R. Przeniosło

Atomic displacements in BiFeO3 as a function of temperature: neutron diffraction study

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.

Neutron diffraction studies of the crystal and magnetic structures of BiFeO3 and Bi0.93La0.07FeO3

Abstract Polycrystalline materials obtained from the around single crystals of BiFe03 and Bi0.93La0.07Fe03, reported as not having superstructure, show superstructure reflections in X-rays and neutron diffraction patterns. The determined magnetic moment of antiferromagnetically ordered Fe3+ ions is μFe = (3.70 ± 0.03)μBand μFe = (3.79 ± 0.03)μB for BiFe03 and Bi0.93La0.07Fe03, respectively.

Does the modulated magnetic structure of BiFeO3 change at low temperatures

Figure 2 and its caption were incorrect, along with a line of the text. Full details can be found in the PDF; these corrections do not change the conclusions of the paper.

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.

Neutron diffraction study of the magnetic structure of α-Mn2O3

The magnetic ordering of α-Mn2O3 has been studied by neutron powder diffraction measurements. An antiferromagnetic ordering occurs with the magnetic unit cell equivalent to the crystalline unit cell. Our results are compared with the results of previous works. The main antiferromagnetic Bragg peaks have different temperature dependence of their intensities, suggesting that the magnetic ordering in α-Mn2O3 cannot be described by a single order parameter. The temperature dependence of magnetization near 0 K for these peaks differs from Bloch’s T3/2 law. The majority of the magnetic peaks disappear at 80 K, which was reported earlier as the Neel temperature. The (100) peak, forbidden in the crystal structure of α-Mn2O3 is present up to 100 K. A new collinear model of the magnetic ordering of α-Mn2O3 at 10 K is presented.

Crystal and Magnetic Structure in Co-Substituted BiFeO3

Ultra-high-resolution neutron diffraction studies of BiFe(0.8)Co(0.2)O3 show a transition from a cycloidal space modulated spin structure at T = 10 K to a collinear G-type antiferromagnetic structure at T = 120 K. The model of antiparallel directions of Fe(3+) and Co(3+) magnetic moments at the shared Wyckoff position describes well the observed neutron diffraction intensities. On heating above RT, the crystal structure of BiFe(0.8)Co(0.2)O3 changes from a rhombohedral R3c to a monoclinic Cm. At 573 K only the Cm phase is present. The collinear C-type antiferromagnetic structure is present in the Cm phase of BiFe(0.8)Co(0.2)O3 at RT after annealing.

Phase coexistence in the charge ordering transition in CaMn7O12

The structural phase transition in CaMn7O12 has been investigated by using high-resolution synchrotron and neutron powder diffraction. Both measurements show a phase coexistence phenomenon: between 409 and 448 K two different crystallographic phases coexist in the material. The first one is trigonal and it has a charge ordering (CO) of the Mn3+ and Mn4+ ions, while the second one is cubic and charge delocalized (CD). The volume fraction of the CD phase increases with temperature from zero at 400 K up to 100% about 460 K. Both phases have domains of at least 150 nm at each temperature in the PS region. A percolation scenario assuming a growth of the volume of the highly conducting CD regions at the expense of the volume of the insulating CO matrix is discussed and it is found to be in agreement with literature data of the CaMn7O12 resistivity.

Possible deuterium positions in the high-temperature deuterated proton conductor Ba3Ca1+yNb2−yO9−δ studied by neutron and X-ray powder diffraction

Abstract High-temperature proton conductors with perovskite structures are a class of well-known systems with high protonic conductivity, which is of high technological interest in view of the possible applications in solid oxide fuel cells. We present neutron and X-ray diffraction studies of the crystal structure of the Ba 3 Ca 1.18 Nb 1.82 O 9− δ + z D 2 O (BCN18) system that exhibits high proton conductivity. It is assumed that the mechanism of proton conductivity in BCN18 is connected with trapping of protons at some interstitial positions in the crystal lattice as deduced from earlier quasielastic neutron scattering and muon spin rotation studies published in the literature. In order to get more information on the location of deuterium, systematic high-resolution neutron diffraction studies of the stoichiometric Ba 3 Ca 1 Nb 2 O 9 and non-stoichiometric BCN18 compounds with and without D 2 O were performed. The ROTAX time-of-flight (TOF) diffractometer at the ISIS neutron spallation source and the SV7 double-axis diffractometer at the DIDO reactor at Julich were used. Refinements of the crystal structures were performed by using the FULLPROF and GSAS programs. The possible deuterium positions in the crystallographic unit cell are discussed.

Modulation in Multiferroic BiFeO3: Cycloidal, Elliptical or SDW?

We present several modulated magnetic ordering models which all describe the high resolution neutron powder diffraction patterns of BiFeO 3 with the same accuracy as the circular cycloid one proposed in [J. Phys. C 15 (1982) 4835]. These orderings are: the elliptical cycloid and the spin density wave (SDW). The ambiguity of the magnetic ordering in BiFeO 3 is important in the context of recent models of the magnetoelectric coupling in perovskites [Phys. Rev. Lett. 95 (2005) 057205 and 96 (2006) 067601].

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)).