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Register a new userA New Iron Pnictide Oxide (Fe2As2)(Ca5(Mg,Ti)4Oy) and a New Phase in Fe-As-Ca-Mg-Ti-O system
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A new layered iron arsenide oxide (Fe2As2)(Ca5(Mg,Ti)4Oy) and its structural derivative were found in the Fe-As-Ca-Mg-Ti-O system. The crystal structure of (Fe2As2)(Ca5(Mg,Ti)4Oy) is identical to that of (Fe2As2)(Ca5(Sc,Ti)4Oy), which was reported in our previous study. The lattice constants of this compound are a = 3.86(4) A and c = 41.05(2) A. In addition, another phase with a thicker blocking layer was found. The structure of the compound and its derivative was tentatively assigned through STEM observation as (Fe2As2)(Ca8(Mg,Ti)6Oy) with sextuple perovskite-type sheets divided by a rock salt layer. The interlayer Fe-Fe distance of this compound is ~30 A. The compound and its derivative exhibited bulk superconductivity, as found from magnetization and resistivity measurements.
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A new layered iron arsenide oxide (Fe2As2)(Ca4(Mg,Ti)3Oy) was discovered. Its crystal structure is tetragonal with a space group of I4/mmm consisted of the anti-fluorite type FeAs layer and blocking layer of triple perovskite cells and is identical with (Fe2As2)(Sr4(Sc,Ti)3O8) discovered in our previous study. The lattice constants of (Fe2As2)(Ca4(Mg,Ti)3Oy) are a = 3.877 A and c = 33.37 A. This compound exhibited bulk superconductivity up to 43 K in susceptibility measurement without intentional carrier doping. A resistivity drop was observed at ~47 K and zero resistance was achieved at 42 K. These values correspond to the second highest Tc among the layered iron-based superconductors after REFeAsO system.
$^{75}$As and $^{45}$Sc NMR measurements unravel the electronic state for Fe-based superconductors with perovskite-type blocking layers Ca$_4$(Mg,Ti)$_3$Fe$_2$As$_2$O$_{8-y}$ ($T_c^{onset}=47$ K) and Ca$_5$(Sc,Ti)$_4$Fe$_2$As$_2$O$_{11-y}$ ($T_c^{onset}=41$ K). In Ca$_5$(Sc,Ti)$_4$Fe$_2$As$_2$O$_{11-y}$, the nuclear spin relaxation rate $1/T_1$ shows pseudogap behavior below $sim80$ K, suggesting that the electronic state is similar to that of LaFeAs(O,F) system with moderate electron doping. The presence of the pseudogap behavior gives an interpretation that the hole-like band (so-called $gamma$ pocket) is located just below the Fermi level from the analogy to LaFeAs(O,F) system and the disappearance of the $gamma$ pocket yields the suppression of the low-energy spin fluctuations. On the other hand, in Ca$_4$(Mg,Ti)$_3$Fe$_2$As$_2$O$_{8-y}$ satisfying the structural optimal condition for higher $T_c$ among the perovskite systems, the extrinsic contribution, which presumably originates in the Ti moment, is observed in $1/T_1T$; however, the moderate temperature dependence of $1/T_1T$ appears by its suppression under high magnetic field. In both systems, the high $T_c$ of $sim40$ K is realized in the absence of the strong development of the low-energy spin fluctuations. The present results reveal that the structural optimization does not induce the strong development of the low-energy spin fluctuations. If we consider that superconductivity is mediated by spin fluctuations, the structural optimization is conjectured to provide a benefit to the development of the high-energy spin fluctuations irrespective to the low-energy part.
We synthesized new layered iron arsenide oxides (Fe2As2)(Sr4(Sc,Ti)3O8),(Fe2As2)(Ba4Sc3O7.5), and (Fe2As2)(Ba3Sc2O5). The crystal structures of these compounds are tetragonal with a space group of I4/mmm. The structure of (Fe2As2)(Sr4(Sc,Ti)3O8) and (Fe2As2)(Ba4Sc3O7.5) consists of the alternate stacking of antifluorite Fe2As2 layers and triple perovskite-type oxide layers. The interlayer distance between the Fe planes of (Fe2As2)(Ba4Sc3O7.5) is ~18.7 A. Moreover, the a-axis of (Fe2As2)(Ba3Sc2O5) is the longest among the layered iron pnictides, indicating the structural flexibility of the layered iron pnictide containing perovskite-type layers. The bulk sample of (Fe2As2)(Sr4(Sc0.6Ti0.4)3O8) exhibited diamagnetism up to 28 K in susceptibility measurements.
We have discovered a new homologous series of iron pnictide oxides (Fe2As2)(Can+2(Al,Ti)nOy)[n = 2,3,4]. These compounds have perovskite-like blocking layers between Fe2As2 layers. The structure of new compounds are tetragonal with space groups of P4/nmm for n = 2 and 4 and P4mm for n = 3, which are similar to those of (Fe2As2)(Can+1(Sc,Ti)nOy)[n = 3,4,5] found in our previous study. Compounds with n = 3 and 4 have new crystal structures with 3 and 4 sheets of perovskite layers, respectively, including a rock salt layer in each blocking layer. The a-axis lengths of the three compounds are approximately 3.8 A, which are close to those of FeSe and LiFeAs. (Fe2As2)(Ca6(Al,Ti)4Oy) exhibited bulk superconductivity in magnetization measurement with Tc(onset)~36 K and resistivity drop was observed at ~39 K. (Fe2As2)(Ca5(Al,Ti)3Oy) also showed large diamagnetism at low temperatures. These new compounds indicate considerable rooms are still remaining for new superconductors in layered iron pnictides.
Combining crystal structure search and first-principles calculations, we report a series of two-dimensional (2D) metal borides including orthorhombic (ort-) MB6 (M=Mg, Ca) and hexagonal (hex-) MB6 (M=Mg, Ca, Sc, Ti, Sr, Y). Then, we investigate their geometrical structures, bonding properties, electronic structures, mechanical properties, phonon dispersions, thermal stability, dynamic stability, electron-phonon coupling (EPC), superconducting properties and so on. Our ab initio molecular dynamics simulation results show that these MB6 can maintain their original configurations up to 700/1000 K, indicating their excellent thermal stability. All their elastic constants satisfy the Born mechanically stable criteria and no visible imaginary frequencies are observed in their phonon dispersions. The EPC results show that these 2D MB6 are all intrinsic phonon-mediated superconductors with the superconducting transition temperature (Tc??) in the range of 2.2-21.3 K. Among them, the highest Tc (21.3 K) appears in hex-CaB6, whose EPC constant () is 0.94. By applying tensile/compressive strains on ort-/hex-CaB6, we find that the compressive strain can obviously soften the acoustic phonon branch and enhance the EPC as well as Tc. The Tc of the hex-CaB6 can be increased from 21.3 K to 28 K under compressive strain of 3%. These findings enrich the database of 2D superconductors and should stimulate experimental synthesizing and characterizing of 2D superconducting metal borides.
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