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Disappearance of superconductivity in the solid solution between (Ca4Al2O6)(Fe2As2) and (Ca4Al2O6)(Fe2P2) superconductors Parasharam M. Shirage, Kunihiro Kihou, Chul-Ho Lee, Nao Takeshita, Hiroshi Eisaki, and Akira Iyo J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja305548s • Publication Date (Web): 30 Aug 2012 Downloaded from http://pubs.acs.org on August 31, 2012

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Journal of the American Chemical Society

Disappearance of superconductivity in the solid solution between (Ca4Al2O6)(Fe2As2) and (Ca4Al2O6)(Fe2P2) superconductors Parasharam M. Shirage†*, Kunihiro Kihou†, Chul-Ho Lee, Nao Takeshita†, Hiroshi Eisaki†, Akira Iyo†* †National

Institute of Advance Industrial Science and Technology (AIST), Central-2, 1-1-1 Umezono, Tsukuba, Ibaraki-305 8568, Japan. *E-mail: [email protected] and [email protected] Supporting Information Placeholder Al, Sc, Ti, V, Cr etc., Pn = P, As. 12-19 Tc of 47 K is marked for ABSTRACT: Alloying effect of the two perovskite-type iron(Ca4(Ti,Mg)3O8)(Fe2As2) ((Ti,Mg)-43822(As)), making this based superconductors, (Ca4Al2O6)(Fe2As2) and system the second-highest Tc family among the Fe-based su(Ca4Al2O6)(Fe2P2), is examined. While the two stoichiometric perconductors.16 Also, (Sr4Sc2O6)(Fe2P2) (Sc-42622(P))12 and compounds possess relatively high-Tc’s of 28 K and 17 K, (Ca4Al2O6)(Fe2P2) (Al-42622(P)) 18 possess Tc of 17 K, which respectively, their solid solution, in a form of is the highest value among all the FeP-based superconductors. (Ca4Al2O6)(Fe2(As1-xPx)2), does not show superconductivity for From the structural point of view, alloying between a wide range from x = 0.50 to x = 0.95. The resultant phase (Ca Al 4 2O6)(Fe2As2) (Al-42622(As)), Tc = 28 K and Aldiagram is thus completely different from those of other typi42622(P), Tc = 17 K, is most intriguing. In the iron-based sucal iron-based superconductors such as BaFe2(As,P)2 and perconductors, there is a wide consensus that the shape of the LaFe(As,P)O, in which superconductivity shows up by substiFePn4 tetrahedron is crucial for determining Tc, and the maxituting P for As on the non-superconducting ‘parent’ commal Tc is attained when the FePn4 tetrahedron become regular pounds. Notably, the solid solutions in the nonshape, (As-Fe-As bond angle α =109.5° in Fig. 1).20 In the superconducting range exhibit resistivity anomalies at the case of Al-42622(As), α is estimated to be 102.1°, which is temperatures around 50 - 100 K. The behavior is reminiscent the smallest value among all the existing FeAsof the resistivity kink commonly observed in various nonsuperconductors.18 Here the small α comes from the short asuperconducting parent compounds, which signals the onset of axis lattice parameter (3.71 Å) due to the small size of the antiferromagnetic/orthorhombic long-range order. The similarCaAlO3 - based perovskite block layers, which results in the ity suggests that the suppression of the superconductivity in elongation of the FeAs4 tetrahedron along the c-axis. On the the present case is also of the magnetic and/or structural other hand, Al-42622(P) possesses a FeP4 tetrahedron close to origin. the regular shape (α = 109.4°). Accordingly, in the present system, substitution of P for As systematically changes the shape of the FeAs4 tetrahedron towards the direction which Besides exhibiting the second highest transition temperafavors higher Tc. ture (Tc) next to the cuprates, the newly discovered iron-based superconductors possess variety of interesting properties 1. In Pn Pn = As Pn = P particular, it is well recognized that the superconducting order Fe Tc (K) 28.3 17.1 resides proximity to the antiferromagnetic (AFM) order, and a (Å) 3.7133 3.6928 in most cases superconductivity emerges by suppressing the c (Å) 15.404 14.927 AFM long-range order existing in the stoichiometric ‘parent’ O lFe-Pn (Å) 2.387 2.262 compounds. There are various ways to suppress these orders. Ca Typical examples are (1) doping electrons/holes by chemical 109.4 αPn-Fe-Pn (°) 102.1 Al substitution, such as Co/K doping into BaFe2As2 2,3 or F dophFe-Pn (Å) 1.500 1.306 ing/introduction of O deficiency in LnFeAsO (Ln = lantha4-6 nides), and (2) application of physical/chemical pressure, α such as the substitution of smaller P atoms at the As sites in h 7-8 9-10 l BaFe2As2 and LnFeAsO. On the other hands, there are several exceptional ironbased compounds that exhibit superconductivity without any doping or pressure. The most well-known material is LiFeAs with Tc = 18 K.11 Another class of materials contain perovFig. 1. A schematic crystal structure of (Ca4Al2O6)(Fe2Pn2). A skite-based block layers, which are expressed either as right hand side table shows the crystal structure parameters for Pn (AEn+1TMnO3n-1)(Fe2Pn2) (abbreviated as TM-(n+1)n(3n= As and P. 1)22(Pn)) or as (AEn+2TMnO3n)(Fe2Pn2) (TM(n+2)n(3n)22(Pn)), in which AE = Ca, Sr, Ba, and TM = Mg,

