Spin-glass behavior in LaFe(x)Co(2-x)P2 solid solutions: interplay between magnetic properties and crystal and electronic structures

Tuning Fe-Se Tetrahedral Frameworks by a Combination of [Fe(en)3]2+ Cations and Cl- Anions

A one-dimensional (1D) chain compound [Fe(en)3]3(FeSe2)4Cl2 (en = ethylenediamine), featuring tetrahedral FeSe₂ chains separated by [Fe(en)₃]²⁺ cations and Cl^- anions, has been synthesized by a low temperature solvothermal method using simple starting materials. The degree of distortion in the Fe-Se backbone is similar to previously reported compounds with isolated 1D FeSe₂ chains. ⁵⁷Fe Mössbauer spectroscopy reveals the mixed-valent nature of [Fe(en)₃]₃(FeSe₂)₄Cl₂ with Fe³⁺ centers in the [FeSe₂]^- chains and Fe²⁺ centers in the [Fe(en)₃]²⁺ complexes. SQUID magnetometry indicates that [Fe(en)3]3(FeSe2)4Cl2 is paramagnetic with a reduced average effective magnetic moment, μ_eff = 9.51 μ_B per formula unit, and a negative Weiss constant, θ = -10.9(4) K, indicating antiferromagnetic (AFM) nearest neighbor interactions within the [FeSe₂]^- chains. Weak antiferromagnetic coupling between chains, combined with rather strong intrachain AFM coupling, leads to spin-glass behavior at low temperatures, as indicated by a frequency shift of the peak observed at 3 K in AC magnetic measurements. A combination of [Fe(en)₃]²⁺ and Cl^- ions is also capable of stabilizing mixed-valent 2D Fe-Se puckered layers in the crystal structure of [Fe(en)₃]₄(Fe₁₄Se₂₁)Cl₂, where Fe₁₄Se₂₁ layers have a unique topology with large open pores. Property measurements of [Fe(en)₃]₄(Fe₁₄Se₂₁)Cl₂ could not be performed due to the inability to either grow large crystals or synthesize this material in single-phase form.

Flux Growth of Phosphide and Arsenide Crystals

Flux crystal growth has been widely applied to explore new phases and grow crystals of emerging materials. To accommodate the needs of high-quality single crystals, the flux crystal growth should be reliable, controllable, and predictable. The selections of suitable flux and growth conditions remain empirical due to the lack of systematic investigation especially for reactions, which involve highly volatile components, such as P and As. Considering the flux elements, often the system in question is a quaternary or a higher multinary system, which drastically increases complexity. In this manuscript, on the examples of flux growth of phosphides and arsenides, guidelines of flux selections, existing challenges, and future directions are discussed. We expect that the field will be further developed by applying in situ techniques and computational modeling of the nucleation and growth kinetics. Additionally, leveraging variables other than temperature, such as applied pressure, will make flux growth a more powerful tool in the future.

Synthesis, Crystal Structure, and Magnetic Properties of R2Mg3SiPn6 (R = La, Ce; Pn = P, As)

Four new quaternary pnictides, R₂Mg₃SiPn₆ (R = La, Ce; Pn = P, As), were synthesized via high-temperature solid-state reactions and gas-phase transport reactions with iodine. Their crystal structures were determined by single crystal X-ray diffraction. All four compounds are isostructural and crystallize in a new structure type in the orthorhombic space group Pnma (No. 62, Z = 4), Pearson symbol oP48. The crystal structures of R₂Mg₃SiPn₆ are composed of two-dimensional puckered MgP₃ layers, which are connected in a three-dimensional framework by P-P dimers and MgSiP₄ double-tetrahedral chains. Rare-earth cations are encapsulated inside the channels of the framework running along [010]. Quantum-chemical calculations predict that La₂Mg₃SiP₆ is an indirect narrow bandgap semiconductor. The Mg-P bonding in MgP₄ tetrahedra and MgP₆ octahedra was analyzed by means of crystal orbital Hamilton population (COHP) analysis. Magnetic characterization of Ce-containing compounds confirmed the trivalent nature of cerium atoms and revealed complex ferrimagnetic ordering at low temperatures.

Synthesis, Crystal Structure, and Properties of La4Zn7P10 and La4Mg1.5Zn8.5P12

Two new zinc phosphides, La₄Zn₇P₁₀ and La₄Mg_1.5Zn_8.5P₁₂, were synthesized via transport reactions, and their crystal structures were determined by single crystal X-ray diffraction. La₄Zn₇P₁₀ and La₄Mg_1.5Zn_8.5P₁₂ are built from three-dimensional Zn-P and Zn-Mg-P anionic frameworks that encapsulate lanthanum atoms. The anionic framework of La₄Zn₇P₁₀ is constructed from one-dimensional Zn₄P₆, Zn₂P₄, and ZnP₄ chains. The Zn₄P₆ chains are also the main building units in La₄Mg_1.5Zn_8.5P₁₂. In La₄Zn₇P₁₀, the displacement of a zinc atom from the origin of the unit cell causes the Zn4 position to split into two equivalent atomic sites, each with 50% occupancy. The splitting of the atomic position substantially modifies the electronic properties, as suggested by theoretical calculations. The necessity of splitting can be overcome by replacement of zinc with magnesium in La₄Mg_1.5Zn_8.5P₁₂. Investigation of the transport properties of a densified polycrystalline sample of La₄Zn₇P₁₀ demonstrates that it is an n-type semiconductor with a small bandgap of ∼0.04 eV at 300 K. La₄Zn₇P₁₀ also exhibits low thermal conductivity, 1.3 Wm^-1 K^-1 at 300 K, which mainly originates from the lattice thermal conductivity. La₄Zn₇P₁₀ is stable in a sealed evacuated ampule up to 1123 K as revealed by differential scanning calorimetry.

Synthesis, crystal and electronic structures, and physical properties of a new quaternary phosphide Ba4Mg2+δCu12−δP10(0 < δ < 2)

A new quaternary phosphide composed of a Cu–Mg–P framework hosting Ba and Mg cations is synthesized and characterized.

Challenges in intermetallics: synthesis, structural characterization, and transitions

Intermetallics that contain rare-earth elements are particularly interesting because of their temperature- and pressure-dependent structural and physical transitions that make them potential candidates for magnetic applications. This article highlights synthetic routes and structural characterization advancements used to investigate intermetallic materials. Experimental and theoretical examples of three intermetallic structure types--ThCr(2)Si(2), Heusler and Laves--are discussed to present a historical review and to illustrate the grand challenges in unravelling structure-property relationships of intermetallic compounds.