Magnetic phase diagram of the solid solution LaMn2(Ge1-xSix)2 (0 ≤ x ≤ 1) unraveled by powder neutron diffraction.

The structural and magnetic properties of the ThCr2Si2-type solid solution LaMn2(Ge1-xSix)2 (x = 0.0 to 1.0) have been investigated employing a combination of X-ray diffraction, magnetization and neutron diffraction measurements, which allowed establishing a magnetic composition-temperature phase diagram. Substitution of Ge by Si leads to a compression of the unit cell, which affects the magnetic exchange interactions. In particular, the magnetic structure of LaMn2(Ge1-xSix)2 is strongly affected by the unit cell parameter c, which is related to the distance between adjacent Mn layers. Commensurate antiferromagnetic layers and a canted ferromagnetic structure dominate the Si-rich part of the solid solution, whilst an incommensurate antiferromagnetic flat spiral and a conical magnetic structure are observed in the Si-poor part.

An experimental and computational study of CO2 adsorption in the sodalite-type M-BTT (M = Cr, Mn, Fe, Cu) metal-organic frameworks featuring open metal sites.

We present a comprehensive investigation of the CO2 adsorption properties of an isostructural series of metal-organic frameworks, M-BTT (M = Cr, Mn, Fe, Cu; BTT3- = 1,3,5-benzenetristetrazolate), which exhibit a high density of open metal sites capable of polarizing and binding guest molecules. Coupling gas adsorption measurements with in situ neutron and X-ray diffraction experiments provides molecular-level insight into the adsorption process and enables rationalization of the observed adsorption isotherms. In particular, structural data confirms that the high initial isosteric heats of CO2 adsorption for the series are directly correlated with the presence of open metal sites and further reveals the positions and orientations of as many as three additional adsorption sites. Density functional theory calculations that include van der Waals dispersion corrections quantitatively support the observed structural features associated with the primary and secondary CO2 binding sites, including CO2 positions and orientations, as well as the experimentally determined isosteric heats of CO2 adsorption.

Cubic Re6+ (5d1) Double Perovskites, Ba2MgReO6, Ba2ZnReO6, and Ba2Y2/3ReO6: Magnetism, Heat Capacity, µSR, and Neutron Scattering Studies and Comparison with Theory.

Double perovskites (DP) of the general formula Ba2MReO6, where M = Mg, Zn, and Y2/3, all based on Re6+ (5d1, t2g1), were synthesized and studied using magnetization, heat capacity, muon spin relaxation, and neutron-scattering techniques. All are cubic, Fm3̅m, at ambient temperature to within the resolution of the X-ray and neutron diffraction data, although the muon data suggest the possibility of a local distortion for M = Mg. The M = Mg DP is a ferromagnet, Tc = 18 K, with a saturation moment ∼0.3 bohr magnetons at 3 K. There are two anomalies in the heat capacity: a sharp feature at 18 K and a broad maximum centered near 33 K. The total entropy loss below 45 K is 9.68 e.u., which approaches R ln 4 (11.52 e.u.) supporting a j = 3/2 ground state. The unit cell constants of Ba2MgReO6 and the isostructural, isoelectronic analogue, Ba2LiOsO6, differ by only 0.1%, yet the latter is an anti-ferromagnet. The M = Zn DP also appears to be a ferromagnet, Tc = 11 K, µsat(Re) = 0.1 µB. In this case the heat capacity shows a somewhat broad peak near 10 K and a broader maximum at ∼33 K, behavior that can be traced to a smaller particle size, ∼30 nm, for this sample. For both M = Mg and Zn, the low-temperature magnetic heat capacity follows a T3/2 behavior, consistent with a ferromagnetic spin wave. An attempt to attribute the broad 33 K heat capacity anomalies to a splitting of the j = 3/2 state by a crystal distortion is not supported by inelastic neutron scattering, which shows no transition at the expected energy of ∼7 meV nor any transition up to 100 meV. However, the results for the two ferromagnets are compared to the theory of Chen, Pereira, and Balents, and the computed heat capacity predicts the two maxima observed experimentally. The M = Y2/3 DP, with a significantly larger cell constant (3%) than the ferromagnets, shows predominantly anti-ferromagnetic correlations, and the ground state is complex with a spin frozen component Tg = 16 K from both direct current and alternating current susceptibility and µSR data but with a persistent dynamic component. The low-temperature heat capacity shows a T1 power law. The unit cell constant of B = Y2/3 is less than 1% larger than that of the ferromagnetic Os7+ (5d1) DP, Ba2NaOsO6.

