Inserting both polar A and magnetic B ions in a same crystalline phase, such as A = Bi3+, B = Fe3+ or Mn3+ in simple perovskites ABO3, has been successful in achieving multiferroic properties with large ferroelectric and magnetic orders. However, modest magnetoelectric couplings have been hitherto reported, thus preventing any application for future electronics. By means of neutron diffraction, we found a large uniform C-type modulation of an E-type antiferromagnetic structure of the Mn3+ ions in the quadruple perovskite BiMn3Mn4O12. A symmetry analysis indicates that this modulation is induced by the internal strain created by the polar Bi3+ ion, which gives evidence of a large magnetoelectric coupling driven by inverse magnetostriction. This modulation is indeed absent in the isomorphic and isovalent compound LaMn3Mn4O12 containing the nonpolar La3+ ion. Our analysis indicates that this coupling mechanism is effective owing to the symmetry-limited structural distortions and inhomogeneities characteristic of the quadruple perovskite structure, thus preventing the release of the strain. We conclude that internal strain is a key control parameter to achieve large magnetoelectric couplings in proper ferroelectrics.

Neutron powder diffraction has been performed on the MTbF6 fluorides (M=Cd, Ca, Sr, (α/β)-Ba). Four of these fluorides (Cd, Ca, Sr, β-Ba) are built of a (pseudo-) tetragonal packing of [TbF6]2− chains and only differs by the chains relative orientations. Thus this series represents a valuable opportunity to evaluate the Tb4+-Tb4+ magnetic interactions. All the compounds displayed antiferromagnetic order (TN=2.70K (Cd), 2.15K (Ca), 2.60K (Sr), 2.10K (β-Ba)), except for the α form of BaTbF6. The crystal structure of this latter fluoroterbate has also been investigated by means of high-resolution neutron powder diffraction. From Neutron Powder Diffraction data, CdTbF6 and β-BaTbF6 magnetic structures were determined, together with the metamagnetic behaviour of β-BaTbF6 as a function of an external magnetic field. A tentative phase diagram is then given for β-BaTbF6. Advantage was taken of the polymorphism of the BaTbF6 fluoroterbate to analyse, on the basis of topological parameters such as bond distances and angles, the magnetic behaviour of its α and β forms. It was shown that superexchange interactions are present in β-BaTbF6, and that these interactions may also rule the magnetic behaviour of the other MTbF6 (M=Ca, Sr, Cd) tetravalent terbium fluorides.

The crystallographic approach to magnetic structures is largely based on two kinds of descriptions: symmetry invariance of magnetic configurations (Magnetic Space Groups, often called Shubnikov groups) and group representation theory applied to conventional crystallographic space groups. The first approach is nearly exclusively used for the case of commensurate magnetic structures and usually is limited to a description of the invariance symmetry properties for this kind of configurations [1–5]. The representation analysis is more general and can be applied to all kinds of magnetic structures. The literature on this field is broad and one can consult the papers of E.F. Bertaut [6–9] and Y. Izyumov and co-workers [10–15] to get a deeper insight into the problem. A possible extension of the Shubnikov approach may be developed in the sense of Magnetic Superspace Groups as in crystallographic incommensurate structures. This more general approach extends the invariance concept to incommensurate magnetic structures and may be a better approach when macroscopic properties have to be deduced from the spin configurations. At present not too much work has been done in that sense so it will not be considered in the present notes.

