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.

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X-ray diffraction study of solid CO(2) at room temperature has shown that the powder pattern of the high-pressure phase, which supersedes the low-pressure cubic Pa3 phase at about 10 gigapascals, is consistently interpreted in terms of an orthorhombic Cmca structure. The orthorhombic cell at 11.8 gigapascals has dimensions of 4.330 +/- 0.015, 4.657 +/- 0.005, 5.963 +/- 0.009 angstroms for its a, b, and c faces, respectively, and a volume of 120.3 +/- 0.5 cubic angstroms. Four molecules contained in the unit cell are located at the base-centered positions with their molecular axes inclined at about 52 degrees with respect to the crystallographic c axis. The volume change associated with the Pa3-Cmca transition is close to zero. The structural dimensions obtained for the high-pressure crystalline phase of CO(2) are of great importance for a theoretical understanding of the role of intermolecular interactions, including quadrupole-quadrupole interactions, in molecular condensation.

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ABSTRACT:A new iron-based superconductor (Ca,Pr)FeAs 2 was discovered. Plate-like crystals of the new phase were obtained and crystal structure was investigated by single-crystal X-ray diffraction analysis. The structure was identified as the monoclinic system with space group P2 1 /m, and is composed of two Ca(Pr) planes, anti-fluorite Fe 2 As 2 layers, and As 2 zigzag chain layers. Plate-like crystals composed of the new phase showed superconductivity with T c ~20 K in both magnetization and resistivity measurements.Several groups of iron-based superconductors, such as REFeAs(O,F) 1 (RE = rare earth elements), AEFeAsF 2 (AE = alkaline earth metals), AEFe 2 As 2 3 , LiFeAs 4 , FeSe 5 , and compounds having perovskite-type oxide layer (ex. Fe 2 P 2 Sr 4 Sc 2 O 6 6 ) have been discovered since 2008. Developments of superconducting tapes and wires have been already attempted for potassium doped AEFe 2 As 2 and fluorine doped REFeAsO, because of their high T c and high H c2 . On the other hand, discovery of new superconductor with high T c and high chemical stability has been still desirable. Since iron-based superconductors are composed of stacking of superconducting layers of Fe 2 Pn 2 or Fe 2 Ch 2 ) (Pn = P, As, Ch = S, Se, Te) and blocking layers, design and search of new blocking layers are promising for discovery of new superconductors. There are several compounds having anti-fluorite blocks and As-based blocks such as UCuAs 2 7 . Recently new iron based superconductors Ca 10 (Pt 3 As 8 )(Fe 2 As 2 ) 5 and Ca 10 (Pt 4 As 8 )(Fe 2 As 2 ) 5 8-10 were reported. These compounds have As-based blocking layers between Fe 2 As 2 layers. The structures of these compounds indicate that there are possibilities to find new iron-based superconductors with arsenide blocking layers. In addition, Saha et al. reported that single crystalline (Ca,RE)Fe 2 As 2 (RE = La, Ce, Pr, Nd) showed superconductivity with high T c exceeding 40 K in resistivity measurement 11 . After this report, superconductivity in RE-doped CaFe 2 As 2 has been reported by several groups 12 . On the other hand, there are several works that report coexistence of two superconducting phases or interface superconductivity in this system 13,14 . Therefore, new superconducting phase is expected in Ca-RE-Fe-As system.In the present study, we have explored new iron-based superconductors in Ca-Pr-Fe-As system and found a new compound (Ca,Pr)FeAs 2 . We report a crystal structure and physical properties of (Ca,Pr)FeAs 2 .All samples were synthesized by the solid-state reaction starting from FeAs(3N), PrAs(3N), Ca(2N), As(4N), and CaO(3N). Since the starting reagents, PrAs and Ca are sensitive to moisture and/or oxygen in air, manipulations were carried out in an argon-filled glove box. Powder mixtures were pelletized, sealed in evacuated quartz ampoules, and reacted at 1000~1200°C for 24 h. Constituent phases were studied by the powder XRD measurements using RIGAKU Ultima-IV diffractometer and intensity date were collected in the 2 range of 5°-80° at a step of 0....