Doubling Of Unit Cell Research Articles

Structural Complexities in Sodium Ion Conductive Antiperovskite Revealed by Cryogenic Transmission Electron Microscopy.

We use low-dose cryogenic transmission electron microscopy (cryo-TEM) to investigate the atomic-scale structure of antiperovskite Na2NH2BH4 crystals by preserving the room-temperature cubic phase and carefully monitoring the electron dose. Via quantitative analysis of electron beam damage using selected area electron diffraction, we find cryogenic imaging provides 6-fold improvement in beam stability for this solid electrolyte. Cryo-TEM images obtained from flat crystals revealed the presence of a new, long-range-ordered supercell with a cubic phase. The supercell exhibits doubled unit cell dimensions of 9.4 Å × 9.4 Å as compared to the cubic lattice structure revealed by X-ray crystallography of 4.7 Å × 4.7 Å. The comparison between the experimental image and simulated potential map indicates the origin of the supercell is a vacancy ordering of sodium atoms. This work demonstrates the potential of using cryo-TEM imaging to study the atomic-scale structure of air- and electron-beam-sensitive antiperovskite-type solid electrolytes.

Structural Phase Transitions in the Double Salts (NH4)2PO3F·NH4NO3 and (NH4)2XO4·3NH4NO3 (X = Se, Cr)

In the context of investigating isostructural relationships between sulfates and monofluorophosphates, crystals of the double salts (NH4)2PO3F·NH4NO3 (AFP·AN) and (NH4)2XO4·3NH4NO3 (AX·3AN; X = Se, Cr) were grown from aqueous solutions and structurally characterized using X-ray diffraction and thermal analysis. Whereas the high-temperature forms of the two AX·3AN double salts are in fact isostructural with the sulfate analogue, AFP·AN crystallizes with a reduced amount of NH4NO3 and thus has a unique crystal structure. Both AFP·AN and the two AX·3AN compounds exhibit reversible structural phase transitions. Upon cooling, the monofluorophosphate double salt transforms from the monoclinic room-temperature polymorph (I; P21/n, Z = 4) to the intermediate triclinic polymorph (II; P1, Z = 4) that in turn transforms to the monoclinic low-temperature polymorph (III; P21/n, Z = 4). The two phase transitions (I) → (II) and (II) → (III) are characterized by a significant increase of the unit cell volumes upon cooling. The two AX·3AN double salts transform upon cooling from a disordered monoclinic crystal structure (P21, Z = 2) to a monoclinic polymorph with a doubled unit cell (P21/c, Z = 4). Such a phase transition is not observed for the sulfate analogue. All molecular moieties are fully ordered at −93 °C for the selenate double salt, whereas one of the nitrate anions remains disordered for the chromate double salt even at −173 °C. In all AFP·AN and AX·3AN crystal structures, the nitrate anions play a crucial role during the phase transitions, and an extensive network of N–H···O hydrogen-bonding interactions is responsible for the cohesion of the crystal.

Ordered deficient perovskite La2/3TiO3 films grown via molecular beam epitaxy

As the parent compound of a promising solid electrolyte material Li3xLa2/3−xTiO3, the perovskite La2/3TiO3 has potential for advancing research on Li-intercalated ionic conductors. Epitaxial La2/3TiO3 films have been grown by molecular beam epitaxy using a growth process consisting of deposition and annealing cycles, with in situ monitoring by electron diffraction. X-ray absorption spectroscopy confirms the tetravalent state of Ti in La2/3TiO3, and the as-grown films are insulating. X-ray diffraction reveals the presence of half-order peaks, indicating a doubling of the pseudocubic perovskite unit cell due to the ordering of La vacancies in alternating A-site layers. These results demonstrate that single-phase, vacancy-ordered epitaxial films of La2/3TiO3 can be stabilized with excellent crystalline and electronic properties over wafer-sized areas, making possible Li-ion intercalation studies in films with well-defined domain boundary properties. Such boundaries are known to profoundly influence Li-ion conduction within the material. Understanding the effects of domain boundaries on Li-ion conduction could lead to improvements in solid-state battery technology and pave the way for the development of more efficient and safer energy storage devices.

