Pressure-Driven Metallization in Hafnium Diselenide.

The quest for new transition metal dichalcogenides (TMDs) with outstanding electronic properties operating under ambient conditions draws us to investigate the 1T-HfSe2 polytype under hydrostatic pressure. Diamond anvil cell (DAC) devices coupled to in situ synchrotron X-ray, Raman, and optical (VIS-NIR) absorption experiments along with density functional theory (DFT)-based calculations prove that (i) bulk 1T-HfSe2 exhibits strong structural and vibrational anisotropies, being the interlayer direction especially sensitive to pressure changes, (ii) the indirect gap of 1T-HfSe2 tends to vanish by a -0.1 eV/GPa pressure rate, slightly faster than MoS2 or WS2, (iii) the onset of the metallic behavior appears at Pmet ∼10 GPa, which is to date the lowest pressure among common TMDs, and finally, (iv) the electronic transition is explained by the bulk modulus B0-Pmet correlation, along with the pressure coefficient of the band gap, in terms of the electronic overlap between chalcogenide p-type and metal d-type orbitals. Overall, our findings identify 1T-HfSe2 as a new efficient TMD material with potential multipurpose technological applications.

Characterization and Decomposition of the Natural van der Waals SnSb2Te4 under Compression.

High pressure X-ray diffraction, Raman scattering, and electrical measurements, together with theoretical calculations, which include the analysis of the topological electron density and electronic localization function, evidence the presence of an isostructural phase transition around 2 GPa, a Fermi resonance around 3.5 GPa, and a pressure-induced decomposition of SnSb2Te4 into the high-pressure phases of its parent binary compounds (α-Sb2Te3 and SnTe) above 7 GPa. The internal polyhedral compressibility, the behavior of the Raman-active modes, the electrical behavior, and the nature of its different bonds under compression have been discussed and compared with their parent binary compounds and with related ternary materials. In this context, the Raman spectrum of SnSb2Te4 exhibits vibrational modes that are associated but forbidden in rocksalt-type SnTe; thus showing a novel way to experimentally observe the forbidden vibrational modes of some compounds. Here, some of the bonds are identified with metavalent bonding, which were already observed in their parent binary compounds. The behavior of SnSb2Te4 is framed within the extended orbital radii map of BA2Te4 compounds, so our results pave the way to understand the pressure behavior and stability ranges of other "natural van der Waals" compounds with similar stoichiometry.

Large Magnetoelectric Coupling Near Room Temperature in Synthetic Melanostibite Mn2 FeSbO6.

Multiferroic materials exhibit two or more ferroic orders and have potential applications as multifunctional materials in the electronics industry. A coupling of ferroelectricity and ferromagnetism is hereby particularly promising. We show that the synthetic melanostibite mineral Mn2 FeSbO6 (R3‾ space group) with ilmenite-type structure exhibits cation off-centering that results in alternating modulated displacements, thus allowing antiferroelectricity to occur. Massive magnetoelectric coupling (MEC) and magnetocapacitance effect of up to 4000 % was detected at a record high temperature of 260 K. The multiferroic behavior is based on the imbalance of cationic displacements caused by a magnetostrictive mechanism, which sets up an unprecedented example to pave the way for the development of highly effective MEC devices operational at or near room temperature.

Low-Temperature Cationic Rearrangement in a Bulk Metal Oxide.

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2 FeMoO6 at unparalleled low temperatures of 150-300 (o) C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3 TeO6 -type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

Magnetic Interactions in the Double Perovskites R2NiMnO6 (R = Tb, Ho, Er, Tm) Investigated by Neutron Diffraction.

R2NiMnO6 (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic P21/n space group, with an almost complete order between Ni(2+) and Mn(4+) cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for La2NiMnO6, with a ferromagnetic structure involving Mn(4+) and Ni(2+) moments. This spin alignment can be rationalized taking into account the Goodenough-Kanamori rules. The magnetic ordering temperature (TCM) decreases abruptly as the size of the rare earth decreases, since TCM is mainly influenced by the superexchange interaction between Ni(2+) and Mn(4+) (Ni(2+)-O-Mn(4+) angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below TCM.

