Phase Transitions and Physical Properties of the Mixed Valence Iron Phosphate Fe3(PO3OH)4(H2O)4.

Iron phosphate materials have attracted a lot of attention due to their potential as cathode materials for lithium-ion rechargeable batteries. It has been shown that lithium insertion or extraction depends on the Fe mixed valence and reduction or oxidation of the Fe ions' valences. In this paper, we report a new synthesis method for the Fe3(PO3OH)4(H2O)4 mixed valence iron phosphate. In addition, we perform temperature-dependent measurements of structural and physical properties in order to obtain an understanding of electronic-structural interplay in this compound. Scanning electron microscope images show needle-like single crystals of 50 µm to 200 µm length which are stable up to approximately 200 °C, as revealed by thermogravimetric analysis. The crystal structure of Fe3(PO3OH)4(H2O)4 single crystals has been determined in the temperature range of 90 K to 470 K. A monoclinic isostructural phase transition was found at ~213 K, with unit cell volume doubling in the low temperature phase. While the local environment of the Fe2+ ions does not change significantly across the structural phase transition, small antiphase rotations occur for the Fe3+ octahedra, implying some kind of electronic order. These results are corroborated by first principle calculations within density functional theory, which also point to ordering of the electronic degrees of freedom across the transition. The structural phase transition is confirmed by specific heat measurements. Moreover, hints of 3D antiferromagnetic ordering appear below ~11 K in the magnetic susceptibility measurements. Room temperature visible light absorption is consistent with the Fe2+/Fe3+ mixed valence.

Site dependence of the magnetocaloric effect in Mn5-x Fe x Si3.

The nuclear and magnetic structures of Mn3Fe2Si3 are investigated in the temperature range from 20 to 300 K. The magnetic properties of Mn3Fe2Si3 were measured on a single crystal. The compound undergoes a paramagnetic to antiferromagnetic transition at T N2 ≃ 120 K and an antiferromagnetic to antiferromagnetic transition at T N1 ≃ 69 K. A similar sequence of magnetic phase transitions is found for the parent compound Mn5Si3 upon temperature variation, but the field-driven transition observed in Mn5Si3 is not found in Mn3Fe2Si3, resulting in a strongly reduced magnetocaloric effect. Structurally, the hexagonal symmetry found for both compounds under ambient conditions is preserved in Mn3Fe2Si3 through both magnetic transitions, indicating that the crystal structure is only weakly affected by the magnetic phase transition, in contrast to Mn5Si3 where both transitions distort the nuclear structure. Both compounds feature a collinear high-temperature magnetic phase AF2 and transfer into a non-collinear phase AF1 at low temperature. While one of the distinct crystallographic sites remains disordered in the AF2 phase in the parent compound, the magnetic structure in the AF2 phase involves all magnetic atoms in Mn3Fe2Si3. These observations imply that the distinct sites occupied by the magnetic atoms play an important role in the magnetocaloric behaviour of the family.

Orthorhombic charge density wave on the tetragonal lattice of EuAl4.

EuAl4 possesses the BaAl4 crystal structure type with tetragonal symmetry I4/mmm. It undergoes a charge density wave (CDW) transition at T CDW = 145 K and features four consecutive antiferromagnetic phase transitions below 16 K. Here we use single-crystal X-ray diffraction to determine the incommensurately modulated crystal structure of EuAl4 in its CDW state. The CDW is shown to be incommensurate with modulation wave vector q = (0,0,0.1781 (3)) at 70 K. The symmetry of the incommensurately modulated crystal structure is orthorhombic with superspace group Fmmm(00σ)s00, where Fmmm is a subgroup of I4/mmm of index 2. Both the lattice and the atomic coordinates of the basic structure remain tetragonal. Symmetry breaking is entirely due to the modulation wave, where atoms Eu and Al1 have displacements exclusively along a, while the fourfold rotation would require equal displacement amplitudes along a and b. The calculated band structure of the basic structure and interatomic distances in the modulated crystal structure both indicate the Al atoms as the location of the CDW. The tem-per-ature dependence of the specific heat reveals an anomaly at T CDW = 145 K of a magnitude similar to canonical CDW systems. The present discovery of orthorhombic symmetry for the CDW state of EuAl4 leads to the suggestion of monoclinic instead of orthorhombic symmetry for the third AFM state.

Commensurate Stacking Phase Transitions in an Intercalated Transition Metal Dichalcogenide.

Intercalation and stacking-order modulation are two active ways in manipulating the interlayer interaction of transition metal dichalcogenides (TMDCs), which lead to a variety of emergent phases and allow for engineering material properties. Herein, the growth of Pb-intercalated TMDCs-Pb(Ta1+x Se2 )2 , the first 124-phase, is reported. Pb(Ta1+x Se2 )2 exhibits a unique two-step first-order structural phase transition at around 230 K. The transitions are solely associated with the stacking degree of freedom, evolving from a high-temperature (high-T) phase with ABC stacking and R3m symmetry to an intermediate phase with AB stacking and P3m1, and finally to a low-temperature (low-T) phase again with R3msymmetry, but with ACB stacking. Each step involves a rigid slide of building blocks by a vector [1/3, 2/3, 0]. Intriguingly, gigantic lattice contractions occur at the transitions on warming. At low-T, bulk superconductivity with Tc  ≈ 1.8 K is observed. The underlying physics of the structural phase transitions are discussed from first-principle calculations. The symmetry analysis reveals topological nodal lines in the band structure. The results demonstrate the possibility of realizing higher-order metal-intercalated phases of TMDCs and advance the knowledge of polymorphic transitions, and may inspire stacking-order engineering in TMDCs and beyond.

