The parameters of the crystal structure of BiFeO(3), described within the space group R3c, have been determined by high-resolution neutron powder diffraction for temperatures from 293 to 923 K. It was found that there is a local minimum for the rhombohedral angle alpha(rh), near the Néel temperature T(N) approximately 640 K, a gradual rotation of the FeO(6) octahedra and an increase of the Fe-O-Fe angle. The displacement of the Bi(3+) ions from the FeO(6) octahedra which influence the electric polarization decreases with temperature. One of the Bi-Fe distances also has a local maximum near T(N). The atomic vibrations of Bi(3+) and O(2-) ions show a significant anisotropy.

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.

The crystal and magnetic structure of BiFeO 3 have been studied with the use of high resolution neutron diffraction between 5 K and 300 K. The atomic coordinates in BiFeO 3 are almost unchanged between 5 K and 300 K.

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)).

The temperature variation of the ͑100͒ and ͑010͒ neutron diffraction peak intensities, related only to the Nd magnetic moments, have been measured on a NdFeO 3 single crystal, at temperatures down to 70 mK. The ͑100͒ peak becomes noticeable below 25 K while the ͑010͒ peak only gives an appreciable contribution below 1 K. Above T N2 Ϸ1 K the ͑100͒ peak intensity is accounted for by the electronic magnetic moments polarized by the Nd-Fe exchange field. Near T N2 a change of slope is observed in the temperature dependence of the ͑100͒ reflection intensity, demonstrating the crossover from the above polarization of Nd under the Nd-Fe exchange to proper long-range ordering due to Nd-Nd interaction. Below ϳ0.4 K another mechanism, polarization of Nd nuclear moments by hyperfine field, contributes to the intensity of the ͑100͒ and ͑010͒ peaks. A simple mean-field model explains consistently the observed temperature dependence of the diffraction intensities as well as earlier specific-heat data. The main feature of this model is allowance for Van Vleck susceptibility, which appears to play an important role in the overall polarization of Nd. The values of the hyperfine field at the Nd nuclei H hf ϭ1.0Ϯ0.15 MOe and of the Nd electronic magnetic moment Nd ϭ0.9 B are deduced, the ratio H hf / Nd being the same as in other Nd compounds.

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