Formation and structure of 1/1 cubic approximants in the Cd-Mg-Ce system: The identification of a new structural variant

The stability region of the 1/1 cubic crystalline approximant to icosahedral quasicrystal in the Cd-Mg-Ce system was investigated in detail. The partial isothermal section of the ternary phase diagram at 673 K was determined in the range from 0 to 18 at% Ce, where it was shown that the approximant spans a wide compositional range of Cd57−86Mg0−29Ce14–15. Mg addition was found to shift the stability region to lower temperatures. Besides, the approximant was found to be in equilibrium not with the Cd-Mg solid solution but with (Cd,Mg)17Ce2 and (Cd,Mg)11Ce compounds as opposed to the previous work [J. Alloys Compd. 822 (2020) 153541]. Based on the diffraction data (both X-ray and electron), to our surprise, a new variant of 1/1 cubic approximant was identified within the compositional range. The prototype Cd37Ce6 structure (space group Pn3̅) which dominates near the Cd-rich end was found to be partly taken over by the Cd25Eu4-type structure (space group Fd3̅) with a doubled unit cell in the higher-Mg part of the compositional range. The atomic structure with both Pn3̅ and Fd3̅ symmetries were determined by means of single-crystal X-ray diffraction measurement. Their refined atomic structures were compared and the preferential sites for Mg substitution are revealed.

Evidence of correlated incommensurate structural and magnetic order in highly oxygen-doped layered nickelate Nd2NiO4.23

$\mathrm{Nd}{}_{2}\mathrm{NiO}{}_{4+\ensuremath{\delta}}$ is a nonstoichiometric oxide, crystallizing in the Ruddlesden-Popper-type framework and exhibiting a wide range of oxygen nonstoichiometry $\ensuremath{\delta}$ with a complex structural phase diagram. Excess oxygen atoms insert holes, directly influencing the Ni valence state and associated electronic structure. We report here the existence of a correlated incommensurate structural and antiferromagnetic order below $T{}_{N}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 150 K in highly oxygen-doped $\mathrm{Nd}{}_{2}\mathrm{NiO}{}_{4+\ensuremath{\delta}}$ with $\ensuremath{\delta} \ensuremath{\approx}$ 0.23. The crystal and magnetic structures are investigated by polarized and unpolarized single-crystal neutron diffraction studies together with synchrotron x-ray powder diffraction and macroscopic magnetization measurements. Due to the long-range ordering of excess oxygen atoms, the real structure of the compound is incommensurately modulated, represented by $\mathbf{Q}{}_{n}=\ifmmode\pm\else\textpm\fi{}0.813{\mathbf{a}}^{*}\ifmmode\pm\else\textpm\fi{}0.519{\mathbf{b}}^{*}$ wave vectors. The antiferromagnetic order is characterized by the presence of incommensurate Bragg peaks of type ($h\phantom{\rule{4pt}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{4pt}{0ex}}\ensuremath{\varepsilon}$, 0, $l$/2), with $h$ and $l$ being odd-integer numbers and the magnetic incommensurability $\ensuremath{\varepsilon}\ensuremath{\approx}$ 0.36. The in-plane magnetic correlation length is found to be $\ensuremath{\xi}{}_{ab}$ = 184(1) \AA{} but is significantly reduced along the $c$ axis to $\ensuremath{\xi}{}_{c}$ = 39(2) \AA{}, indicating the quasi-two-dimensional nature of the antiferromagnetic correlations. Our study indicates that the excess oxygen atoms nevertheless strongly enhance the magnetic correlations between the ${\mathrm{NiO}}_{2}$ planes, resulting in a doubling of the magnetic unit cell along the $c$ axis. In contrast to $\mathrm{Nd}{}_{2}\mathrm{NiO}{}_{4.1}$, in which the incommensurate periodicity of oxygen and magnetic ordering was found to be identical, the structural and electronic orderings in heavily oxygen-doped ${\mathrm{Nd}}_{2}{\mathrm{NiO}}_{4.23}$ show two independent modulation vectors. Our study thus unravels a strong correlation between structural and electronic orderings in the highly oxygen-doped $\mathrm{Ln}{}_{2}\mathrm{NiO}{}_{4+\ensuremath{\delta}}$ system.

