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A new oxide, sodium-iron antimonate Na 2 FeSbO 5 , was synthesized and structurally characterized, and its static and dynamic magnetic properties were comprehensively studied both experimentally by dc and ac magnetic susceptibility, magnetization, specific heat, ESR and Mössbauer measurements and theoretically by density functional calculations. The resulting single crystal structure (a = 15.6991(9) Å; b = 5.3323 (4) Å; c = 10.8875(6) Å; S.G. Pbna) consists of edge-shared SbO 6 octahedral chains, which alternate with vertex-linked, magnetically active FeO 4 tetrahedral chains. The 57 Fe Mössbauer spectra confirmed the presence of high spin Fe 3+ (3d 5) ions in distorted tetrahedral oxygen coordination. The magnetic susceptibility and specific heat data show absence of a long-range magnetic ordering in Na 2 FeSbO 5 down to 2 K, but ac magnetic susceptibility unambigously demonstrates spin-glass-type behavior with unique two-step freezing at T f1 ≈ 80 K and T f2 ≈ 35 K. Magnetic hyperfine splitting of 57 Fe Mössbauer spectra was observed below T * ≈ 104 K (T f1 < T *). The spectra just below T * (T f1 < T < T *) exhibit a relaxation behavior caused by critical spin fluctuations indicating the existence of short-range correlations. The stochastic model of ionic spin relaxation was used to account for the shape of the Mössbauer spectra below the freezing temperature. A complex slow dynamics is further supported by ESR data revealing two different absorption modes presumably related to ordered and disordered segments of spin chains. The data imply a spin-cluster ground state for Na 2 FeSbO 5 .

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A potassium salt of the N 2 S 2 O 2 -coordination Fe(III) anion K[Fe(5Clthsa) 2 ] (1) (5Cl-thsa − 5-chlorosalicylaldehyde thiosemicarbazone) is synthesized and characterized structurally and magnetically over a wide temperature range. Two polymorphs of salt 1 characterized by the common 2D polymer nature and assigned to the same orthorhombic Pbcn space group have been identified. The molecular structure of the minor polymorph of 1 was solved and refined at 100, 250, and 300 K is shown to correspond to the LS configuration. The dominant polymorph of 1 features K + cations disordered over a few crystallographic sites, while the minor polymorph includes fully ordered K + cations. The major polymorph exhibits a complete three-step cooperative spin-crossover transition both in the heating and cooling modes: The first step occurs in a temperature range from 2 to 50 K; the second abrupt hysteretic step occurs from 200 to 250 K with T 1/2 = 230 K and a 6 K hysteresis loop. The third gradual step occurs from 250 to 440 K. According to 57 Fe Mossbauer, XRPD, and EXAFS data, the spin-crossover transition for the dominant polymorph is quite peculiar. Indeed, the increase in the HS concentration by 57% at the second step does not result in the expected significant increase in the iron(III)−ligand bond lengths. In addition, the final step of the spin conversion (Δγ HS = 26%) is associated with a structural phase transition with a symmetry lowering from the orthorhombic (Pbcn) to the monoclinic (P2 1 /n) space group. This nontrivial phenomenon was investigated in detail by applying magnetization measurements, electron spin resonance, 57 Fe Mossbauer spectroscopy, and DFT calculations. These results provide a new platform for understanding the multistep spin-crossover character in the Fe(III) thsacomplexes and related compounds.

MSbO compounds (M = Mg, Co, Ni, Cu, Zn) are known in the tetragonal trirutile forms, slightly distorted monoclinically with M = Cu due to the Jahn-Teller effect. In this study, using a low-temperature exchange reaction between ilmenite-type NaSbO and molten MSO-KCl (or MgCl-KCl) mixtures, these five compositions were prepared for the first time as trigonal layered rosiaite (PbSbO)-type phases. Upon heating, they irreversibly transform to the known phases via amorphous intermediates, in contrast to previously studied isostructural MnSbO, where the stable phase is structurally related to the metastable phase. The same method was found to be applicable for preparing stable rosiaite-type CdSbO. The formula volumes of the new phases show an excellent correlation with the ionic radii (except for M = Cu, for which a Jahn-Teller distortion is suspected) and are 2-3% larger than those for the known forms although all coordination numbers are the same. The crystal structure of CoSbO was refined via the Rietveld method: P3[combining macron]1m, a = 5.1318(3) Å, and c = 4.5520(3) Å. Compounds with M = Co and Ni antiferromagnetically order at 11 and 15 K, respectively, whereas the copper compound does not show long-range magnetic order down to 1.5 K. A comparison between the magnetic behavior of the metastable and stable polymorphs was carried out. FeSbO could not be prepared because of the 2Fe + Sb = 2Fe + Sb redox reaction. This electron transfer produces an additional 5s shell for Sb and results in a volume increase. A comparison of the formula volume for the stable mixture FeSbO + 0.5SbO with that extrapolated for FeSbO predicted that the trirutile-type FeSbO can be stabilized at high pressures.