Giant magnetoresistance in the half-metallic double-perovskite ferrimagnet Mn2FeReO6.

The first transition-metal-only double perovskite compound, Mn(2+) 2 Fe(3+) Re(5+) O6 , with 17 unpaired d electrons displays ferrimagnetic ordering up to 520 K and a giant positive magnetoresistance of up to 220 % at 5 K and 8 T. These properties result from the ferrimagnetically coupled Fe and Re sublattice and are affected by a two-to-one magnetic-structure transition of the Mn sublattice when a magnetic field is applied. Theoretical calculations indicate that the half-metallic state can be mainly attributed to the spin polarization of the Fe and Re sites.

Magnetic-structure-stabilized polarization in an above-room-temperature ferrimagnet.

Above-room-temperature polar magnets are of interest due to their practical applications in spintronics. Here we present a strategy to design high-temperature polar magnetic oxides in the corundum-derived A2BB'O6 family, exemplified by the non-centrosymmetric (R3) Ni3TeO6-type Mn(2+)2Fe(3+)Mo(5+)O6, which shows strong ferrimagnetic ordering with TC = 337 K and demonstrates structural polarization without any ions with (n-1)d(10)ns(0), d(0), or stereoactive lone-pair electrons. Density functional theory calculations confirm the experimental results and suggest that the energy of the magnetically ordered structure, based on the Ni3TeO6 prototype, is significantly lower than that of any related structure, and accounts for the spontaneous polarization (68 µC cm(-2)) and non-centrosymmetry confirmed directly by second harmonic generation. These results motivate new directions in the search for practical magnetoelectric/multiferroic materials.

Enhancement of the Curie temperature along the perovskite series RCu3Mn4O12 driven by chemical pressure of R3+ cations (R = rare earths).

The compounds of the title series have been prepared from citrate precursors under moderate pressure conditions (P = 2 GPa) and 1000 degrees C in the presence of KClO(4) as oxidizing agent. The crystal structures are cubic, space group Im3 (No. 204); the unit cell parameters linearly vary from a = 7.3272(4) A (R = La) to a = 7.2409(1) A (R = Lu) at room temperature. A neutron or synchrotron X-ray diffraction study of all the members of the series reveals an interesting correlation between some structural parameters and the magnetic properties. The electron injection effect upon replacement of Ca(2+) with R(3+) cations in the parent CaCu(3)Mn(4)O(12) oxide leads to a substantial increment of the ferrimagnetic Curie temperature (T(C)). An essential ingredient is supplied by the internal pressure of the R(3+) cations upon a decrease in size along the rare-earth series, from La to Lu: the concomitant compression of the MnO(6) octahedral units for the small rare earths provides progressively shorter Mn-O distances and improves the overlapping between Mn and O orbitals, thereby promoting superexchange and enhancing T(C) by 50 K along the series. This interaction is also reinforced by a ferromagnetic component that depends on the local distortion of the MnO(6) octahedra, which also increases along the series, constituting an additional factor, via intersite virtual charge transfer t-e orbital hybridization, for the observed increment of T(C).

Cobalt(II) citrate cubane single-molecule magnet.

An investigation of the magnetic properties of the cobalt(II) citrate cubane [C(NH 2) 3] 8{Co 4(cit) 4}.4H 2O reveals that the cluster is a new cobalt(II) single-molecule magnet, with an energy barrier to reorientation of the magnetization, Delta E/ k B = 21 K, and tau 0 = 8 x 10 (-7) s. The compound displays distinct, frequency-dependent peaks in the out-of-phase (chi'') component of the ac magnetic susceptibility and magnetization versus field hysteresis loops that are temperature and sweep rate dependent. The hysteresis loops collapse at zero field due to very fast quantum tunneling of the magnetization (QTM).