Auxeticity modifications and unit cell doubling in Yukawa fcc crystals with [001]-nanochannels filled by hard spheres

In recent years, the investigation of auxetic materials is receiving more and more attention due to their wide range of applications which follow enhancing indentation resistance, toughness, shear resistance, and other advantages of such materials. This work reports results of studies of models of auxetic metamaterials with nanoinclusions. Yukawa crystals with nanoinclusions in the form of nanochannels (NCs) in the [001] crystallographic direction, filled by hard spheres, were simulated by Monte Carlo in a wide range of pressures to determine their elastic properties. Particular attention has been devoted to the Poisson’s ratio (PR). It has been found that depending on the NCs’ type and pressure, the value of PR can vary from −0.302(12) to 1.083(14). The microscopic structures of the crystals were also examined in detail. A solid-solid phase transition in a host-guest system (the Yukawa crystal with hard spheres) was observed. Interestingly, this phase transition generates a unit cell doubling along the NCs. To localize this phase transition, apart from studies of the structure, the PR as a sensitive indicator of the phase transition was applied. In addition, it was found that the studied Yukawa systems with nanoinclusions for certain pressure values are completely non-auxetic, despite both the Yukawa and hard sphere crystals without inclusions are partially auxetic at the same conditions. This indicates that the presence of [001] NCs in the system not only can enhance auxeticity in comparison to the system without NCs but also, at some thermodynamic conditions, can lead to a completely non-auxetic behavior of the system which is partially auxetic without the NCs. Hence, one can use NCs to tune auxetic properties of crystals.

Hidden magnetic texture in the pseudogap phase of high-Tc YBa2Cu3O6.6

Despite decades of intense research, the enigmatic pseudo-gap (PG) phase of superconducting cuprates remains unsolved. In the last 15 years, various symmetry breaking states were discovered in the PG phase, including an intra-unit cell (IUC) magnetism, which preserves the lattice translational (LT) symmetry but breaks the time-reversal and parity symmetries, and an additional incipient charge density wave breaking the LT symmetry. However, none of these states can (alone) account for the partial gapping of the Fermi surface. Here we report a hidden LT-breaking magnetism using polarized neutron diffraction. Our measurements reveal magnetic correlations, in two different underdoped YBa2Cu3O6.6 single crystals that set in at the PG onset temperature with (i) a planar propagation wave vector (π, 0) ≡ (0, π), yielding a doubling or quadrupling of the magnetic unit cell and (ii) magnetic moments mainly pointing perpendicular to the CuO2 layers. The LT-breaking magnetism is at short-range suggesting the formation of clusters of 5–6 unit cells. Together with the previously reported IUC magnetism, it yields a hidden magnetic texture of the CuO2 unit cells hosting loop currents, forming large supercells that may be helpful for elucidating the PG puzzle.

Observation of cation-specific critical behavior at the improper ferroelectric phase transition in Gd2(MoO4)3

Gadolinium molybdate is a classical example of an improper ferroelectric and ferroelastic material. It is established that the spontaneous polarization arises as a secondary effect, induced by a structural instability in the paraelectric phase, which leads to a unit cell doubling and the formation of a polar axis. However, previous x-ray diffraction (XRD) studies on gadolinium molybdate have been restricted by the limited ability to include a wide $2\ensuremath{\theta}$ range in the analysis, and thus, at atomic scale, much remains to be explored. By applying temperature-dependent XRD, we observe the transition from the paraelectric tetragonal phase to the orthorhombic ferroelectric phase. The ferroelastic strain is calculated based on the thermal evolution of the lattice parameters, and Rietveld refinement of the temperature-dependent data reveals that the displacement of different cations follows different critical behavior, providing insight into the structural changes that drive the improper ferroelectricity in gadolinium molybdate.

Vibrational properties of SrVO2H with large spin-phonon coupling

The antiferromagnetic transition metal oxyhydride ${\mathrm{SrVO}}_{2}\mathrm{H}$ is distinguished by its stoichiometric composition and an ordered arrangement of H atoms. The tetragonal structure is related to the cubic perovskite and consists of alternating layers of ${\mathrm{VO}}_{2}$ and SrH. ${d}^{2}$ V(III) attains a sixfold coordination by four O and two H atoms. The latter are arranged in a trans fashion, which produces H--V--H chains along the tetragonal axis. Here, we investigate the vibrational properties of ${\mathrm{SrVO}}_{2}\mathrm{H}$ by inelastic neutron scattering and infrared spectroscopy combined with phonon calculations based on density functional theory. The H-based vibrational modes divide into a degenerate bending motion perpendicular to the H--V--H chain direction and a highly dispersed stretching motion along the H--V--H chain direction. The bending motion, with a vibrational frequency of approximately 800 ${\mathrm{cm}}^{\ensuremath{-}1}$, is split into two components separated by about 50 ${\mathrm{cm}}^{\ensuremath{-}1}$, owing to the doubled unit cell from the antiferromagnetic structure. Interestingly, spin-phonon coupling stiffens the H-based modes by $50\ensuremath{-}100\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ although super-exchange coupling via H is very small. Frequency shifts of the same order of magnitude also occur for V--O modes. It is inferred that ${\mathrm{SrVO}}_{2}\mathrm{H}$ displays the hitherto largest recognized coupling between magnetism and phonons in a material.

