Antiferromagnetic Spin Correlations Between Corner-Shared [FeO₅]^7– and [FeO₆]^9– Units, in the Novel Iron-Based Compound: BaYFeO₄

A novel quaternary compound in the Ba–Y–Fe-O phase diagram was synthesized by solid-state reaction and its crystal structure was characterized using powder X-ray diffraction. The crystal structure of BaYFeO₄ consists of a unique arrangement of Fe³⁺ magnetic ions, which is based on alternate corner-shared units of [FeO₅]^7– square pyramids and [FeO₆]^9– octahedra. This results in the formation of stairwise channels of FeO polyhedra along the <i>b</i> crystallographic axis. The structure is described in an orthorhombic crystal system in the space group <i>Pnma</i> with lattice parameters <i>a</i> = 13.14455(1) Å, <i>b</i> = 5.694960(5) Å, and <i>c</i> = 10.247630(9) Å. The temperature-dependent magnetic susceptibility data reveal two antiferromagnetic (AFM) transitions at 33 and 48 K. An upturn in the magnetic susceptibility data above these transitions is observed, which does not reach its maximum even at 390 K. The field-dependent magnetization data at both 2 and 300 K show a nearly linear dependence and do not exhibit significant hysteresis. Heat capacity measurements between 2 and 200 K reveal only a broad anomaly without any indication of long-range ordering. The latter data set is not in good agreement with the magnetic susceptibility data, which makes it difficult to exactly determine the magnetic ground state of BaYFeO₄. Accordingly, a temperature-dependent neutron diffraction study is in order, which will enable resolving this issue. The theoretical study of the relative strengths of magnetic exchange interactions along various possible pathways, using extended Hückel spin dimer analysis, shows that only interactions between square pyramidal and octahedral centers are significant, and among them, the intrachannel correlations are stronger than interchannel interactions. This is the first physical property study in such a magnetic ion substructure

Long-Range Antiferromagnetic Ordering in B‑Site Ordered Double Perovskite Ca₂ScOsO₆

A new Os-based B-site ordered double perovskite with the chemical composition of Ca₂ScOsO₆ was successfully synthesized. The crystal structure of the title compound was determined by employing the powder X-ray diffraction method and was found to crystallize in the monoclinic <i>P</i>2₁/<i>n</i> space group with the cell constants of <i>a</i> = 5.4716(1) Å, <i>b</i> = 5.6165(1) Å, <i>c</i> = 7.8168 (1) Å, and β = 89.889 (2)°. The temperature-dependent magnetic susceptibility data suggest that this novel <i>S</i> = ³/₂ compound undergoes an antiferromagnetic transition at ∼69 K. Fitting the high-temperature susceptibility data (100–300 K) to Currie–Weisse behavior showed <i>C</i> = 1.734 emu·K/mol (μ_eff = 3.72 bohr magnetons) and θ = −341 K, which is indicative of dominant antiferromagnetic interactions. Temperature-dependent specific heat measurements exhibit a λ shape anomaly at 69 K, which is consistent with a long-range ordering of the spins. Because of a triangular arrangement of antiferromagnetically ordered magnetic ions, the system exhibits some degree of geometric magnetic frustration (GMF), but not strongly. Spin-dimer analysis, employing extended Hückel theory, reveals that a dominant exchange interaction exists (along the <i>a</i> crystallographic axis in perovskite layer), which violates the perfect condition for GMF

Synthesis, Crystal Structure, and Magnetic Properties of Li₃Mg₂OsO₆, a Geometrically Frustrated Osmium(V) Oxide with an Ordered Rock Salt Structure: Comparison with Isostructural Li₃Mg₂RuO₆

The novel osmium-based oxide Li₃Mg₂OsO₆ was synthesized in polycrystalline form by reducing Li₅OsO₆ by osmium metal and osmium­(IV) oxide in the presence of stoichiometric amounts of magnesium oxide. The crystal structure was refined using powder X-ray diffraction data in the orthorhombic <i>Fddd</i> space group with <i>a</i> = 5.88982(5) Å, <i>b</i> = 8.46873(6) Å, and <i>c</i> = 17.6825(2) Å. This compound is isostructural and isoelectronic with the ruthenium-based system Li₃Mg₂RuO₆. The magnetic ion sublattice Os⁵⁺ (<i>S</i> = ³/₂) consists of chains of interconnected corner- and edge-shared triangles, which brings about the potential for geometric magnetic frustration. The Curie–Weiss law holds over the range 80–300 K with <i>C</i> = 1.42(3) emu·K/mol [μ_eff = 3.37(2) μ_B] and θ_C = −105.8(2) K. Below 80 K, there are three anomalies at 75, 30, and 8 K. Those at 75 and 30 K are suggestive of short-range antiferromagnetic correlations, while that at 8 K is a somewhat sharper maximum showing a zero-field-cooled/field-cooled divergence suggestive of perhaps spin freezing. The absence of magnetic Bragg peaks at 3.9 K in the neutron diffraction pattern supports this characterization, as does the absence of a sharp peak in the heat capacity, which instead shows only a very broad maximum at ∼12 K. A frustration index of <i>f</i> = 106/8 = 13 indicates a high degree of frustration. The magnetic properties of the osmium phase differ markedly from those of the isostructural ruthenium material, which shows long-range antiferromagnetic order below 17 K, <i>f</i> = 6, and no unusual features at higher temperatures. Estimates of the magnetic exchange interactions at the level of spin-dimer analysis for both the ruthenium and osmium materials support a more frustrated picture for the latter. Errors in the calculation and assignment of the exchange pathways in the previous report on Li₃Mg₂RuO₆ are identified and corrected

