We report on high-resolution x-ray synchrotron powder-diffraction, magnetic-susceptibility, sound-velocity, thermal-expansion, and heat-capacity studies of the stoichiometric spinel ${\text{FeCr}}_{2}{\text{S}}_{4}$. We provide clear experimental evidence of a structural anomaly which accompanies an orbital-order transition at low temperatures due to a static cooperative Jahn-Teller effect. At 9 K, magnetic susceptibility, ultrasound velocity, and specific heat reveal pronounced anomalies that correlate with a volume contraction as evidenced by thermal-expansion data. The analysis of the low-temperature heat capacity using a mean-field model with a temperature-dependent gap yields a gap value of about 18 K and is interpreted as the splitting of the electronic ground state of ${\text{Fe}}^{2+}$ by a cooperative Jahn-Teller effect. This value is close to the splitting of the ground state due to spin-orbit coupling for isolated ${\text{Fe}}^{2+}$ ions in an insulating matrix, indicating that Jahn-Teller and spin-orbit coupling are competing energy scales in this system. We argue that due to this competition, the spin-reorientation transition at around 60 K marks the onset of short-range orbital ordering accompanied by a clear broadening of Bragg reflections, an enhanced volume contraction compared to usual anharmonic behavior, and a softening of the lattice observed in the ultrasound measurements.

/pdf/structural-anomalies-and-the-orbital-ground-state-in-fecr-2-37x34635ce.pdf

Structural anomalies and the orbital ground state in FeCr 2 S 4

Intercalation of organic cations in superconducting iron selenide can significantly increase the critical temperature (Tc). We present an electrochemical method using beta-FeSe crystals (Tc ~ 8 K) floating on a mercury cathode to intercalate tetramethylammonium ions (TMA+) quantitatively to obtain bulk samples of (TMA)0.5Fe2Se2 with Tc = 43 K. The layered crystal structure is closely related to the ThCr2Si2-type with disordered TMA+ ions between the FeSe layers. Although the organic ions are not detectable by X-ray diffraction, packing requirements as well as first principle DFT calculations constrain the specified structure. Our synthetic route enables electrochemical intercalations of other organic cations with high yields to greatly optimize the superconducting properties, and to expand this class of high-Tc materials.

Electrochemical synthesis and crystal structure of the organic ion intercalated superconductor (TMA)0.5Fe2Se2 with Tc = 43 K

We report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures $T_{\mathrm{C}}\approx 170$ K and the transition at $T_{\mathrm{OO}}\approx 10$ K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the state below $T_{\mathrm{OO}}$. The magnetic moments of the Cr- and Fe-ions are reduced from the spin-only values throughout the magnetically ordered regime, but approach the spin-only values for fields $>$5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60 K. Below $T_{\mathrm{OO}}$ this contraction is followed by a moderate expansion of the sample for fields larger than $\sim$4.5 T. The transition at $T_{\mathrm{OO}}$ is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for $T<T_{\mathrm{OO}}$. A linear correlation of the magnetic-field-induced change of the dielectric constant and the magnetic-field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10 K.

/pdf/fecr-2-s-4-in-magnetic-fields-possible-evidence-for-a-4obi3kw8rd.pdf

FeCr$_2$S$_4$ in magnetic fields: possible evidence for a multiferroic ground state

Recent results in physics and technology of ternary magnetic oxide and chalcogenide spinels AB 2 X 4 (A = Fe, Mn, Co, Cd, Zn, Hg; B = Cr, Co, Sc, Al; X = O, S, Se) are reviewed. Using magnetic susceptibility, specific heat, thermal expansion, electron-spin resonance, neutron diffraction, broad-band dielectric spectroscopy, and infrared optical spectroscopy, the spin, charge, orbital, and lattice correlations in these compounds were investigated. The novel magnetic ground states, e.g., spin liquid, spin―orbital liquid, and orbital glass, and in triguing effects, like colossal magnetocapacitive coupling, negative thermal expansion, and spin-driven Jahn―Teller structural transformations were revealed. In considering the origin of these phenomena, the concept of geometrical and bond frustration is explored. It relates the interplay of charge, spin, and orbital degrees of freedom with the inherent topological frustration and competing exchange interactions between the magnetic ions.

Ternary magnetic semiconductors: recent developments in physics and technology

We report a novel facile route for synthesis of iron-based ternary nanocrystals (NCs), i.e., FeCr2Se4 and FeCr2S4, using a wet chemistry method. The electronic structure with a narrow band gap of Fe-based ternary semiconducting NCs with different dopants offers promising conductive properties. Owing significantly to the non-toxicity of their constituent materials, they exhibit higher potential for being utilized in solar cells than Cd and Pb-based compounds. The general route for synthesis of ternary compounds includes mixing and reacting of different elemental powders in evacuated silica ampoules for long periods of time under high temperature (700–1000 °C). However, this process results in an uncontrolled size and morphology of the colloidal NCs. In our proposed method, the NC morphology was easily controlled by the solvent and heating temperature, utilizing oleylamine as a surfactant, solvent, and reducing agent. The synthesized NCs possess excellent mono-dispersity of size and shape without any aggregation and the conductivities of the deposited layer of FeCr2Se4 and FeCr2S4 NCs are 3.25 μA and 2.33 μA, respectively. Therefore, these iron-based NCs may replace the use of chalcogenides that are known to be environmentally toxic, and may contribute to low efficiency of energy transfer.

Facile synthesis of phase-pure FeCr2Se4 and FeCr2S4 nanocrystals via a wet chemistry method

We report on the fast preparation of the ternary magnetic semiconductor spinel FeCr2S4 using the Field-Activated Sintering Technique (FAST). The structural (X-ray) and magnetic (Squid) characterization demonstrates a similarity of the properties of the FAST crystals and those prepared by conventional solid-state synthesis. Residual structural disorder in FAST samples has a pronounced influence on the ferrimagnetic–paramagnetic transition and the orbital ordering at low temperatures.

Structural and magnetic properties of FeCr2S4 spinel prepared by field-activated sintering and conventional solid-state synthesis

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Structural and magnetic properties of FeCr2S4 spinel prepared by field-activated sintering and conventional solid-state synthesis