains. Competing
interchain exchange interactions are discussed as a possible origin of the
incommensurate magnetic order
Electron density distribution in paramagnetic chromium A gamma ray diffraction study
High accuracy single crystal structure factors, complete up to sin amp; 61553; amp; 61548; amp; 61472; amp; 61501; amp; 61472; amp; 61489; amp; 61486; amp; 61495; amp; 61496; amp; 61472; amp; 61485; amp; 61489; amp; 61484; amp; 61472; have been measured from paramagnetic chromium at 333 K using 316.5 keV gamma radiation. A detailed description of the electron density distribution is derived in terms of a multipolar atomic deformation model. There is pronounced charge asphericity in the valence region arising from preferential occupancy of the t2g subshell. The 3d charge distribution is contracted by 12.6 relative to the free atom, in accordance with magnetic synchrotron x ray and neutron measurements. By contrast, the atomic crystal scattering factor deduced from experiment is found to be in contradiction with earlier experimental and theoretical work. Achievement of a reliable Debye Waller factor is of vital importance in this context. There is no evidence for an anharmonic term in the atomic potential. Real space and energetic features of the charge density topology are used to characterize the directed metallic bonds. Special attention is paid to the form factor approximation in diffraction data analysis
Reassessment of the electron density in Cu2O using gamma ray diffraction
The electron density distribution in Cu2O has been critically reexamined to test controversial conclusions from earlier experimental and theoretical studies. The electron density is derived via multipole refinement of high quality single crystal diffraction data, collected at room temperature with 316.5 keV gamma radiation. Four gamma lines in the energy range 200 600 keV have been used to extrapolate extinction free low order structure factors. The remaining extinction corrections refine to a crystal mosaicity identical to the observed one. There is no support for anharmonic contributions to the thermal parameters. Important features of the derived electron density are i a partially filled dz2 orbital, ii an incomplete ionization of Cu and O, iii no interstitial Cu Cu charge pileup, thereby refuting the covalent bonding hypothesi
Magnetic structure of the edge-sharing copper oxide chain compound NaCu2O2
Single-crystal neutron diffraction has been used to determine the
incommensurate magnetic structure of NaCu2O2, a compound built up of chains of
edge-sharing CuO4 plaquettes. Magnetic structures compatible with the lattice
symmetry were identified by a group-theoretical analysis, and their magnetic
structure factors were compared to the experimentally observed Bragg
intensities. In conjunction with other experimental data, this analysis yields
an elliptical helix structure in which both the helicity and the polarization
plane alternate among copper-oxide chains. This magnetic ground state is
discussed in the context of the recently reported multiferroic properties of
other copper-oxide chain compounds
Magnetic properties of PdAs2O6: a dilute spin system with an unusually high N\'eel temperature
The crystal structure and magnetic ordering pattern of PdAs2O6 were
investigated by neutron powder diffraction. While the magnetic structure of
PdAs2O6 is identical to the one of its isostructural 3d-homologue NiAs2O6, its
N\'{e}el temperature (140 K) is much higher than the one of NiAs2O6 (30 K).
This is surprising in view of the long distance and indirect exchange path
between the magnetic Pd ions. Density functional calculations yield
insight into the electronic structure and the geometry of the exchange-bond
network of both PdAs2O6 and NiAs2O6, and provide a semi-quantitative
explanation of the large amplitude difference between their primary exchange
interaction parameters
Magnetic phase diagram of
Magnetometry, electrical transport, and neutron scattering measurements were
performed on single crystals of the Fe^{4+}-containing perovskite-related phase
Sr_3Fe_2O_7-x as a function of oxygen content. Although both the crystal
structure and electron configuration of this compound are closely similar to
those of well-studied ruthenates and manganates, it exhibits very different
physical properties. The fully-oxygenated compound (x=0) exhibits a
charge-disproportionation transition at T_D = 340 K, and an antiferromagnetic
transition at T_N = 115 K. For temperatures T \leq T_D, the material is a
small-gap insulator; the antiferromagnetic order is incommensurate, which
implies competing exchange interactions between the Fe^{4+} moments. The
fully-deoxygenated compound (x=1) is highly insulating, and its Fe^{3+} moments
exhibit commensurate antiferromagnetic order below T_N ~ 600 K. Compounds with
intermediate x exhibit different order with lower T_N, likely as a consequence
of frustrated exchange interactions between Fe^{3+} and Fe^{4+} sublattices. A
previous proposal that the magnetic transition temperature reaches zero is not
supported.Comment: 8 pages, 6 figure
Competing Jahn Teller distortions and ferrimagnetic ordering in the geometrically frustrated system Ni1 xCuxCr2O4
Competing Jahn Teller distortions combined with geometrical frustration give rise to a rich phase diagram as a function of x Cu and temperature in the spinel system Ni1 xCuxCr2O4. The Jahn Teller distortion of the end members acts in opposite ways, with an elongation of the NiO4 tetrahedra resulting in a structural transition at TS1 317K in NiCr2O4, but a flattening in the CuO4 tetrahedra at TS1 846K in CuCr2O4. In both cases the symmetry is lowered from cubic Fd 3m to tetragonal I41 amd on cooling. In order to follow the influence of Jahn Teller active Ni2 and Cu2 ions on the structural and magnetic properties of chromium spinels, we have investigated a series of samples of Ni1 xCuxCr2O4 by x ray and neutron powder diffraction. In the critical range 0.10 lt; x Cu lt; 0.20, strong orthorhombic distortions were observed, where competing Jahn Teller activities between the Cu2 and Ni2 ions result in distortions along both the a and c axes. For Ni0.85Cu0.15Cr2O4, the orthorhombic structure Fddd is stabilized up to TS2 368 2 K, close to the first structural phase transition at TS1 374 2 K. A ferrimagnetic spin alignment of the Ni Cu and chromium atoms sets in at much lower temperature TC 95K in this compound. The end members NiCr2O4 and CuCr2O4 undergo this ferrimagnetic transition at TC 74 and 135 K, respectively. These transitions are accompanied by the structural change to the orthorhombic symmetry which relieves the frustration. NiCr2O4 and Ni0.85Cu0.15Cr2O4 undergo a second magnetic transition at TM2 24 and 67K due to a superimposed antiferromagnetic ordering of the Cr moments resulting in a noncollinear magnetic structure. In the system Ni1 xCuxCr2O4, the magnetic transitions TC and TM2 merge with increasing copper content up to x Cu similar to 0.5. For the Ni rich chromites, geometrical frustration causes a strong reduction of the chromium moments, where magnetic long range order coexists with a disordered spin liquid like or a reentrant spin glass like state. This paper provides insight into the interplay between the Jahn Teller effect, geometrical frustration, and long range magnetic order in these complex system
Magnetic neutron scattering study of YVO3: Evidence for an orbital Peierls state
Neutron spectroscopy has revealed a highly unusual magnetic structure and
dynamics in YVO, an insulating pseudocubic perovskite that undergoes a
series of temperature induced phase transitions between states with different
spin and orbital ordering patterns. A good description of the neutron data is
obtained by a theoretical analysis of the spin and orbital correlations of a
realistic one-dimensional model. This leads to the tentative identification of
one of the phases of YVO with the ``orbital Peierls state'', a
theoretically proposed many-body state comprised of orbital singlet bonds.Comment: final version, to appear in PR
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