By means of neutron powder diffraction, we investigated the effect of the polar Bi^3+ ion on the magnetic ordering of the Mn^3+ ions in BiMn_3Mn_4O_12, the counterpart with quadruple perovskite structure of the simple perovskite BiMnO_3. The data are consistent with a noncentrosymmetric spacegroup Im which contrasts the centrosymmetric one I2/m previously reported for the isovalent and isomorphic compound LaMn_3Mn_4O_12, which gives evidence of a Bi^3+-induced polarization of the lattice. At low temperature, the two Mn^3+ sublattices of the A' and B sites order antiferromagnetically (AFM) in an independent manner at 25 and 55 K, similarly to the case of LaMn_3Mn_4O_12. However, both magnetic structures of BiMn_3Mn_4O_12 radically differ from those of LaMn_3Mn_4O_12. In BiMn_3Mn_4O_12 the moments M_A' of the A' sites form an anti-body AFM structure, whilst the moments M_B of the B sites result from a large and uniform modulation ±M_B,b along the b-axis of the moments M_B,ac in the ac-plane. The modulation is strikingly correlated with the displacements of the Mn^3+ ions induced by the Bi^3+ ions. Our analysis unveils a strong magnetoelastic coupling between the internal strain created by the Bi^3+ ions and the moment of the Mn^3+ ions in the B sites. This is ascribed to the high symmetry of the oxygen sites and to the absence of oxygen defects, two characteristics of quadruple perovskites not found in simple ones, which prevent the release of the Bi^3+-induced strain through distortions or disorder. This demonstrates the possibility of a large magnetoelectric coupling in proper ferroelectrics and suggests a novel concept of internal strain engineering for multiferroics design.

The ternary silicides RE6Ti1.67Si3 (RE = Ce, Pr, Nd, Gd, Tb and T = Co, Ni) crystallizing with the hexagonal Ce6Ni1.67Si3-type structure has been recently extensively studied. It has been shown that some of these compounds exhibit interesting magnetocaloric properties. In this work, the magnetic structures of the Nd6Ni1.67Si3 compound were determined by means of neutron powder diffraction. According to previous magnetization and specific heat measurements, this compound exhibits two successive magnetic transitions at 84K and 38K. The first transition corresponds to a ferromagnetic arrangement of the Nd-moments along the c-axis and the second one to a non-collinear ferromagnetic arrangement yielding a conical structure. The magnetic structures of Nd6Ni1.67Si3 are compared to those of the Nd6Co1.67Si3homologous ternary silicide.

The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and x-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T, showing a minor but significant dependence of the dielectric constant on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the roomtemperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the CurieWeiss constant.

The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and x-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T, showing a minor but significant dependence of the dielectric constant on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the Curie-Weiss constant.

The magnetic structures of the two bismuth oxy-phosphate compounds BiMPO5 (M2+ = Ni2+, Co2+) have been determined by neutron powder diffraction using group theory analysis as a preliminary tool. Both compounds adopt a monoclinic crystal structure (S.G. P 21/n, a = 7.1642(2) Å, b = 11.2038(3) Å, c = 5.1740(2) Å and β = 107.296(2)° for Ni2+ and a = 7.2441(1) Å, b = 11.2828(1) Å, c = 5.2258(1) Å and β = 107.841(1)° for Co2+). The refinement of the magnetic structures below TN = 17.5 and 15 K, respectively, for both compounds show that the magnetic structure is characterized by the propagation vector k = (−1/2, 0, 1/2), with components given with respect to the reciprocal lattice of the nuclear structure. This means a magnetic unit cell that is a multiple of the nuclear cell. The magnetic structure is constituted of ferromagnetic pairs of metal ions antiferromagnetically coupled within double chains. The relative strength of the intra and inter double chains exchange interactions has been examined by establishing a theoretical magnetic phase diagram. Most of the interactions come from M–O–O–M super–super-exchange paths. At its ground state, BiNiPO5 shows a nearly collinear arrangement of magnetic moments with m1.5 K = 2.13(3) μB/Ni. Due to the strong magnetic anisotropy of Co2+ (m1.5 K = 3.52(3) μB/Co), the collinear character is largely lost while the magnetic structure remains analysable on the basis of the greatest isotropic component of the local moments.

Magnetic structure determination (description of the microscopic arrangement of magnetic moments in a crystal) requires neutron diffraction studies. Magnetic neutron powder diffraction is, and will remain in the future, the most straightforward technique to determine magnetic structures as a function of temperature, pressure ...W e will show here the successive steps of a magnetic structure determination, from the experiment (2-axis diffractometer) to the results: commensurate and/or incommensurate long- range magnetic order, via data analysis. A special attention is devoted to symmetry analysis. Examples are selected in the R2T2X system (with R = Lanthanide or Uranium; T = Transition Metal, X = In or Sn).