Effects of high pressure on the luminescence spectra of Eu(SO4)2.NH4 microcrystals: anisotropically induced structural distortions.

The possibilities of the use of Eu3+ in extracting information of the pressure effects on the nature of its crystal site in the NH4.Eu(SO4)2 catalytic host are closely inspected through the study of emission spectra for applied pressures up to 87 kbar. The phenomenological crystal field analysis of these spectra reveals clear discontinuities, at approximately 30 kbar, the sharper ones, and then at approximately 70 kbar, in crystal field strength trends, which taken together with structure-based simulations of crystal field interactions indicate well-defined pressure-induced anisotropic distortions in Eu3+ local environments.

A mixed-valence Co7 single-molecule magnet with C3 symmetry.

The synthesis, structure and magnetic properties of [Co(II)(4)Co(III)(3)(HL)(6)(NO(3))(3)(H(2)O)(3)](2+) [H(3)L = H(2)NC(CH(2)OH)(3)] are reported: the complex is an exchange-biased single molecule magnet.

Correlation between the crystal structure and the Curie temperature in RCu3(Mn3Fe)O12 (R = rare-earth) complex perovskites.

New members of the family of complex-perovskite oxides with the formula RCu(3)(Mn(3)Fe)O(12) (R = Ce, Pr, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) have been synthesized and characterized. Polycrystalline samples have been prepared from citrate precursors treated under moderate pressure conditions (2-3.5 GPa) and 1000 °C in the presence of KClO(4) as an oxidizing agent. All the samples have been studied by neutron powder diffraction (NPD) at 300 K and 2 K. These oxides crystallize in the cubic space group Im3 (no. 204). Mn(4+)/Mn(3+) and Fe(3+) occupy at random the octahedral B positions of the perovskite structure. These materials have also been characterized by magnetic and magnetotransport measurements. The observed enhancement of T(C) along the RCu(3)(Mn(3)Fe)O(12) series is understood as an effect of the chemical pressure on the (Mn,Fe)-O bonds as R(3+) size decreases. The semiconducting behaviour observed in all of the samples is related with the introduction of Fe at B position. Despite the drastic change of the transport properties, significant negative magnetoresistance values are observed in the Fe-containing compounds both at 10 K and 300 K.

Slow magnetic relaxation in a 3D network of cobalt(II) citrate cubanes.

The synthesis, structure and magnetic properties of a 3D network based on {Co4(cit)4}8⁻ (H4cit = citric acid) cubane units linked by octahedral Co(II) centres is reported.

Pressure-induced Jahn-Teller switching in a Mn12 nanomagnet.

Pressure-induced switching of a fast-relaxing single-molecule magnet to a slow-relaxing isomer is observed for the first time by using a combination of high pressure single-crystal X-ray diffraction and high pressure magnetic measurements.

High pressure studies of hydroxo-bridged Cu(II) dimers.

A combination of high pressure single crystal X-ray diffraction and high pressure SQUID magnetometry has been used to study three hydroxo-bridged copper(II) dimers. [Cu2(OH)2(H2O)2(tmen)2](ClO4)2 (1; tmen = tetramethylethylenediamine), [Cu2(OH)2(tben)2](ClO4)2 (2; tben = di-tbutylethylenediamine) and [Cu2(OH)2(bpy)2](BF4)2 (3; bpy = 2,2'-bipyridine) have been structurally determined to 2.5, 0.9 and 4.7 GPa, respectively. The application of hydrostatic pressure imposes significant distortions and modifications in the structures of all three complexes. This is particularly true of the bond distances and angles between the metal centres and the bridging hydroxo groups. Compound 1 undergoes a phase transition between 1.2 and 2.5 GPa caused by the loss of a coordinated water molecule. This leads to a loss of symmetry and dramatic changes in the molecular structure of the complex. The structural changes are manifested in changes in the magnetic behaviour of the complexes as seen in dc susceptibility measurements up to approximately 0.9 GPa for 1, 2 and 3: the exchange becomes less antiferromagnetic in 1 and 2 and more ferromagnetic in 3.