Single-crystal-to-single-crystal phase transitions of commensurately modulated sodium saccharinate 1.875-hydrate.

This work reports reversible, single-crystal-to-single-crystal phase transitions of commensurately modulated sodium saccharinate 1.875-hydrate [Na(sac)(15/8)H2O]. The phases were studied in the temperature range 298 to 20 K. They exhibit complex disordered states. An unusual reentrant disorder has been discovered upon cooling through a phase transition at 120 K. The disordered region involves three sodium cations, four water molecules and one saccharinate anion. At room temperature, the structure is an eightfold superstructure that can be described by the superspace group C2/c(0σ20)s0 with q = (0, 3/4, 0). It demonstrates maximum disorder with the disordered chemical entities having slightly different but close to 0.50:0.50 disorder component ratios. Upon cooling, the crystal tends to an ordered state, smoothly reaching a unified disorder component ratio of around 0.90:0.10 for each of the entities. Between 130 and 120 K a phase transition occurs involving a sudden increase of the disorder towards the disorder component ratio 0.65:0.35. Meanwhile, the space group and general organization of the structure are retained. Between 60 and 40 K there is another phase transition leading to a twinned triclinic phase. After heating the crystal back to room temperature its structure is the same as before cooling, indicating a complete reversibility of the phase transitions.

Robust spin-ice freezing in magnetically frustrated Ho2Ge x Ti2- x O7 pyrochlore.

Structural analysis of spin frustrated Ho2Ge x Ti2-x O7 (x = 0, 0.1, 0.15 & 0.25) pyrochlore oxides has been performed using high resolution x-ray diffraction pattern and low temperature synchrotron x-ray diffraction pattern. The effect of positive chemical pressure on the spin dynamics of Ho2Ge x Ti2-x O7 has been analysed through the study of static (M-T and M-H; magnetisation against temperature & magnetisation against magnetic field) and dynamical (ac susceptibility) magnetic measurements. In lower temperature regime (∼2 K), such systems are predominantly governed by competing exchange (J nn) and dipolar (D nn) magnetic interactions. Magnetic measurements indicate that the application of increased chemical pressure in Ho2Ti2O7 matrix propels the system towards diminished ferromagnetic interaction. Dipolar coupling constant remains almost unchanged but Curie-Weiss temperature (θ cw) reduces to -0.04 K from 0.33 K (for an applied magnetic field; H = 100 Oe) with increasing x in Ho2Ge x Ti2-x O7. Positive chemical pressure establishes the dominance of Ho-Ho antiferromagnetic interaction J nn over dipolar interaction D nn. Spin relaxation feature corresponding to thermally activated single ion freezing (T s∼15 K) is shifted towards lower temperature. This chemical pressure-driven T s shift is ascribed to the alteration in crystal field effect, which reduces the activation energy for singe ion spin freezing. The reduction in the activation energy indicates crystal field-phonon coupling in Ho2Ge x Ti2-x O7 system. The robustness in spin ice freezing (second spin relaxation feature in ac susceptibility curve) remains unaffected with increasingly chemical pressure. This spin freezing ('2 in-2 out' spin arrangement in tetrahedra) is related to quantum tunnelling phenomenon, at T ice ∼ 2 K. It indicates that majority of spins still follows the 'ice rule' in Ho2Ge x Ti2-x O7 even after the application of chemical pressure.

Spectral Control of an X-Ray L-Edge Transition via a Thin-Film Cavity.

By embedding a thin layer of tantalum in an x-ray cavity, we observe a change in the spectral characteristics of an inner-shell transition of the metal. The interaction between the cavity mode vacuum and the L_{III}-edge transition is enhanced, permitting the observation of the collective Lamb shift, superradiance, and a Fano-like cavity-resonance interference effect. This experiment demonstrates the feasibility of cavity quantum electrodynamics with electronic resonances in the x-ray range with applications to manipulating and probing the electronic structure of condensed matter with high-resolution x-ray spectroscopy in an x-ray cavity setting.

Structural and kinetic investigation of the hydride composite Ca(BH4)2 + MgH2 system doped with NbF5 for solid-state hydrogen storage.

