A New Layered Iron Arsenide Superconductor: (Ca,Pr)FeAs₂

Abstract

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....

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1−xLaxFeAs2

“…the space group P 2 1 or P 2 1 /m rather than the originally assumed tetragonal structure [13,14]. Second harmonic generation experiments confirmed the space group P 2 1 for La doped compounds [15].…”

“…[14], and the latter is reported, for example, for the Pr doped compound in Ref. [13]. While second harmonic generation verifies the lack of inversion symmetry in the La doped compound [15], no similar study exists for the Pr doped compound.)…”

“…The other X-ray diffraction experiment on Pr-doped compounds (Ca 1−x Pr x FeAs 2 ) were reported to have the monoclinic structure with P 2 1 /m having the inversion symmetry [13]. (The measured lattice constants are a = 3.9163Å, b = 3.8953Å, c = 10.311Å, and β = 90.788 • .)…”

“…Attempts to synthesize iron pnictide compounds in this structure were not originally successful, but new Mn based materials in this structure were found [7,8] and it was observed theoretically [7] and experimentally [8][9][10][11] that the spacer layers exhibit Dirac cones [12]. Recently, Fe superconductors in the 112 structure were synthesized, (Ca,Pr)FeAs 2 [13], and Ca 1−x La x FeAs 2 [14].…”

Dependences on RE of superconducting properties of transition metal co-doped (Ca,RE)FeAs2 with RE= La–Gd

“…Searches for new iron-based superconductors have been still continuing, resulted in a recent discovery of new series of iron-based superconductors (Ca,RE)FeAs 2 ((Ca,RE)112) [5][6][7][8][9] . (Ca,RE)112 has several unique properties, such as monoclinic structure with space group of P2 1 or P2 1 /m, and co-existence of As -1 and As -3 in the crystal lattice [5,6] . Moreover, anisotropy of (Ca,La)112 was reported to be 2.0-4.2 [10] , which is smaller than that of 1111 type superconductors, while interlayer distance between Fe layer (d Fe-Fe ) is longer in (Ca,La)112.…”