DyMn2D6 has been prepared by applying high gaseous deuterium pressure on DyMn2. This phase is isostructural with other RMn2D6 (R = Y, Er) compounds and crystallizes with a K2PtCl6 type structure having an ordered anion and a partially disordered cation arrangement because Dy and half the Mn atoms are randomly substituted in the same 8c site. The reverse susceptibility follows a Curie–Weiss law with an effective moment of 10 μB similar to that of DyMn2. Short range magnetic order, corresponding to ferromagnetic correlations, is observed in the neutron patterns up to 10 K and can be attributed to Dy–Dy interactions. The decomposition of the deuteride into Mn and DyD2, studied by thermal gravimetric analysis, occurs between 470 and 650 K. A further deuterium desorption takes place above 920 K.

New titanyl phosphate Ti2O(H2O)(PO4)2 has been prepared and characterized by X-ray and neutron diffraction, nuclear magnetic resonance, infrared and Raman spectroscopies and thermogravimetric analysis. The crystal structure has been solved from neutron powder diffraction data at 300K by Rietveld method in P21 space group. The refinement led to satisfactory profile factors (Rp=2.7%, Rwp=3.2%) and crystal structure model indicators (RB=5.8%, RF=3.2%). The cell is monoclinic with a=7.3735Å, b=7.0405Å, c=7.6609Å and β=121.48°, Z=4. The structure can be described as a three-dimensional framework built up by chains of [TiO5(OH2)] octahedra with alternative short bonds [Ti(1)–O(12); Ti(2)–O(12), 1.88–1.84Å] and long ones [Ti(1)–OW; Ti(2)–OW, 2.25–2.23Å] along c-axis and connected via [PO4] tetrahedra. Oxygen atom denoted O(12) is only linked to two titanium atoms and Oxygen atom denoted OW is linked to two titanium atoms and two hydrogen atoms. O(12) and OW are not linked to P atoms and justify the titanyl phosphate formulation Ti2O(H2O)(PO4)2. The infrared and Raman spectra presents peaks due to vibrations of Ti–O, P–O and O–H bonds. The 31P MAS NMR spectrum reveals two 31P resonance lines, in agreement with the structure which showed two crystallographic sites for phosphorus. The thermogravimetric analysis show that Ti2O(H2O)(PO4)2 is thermally stable until 400°C. Above this temperature, it losses water and decomposes to Ti5O4(PO4)4 and TiP2O7.

Effects of Pr substitution on the magnetic properties of perovskite manganite Nd0.5Sr0.5MnO3 have been investigated by measurements of electron spin resonance, magnetization, and powder neutron dif...

Two new titanates of strontium, lithium and a 3d metal, with the composition SrLiMTi4O11 (M=Cr, Fe), have been discovered. Single crystals were obtained by spontaneous crystallization from a high-temperature solution with LiBO2 as the solvent. The structure of SrLiCrTi4O11 was refined in the orthorhombic space group Pnma (Z=4), while SrLiFeTi4O11 appeared to adopt a four-times larger orthorhombic unit cell with Pbcn (Z=16). The structures can be described by a close-packed arrangement of Sr and O atoms. The unit cell contains six ;compact planes' perpendicular to [100] in the layer sequence ABACBC [(chc)2]. Ti and Cr or Fe atoms occupy some of the interstitial octahedral sites created, whereas Li atoms are in tetrahedral sites. Depending on the synthesis conditions of SrLiCrTi4O11, the distribution of Cr and Ti atoms on the four possible crystallographic sites is not the same. Having a similar compact-planes sequence, SrLiCrTi4O11 and SrLiFeTi4O11 structures differ in the arrangement of Sr and O atoms per layer of close packing, which also induces a correlated variation in the Li-tetrahedra distribution.

