Growth of whisker-like and bulk single crystals of PrFeAs(O,F) under high pressure

Superconducting whisker-like and bulk single crystals of PrFeAs(O,F) were grown at 3 GPa and~1500 °C from NaAs flux using the cubic anvil high-pressure and high-temperature technique. The ribbon-shaped whisker-like crystals were 400-1300 μm in length, 40-70 μm in width, and 5-10 μm in thickness with the long sharp edge in the ab-plane and the c-axis perpendicular to the face. The grown bulk crystals were found to have irregular plate-like shapes with linear sizes up to 1 mm and 120 μm in thickness. The both type of single-crystalline objects are crystallized in the same tetragonal structure and showed a single-phase behavior in the temperature dependence of magnetization with a superconducting transition temperature of about 31 K. The presence of temperature gradient and reactive flux seems to play an important role for the growth of single-crystalline whiskers and bulk single crystals of PrFeAs(O,F). These observations demonstrate that the morphological properties of the crystals are depending not only on the symmetry of the crystal structure of the desired material, but also of the growth medium.