Flux requirements for the growth of RFeAsO (R=rare earth) superconductors

Abstract

Millimeter-sized LaFeAsO single crystals have been successfully grown out of NaAs flux starting with ͑LaAs+ 1 / 2Fe 2 O 3 ͒ : 20NaAs. The factors which allow the growth of these crystals out of NaAs but not out of many other fluxes, such as FeAs, have been investigated. X-ray powder diffraction found that NaAs synthesized for the growth of RFeAsO superconductors has monoclinic LiAs structure. Thermal analysis confirmed that NaAs melts congruently at about 600 °C. The ability to extract RFeAsO crystals from this NaAs flux suggests that NaAs has a significant oxygen solubility, possibly due to the formation of the ternary compound NaAsO 2 , and enough oxygen diffusivity to transport solute to the growth front. Oxygen solubility and diffusivity are two important factors in searching for an alternative environmentally benign flux for the growth of RFeAsO superconductors.

High-pressure flux growth, structural, and superconducting properties ofLnFeAsO (Ln=Pr, Nd, Sm) single cryst

“…Thus, one can conclude that the 1111 phase formation is much easier in molten salts than that in NaAs and KAs. Therefore, it is not surprising that doping control and obtaining high T c superconductors through substituting of F for O is very difficult not only at ambient pressure [11][12][13] but also under HPHT crystal growth conditions. The synthesis temperature is the second very important parameter since once it exceeds 1450 °C, the 1111-type phase tends to decompose into various phases, such as LnAs, Ref.…”

“…However, as many groups have proved through trial and error [1][2][3][4][5][6][7][8][9][10][11][12][13], the crystal growth of LnFePnO (Ln1111, Ln: lanthanide, Pn: pnictogen) oxypnictides is a difficult task. A number of exciting features with respect to structural, electronic, magnetic, and superconducting properties are still unresolved or have not been adequately studied yet.…”

Growth and characterizations of Ba2Ti2Fe2As4O single crystals

“…Recently, by employing a NaAs-flux method, the ‘1111’ oxypnictides were successfully grown [15, 16]. To avoid introducing extra elements, Ba 2 As 3 was chosen as the flux for growing Ba 2 Ti 2 Fe 2 As 4 O single crystals, since it has been used to grow sizable single crystals of doped BaFe 2 As 2 [17].…”

Flux growth utilizing the reaction between flux and crucible

“…NaAsO 2 has a low melting temperature of about 600 o C, which is similar to that of NaAs. [21] After crystal growth, the Al 2 O 3 crucible turns to be grey and reaction between the crucible and flux was always observed. Sometimes, the Al 2 O 3 growth crucible cracks after growth.…”