Crystallization of sulfides from alkali polysulfide fluxes

The possibilities of growing crystals of metals, alloys, chalcogenides, and pnictides in halide melts using a steady-state temperature gradient are analyzed. Halides of alkali metals and aluminum can be used as transport media. The choice is determined by the melting temperature of salt mixtures. A conducting contour can also be applied to increase transport efficiency. This technique of crystal growth is similar to the electrochemical method. To eliminate interference during migration, some elements can be isolated and forced to migrate through independent channels to the crystal formation region. The technique considered here makes it possible to grow crystals of necessary quality without special equipment; the small crystal sizes are sufficient for laboratory study.

General principles of the synthesis of chalcogenides and pnictides in salt melts using a steady-state temperature gradient

Single crystals of sulfur-substituted iron selenide, FeSe1−xSx, were grown within eutectics of molten halides, AlCl3/KCl, AlCl3/KCl/NaCl or AlCl3/KBr, under permanent temperature gradient. The innovative “ampoule in ampoule” design of a crystallization vessel allows obtaining mm-sized plate-like single crystals with a sulfur content up to x ∼ 0.19. The sharp anomalies in the physical properties indicate the superconducting and nematic phase transitions in FeSe0.96 at TC = 8.4 K and TN = 90 K, respectively. Scanning tunneling microscopy reveals the presence of dumbbell defects associated with Fe vacancies and dark defects at the chalcogen site associated with S within the FeSe1−xSx series of compounds. Scanning tunneling spectroscopy shows the presence of two different superconducting gaps at both hole and electron pockets of the Fermi surface for low S content levels. As a function of sulfur content, TC follows the conventional dome-shaped curve while TN decreases with x. The overall appearance of the T–x phase diagram of FeSe1−xSx suggests the importance of nematic fluctuations for the formation of the superconducting state in these compounds.

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Single crystal growth, transport and scanning tunneling microscopy and spectroscopy of FeSe1−xSx

Sphalerite crystals (Fe,Zn)S containing up to 56 mol% of FeS have been synthesized by gas transport method and in molten salts in the temperature range 340–780 °C at various sulfur fugacities. It is shown that lattice parameter of Fe-bearing sphalerite changes with temperature and composition (x, mol% FeS in sphalerite) according to parabolic equation: $$a\, \pm \,0.0004/{\text{\AA}} = \, \left( {5.4099\, \pm \,0.0008} \right) \, + \, \left( {5.82\, \pm \,0.36} \right) \cdot 10^{ - 4} \cdot x \, + \, \left( { - 4.7\, \pm \,0.6} \right) \cdot 10^{ - 6} \cdot x^{2} + \, \left( {4.2\, \pm \,0.4} \right) \cdot 10^{ - 5} \cdot \left( {T{-} \, 298.15{\text{ K}}} \right)$$
 . This relationship is independent from synthesis temperature and sulfur fugacity. Temperature expansion coefficient is independent from temperature or composition of the sphalerite solid solution. It is shown that increase in Fe content of the synthesis charge leads to larger deviations between the target and real composition of the obtained crystals. Vickers microhardness of sphalerite increases in the composition range 0–1 mol% of FeS, has broad maximum in the range 1–5 mol% and decreases at higher Fe content.

Sidnwy G. Parker

Papers

Molten flux growth of cubic zinc sulfide crystals