Observation of Spin‐Induced Ferroelectricity in a Layered van der Waals Antiferromagnet CuCrP2S6

AbstractCuCrP2S6, a van der Waals magnet having stacked layers of 2D honeycomb lattice made of CuS3 triangles and CrS6 octahedra, exhibits an A‐type antiferromagnetic order with the Néel temperature (TN) = 32 K. Upon in‐plane magnetic field (H) being applied below TN, H‐induced modulation of the c*‐axis electric polarization (ΔPc*) is found at fields lower than the saturation field µ0HS = 6.1 T, at which a forced ferromagnetic alignment sets in. Based on the symmetry analyses and dependence of ΔPc* on H and the azimuthal angle of applied H direction, a microscopic origin of the magnetoelectric (ME) coupling is attributed to the spin‐direction‐dependent p–d hybridization that is allowed due to the presence of off‐centered Cr3+ octahedra. A comparative study on CuCrP2Se6, however, finds no H‐induced P modulation due to cancellation of P between neighboring layers with the doubling of a crystallographic unit cell at TN. As the p–d hybridization mechanism allows generation of P in a single Cr atom–ligand pair, the results imply that large ME coupling should exist even in a single layer limit of CuCrP2S6.

Theory of the charge density wave in AV3Sb5 kagome metals

The family of metallic kagome compounds $A$V$_3$Sb$_5$ ($A$=K, Rb, Cs) was recently discovered to exhibit both superconductivity and charge order. The nature of the charge-density wave (CDW) phase is presently unsettled, which complicates the interpretation of the superconducting ground state. In this paper, we use group-theory and density-functional theory (DFT) to derive and solve a phenomenological Landau model for this CDW state. The DFT results reveal three unstable phonon modes with the same in-plane momentum but different out-of-plane momenta, whose frequencies depend strongly on the electronic temperature. This is indicative of an electronically-driven CDW, stabilized by features of the in-plane electronic dispersion. Motivated by the DFT analysis, we construct a Landau free-energy expansion for coupled CDW order parameters with wave-vectors at the $M$ and $L$ points of the hexagonal Brillouin zone. We find an unusual trilinear term coupling these different order parameters, which can promote the simultaneous condensation of both CDWs even if the two modes are not nearly-degenerate. We classify the different types of coupled multi-$\bf{Q}$ CDW orders, focusing on those that break the sixfold rotational symmetry and lead to a unit-cell doubling along all three crystallographic directions, as suggested by experiments. We determine a region in parameter space, characterized by large nonlinear Landau coefficients, where these phases - dubbed staggered tri-hexagonal and staggered Star-of-David - are the leading instabilities of the system. Finally, we discuss the implications of our results for the kagome metals.

Topological charge density waves at half-integer filling of a moiré superlattice

At partial filling of a flat band, strong electronic interactions may favor gapped states harboring emergent topology with quantized Hall conductivity. Emergent topological states have been found in partially filled Landau levels and Hofstadter bands; in both cases, a large magnetic field is required to engineer the underlying flat band. The recent observation of quantum anomalous Hall effects (QAH) in narrow band moir\'e systems has led to the theoretical prediction that such phases may be realized even at zero magnetic field. Here we report the experimental observation of insulators with Chern number $C=1$ in the zero magnetic field limit at $\nu=3/2$ and $7/2$ filling of the moir\'e superlattice unit cell in twisted monolayer-bilayer graphene (tMBG). Our observation of Chern insulators at half-integer values of $\nu$ suggests spontaneous doubling of the superlattice unit cell, in addition to spin- and valley-ferromagnetism. This is confirmed by Hartree-Fock calculations, which find a topological charge density wave ground state at half filling of the underlying $C=2$ band, in which the Berry curvature is evenly partitioned between occupied and unoccupied states. We find the translation symmetry breaking order parameter is evenly distributed across the entire folded superlattice Brillouin zone, suggesting that the system is in the flat band, strongly correlated limit. Our findings show that the interplay of quantum geometry and Coulomb interactions in moir\'e bands allows for topological phases at fractional superlattice filling that spontaneously break time-reversal symmetry, a prerequisite in pursuit of zero magnetic field phases harboring fractional statistics as elementary excitations or bound to lattice dislocations.

