W. Sławiński

Spin reorientation and structural changes in NdFeO3

The crystal structure and the Fe3+ magnetic moment ordering in NdFeO3 have been studied by high-resolution neutron powder diffraction at temperatures ranging from 1.5 to 300 K. Between 100 and 200 K a spin reorientation transition is observed with gradual changes of the directions of the Fe3+ ordered magnetic moments. The spin reorientation temperature range is associated with changes of the crystal structure. The b lattice parameter has a broad local minimum in the spin reorientation region. There is also a coherent rotation of the FeO6 octahedra with an increase of the Fe–O–Fe angles with increasing temperature. These structural changes tend to increase the strength of the in-plane (a,b) Fe–Fe interactions and to decrease the strength of Fe–Fe interactions along the c-axis as the temperature increases. The Fe3+ magnetic moment ordering above 200 K is close to the antiferromagnetic Gx type. The total Fe3+ ordered magnetic moment at room temperature equals 3.87(5) μB. Below 100 K the Fe3+ magnetic moment ordering is a combination of the antiferromagnetic Gx and Gz type. The ordered Fe3+ magnetic moment components at 1.5 K are Mx = 1.30(15) μB and Mz = 3.97(5) µB. There is a C-type antiferromagnetic ordering of the Nd3+ magnetic moments at 1.5 K with the ordered Nd3+ moment value of 1.10(7) μB.

Modulation of atomic positions in CaCuxMn7−xO12 (x ≤ 0.1)

The modulation of atomic positions in CaCu(x)Mn(7-x)O12 (x = 0 and 0.1) was studied using synchrotron radiation powder diffraction below 250 and 220 K, respectively. The copper-rich member CaCu(x)Mn(7-x)O12 (x = 0.23) does not show any modulation of the atomic positions at temperatures as low as 10 K. Using low-temperature neutron powder diffraction the modulation of the magnetic moments of Mn ions in CaCu(x)Mn(7-x)O12 (x = 0, 0.1 and 0.23) has been investigated. Long-range modulated magnetic ordering in CaCu(x)Mn(7-x)O12 (x = 0, 0.1 and 0.23) is observed below 90.4, 89.2 and 78.1 K. (0,0,q(p)) and (0,0,q(m)) are the propagation vectors describing the modulations of the atomic positions and the magnetic moments. For CaCu(x)Mn(7-x)O12 (x = 0 and 0.1) the magnetic modulation and atomic modulation lengths are related by a factor of 2 satisfying the relation (1-q(p)) = 2(1-q(m)).

Structural and magnetic modulations in CaCuxMn7 − xO12

Low temperature atomic position modulations and magnetic moment modulations are reported for CaCu(x)Mn(7 - x)O(12) (x = 0.0, 0.1 and 0.23) using neutron powder diffraction. Both modulations are described with propagation vectors (0, 0, q) parallel to the c-axis in the hexagonal setting. The present neutron diffraction studies confirm the quantitative model describing the atomic position modulations in CaCu(x)Mn(7 - x)O(12) (x = 0.0 and 0.1) as derived from synchrotron based powder x-ray diffraction studies (Sławiński et al 2009 Acta Crystallogr. B 65 535). Neutron diffraction studies confirm the relation between the atomic position modulation length L(p) and the magnetic modulation length L(m) = 2L(p) between 50xa0K and the Néel temperature T(N). CaCu(x)Mn(7 - x)O(12) (x = 0.1 and 0.23) shows a magnetic phase transition near 50xa0K associated with considerable changes of the magnetic modulation length and the magnetic coherence length, similar to that observed in the parent CaMn(7)O(12).

Helical screw type magnetic structure of the multiferroic CaMn7O12 with low Cu-doping.