Hydrogen Storage and Selective, Reversible O2 Adsorption in a Metal-Organic Framework with Open Chromium(II) Sites.

A chromium(II)-based metal-organic framework Cr3 [(Cr4 Cl)3 (BTT)8 ]2 (Cr-BTT; BTT(3-) =1,3,5-benzenetristetrazolate), featuring coordinatively unsaturated, redox-active Cr(2+) cation sites, was synthesized and investigated for potential applications in H2 storage and O2 production. Low-pressure H2 adsorption and neutron powder diffraction experiments reveal moderately strong Cr-H2 interactions, in line with results from previously reported M-BTT frameworks. Notably, gas adsorption measurements also reveal excellent O2 /N2 selectivity with substantial O2 reversibility at room temperature, based on selective electron transfer to form Cr(III) superoxide moieties. Infrared spectroscopy and powder neutron diffraction experiments were used to confirm this mechanism of selective O2 binding.

Accelerated Removal of Fe-Antisite Defects while Nanosizing Hydrothermal LiFePO4 with Ca(2).

Based on neutron powder diffraction (NPD) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), we show that calcium ions help eliminate the Fe-antisite defects by controlling the nucleation and evolution of the LiFePO4 particles during their hydrothermal synthesis. This Ca-regulated formation of LiFePO4 particles has an overwhelming impact on the removal of their iron antisite defects during the subsequent carbon-coating step since (i) almost all the Fe-antisite defects aggregate at the surface of the LiFePO4 crystal when the crystals are small enough and (ii) the concomitant increase of the surface area, which further exposes the Fe-antisite defects. Our results not only justify a low-cost, efficient and reliable hydrothermal synthesis method for LiFePO4 but also provide a promising alternative viewpoint on the mechanism controlling the nanosizing of LiFePO4, which leads to improved electrochemical performances.

Multiferroicity Broken by Commensurate Magnetic Ordering in Terbium Orthomanganite.

TbMnO3 is an important multiferroic material with strong coupling between magnetic and ferroelectric orderings. Incommensurate magnetic ordering is suggested to be vital for this coupling in TbMnO3 , which can be modified by doping at the site of Tb and/or Mn. Our study shows that a self-doped solid solution Tb1-x Mny MnO3 (y≤x) can be formed with Mn doped into the site of Tb of TbMnO3 . When y is small Tb1-x Mny MnO3 shows both ferroelectric and incommensurate magnetic orders at low temperature, which is similar to TbMnO3 . However, if y is large enough, a commensurate antiferromagnetic ordering appears along with the incommensurate magnetic ordering to prevent the appearance of multiferroicity in Tb1-x Mny MnO3 . That is to say, the magnetoeletric coupling can be broken by the co-existence of a commensurate antiferromagnetic ordering. This finding may be useful to the study of TbMnO3 .

Synthesis, structure, and magnetic properties of (Tb(1-x)Mn(y))MnO(3-δ).

Two compounds (Tb(1-x)Mn(y))MnO(3-δ) (W1, x = 0.089, y = 0.063; W2, x = 0.122, y = 0.102) have been synthesized by solid-state method and characterized using neutron diffraction and magnetic measurements. They crystallize in space group Pnma at room temperature and Pna21 at low temperature. W1 shows a sinusoidal antiferromagnetic order, and W2 shows both sinusoidal and canted commensurate antiferromagnetic orders. The magnetic moments of the commensurate antiferromagnetic order for W2 are antiferromagnetically coupled along the a- and c-axes, and ferromagnetically coupled along the b-axis in the Pna21 setting. Strong ferromagnetic response is induced by doping more Mn into the Tb site of (Tb(1-x)Mn(y))MnO(3-δ).

