A single-crystal neutron diffraction study of wardite, NaAl3(PO4)2(OH)4·2H2O

The crystal structure and crystal chemistry of wardite, ideally NaAl3(PO4)2(OH)4\ub72H2O, was investigated by single-crystal neutron diffraction (data collected at 20 K) and electron microprobe analysis in wavelength-dispersive mode. The empirical formula of the sample used in this study is: (Na0.91Ca0.01)\u3a3 = 0.92(Al2.97Fe3+0.05Ti0.01)\u3a3 = 3.03(P2.10O8)(OH)4\ub71.74H2O. The neutron diffraction data confirm that the crystal structure of wardite can be described with a tetragonal symmetry (space group P41212, a = b = 7.0577(5) and c = 19.0559(5) \uc5 at 20 K) and consists of sheets made of edge-sharing Na-polyhedra and Al-octahedra along with vertex-sharing Al-octahedra, parallel to (001), connected by P-tetrahedra and H bonds to form a (001) layer-type structure, which well explains the pronounced {001} cleavage of the wardite crystals. The present data show that four crystallographically independent H sites occur in the structure of wardite, two belonging to a H2O molecule (i.e., H1\u2013O6\u2013H2) and two forming hydroxyl groups (i.e., O5\u2013H3 and O7\u2013H4). The location of the hydrogen atoms allows us to define the extensive network of H bonds: the H atoms belonging to the H2O molecule form strong H bonds, whereas both the H atoms belonging to the two independent hydroxyl groups form weak interactions with bifurcated bonding schemes. As shown by the root-mean-square components of the displacement ellipsoids, oxygen and hydrogen atoms have slightly larger anisotropic displacement parameters compared to the other sites (populated by P, Al and Na). The maximum ratio of the max and min root-mean-square components of the displacement ellipsoids is observed for the protons of the hydroxyl groups, which experience bifurcated H-bonding schemes. A comparative analysis of the crystal structure of wardite and fluorowardite is also provided

Switching of the Chiral Magnetic Domains in the Hybrid Molecular/Inorganic Multiferroic (ND4)2[FeCl5(D2O)]

(ND4)2[FeCl5(D2O)] represents a promising example of the hybrid molecular/inorganic approach to create materials with strong magneto-electric coupling. Neutron spherical polarimetry, which is directly sensitive to the absolute magnetic configuration and domain population, has been used in this work to unambiguously prove the multiferroicity of this material. We demonstrate that the application of an electric field upon cooling results in the stabilization of a single-cycloidal magnetic domain below 6.9 K, while poling in the opposite electric field direction produces the full population of the domain with opposite magnetic chirality. We prove the complete switchability of the magnetic domains at low temperature by the applied electric field, which constitutes a direct proof of the strong magnetoelectric coupling. Additionally, we refine the magnetic structure of the ordered ground state, deducing the underlying magnetic space group consistent with the direction of the ferroelectric polarization, and we provide evidence of a collinear amplitude-modulated state with magnetic moments along the a-axis in the temperature region between 6.9 and 7.2 K

Natural ferroelectric order near ambient temperature in HoFeO3: A member of RFeO3 orthoferrites

Current scenario in multiferroics demands a breakthrough discovery of promising materials after BiFeO3. Recently, the controversial discovery of room temperature ferroelectricity (FE) in SmFeO3 [PRL 107, 117201 (2011); 113, 217203 (2014)] inspires the investigation of HoFeO3. Here, we report a natural ferroelectric order below 210 K (TFE) along c-axis with reasonably large polarization and low-field strong magnetoelectric coupling. Synchrotron and neutron diffraction results confirm that a shift of O atoms along c-axis of polar Pbn21 structure causes FE in HoFeO3. The exchange striction mechanism is suggested to elucidate the ferroelectric order. The results create a renewed attention for searching promising candidates with a natural ferroelectric order and higher TFE in the rest of the RFeO3 series

Orthorhombic distortion and orbital order in the vanadium spinel FeV2 O4

Using synchrotron and neutron diffraction measurements, we find a low-temperature orthorhombic phase in vanadium spinel FeV2O4. The orbital order of V3+ ions with tetragonal normal modes occurs at 68 K, and this leads to an appearance of the pseudotetragonal phase at a noncollinear ferrimagnetic transition temperature. Below the magnetic transition temperature, unconventional behavior of the orbital state of Fe2+ ions accompanied by the emergence of the orthorhombic phase was observed by using the normal mode analysis. We have also studied the structural properties of orbitally diluted materials. The orthorhombic phase, which is significantly affected by the other ions, is intrinsic in FeV2O4. We suggest the orthorhombic phase is strongly related with the double orbital states of Fe2+ and V3+ ions

Strain control of a bandwidth-driven spin reorientation in Ca₃Ru₂O₇

The layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca3Ru2O7, which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a 90∘ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca3Ru2O7, using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO6 octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them