We report superconductivity in the novel 112-type iron-based compound Ca 1Àx La x FeAs 2 . Single-crystal X-ray diffraction analysis revealed that the compound crystallizes in a monoclinic structure (space group P2 1 ), in which Fe 2 As 2 layers alternate with Ca 2 As 2 spacer layers such that monovalent arsenic forms zigzag chains. Superconductivity with a transition temperature (T c ) of 34 K was observed for the x ¼ 0:1 sample, while the x ¼ 0:21 sample exhibited trace superconductivity at 45 K. First-principles band calculations demonstrated the presence of almost cylindrical Fermi surfaces, favorable for the high T c in La-doped CaFeAs 2 .KEYWORDS: iron-based superconductors, Ca-La-Fe-As, 112-type, CaFeAs 2Since the discovery of superconductivity with a transition temperature (T c ) of 26 K in LaFeAsO 1Àx F x , 1) there has been tremendous effort towards synthesizing novel iron pnictide superconductors. [2][3][4][5][6][7][8][9][10][11][12][13][14] All of the iron pnictide superconductors identified so far consist of a common structural motif, i.e., Fe 2 As 2 layers that are alternately stacked with various kinds of spacer layers. Therefore, the central goal for realizing a higher T c has been finding a novel spacer layer that can suitably tune the electronic states of Fe 2 As 2 layers.Recently, superconductivity has been discovered in Ca 10 (Pt n As 8 )(Fe 2Àx Pt x As 2 ) 5 , which consists of As-As dimers with a formal electron count of As 2À in the spacer layer. [15][16][17][18] Because of the 4p 3 electron configuration of elemental arsenic, arsenic can form various bonding structures: (i) Isolated arsenic with a formal electron count of As 3À . Examples include A 3 As (A ¼ Li, Na, and K) and iron-based superconductors. (ii) Dimerized As-As with a single bond. Its formal electron count is As 2À . Sr 2 As 2 and Ca 10 (Pt n As 8 )(Fe 2Àx Pt x As 2 ) 5 with As-As dimer bonds in the spacer layer can be categorized here. (iii) A one-dimensional chain connected by arsenic single bonds with a formal electron count of As À . This category includes KAs as an example. Realizing novel iron-based superconductors with spacer layers composed of complex bonding networks of arsenic such as (iii) has been a longstanding challenge: Shim et al. have theoretically proposed the hypothetical compound BaFeAs 2 (112-type) with spacer layers of the arsenic square network, and suggested that such compounds can be used to examine the role of charge and polarization fluctuations as well as the importance of two-dimensionality in the mechanism of superconductivity. 19) Although the 112-type iron pnictides AEFeAs 2 (AE ¼ Ca, Sr, Ba) have not yet been synthesized, the isostructural compounds RET As 2 (RE = rare-earth elements; T ¼ Cu, Ag, Au) have been studied intensively. 20,21) In this letter, we present a report on the novel 112-type iron-based superconductor Ca 1Àx La x FeAs 2 . Although pure CaFeAs 2 was not obtained, we found that the substitution of a small amount of La for Ca stabilizes the 112 phase. Thus, Ca 1Àx La x FeAs 2 ...

A number of iron-based superconductors have been discovered, 1-3 which include LaFeAsO (1111-type structure), 4 BaFe 2 As 2 (122-type), 5 LiFeAs (111-type), 6 and FeSe (11-type), 7 as well as compounds with complex oxide spacer layers [8][9][10][11] and arsenide spacer layers such as Ca 10 (Pt 4 As 8 )(Fe 2 As 2 ) 5 . [12][13][14][15][16][17] The maximum superconducting transition temperature T c is 55 K of the 1111-type structure. 18 In order to further increase T c , an exploration of novel structure types should be performed.Very recently, Katayama et al. 19 and Yakita et al. 20 have reported superconductivity in Ca 1−x La x FeAs 2 and Ca 1−x Pr x FeAs 2 , respectively, with a novel 112-type structure. Ca 1−x La x FeAs 2 crystalizes in a monoclinic structure with the space group P2 1 (No. 4) and consists of alternately stacked Fe 2 As 2 and arsenic zigzag bond layers. 19 Although pure CaFeAs 2 was not obtained, Katayama et al. found that the substitution of a small amount of La for Ca stabilizes the 112 phase and induces superconductivity at T c = 34 K for x = 0.16. Interestingly, Katayama et al. 19 suggested that the trace superconductivity of Ca 1−x La x FeAs 2 could exhibit T c = 45 K.In this paper, we report that a large increase in T c occurs with the phosphorus or antimony doping of Ca 1−x La x FeAs 2 . P-doped Ca 0.84 La 0.16 FeAs 2 and Sb-doped Ca 0.85 La 0.15 FeAs 2 exhibited T c values of 41 and 43 K, respectively, while P/Sbfree Ca 0.85 La 0.15 FeAs 2 exhibited T c = 35 K.Single crystals of Ca 1−x La x Fe(As 1−y Pn y ) 2 (Pn = P and Sb) were grown by heating a mixture of Ca, La, FeAs, As, P, and Sb powders. A stoichiometric amount of the mixture was placed in an aluminum crucible and sealed in an evacuated quartz tube. The preparation was carried out in a glove box filled with argon gas. Ampules were heated at 700 • C for 3 h, heated to 1100 • C at a rate of 46 • C/h, and cooled to 1050 • C at a rate of 1.25 • C/h, followed by furnace cooling. The obtained samples were characterized by powder Xray diffraction (XRD) analysis, performed using a Rigaku RINT-TTR III X-ray diffractometer with CuK α radiation. The Ca 1−x La x Fe(As 1−y Pn y ) 2 was obtained together with a powder mixture of LaAs, FeAs, FeAs 2 , and CaFe 2 As 2 . We separated platelike single crystals of the present system with typ-

The effects of simultaneous Sb doping on the superconductivity of 112-type Ca 1¹x RE x FeAs 2 (RE = La, Ce, Pr, and Nd) were studied through measurements of the magnetization and electrical resistivity. In Sb-free materials, the superconducting transition temperature T c of the La-doped sample was 35 K, while those of the Pr-and Nd-doped samples were ³10 K; no superconductivity was observed in the Ce-doped sample. Sb doping increased the T c of all REdoped samples: T c increased to 47, 43, 43, and 43 K for RE = La, Ce, Pr, and Nd, respectively. We also found that the enhanced superconductivity results from the increase in the lattice parameter b, which increases the As-Fe-As bond angle to be closer to the ideal tetrahedron value. These observations provide insight for further increasing the T c of the 112 phase.

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Single crystal X-ray diffraction studies were performed for the Sbdoped 112-type iron-based superconductor Ca 1−x La x FeAs 2 with the superconducting transition temperature T c of 47 K. Doped Sb preferably substituted not for As(1) in the FeAs layers but for As(2) in the layers of As zigzag chains. Structural reasons for T c enhancement by Sb doping were discussed.