The modulated crystal structure and modulated magnetic ordering of the multiferroic CaCu(x)Mn(7-x)O(12) is studied by analysing neutron and synchrotron-radiation (SR) powder diffraction data with a model based on the magnetic superspace group R31(00γ)ts. Both atomic position modulations and magnetic modulations are described with the modulation vector (0, 0, q). The magnetic ordering is a screw-type circular helix where the magnetic moments are perpendicular to the c direction. The temperature dependence of the modulation vector length and the ordered magnetic moments of Mn(3+) and Mn(4+) ions is given between T = 50 K and the Néel temperature T(N) is approximately equal to 90 K. The atomic position modulation length L(p) and the magnetic modulation length L(m) fulfil the relation L(m) = 2L(p) at all temperatures between 50 K and T(N).

Beats in the Magnetic Modulation of Multiferroic CaMn7O12

The modulated magnetic ordering and the atomic position modulation are studied in the case of the multiferroic CaCu x Mn 7- x O 12 by neutron and SR powder diffraction methods. Below the magnetic transition temperature of T N2 =50 K both orderings are described by using the magnetic superspace group R31(00γ)ts. For the magnetic ordering two different incommensurate modulation vectors are used: (0,0, q 1 m ) and (0,0, q 2 m ). For the atomic position modulation only one modulation vector is needed (0,0, q p ). The magnetic ordering is of helical screw-type showing beats in the magnetic moment spatial distribution. The values of q 1 m and q 2 m change with temperature below T N2 =50 K but the average value fulfil the relation (1/2)( q 1 m + q 2 m ) = (1/2) q p = const. This relation underlines the importance of the magnetoelastic coupling in CaCu x Mn 7- x O 12 compounds below T N2 =50 K.

Dilemma on the crystal structure of CaCu3Ti4O12

The crystal structure of CaCu3Ti4O12 has been studied using high resolution synchrotron radiation based x-ray powder diffraction. The observed x-ray diffraction patterns show pronounced Bragg peak asymmetries which should not be present assuming the commonly accepted cubic crystal structure of CaCu3Ti4O12 described by the space group Im-3. Several structural models are discussed. The first model assumes a coexistence of two phases with the cubic symmetry (both space group Im-3) and different lattice constants. The next models are based on subgroups of the cubic space group Im-3. The best agreement with the experimental data is found for the two-phase cubic model.

Charge ordering in CaCuxMn7−xO12 (x = 0.0 and 0.1) compounds

The crystal structure of CaMn7O12 and CaCu0.1Mn6.9O12 has been studied by synchrotron radiation (SR) based powder x-ray diffraction and neutron powder diffraction in the temperature range from 10 K up to 290 K. The lattice parameter a exhibits a minimum at 45 K in CaMn7O12. The c lattice parameter in CaMn7O12 and CaCu0.1Mn6.9O12 has a maximum at the same temperature. Additional Bragg peaks have been found in the SR diffraction patterns in CaMn7O12 and CaCu0.1Mn6.9O12 below 250 K and 220 K, respectively. All diffraction peaks have been indexed as (h,k,l ± κ), where κ was equal to 0.079(15) for CaMn7O12 and 0.093(15) for CaCu0.1Mn6.9O12. The incommensurate low-temperature diffraction peaks are not observed in neutron diffraction patterns. This leads to the conclusion that the phase transition to the incommensurate structure is due to charge ordering rather than atomic position modulation. The charge ordering temperature coincides with dielectric constant changes of four orders of magnitude for CaMn7O12.

Particle and crystallite size effects on the modulated structure of multiferroic CaMn{sub 7}O{sub 12}

Abstract A systematic study of the atomic position modulation in polycrystalline CaMn 7 O 12 with different particle sizes (determined from scanning electron microscopy) and crystallite sizes (determined from X-ray diffraction) is reported. The morphology and phase composition are characterised by scanning electron microscopy, selected area electron diffraction and energy dispersive X-ray spectroscopy. The crystal structure and the atomic position modulation in polycrystalline CaMn 7 O 12 samples are explored with high-resolution synchrotron radiation based powder X-ray diffraction. The modulation vector and the modulation amplitudes observed at T =10xa0K do not change as a function of particle size, crystallite size and microstrain fluctuations. In addition, we report and discuss the formation of the additional CaMn 4 O 8 nanowire phase during high temperature annealing.