Designing safe, compact and high capacity hydrogen storage systems is the key step towards introducing a pollutant free hydrogen technology into a broad field of applications. Due to the chemical bonds of hydrogen-metal atoms, metal hydrides provide high energy density in safe hydrogen storage media. Reactive hydride composites (RHCs) are a promising class of high capacity solid state hydrogen storage systems. Ca(BH4)2 + MgH2 with a hydrogen content of 8.4 wt% is one of the most promising members of the RHCs. However, its relatively high desorption temperature of ∼350 °C is a major drawback to meeting the requirements for practical application. In this work, by using NbF5 as an additive, the dehydrogenation temperature of this RHC was significantly decreased. To elucidate the role of NbF5 in enhancing the desorption properties of the Ca(BH4)2 + MgH2 (Ca-RHC), a comprehensive investigation was carried out via manometric measurements, mass spectrometry, Differential Scanning Calorimetry (DSC), in situ Synchrotron Radiation-Powder X-ray Diffraction (SR-PXD), X-ray Absorption Spectroscopy (XAS), Anomalous Small-Angle X-ray Scattering (ASAXS), Scanning and Transmission Electron Microscopy (SEM, TEM) and Nuclear Magnetic Resonance (NMR) techniques.

High-contrast sub-millivolt inelastic X-ray scattering for nano- and mesoscale science.

Photon and neutron inelastic scattering spectrometers are microscopes for imaging condensed matter dynamics on very small length and time scales. Inelastic X-ray scattering permitted the first quantitative studies of picosecond nanoscale dynamics in disordered systems almost 20 years ago. However, the nature of the liquid-glass transition still remains one of the great unsolved problems in condensed matter physics. It calls for studies at hitherto inaccessible time and length scales, and therefore for substantial improvements in the spectral and momentum resolution of the inelastic X-ray scattering spectrometers along with a major enhancement in spectral contrast. Here we report a conceptually new spectrometer featuring a spectral resolution function with steep, almost Gaussian tails, sub-meV (≃620 µeV) bandwidth and improved momentum resolution. The spectrometer opens up uncharted space on the dynamics landscape. New results are presented on the dynamics of liquid glycerol, in the regime that has become accessible with the novel spectrometer.

Hydrogen bond dynamics in crystalline ß-9-anthracene carboxylic acid--a combined crystallographic and spectroscopic study.

We compare results from single crystal X-ray diffraction and FTIR spectroscopy to elucidate the nature of hydrogen bonding in ß-9-anthracene carboxylic acid (ß-9AC, C(15)H(10)O(2)). The crystallographic studies indicate a disorder for the protons in the cyclic hydrogen bond. This disorder allows the determination of the energy difference between two proton sites along the hydrogen bond. The temperature dependent Fourier transform infrared spectroscopy (FTIR) underpins the crystallographic results. The combination of both methods allows the estimation of a one-dimensional potential curve describing the OH-stretching motion. The dynamical properties of the proton transfer along the hydrogen bond are extracted from this potential. The work presented here has profound implication on future studies of photochemical dynamics of crystalline ß-9AC, which can deliver a deeper understanding of the mechanism of photochemical driven molecular machines and the optical and electronic properties of molecular organic semiconductors.

High-pressure phase transition of Bi2Fe4O9.

The high-pressure behaviour of Bi2Fe4O9 was analysed by in situ powder and single-crystal x-ray diffraction and Raman spectroscopy. Pressures up to 34.3(8) GPa were generated using the diamond anvil cell technique. A reversible phase transition is observed at approximately 6.89(6) GPa and the high-pressure structure is stable up to 26.3(1) GPa. At higher pressures the onset of amorphization is observed. The crystal structures were refined from single-crystal data at ambient pressure and pressures of 4.49(2), 6.46(2), 7.26(2) and 9.4(1) GPa. The high-pressure structure is isotypic to the high-pressure structure of Bi2Ga4O9. The lower phase transition pressure of Bi2Fe4O9 with respect to that of Bi2Ga4O9 (16 GPa) confirms the previously proposed strong influence of cation substitution on the high-pressure stability and the misfit of Ga3+ and Fe3+ in tetrahedral coordination at high pressure. A fit of a second-order Birch­Murnaghan equation of state to the p­V data results in K0 = 74(3) GPa for the low-pressure phase and K0 = 79(2) GPa for the high-pressure phase. The mode Grüneisen parameters were obtained from Raman-spectroscopic measurements.

High-pressure behaviours of HoMn(2)O(5) and BiMn(2)O(5).

High-pressure behaviours of isomorphous HoMn(2)O(5) and BiMn(2)O(5) (Pbam, Z = 4) were studied, using Raman spectroscopy and single-crystal x-ray diffraction, with the samples in diamond anvil cells pressurized to 10.09 GPa and 8.54 GPa, respectively, at room temperature. No phase transition was detected in either material. The two manganates are most compressible along the a axis with the lattice contractions along the b and c directions nearly the same. Their P-V data fitted with the second-order Birch-Murnaghan equation of state (B' = 4.0) give the bulk moduli B(0) = 138(2) GPa for BiMn(2)O(5) and B(0) = 173(3) GPa for HoMn(2)O(5). The difference in bulk moduli results from BiMn(2)O(5) being more compressible than HoMn(2)O(5) along the a axis. Our study shows that the Mn-O frameworks in isomorphous RMn(2)O(5) compounds (R = Bi and rare earth) are not rigid at high pressures, but rather adapt to the behaviour of the coordination spheres around the R atoms. Bond valence calculations indicate that the pressure-induced charge transfers between the two Mn sites have different characters for the two compounds.