Structure and Magnetic Properties of KRuO₄

The crystal structure of KRuO₄ is refined at both 280 and 3.5 K from neutron powder data, and magnetic properties are reported for the first time. The scheelite structure, <i>I</i>4₁/<i>a</i>, is confirmed at both temperatures. Atomic positions of greater accuracy than the original 1954 X-ray study are reported. The rare Ru⁷⁺ ion resides in a site of distorted tetrahedral symmetry with nominal electronic configuration 4d¹(e¹). Curie–Weiss parameters are near free ion values for the effective moment and θ = −77 K, indicating dominant antiferromagnetic (AF) correlations. A broad susceptibility maximum occurs near 34 K, but long-range AF order sets in only below 22.4 K as determined by magnetization and heat capacity data. The entropy loss below 50 K is only 44% of the expected <i>R</i> ln 2, indicating the presence of short-range spin correlations over a wide temperature range. The Ru sublattice consists of centered, corner-sharing tetrahedra which can lead to geometric frustration if both the nearest-neighbor, <i>J</i>₁, and the next-nearest-neighbor, <i>J</i>₂, exchange constants are AF and of similar magnitude. A spin dimer analysis finds <i>J</i>₁/<i>J</i>₂ ≈ 25, indicating weak frustration, and a (d_<i>z</i>²)¹ ground state. A single, weak magnetic reflection was indexed as (110). The absence of the (002) magnetic reflection places the Ru moments parallel to the <i>c</i> axis. The Ru⁷⁺ moment is estimated to be 0.57(7) μ_B, reduced from an expected value near 1 μ_B. A recent computational study of isostructural, isoelectronic KOsO₄ predicts a surprisingly large orbital moment due to spin–orbit coupling (SOC). However, the free ion SOC constant for Ru⁷⁺ is only ∼30% that of Os⁷⁺, so it is unclear that this effect can be implicated in the low ordered moment for KRuO₄. The origin of the short-range spin correlations is also not understood

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 <i>Pnma</i> space group with two distinct Fe³⁺ 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 <i>b</i>-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, <b>k</b> = (0, 0, ¹/₃), and the moments along the <i>b</i>-axis, whereas the structure becomes an incommensurate cycloid [<b>k</b> = (0, 0, ∼0.35)] below 36 K with the moments within the <i>bc</i>-plane. However, for both cases the ordered moments on Fe³⁺ are only of the order ∼3.0 μ_B, smaller than the expected values near 4.5 μ_B, 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 <b>k</b> = (0, 0, 0) magnetic structure, and allow for neither low dimensionality nor geometric frustration, are inadequate to explain the observed complex magnetic structure

Structure and Magnetic Properties of KRuO₄

The crystal structure of KRuO₄ is refined at both 280 and 3.5 K from neutron powder data, and magnetic properties are reported for the first time. The scheelite structure, <i>I</i>4₁/<i>a</i>, is confirmed at both temperatures. Atomic positions of greater accuracy than the original 1954 X-ray study are reported. The rare Ru⁷⁺ ion resides in a site of distorted tetrahedral symmetry with nominal electronic configuration 4d¹(e¹). Curie–Weiss parameters are near free ion values for the effective moment and θ = −77 K, indicating dominant antiferromagnetic (AF) correlations. A broad susceptibility maximum occurs near 34 K, but long-range AF order sets in only below 22.4 K as determined by magnetization and heat capacity data. The entropy loss below 50 K is only 44% of the expected <i>R</i> ln 2, indicating the presence of short-range spin correlations over a wide temperature range. The Ru sublattice consists of centered, corner-sharing tetrahedra which can lead to geometric frustration if both the nearest-neighbor, <i>J</i>₁, and the next-nearest-neighbor, <i>J</i>₂, exchange constants are AF and of similar magnitude. A spin dimer analysis finds <i>J</i>₁/<i>J</i>₂ ≈ 25, indicating weak frustration, and a (d_<i>z</i>²)¹ ground state. A single, weak magnetic reflection was indexed as (110). The absence of the (002) magnetic reflection places the Ru moments parallel to the <i>c</i> axis. The Ru⁷⁺ moment is estimated to be 0.57(7) μ_B, reduced from an expected value near 1 μ_B. A recent computational study of isostructural, isoelectronic KOsO₄ predicts a surprisingly large orbital moment due to spin–orbit coupling (SOC). However, the free ion SOC constant for Ru⁷⁺ is only ∼30% that of Os⁷⁺, so it is unclear that this effect can be implicated in the low ordered moment for KRuO₄. The origin of the short-range spin correlations is also not understood