Gold-Cage Perovskites: A Three-Dimensional AuIII-X Framework Encasing Isolated MX63- Octahedra (MIII = In, Sb, Bi; X = Cl-, Br-, I-).

The Cs8AuIII4MIIIX23 (M = In3+, Sb3+, Bi3+; X = Cl-, Br-, I-) perovskites are composed of corner-sharing Au-X octahedra that trace the edges of a cube containing an isolated M-X octahedron at its body center. This structure, unique within the halide perovskite family, may be derived from the doubled cubic perovskite unit cell by removing the metals at the cube faces. To our knowledge, these are the only halide perovskites where the octahedral sites do not bear an average 2+ charge. Charge compensation in these materials requires a stoichiometric halide vacancy, which is disordered around the Au atom at the unit-cell corner and orders when the crystallization is slowed. Using X-ray crystallography, X-ray absorption spectroscopy, and pair distribution function analysis, we elucidate the structure of this unusual perovskite. Metal-site alloying produces further intricacies in this structure, which our model explains. Compared to other halide perovskites, this class of materials shows unusually low absorption onset energies ranging between ca. 1.0 and 2.4 eV. Partial reduction of Au3+ to Au+ affords an intervalence charge-transfer band, which redshifts the absorption onset of Cs8Au4InCl23 from 2.4 to 1.5 eV. With connected Au-X octahedra and isolated M-X octahedra, this structure type combines zero- and three-dimensional metal-halide sublattices in a single material and stands out among halide perovskites for its ordering of homovalent metals, ordering of halide vacancies, and incorporation of purely trivalent metals at the octahedral sites.

Doping the chiral spin liquid: Topological superconductor or chiral metal

We point out that there are two different chiral spin liquid states on the triangular lattice and discuss the conducting states that are expected on doping them. These states labeled CS1 and CS2 are associated with two distinct topological orders with different edge states, although they both spontaneously break time reversal symmetry and exhibit the same quantized spin Hall conductance. While CSL1 is related to the Kalmeyer-Laughlin state, CSL2 is the $\nu =4$ member of Kitaev's 16 fold way classification. Both states are described within the Abrikosov fermion representation of spins, and the effect of doping can be accessed by introducing charged holons. On doping CSL2, condensation of charged holons leads to a topological d+id superconductor. However on doping CSL1 , in sharp contrast , two different scenarios can arise: first, if holons condense, a chiral metal with doubled unit cell and finite Hall conductivity is obtained. However, in a second novel scenario, the internal magnetic flux adjusts with doping and holons form a bosonic integer quantum Hall (BIQH) state. Remarkably, the latter phase is identical to a $d+id$ superconductor. In this case the Mott insulator to superconductor transition is associated with a bosonic variant of the integer quantum Hall plateau transition for the holon. We connect the above two scenarios to two recent numerical studies of doped chiral spin liquids on triangular lattice. Our work clarifies the complex relation between topological superconductors, chiral spin liquids and quantum criticality .

About the ground state structure and superconductivity of the [formula omitted] compound

In the present investigation, based on density functional theory computational approach, we show how the experimentally accepted AlB2-like structure of the SrAlGe compound is suggested to be dynamically unstable at 0 K. We propose a stable configuration that is deduced from the detailed analysis of the phonon spectrum with a doubled unit cell and buckled AlGe layers. It allows to closely reproduce the experimental critical superconducting temperature assuming a phonon mediated superconductivity as described within BCS theory. A soft puckering mode is identified both as the cause for the structural instability and as the main contributor in the electron-phonon coupling constant. From the detailed study of the electronic structure, we show how a peierls-like distortion with the crystal symmetry lowering, affecting mainly the Al-pz orbitals, is correlated with the puckering and with the electron-phonon coupling mechanism.

