In 2008 Kamihara et al. found a new class of high-T_C superconductors: iron-based oxypnictides. A transition temperature of 26 K in F-doped LaFeAsO (1111-type compound) was reported [1]. Replacing the nonmagnetic La ion by magnetic rare-earth elements in 1111-compounds leads to an increase of the superconducting transition temperature (T_C) up to ~ 50 K. However, the application of insitu high pressure or sintering the 1111-compounds at high pressure/high temperature (HP/HT) conditions has been proven to be more effective for the increase of the T_C [2,3].

Although the synthesis of REFeAsO_1-xF_x of polycrystalline material has been reported in many papers, the single crystal growth of these compounds is very difficult and still a challenge.

For single crystal growth of LaFeAsO_1-xF_x and NdFeAsO_1-xF_x, grain growth- and flux- method at HP/HT-conditions were performed using a multi-anvil apparatus. This multi-anvil apparatus Walker Type can achieve pressure up to 8 GPa and temperature up to 1774 K. In this contribution the main emphasis is put on the difficulties and the progresses made so far in synthesis and crystal growth at HP/HT conditions. 

References:

[1] Y. Kamihara et al., J. Am. Chem. Soc. 130, 3297 (2008)

[2] J. Karpinski et al., Physica C 469, 370–380 (2009)

[3] Z. Ren et al., Adv. Mater 21, 4584–4592 (2009)

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1. Single crystals by vapor and solution growth of organic semiconductors
2. Ferromagnetic QCP in YbNi₄(P_1-xAs_x)₂