The new oxyarsenate Li0.5Ni0.25TiOAsO4 has been synthesized and studied by a combination of X-ray powder diffraction, neutrons powder diffraction and vibrational spectroscopy. Li0.5Ni0.25TiOAsO4 crystallizes in the monoclinic P21/c space group with the unit cell parameters: a=6.5854(3)Å, b=7.4665(4)Å, c=7.4969(4)Å, β=89.884(6)°, V=368.62(1)Å3 and Z=4. The structure has been determined at room temperature from neutrons diffraction by the Rietveld method analysis. It is formed by a 3D network of TiO6 octahedra and AsO4 tetrahedra sharing corners. Structural refinement shows a partial and a statistical occupancy of 2a and 2b sites by Li+ and Ni2+ ions. TiO6 octahedra are linked together by corners and form infinite chains along c-axis. Raman and infrared studies confirm the existence of -TiOTi- chains. Diffuse reflectance spectrum indicates the presence of octahedrally coordinated Ni2+ ions.

The structural and magnetic properties of the complex metal oxides Pb(Mn1/2Nb1/2)O3 (PMNO) and Pb(Mn1/4Fe1/4Nb1/2)O3 (PMFNO), which belong to a class of disordered perovskites have been studied. The magnetic susceptibilities of PMNO showed hysteresis between field cooled and zero-field cooled conditions below the transition of 15K, suggesting that the material has a spin-glass feature. Neutron diffraction patterns of PMNO showed no evidence of a long-range magnetic ordering at 1.5K, which is consistent with spin-glass behavior. Rietveld refinements of neutron powder diffraction data collected at different temperatures between 1.5 and 700K have been carried out in order to extract structural information. The crystal structure of this compound is cubic (space group Pm3¯m) within the whole temperature interval. The Mn and Nb ions were found to be disordered over the perovskite B-sites. The main feature of this structure is the positional disorder at the Pb site, the importance of which in connection with the ferroic transitions is briefly discussed. The Pb cations show a positional disorder shifting from their high-symmetry positions along the [111] direction. The effect of Fe-doping on PMNO has been studied. The substitution of Fe at the Mn site in PMFNO results in a small changes of the magnetic properties without significant differences in the crystal structures. The factors governing the observed structural and magnetic properties of PMNO and PMFNO are discussed and compared with those of other quaternary Mn-containing perovskites. For the PbB3+1/2Nb1/2O3 series with the isomorphous substitution B3+, graphs of average lattice parameters of the perovskite phase and the temperatures of ferroelectric and magnetic phase transitions as functions of the B3+ cation radius were constructed and are discussed. Influence of A-cation sublattice on magnetic properties is also considered.

A novel hydride YMn2H6 has been synthesized from C15 Laves YMn2 under hydrogen pressure. According to neutron powder diffraction experiments, YMn2H6 crystallizes in the Fm-3m space group with a = 6.708 angstrom at 300 K. Thus, the initial C15 structure is strongly rearranged to form a structure in which the Y atoms and half of the Mn atoms statistically occupy the 8c site, whereas the other Mn atoms located in the 4a site are surrounded by 6 H atoms (24e). According to our knowledge, this type of behaviour has never been observed in hydrides derived from Laves phase intermetallics. A corresponding investigation of the synthesis of ErMn2H6 under high hydrogen pressure has been conducted.

A detailed analysis of the crystal structure in RETiO3 with RE = La, Nd, Sm, Gd, and Y reveals an intrinsic coupling between orbital degrees of freedom and the lattice which cannot be fully attributed to the structural deformation arising from bond-length mismatch. The TiO6 octahedra in this series are all irregular with the shape of the distortion depending on the RE ionic radius. These octahedron distortions vary more strongly with temperature than the tilt and rotation angles. Around the Ti magnetic ordering all compounds exhibit strong anomalies in the thermal-expansion coefficients, these anomalies exhibit opposite signs for the antiferromagnetic and ferromagnetic compounds. Furthermore the strongest effects are observed in the materials close to the magnetic cross-over from antiferromagnetic to ferromagnetic order.

The nuclear structure and magnetic properties of perovskite-type samples of compositions LaCr1−yMnyO3 (y=0, 0.1, 0.2, 0.3) have been studied using neutron powder diffraction technique and magnetization measurements. All samples have orthorhombic structure with space group Pnma (No. 62) in the temperature range 1.5–400K. The pure lanthanum chromate LaCrO3 is a G-type antiferromagnet with a Néel temperature of 290K. The Mn substituted samples also order antiferromagnetically at about 290K, with canted magnetic moments yielding a ferromagnetic component.