Thermal conductivity of the quasi-one-dimensional materials TaSe3 and ZrTe3

The high breakdown current densities and resilience to scaling of the metallic transition metal trichalcogenides TaSe3 and ZrTe3 make them of interest for possible interconnect applications, and it motivates this study of their thermal conductivities and phonon properties. These crystals consist of planes of strongly bonded one-dimensional chains more weakly bonded to neighboring chains. Phonon dispersions and the thermal conductivity tensors are calculated using density functional theory combined with an iterative solution of the phonon Boltzmann transport equation. The phonon velocities and the thermal conductivities of TaSe3 are considerably more anisotropic than those of ZrTe3. The maximum LA velocity in ZrTe3 occurs in the cross-chain direction, and this is consistent with the strong cross-chain bonding that gives rise to large Fermi velocities in that direction. The thermal conductivities are similar to those of other metallic two-dimensional transition metal dichalcogenides. At room temperature, a significant portion of the heat is carried by the optical modes. In the low frequency range, the phonon lifetimes and mean free paths in TaSe3 are considerably shorter than those in ZrTe3. The shorter lifetimes in TaSe3 are consistent with the presence of lower frequency optical branches and zone-folding features in the acoustic branches that arise due to the doubling of the TaSe3 unit cell within the plane.

Expanding the Averievite Family, (MX)Cu5O2(T5+O4)2 (T5+ = P, V; M = K, Rb, Cs, Cu; X = Cl, Br): Synthesis and Single-Crystal X-ray Diffraction Study.

Averievite-type compounds with the general formula (MX)[Cu5O2(TO4)], where M = alkali metal, X = halogen and T = P, V, have been synthesized by crystallization from gases and structurally characterized for six different compositions: 1 (M = Cs; X = Cl; T = P), 2 (M = Cs; X = Cl; T = V), 3 (M = Rb; X = Cl; T = P), 4 (M = K; X = Br; T = P), 5 (M = K; X = Cl; T = P) and 6 (M = Cu; X = Cl; T = V). The crystal structures of the compounds are based upon the same structural unit, the layer consisting of a kagome lattice of Cu2+ ions and are composed from corner-sharing (OCu4) anion-centered tetrahedra. Each tetrahedron shares common corners with three neighboring tetrahedra, forming hexagonal rings, linked into the two-dimensional [O2Cu5]6+ sheets parallel to (001). The layers are interlinked by (T5+O4) tetrahedra (T5+ = V, P) attached to the bases of the oxocentered tetrahedra in a “face-to-face” manner. The resulting electroneutral 3D framework {[O2Cu5](T5+O4)2}0 possesses channels occupied by monovalent metal cations M+ and halide ions X−. The halide ions are located at the centers of the hexagonal rings of the kagome nets, whereas the metal cations are in the interlayer space. There are at least four different structure types of the averievite-type compounds: the P-3m1 archetype, the 2 × 2 × 1 superstructure with the P-3 space group, the monoclinically distorted 1 × 1 × 2 superstructure with the C2/c symmetry and the low-temperature P21/c superstructure with a doubled unit cell relative to the high-temperature archetype. The formation of a particular structure type is controlled by the interplay of the chemical composition and temperature. Changing the chemical composition may lead to modification of the structure type, which opens up the possibility to tune the geometrical parameters of the kagome net of Cu2+ ions.

Low temperature structures and magnetic interactions in the organic-based ferromagnetic and metamagnetic polymorphs of decamethylferrocenium 7,7,8,8-tetracyano-p-quinodimethanide, [FeCp*2]˙+[TCNQ]˙.

