Structure and Superconductivity of $FeSe_{1 - x}$ and $FeTe_{1 - y}Se_{y}$ Crystals: Dependence on the Synthesis Methods, Starting Composition, and Growth Conditions

Syntheses of superconducting iron chalcogenides $FeSe_{1 - x}$ (x = 0-0.15) and $FeTe_{1 - y}Se_{y}$ (y = 0.3-0.55) were performed. Superconducting phase of iron selenide was obtained by the solid-state reaction and from liquid phase. The highest values of critical temperature ($T_c$ = 8.2-8.7 K) exhibit $FeSe_{1-x}$ obtained by the crystallization from a melt with excess of iron less than 1 mol%. The samples from a melt contain up to 78% of tetragonal phase, as estimated by the X-ray diffraction. Lattice parameters and unit cell volume for the samples exhibiting highest $T_{c}$ and sharpest transition to superconducting state are limited to narrow range, with c/a ratio close to 1.469. The samples with excess of selenium contain higher amount of hexagonal phase than stoichiometric one. Superconducting single-crystalline samples of $FeTe_{1 - y}Se_{y}$ (up to 100% of tetragonal phase) were obtained using Bridgman's method. When y value increases, the volume of unit cell decreases. The critical temperature $T_{c}$ changes from ≈ 11.5 K for y ≈ 0.3 to ≈ 14.7 K for y ≈ 0.5.