Influence of disorder on the structural phase transition and magnetic interactions in Ba3−x_{3-x}Srx_xCr2_2O8_8

The spin dimer system $\mathrm{Ba}_{3-x}\mathrm{Sr}_x\mathrm{Cr_2O_8}$ is a solid solution of the triplon Bose-Einstein condensation candidates $\mathrm{Ba_3Cr_2O_8}$ and $\mathrm{Sr_3Cr_2O_8}$. The magnetic intradimer interaction constant $J_0$ in this spin system can be tuned by varying the Sr content $x$. Very interestingly, this variation of $J_0$ with $x$ is highly nonlinear. In the present study, we show that this peculiar behavior of $J_0$ can be only partly explained by the changes in the average crystal structure alone. We report on neutron powder diffraction experiments to probe the corresponding structural details. Performing extended H\"{u}ckel tight binding calculations based on those structural details obtained at liquid helium temperatures, we found that the change of the magnetic interaction constant can be well reproduced by taking into account the presence of a structural transition due to the Jahn-Teller active Cr$^{5+}$-ions. This transition, lifting the orbital degeneracy and thereby the magnetic frustration in the system, is heavily influenced by disorder in the system arising from partially exchanging Ba with Sr

Computational Chemistry-Guided Syntheses and Crystal Structures of the Heavier Lanthanide Hydride Oxides DyHO, ErHO, and LuHO

Heteroanionic hydrides offer great possibilities in the design of functional materials. For ternary rare earth hydride oxide REHO, several modifications were reported with indications for a significant phase width with respect to H and O of the cubic representatives. We obtained DyHO and ErHO as well as the thus far elusive LuHO from solid-state reactions of RE2O3 and REH3 or LuH3 with CaO and investigated their crystal structures by neutron and X-ray powder diffraction. While DyHO, ErHO, and LuHO adopted the cubic anion-ordered half-Heusler LiAlSi structure type (F4¯3m, a(DyHO) = 5.30945(10) Å, a(ErHO) = 5.24615(7) Å, a(LuHO) = 5.171591(13) Å), LuHO additionally formed the orthorhombic anti-LiMgN structure type (Pnma; LuHO: a = 7.3493(7) Å, b = 3.6747(4) Å, c = 5.1985(3) Å; LuDO: a = 7.3116(16) Å, b = 3.6492(8) Å, c = 5.2021(7) Å). A comparison of the cubic compounds’ lattice parameters enabled a significant distinction between REHO and REH1+2xO1−x (x 0). Furthermore, a computational chemistry study revealed the formation of REHO compounds of the smallest rare earth elements to be disfavored in comparison to the sesquioxides, which is why they may only be obtained by mild synthesis conditions

Hydrogen induced site depopulation in the LaMgNi4-hydrogen system

The LaMgNi4-hydrogen system was investigated by in-situ neutron powder diffraction and pressure-composition isotherm measurements at 100 °C and hydrogen (deuterium) pressures of up 50 bar. The system displays three hydride phases that have distinctly different hydrogen plateau pressures and H atom distributions. The cubic α-LaMgNi4H0.75 phase forms below 0.01 bar hydrogen pressure and H atoms fill one type of tetrahedral Ni4 interstices. The orthorhombic distorted β-LaMgNiH3.7 phase forms at about 3 bar hydrogen pressure and H atoms fill both tetrahedral LaNi3 and triangular bi-pyramidal La2MgNi2 interstices. Interestingly, tetrahedral Ni4 interstices are no longer occupied. Finally, the most hydrogen rich γ-LaMgNi4H4.85 phase forms above 20 bar. It has again cubic symmetry and H atoms continue to occupy triangular bi-pyramidal La2MgNi2 interstices while filling a new type of tetrahedral Ni4 interstices that are neither occupied in the α- nor in the β-phase. The tetrahedral LaNi3 interstices occupied in the β-phase are empty. Hydrogen induced depopulations of interstitial sites in metal hydrides are relatively rare and consistent with, but not entirely due to, the onset of repulsive H-H interactions at increasing hydrogen concentration

Hydrothermal synthesis of anisotropic alkali and alkaline earth vanadates

In the course of a systematic field study, anisotropic alkali and alkaline earth vanadates have been accessed through a straightforward, one-step hydrothermal process. They are formed quantitatively from V2O5 and alkali- or alkaline earth halide solutions after a few days of autoclave treatment in the temperature range between 100 and 220 °C. The presence of ionic additives leads to an interplay between the formation of isotropic crystalline phases and the production of fibrous oxide materials, such as a novel magnesium vanadate. The influence of the hydrothermal parameters and of the alkali/alkaline earth halides on the emerging phases and morphologies has been investigated in the course of a systematic study. The results are compared with other vanadate- and transition metal oxide-based hydrothermal systems, and the emerging trends are discussed with respect to the development of predictive synthetic concepts for nanostructured vanadium oxide