To identify the genesis of the differing magnetic behaviors for the ferro- (FO) and metamagnetic (MM) polymorphs of [FeCp*2][TCNQ] (Cp* = pentamethylcyclopentadienide; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) the low temperature (18 ± 1 K) structures of each polymorph were determined from high-resolution synchrotron powder diffraction data. Each polymorph possesses chains of alternating S = 1/2 [FeCp*2]˙+ cations and S = 1/2 [TCNQ]˙+, but with differing relative orientations. These as well as an additional paramagnetic polymorph do not thermally interconvert. In addition, the room and low (<70 ± 10 K) temperature structures of the MM polymorph, MMRT and MMLT, respectively, differ from that previously reported at 167 K (-106 °C) MM structure, and no evidence of either phase transition was previously noted even from the magnetic data. This transition temperature and enthalpy of this phase transition for MMRT⇌MM was determined to be 226.5 ± 0.4 K (-46.7 ± 0.4 °C) and 0.68 ± 0.04 kJ mol-1 upon warming, respectively, from differential calorimetry studies (DSC). All three MM phases are triclinic (P1[combining macron]) with the room temperature phase having a doubled unit cell relative to the other two. The lower temperature phase transition involves a small rearrangement of the molecular ions and shift in lattice parameters. These three MM and FO polymorphs have been characterized and form extended 1-D chains with alternating S = 1/2 [FeCp*2]˙+ cations, and S = 1/2 [TCNQ]˙- anions, whereas the fifth, paramagnetic (P) polymorph possesses S = 0 π-[TCNQ]22- dimers. At 18 ± 1 K the intrachain FeFe separations are 10.738(2) and 10.439(3) Å for the FO and MMLT polymorphs, respectively. The key structural differences between FO and MMLT at 18 ± 1 K are the 10% shorter interchain NN and the 2.8% shorter intrachain FeFe separation present for MMLT. Computational analysis of all nearest-neighbor spin couplings for the 18 K structures of FO and MMLT indicates that the intrachain [FeCp*2]˙+[TCNQ]˙- spin couplings (H = -2Si·Sj) are the strongest (4.95 and 6.5 cm-1 for FO and MMLT, respectively), as previously hypothesized, and are ferromagnetic due to their S = 1/2 spins residing in orthogonal orbitals. The change in relative [TCNQ]˙-[TCNQ]˙- orientations leads to a computed change from the ferromagnetic interaction (0.2 cm-1) for FO to an antiferromagnetic interaction (-0.1 cm-1) for MMLT in accord with its observed antiferromagnetic ground state. Hence, the magnetic ground state cannot be solely described by the dominant magnetic interactions.

An in-situ x-ray diffraction and infrared spectroscopic study of the dehydration of AlPO4-54

The dehydration of the large pore aluminophosphate AlPO4-54•xH2O was studied in situ as a function of vacuum pressure at room temperature by infrared spectroscopy and x-ray diffraction. On polycrystalline samples, under primary vacuum, the adsorbed water is removed very rapidly. The structural water in the AlO6 octahedra is then removed slowly with the quantity of the fully dehydrated form gradually increasing from 43 to 65% of the total material. This results in a two-phase mixture of fully dehydrated and a distinct partially dehydrated AlPO4-54•1/2H2O phase containing structural water. These phases have almost identical a cell parameters and very close c parameters. Secondary vacuum increases the amount of fully dehydrated material to more than 76%. Prolonged heating under vacuum increases the amount of fully dehydrated material to 90%. On single crystal samples, removal of adsorbed water is slower and results in a reduction in crystal quality, however prolonged exposure to primary vacuum yielded fully dehydrated material without the need for heating. The monoclinic structure of anhydrous AlPO4-54, space group Cm, with a doubled unit cell was confirmed using single crystal synchrotron x-ray diffraction data.

Complex interplay of magnetic interactions in 5f -electron systems: The case of U2Ni2Sn

Recent experiments on a single crystal of ${\mathrm{U}}_{2}{\mathrm{Ni}}_{2}\mathrm{Sn}$ established a strong uniaxial magnetic anisotropy and an unusual magnetic structure that is antiferromagnetic both inside the U layers and between the layers. We apply fully relativistic first-principles calculations to investigate various magnetic configurations of the system. The calculations confirmed both the character of the magnetic anisotropy and the type of the ground-state magnetic structure. The study of the electron density of states shows that the lowest energy of the ground-state structure is related to the doubling of the magnetic unit cell and corresponding reduction of the Brillouin zone, leading to the formation of the energy gaps in the electronic structure. The mapping of the energies of the magnetic structures on the Heisenberg Hamiltonian of interacting atomic moments leads to a surprising result that only one of the interatomic exchange interactions is antiferromagnetic. We explain how this one antiferromagnetic interaction competing with several ferromagnetic interactions leads to the simultaneous presence of two types of antiferromagnetic behavior. We provide a simple mean-field estimate of the N\'eel temperature and conclude that short-range magnetic order, neglected on the mean-field level, may play an important role at temperatures around the critical temperature. We demonstrate the existence of self-consistent noncollinear magnetic states that limits the accuracy of treating ${\mathrm{U}}_{2}{\mathrm{Ni}}_{2}\mathrm{Sn}$ as a pure Ising magnet. We apply $\mathrm{GGA}+U$ method to examine the influence of the Hubbard parameter $U$ on the magnetic moments and energies of the magnetic configurations. An estimation of the influence of the contraction of the lattice on the magnetic properties is reported.