High pressure effects on a trimetallic Mn(II/III) SMM.

A combined study of the high pressure crystallography and high pressure magnetism of the complex [Mn3(Hcht)2(bpy)4](ClO4)3.Et2O.2MeCN (1.Et2O.2MeCN) (H3cht is cis,cis-1,3,5-cyclohexanetriol) is presented in an attempt to observe and correlate pressure induced changes in its structural and physical properties. At 0.16 GPa the complex 1.Et2O.2MeCN loses all associated solvent in the crystal lattice, becoming 1. At higher pressures structural distortions occur changing the distances between the metal centres and the bridging oxygen atoms making the magnetic exchange between the manganese ions weaker. No significant variations are observed in the Jahn-Teller axis of the only Mn(III) present in the structure. High pressure dc chiMT plots display a gradual decrease in both the low temperature value and slope. Simulations show a decrease in J with increasing pressure although the ground state is preserved. Magnetisation data do not show any change in |D|.

High pressure induced spin changes and magneto-structural correlations in hexametallic SMMs.

The first combined high pressure single-crystal X-ray diffraction and high pressure magnetism study of two polymetallic clusters is presented in an attempt to correlate the observed changes in structure with changes in magnetic response without the need for changes in external ligation. At 1.5 GPa the structure of [Mn(6)O(2)(Et-sao)(6)(O(2)CPh(Me)(2))(2)(EtOH)(6)] (1; Et-saoH(2) = 2-hydroxyphenylpropanone)--a single molecule magnet (SMM) with an effective anisotropy barrier of approximately 86 K--and of [Mn(6)O(2)(Et-sao)(6)(O(2)C-naphth)(2)(EtOH)(4)(H(2)O)(2)] 2 both undergo significant structural distortions of their metallic skeletons, which has a direct effect upon the observed magnetic response. The application of hydrostatic pressure on the two compounds (up to 1.5 GPa) flattens the Mn-N-O-Mn torsion angles weakening the magnetic exchange between the metal centres. In both compounds one interaction switches from ferro- to antiferromagnetic, with the Jahn-Teller (JT) axes compressing (on average) and re-aligning differently with respect to the plane of the three metal centres. High pressure dc chi(M)T plots display a gradual decrease in the low temperature peak height and slope, simulations showing a decrease in |J| with increasing pressure with a second antiferromagnetic J value required to simulate the data. The "ground states" change from S = 12 to S = 11 for 1 and to S = 10 for 2. Magnetisation data for both 1 and 2 suggest a small decrease in |D|, while out-of-phase (chi(M)(//)) ac data show a large decrease in the effective energy barrier for magnetisation reversal.

Synthesis and characterisation of a Ni4 single-molecule magnet with S4 symmetry.

[Ni4Cl4(HL)4] () {H2L=HN(CH2CH2OH)2} has S4 symmetry and crystallises in the tetragonal space group I4(1)/a. Two exchange couplings are observed between the four Ni(II) centres, with J1=7.29 cm(-1) and J2=-2.08 cm(-1), leading to an S=4 ground state. The Ni4 complex shows the onset of frequency dependent signals in the out-of-phase ac susceptibility below 3 K. In single-crystal measurements carried out using a micro-SQUID, hysteresis loops are observed below 0.5 K, confirming that shows slow relaxation of magnetisation. The loops are temperature dependent but only weakly sweep rate dependent due to the presence of small intermolecular interactions, which hinder quantum tunnelling. This exchange bias between Ni4 molecules is also seen in high-frequency high-field EPR measurements, which give the parameters D=-0.75 cm(-1), B4 degrees=-6.7x10(-5) cm(-1) and gz=2.275.