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Based on the above reasoning, we have synthesized the solid solutions of the Al-42622(As) and Al-42622(P) superconductors, in a form of (Ca4Al2O6)(Fe2(As1-xPx)2) (Al42622(As,P)), and characterized their physical properties. While the main purpose of this study is to establish the As-P solid solution phase diagram which possesses two superconducting end members for the first time, we have also expected to enhance Tc in the alloyed system since it results in tuning the FeAs4 local structure. Against the initial expectation, we have instead found that superconductivity vanishes for wide compositions, ranging from x = 0.50 to x = 0.95, in striking contrast to the typical iron-based superconductors, such as BaFe2(As,P)2 and LaFe(As,P)O, in which superconductivity shows up by alloying P and As. Polycrystalline samples of Al-42622(As,P) were synthesized by the solid-state reaction method using high-pressure (HP) synthesis technique．The starting materials, CaO, Al, As, P, Fe and Fe2O3, were weighed with the corresponding compositions and mixed together using an agate mortar within a glove box filled with dry N2 gas. The mixed powder was loaded into a HP cell and then heated beyond 1150°C to 1300 °C, under the external pressure of 4.5 GPa. The detail of the sample synthesis is described in Ref. 18. Powder X-ray diffraction (XRD) patterns were measured with CuKα radiation, and dc magnetic susceptibility was measured with a SQUID magnetometer (Quantum Design MPMS). The resistivity was measured by the four-probe method.

Figs. 2 X-ray diffraction patterns of (Ca4Al2O6)(Fe2(As1-xPx)2) with x = 0, 0.5, 0.75 and 1.0. Open circles indicate a CaO impurity phase.

Fig. 2 shows the XRD patterns of (Ca4Al2O6)(Fe2(As1P ) ) x x 2 samples with x = 0.0, 0.20, 0.50, 0.75, 1.0. Major peaks are indexed based on the tetragonal crystal structure (P4/nmm), while small amount of CaO is detected in some of the XRD patterns. The a- and c-axis lattice parameters are calculated from the XRD patterns using the least square method, as shown in Fig. 3. The lattice parameters change with x almost linearly, following the Vegard’s law. The c-axis lattice parameter of Al-42622(P) shortens by 3.1% compared to that of Al-42622(As), which is comparable to LaFePnO (~ 2.6 %) 1, 21 and BaFe2Pn2 (~ 4.3 %) . 22 On the other hand, the contraction of the a-axis lattice parameter is ~ 0.6 %, much smaller than those of LaFePnO (~ 1.8 %) and BaFe2Pn2 (~ 2.9 %), due

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to the rigid structure of the CaAlO3-based perovskite-type block units. As a result, the FePn4 tetrahedron changes its shape without changing the Fe-Fe distance.

Figs. 3 lattice parameters of a- and c-axis as a function of x in (Ca4Al2O6)(Fe2(As1-xPx)2).