Design of magnetic spirals in layered perovskites: extending the stability range far beyond room temperature

In insulating materials with ordered magnetic spiral phases, ferroelectricity can emerge due to the breaking of inversion symmetry. This property is of both fundamental and practical interest, in particular with a view to exploiting it in low-power electronic devices. Advances towards technological applications have been hindered, however, by the relatively low ordering temperatures $T_\mathrm{spiral}$ of most magnetic spiral phases, which rarely exceed 100 K. We have recently established that the ordering temperature of a magnetic spiral can be increased up to 310 K by the introduction of chemical disorder. Here we explore the design space opened up by this novel mechanism by combining it with a targeted lattice control of some magnetic interactions. In Cu-Fe layered perovskites we obtain $T_\mathrm{spiral}$ values close to 400 K, comfortably far from room temperature and almost 100 K higher than using chemical disorder alone. Moreover, we reveal a linear, universal relationship between the spiral's wave vector and the onset temperature of the spiral phase. This linear law ends at a paramagnetic-collinear-spiral triple point, which defines the highest spiral ordering temperature that can be achieved in this class of materials. Based on these findings, we propose a general set of rules for designing magnetic spirals in layered perovskites using external pressure, chemical substitutions and/or epitaxial strain, which should guide future efforts to engineer magnetic spiral phases with ordering temperatures suitable for technological applications.Comment: 5 figures, 35 pages, to be appeared on Science Advanc

Structural Investigation into Magnetic Spin Orders of a Manganese Phosphatic Oxyhydroxide, Mn5(PO4)2(PO3(OH))2(HOH)4

The ferri- and antiferromagnetic structures of a hureaulite-type synthetic compound, Mn2+5(PO4)2(PO3(OH))2(HOH)4, were elucidated by high-resolution neutron powder diffraction in combination with magnetic susceptibility and heat capacity measurements. At 6.17 K, the paramagnetic phase (space group: C2/c) transforms to inherit a ferrimagnetic order (magnetic space group: C2'/c'), followed at 1.86 K by an incommensurately modulated antiferromagnetic order (magnetic superspace group: P21/c.1'(α0γ)00s with the propagation vector k(0.523(2), 0, 0.055(1)). In the ferrimagnetic state, antiferromagnetic interactions are dominant for both intra and inter pentamers of Mn2+(O, HOH)6 octahedra. Differently aligned spin-canting sublattices seen in the ferrimagnetic models at 3.4, 4.5, and 6.1 K explain a weak ferromagnetism in the title compound. The observation of magnetic moments vigorously changing in a small temperature range of 6.1-1.5 K adumbrates a high complexity of interplaying structural and magnetic orders in this manganese phosphatic oxyhydroxide

Incommensurate magnetic structure of CrAs at low temperatures and high pressures

The magnetic structure of chromium arsenide CrAs is studied with neutron powder diffraction at ambient pressure in the temperature range 1.5–300 K as well as with neutron single-crystal diffraction at 2 K and 0.12 GPa. The material undergoes an anti-isostructural phase transition at TN = 267 K and atmospheric conditions, in which both orthorhombic phases have the same space-group symmetry (Pnma, Z = 4) but different distortions of the parent hexagonal structure of the NiAs type (P63/mmc, Z = 2). The magnetic structure below TN is incommensurate with the propagation vector k = (0, 0, kc). At ambient pressure, the component kc decreases from kc = 0.3807 (7) at 260 K to kc = 0.3531 (6) at 50 K. Below this temperature, it is basically constant. With increasing pressure at 2 K, kc is also constant within standard uncertainties [kc = 0.353 (2)]. For the analysis of the magnetic structure, a group-theoretical approach based on the space group of the nuclear structure and its subgroups is used. To avoid falling into false minima in the refinements, a random search for magnetic moments in the models is implemented. In the literature, the magnetic structure has been determined on the basis of powder diffraction data as a double helix propagating along the c axis. Although this double-helical model leads to satisfactory agreement factors for our powder data, it does not reproduce the intensities of the magnetic satellite reflections measured on single-crystal data in a satisfactory way and can therefore be discarded. Instead, several other models are found that lead to better agreement. Each of them is spiral-like with directional components in all three directions and with no spin-density wave character that would cause a non-constant magnetic moment. In all these models, the ordering of the spins is neither a pure helix nor a pure cycloid. Instead, the unit vectors of the spin rotation planes make an angle α, 0° < α < 90°, with respect to the c* direction. The model in superspace group P21.1′(α0γ)0s yields the best agreement factors in the refinements of the neutron single-crystal and powder diffraction data. This model is unique as it is the only one in which all the magnetic moments rotate with the same chirality