Partial spin ordering and complex magnetic structure in BaYFeO4: a neutron diffraction and high temperature susceptibility study.

The novel iron-based compound, BaYFeO4, crystallizes in the Pnma space group with two distinct Fe(3+) sites, that are alternately corner-shared [FeO5](7-) square pyramids and [FeO6](9-) octahedra, forming into [Fe4O18](24-) rings, which propagate as columns along the b-axis. A recent report shows two discernible antiferromagnetic (AFM) transitions at 36 and 48 K in the susceptibility, yet heat capacity measurements reveal no magnetic phase transitions at these temperatures. An upturn in the magnetic susceptibility measurements up to 400 K suggests the presence of short-range magnetic behavior at higher temperatures. In this Article, variable-temperature neutron powder diffraction and high-temperature magnetic susceptibility measurements were performed to clarify the magnetic behavior. Neutron powder diffraction confirmed that the two magnetic transitions observed at 36 and 48 K are due to long-range magnetic order. Below 48 K, the magnetic structure was determined as a spin-density wave (SDW) with a propagation vector, k = (0, 0, (1)/3), and the moments along the b-axis, whereas the structure becomes an incommensurate cycloid [k = (0, 0, ∼0.35)] below 36 K with the moments within the bc-plane. However, for both cases the ordered moments on Fe(3+) are only of the order ∼3.0 µB, smaller than the expected values near 4.5 µB, indicating that significant components of the Fe moments remain paramagnetic to the lowest temperature studied, 6 K. Moreover, new high-temperature magnetic susceptibility measurements revealed a peak maximum at ∼550 K indicative of short-range spin correlations. It is postulated that most of the magnetic entropy is thus removed at high temperatures which could explain the absence of heat capacity anomalies at the long-range ordering temperatures. Published spin dimer calculations, which appear to suggest a k = (0, 0, 0) magnetic structure, and allow for neither low dimensionality nor geometric frustration, are inadequate to explain the observed complex magnetic structure.

Structural phase transitions induced by pressure in ammonium borohydride.

A combined experimental and theoretical study of hydrogen-rich ammonium borohydride (NH4BH4) subjected to pressures up to 10 GPa indicates two phase transitions, detected by synchrotron radiation powder X-ray diffraction, Raman spectroscopy and Car-Parrinello molecular dynamics calculations, at 1.5 and 3.4 GPa. The ambient pressure, face-centred cubic phase of NH4BH4 transforms into a highly disordered intermediate structure which then evolves upon increasing pressure into an orthorhombic, distorted CsCl structure. The structure of the latter phase was solved using ab initio computational techniques and from a Rietveld full pattern refinement of the powder X-ray diffraction data.

Structure and dynamics of ammonium borohydride.

Synchrotron powder X-ray diffraction, ab initio molecular dynamics calculations and solid state (1)H and (2)H NMR are used to refine the structure of crystalline NH(4)BH(4) including H atoms. Rapid reorientations of both ions mean that on average half-hydrogens occupy the corners of a cube around B or N.

Electron density topology of cubic structure I Xe clathrate hydrate at high pressure.

In this report, we present a detailed powder x-ray diffraction study of the structural properties and charge density topology of structure I Xe clathrate hydrate under high pressure and room temperature. The pressure dependence of the structural parameters was determined by applying a Rietveld analysis to the experimental data. The combined Rietveld/maximum entropy method was used to derive the most probable charge density distribution at each pressure. Our results show that the charge density distribution of the encaged Xe atoms differs depending on the type of host cage at all pressures. Spherical electron density distributions were observed for the Xe atoms in the small cages while the atoms in the large cages showed longitudinal elongated electronic distributions. Along with the observed cage deformations, the change in electronic density distribution represents a clear indication that the guest-host interaction differs significantly between the small and large cages at high pressures. A similar behavior has been previously reported in low-temperature studies of methane clathrate hydrate.