Oxalate-mediated long-range antiferromagnetism order in Fe2(C2O4)3·4H2O

Neutron powder diffraction and susceptibility measurements reveal the long range antiferromagnetic order mediated by oxalate molecules in Fe₂(C₂O₄)₃·4H₂O.

Structure refinement and superspace description of the system Bi(2(n + 2))Mo(n)O(6(n + 1)) (n = 3, 4, 5 and 6)

The system Bi(2(n + 2))Mo(n)O(6(n + 1)) is described within the superspace formalism. Two superspace models are proposed for the different members of this family, depending on the parity of the parameter n. The superspace model for the odd members is constructed through the embedding of the cationic distribution of the member with n = 3, and the modification of a superspace model previously proposed for the compound Bi(2)MoO(6). However, this model cannot be applied to the even members of the family. Performing the appropriate transformations, a suitable superspace model for the even members is obtained. The atomic structure of the different compounds of the family have been refined through the Rietveld method combining synchrotron X-ray and neutron powder diffraction data.

Magnetic structure of Gd5Si2Ge2 and Gd5Si2Ge1.9M0.1 (M = Ga, Cu)

Powder x-ray diffraction patterns of the doped compounds Gd(5)Si(2)Ge(1.9)M(0.1) (M = Ga, Cu) show the same crystal structure, orthorhombic Gd(5)Si(4)-type, in the ferromagnetic and paramagnetic phases. This is different from Gd(5)Si(2)Ge(2), whose paramagnetic phase is monoclinic. The magnetic structure at low temperature, solved from diffraction experiments with hot neutrons, is the same in all the three compounds, collinear ferromagnetic with moments along the crystal b-axis, or F(y)F(By) according to Bertaut's notation. These results, combined with those of heat capacity and magnetocaloric effect, indicate, similarly to Gd(5)Si(4), a second-order, purely magnetic, transition in the doped compounds explaining the absence of hysteresis.

Zener polaron ordering variants induced by A-site ordering in half-doped manganites

We have studied the magnetism of the half-doped charge ordered manganite YBaMn2O6. A formation of ferromagnetic plaquettes of four Mn atoms in the charge ordered phase below T_{CO} approximately 480 K is inferred from high temperature magnetic susceptibility data and the magnetic structure, as determined by neutron powder diffraction at T=1.5 K. The results indicate that new fourfold Mn paramagnetic units form between T_{N}<T<T_{CO}, and that they order in a noncollinear mode below T_{N}=190 K. The magnetism of YBaMn2O6 is consistent with what is expected from the Zener polarons ordering picture of the charge ordering phenomenon in manganites [A. Daoud-Aladine, Phys. Rev. Lett. 89, 097205 (2002)10.1103/PhysRevLett.89.097205].

Orientational ordering in the low-temperature stable phases of deuterated thiophene

The stable structures of deuterated thiophene C4D4S were investigated at 155 (phase III), 115 (phase IV), 100 and 1.5 K (phase V) by neutron powder diffraction. At 155 K, thiophene is orthorhombic with space group Pbnm. Although there is some degree of in-plane orientational disorder, molecules begin to order along two symmetrically equivalent main orientations. At 115 K the structure is incommensurate, with a wavevector q ≃ 0.55a*. At 100 K and below, there is a doubling of the a cell parameter and the structure space group is P21. For the first time, it is shown that, unlike C4H4S, phase V of C4D4S is not an orientational glass: thiophene molecules are perfectly ordered and are oriented within the molecular plane along two alternating directions, corresponding to the two main orientations observed at 155 K. This ordering probably originates in the slowing down of the in-plane reorientational dynamics upon deuteration.

Magnetic structure of the kagome mixed compound (Co(0.5)Ni(0.5))(3)V(2)O(8)

We report the magnetic structure of (Co(0.5)Ni(0.5))(3)V(2)O(8) (CNVO) deduced by single crystal neutron diffraction. This compound exhibits features which differ from that of its parent compounds, which are absolutely collinear along the a axis for Co(3)V(2)O(8) (CVO) or exhibit magnetic moments predominantly in the a-b plane with small components along c in the case of Ni(3)V(2)O(8) (NVO). The averaged magnetic moments of the statistically distributed Ni(2+) and Co(2+) ions in CNVO are oriented in the a-c plane and form loops of quasiferromagnetically coupled spins. These loops are connected along the a axis and separated along the c axis by cross-tie spins forming a quasiferromagnetic wave with the upper part of the respective neighbouring loops. The magnetic moments are sinusoidally modulated by the propagation vector k = (0.49,0,0) with an average amplitude of 1.59(1) μ(B) for a magnetic ion on a cross-tie site and 1.60(1) μ(B) for the spine site. In addition to neutron diffraction, specific heat and magnetization data, which confirm that the only magnetic phase transition above 1.8 K is the onset of antiferromagnetic order at T(N) = 7.4(1) K, are presented.

Investigation of the Electronic and Structural Properties of Potassium Hexaboride, KB6, by Transport, Magnetic Susceptibility, EPR, and NMR Measurements, Temperature-Dependent Crystal Structure Determination, and Electronic Band Structure Calculations

The electronic and structural properties of potassium hexaboride, KB(6), were examined by transport, magnetic susceptibility, EPR, and NMR measurements, temperature-dependent crystal structure determination, and electronic band structure calculations. The valence bands of KB(6) are partially empty, but the electrical resistivity of KB(6) reveals that it is not a normal metal. The magnetic susceptibility as well as EPR and NMR measurements show the presence of localized electrons in KB(6). The EPR spectra of KB(6) have two peaks, a broad ( approximately 320 G) and a narrow (less than approximately 27 G) line width, and the temperature-dependence of the magnetic susceptibility of KB(6) exhibits a strong hysteresis below 70 K. The temperature-dependent crystal structure determination of KB(6) shows the occurrence of an unusual variation in the unit cell parameter hence supporting that the hysteresis of the magnetic susceptibility is a bulk phenomenon. The line width DeltaH(pp) of the broad EPR signal is independent of temperature and EPR frequency. This finding indicates that the line broadening results from the dipole-dipole interaction, and the spins responsible for the broad EPR peak has the average distance of approximately 1.0 nm. To explain these apparently puzzling properties, we examined a probable mechanism of electron localization in KB(6) and its implications.

Zener polaron ordering in half-doped manganites

We have refined the crystal structures of a Pr(0.60)Ca(0.40)MnO(3) single crystal from neutron diffraction data. The result at low temperature gives a superstructure that cannot be interpreted as Mn(3+)/Mn(4+) charge ordering. The pattern of atom displacements suggests the trapping of electrons within pairs of Mn sites, involving both a local double exchange and a polaronic-like distortion. The two mechanisms act together to form vibronic localized electronic states: Zener polarons. We have confirmed this picture by showing how it elucidates the unconventional paramagnetic behavior of half-doped manganites.