New-Structure-Type Fe-Based Superconductors: CaAFe₄As₄ (A = K, Rb, Cs) and SrAFe₄As₄ (A = Rb, Cs)

Abstract

Fe-based superconductors have attracted research interest because of their rich structural variety, which is due to their layered crystal structures. Here we report the new-structure-type Fe-based superconductors CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs), which can be regarded as hybrid phases between AeFe2As2 (Ae = Ca, Sr) and AFe2As2. Unlike solid solutions such as (Ba(1-x)K(x))Fe2As2 and (Sr(1-x)Na(x))Fe2As2, Ae and A do not occupy crystallographically equivalent sites because of the large differences between their ionic radii. Rather, the Ae and A layers are inserted alternately between the Fe2As2 layers in the c-axis direction in AeAFe4As4 (AeA1144). The ordering of the Ae and A layers causes a change in the space group from I4/mmm to P4/mmm, which is clearly apparent in powder X-ray diffraction patterns. AeA1144 is the first known structure of this type among not only Fe-based superconductors but also other materials. AeA1144 is formed as a line compound, and therefore, each AeA1144 has its own superconducting transition temperature of approximately 31-36 K.

Synthesis, Crystal Structure and Superconductivity in RbLn2Fe4As4O2(Ln = Sm, Tb, Dy, and Ho)

“…This fact suggests that RbLn 2 Fe 4 As 4 O 2 is a line compound which bears the same hole-doping level of 25%, similar to the case in 1144-type superconductors. 9 Therefore, it is of meaning to study the possible factors that influence T c . Fig.…”

Anomalous phonon behavior in superconducting CaKFe4As4 : An optical study

“…The mode at lower frequency (phonon A) corresponds to the longer Fe-As(1) bond (see the upper inset of Fig. 1) with a length of 2.395Å, while the other mode (phonon B) corresponds to the Fe-As(2) bond with a shorter bond length of 2.387Å [13]. Considering the fact that CaKFe 4 As 4 can be regarded as the hybrid phase of CaFe 2 As 2 and KFe 2 As 2 , as shown in Fig.…”

“…CaKFe 4 As 4 is a member of the AeAFe 4 As 4 (Ae stands for alkaline earth, and A for alkali metal) family [13], which is a new system of FeSCs. Even though the chemical composition of AeAFe 4 As 4 is the same as that of the heavily investigated (Ae,A)Fe 2 As 2 , the crystalline structure is rather different.…”

Signature of multigap nodeless superconductivity in CaKFe4As4

“…This family has a diverse range of structures all containing a common Fe 2 An 2 (An = P, As, Se, Te) layer, analogous to the CuO 2 layers in high-T c cuprates [6]. A e AFe 4 As 4 (A e = Ca, Sr, Eu and A = K, Rb, Cs) materials, discovered in 2016, are the new family of Fe-based superconductors with T c of ≈ 31−36 K [7]. Until now, only a few experimental studies have been performed which show that this new family of superconductors is structurally identical to CaFe 2 As 2 , with the A e and A sites forming alternating planes along the crystallographic c-axis, separated by FeAs layers [7].…”

“…A e AFe 4 As 4 (A e = Ca, Sr, Eu and A = K, Rb, Cs) materials, discovered in 2016, are the new family of Fe-based superconductors with T c of ≈ 31−36 K [7]. Until now, only a few experimental studies have been performed which show that this new family of superconductors is structurally identical to CaFe 2 As 2 , with the A e and A sites forming alternating planes along the crystallographic c-axis, separated by FeAs layers [7]. Unlike other Fe-based superconductors, A e AFe 4 As 4 does not show any structural phase transition below room temperature [8].…”