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.

Luminescent mechanochromism has been intensively studied in the past few years. However, the difference in the anisotropic grinding and the isotropic compression is not clearly distinguished in many cases, in spite of the importance of this discrimination for the application of such mechanochromic materials. We now report the distinct luminescent responses of a new organic fluorophore, tetrathiazolylthiophene, to these stresses. The multichromism is achieved over the entire visible region using the single fluorophore. The different mechanisms of a blue shift by grinding crystals and of a red shift under hydrostatic pressure are fully investigated, which includes a high-pressure single-crystal X-ray diffraction analysis. The anisotropic and isotropic modes of mechanical loading suppress and enhance the excimer formation, respectively, in the 3D hydrogen-bond network.

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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....