Fig. 4 shows the temperature (T)-dependence of the zero-field-cooled (ZFC) and the field-cooled (FC) magnetic susceptibility (χ) for x = 0.0, 0.20, 0.50, 0.60, 0.75, 0.95, 1.0 samples. The samples with x = 0.0, 0.20, 0.50 and 1.0 show clear superconducting transitions at onset temperatures of 26.8 K, 22.5 K, 4.2 K, and 16.9 K, respectively. Tc of x = 0.20 sample is lower than that of x = 0 one by 4 K. At x = 0.50, Tc exhibit salient sample dependence, ranging from 16 K to < 2 K, suggesting that the phase boundary between the superconducting and non-superconducting regions exists around this composition. On the other hand for the x = 0.60, 0.75 and 0.95 samples, there is no trace of superconductivity above 2 K. Note that superconductivity appears again for x = 1.0. Near x = 1.0, small amount of As is sufficient to suppress the superconductivity completely. In Fig. 5, T-dependent resistivity (ρ) data on the same samples are shown. The samples with x = 0.0, 0.20, 0.50 and 1.0 show superconducting transitions at almost the same temperatures observed in the susceptibility measurements. There is no clear superconducting transition above 2 K for x = 0.60, 0.75 and 0.95. These behaviors are consistent with the susceptibility measurements. Notably, the non-superconducting and the low-Tc samples, x = 0.50, 0.60, 0.75, 0.95, exhibit resistivity anomalies (kinks), which are indicated by black arrows. The kink temperature (Ta) is highest at 97 K for the x = 0.50 samples, and systematically decreases with x, down to 50 K for x = 0.95. The feature becomes obscurer with x, finally indiscernible at x = 1.0. The observed kink behavior is reminiscent of the resistivity anomaly accompanying the AFM order phase transition, which is recognized as the hallmark of the parent compounds of the FeAs-based superconductors. Based on the above results, the electronic phase diagram of the Al-42622(As,P) solid solution system is depicted as Fig. 6. Here Tc and Ta are determined from the magnetic susceptibility and the resistivity data, respectively. With increasing x, Tc gradually decreases up to x = 0.50, then superconductivity suddenly disappears. Non-superconducting region dominates the phase diagram over a wide range from x = 0.50 to x = 0.95, following the sudden recovery of the superconductivity at x = 1.0. This phase diagram is in stark contrast to other typical iron-based superconductors, such as BaFe2(As,P)2 or


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LaFe(As,P)O, where the superconductivity shows up by the P substitution on non-superconducting parent compounds which exist around x = 0.

Figs. 4 Temperature dependence of magnetic susceptibility of (Ca4Al2O6)(Fe2(As1-xPx)2) with x = 0, 0.2, 0.5, 0.6, 0.75, 0.95 and 1.0. Black arrows indicate onset of superconducting transitions.

What is the reason for the disappearance of superconductivity between x = 0.50 to x = 0.95? The immediate candidate is the effect of disorder introduced by the substitution of P at random positions. However, the effect of As-P disorder is considered to be rather weak in iron-based superconductors, considering the following experimental facts. (1) In the solid solution between LaFeAsO0.9F0.1 (Tc = 26 K) and LaFePO0.9F0.1 (Tc = 7 K), Tc changes almost linearly with P content.23 There is no additional decrease in Tc ascribed to the disorder effect in the intermediate region. (2) In BaFe2Pn2, superconductivity appears as a consequence of alloying. The resultant metallic state is highly conductive, as evidenced by the observation of the quantum oscillations.24 The result implies that the effect of P disorder is very weak in the BaFe2Pn2 system. We also note that the existence of the nonsuperconducting region in Al-42622(As,P) cannot be explained by the theoretical band calculation.25, 26 As pointed out, the resistivity of the nonsuperconducting samples is characterized by the kink around 50 - 100 K. This is reminiscent of the behavior observed in the parent compounds, in which case it is related to the onset of the AFM long-range order. The similarity suggests that the lack of the superconductivity in the present As-P alloyed samples is also associated with the AFM order. We remark that the signature of the AFM long-range order in the alloyed samples have been actually confirmed by the measurements of the 31Pnuclear magnetic resonance (NMR) on the same samples.27

Fig. 5 Temperature dependence of resistivity of (Ca4Al2O6)(Fe2(As1-xPx)2) with x = 0, 0.2, 0.5, 0.6, 0.75, 0.95 and 1.0. Resistivity anomalies (kinks) are indicated by black arrows.

Fig. 6 Tc and Ta as a function of x in (Ca4Al2O6)(Fe2(As1-xPx)2) . The results not presented in Fig. 4 or Fig. 5 are also plotted.

As pointed out, the FeAs4 octahedron of Al-42622(As) is elongated along the c-axis, resulting in the small As-Fe-As angle α = 102.1°. Notably, another ‘stoichiometric’ FeAsbased superconductor, LiFeAs, also possesses an elongated FeAs4 tetrahedron with α = 103.1°.28 In terms of the shape of the tetrahedron, these materials are distinct from other stoichi-



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ometric non-superconducting (exhibit the AFM) compounds, such as LaFeAsO (α = 113.5°)1, SmFeAsO (α = 110.5°) 28, BaFe2As2 (α = 111.1°) 29, NaFeAs (α = 108.6°) 30, which commonly possess α much closer to the regular value of 109.5°. The As-Fe-As bond-angle α is considered as a critical parameter which determines the ground state of the stoichiometric FeAs-based compounds because NaFeAs and LiFeAs possess different ground states, in spite of the same crystal structure. Small (large) α favors the superconducting (antiferromagnetic-orthorhombic) ground state. In the case of Al-42622(As,P), P substitution for As results in squeezing the elongated FePn4 tetrahedron towards the regular shape. According to the aforementioned material trend, this deformation results in suppressing the superconducting ground state and favoring the AFM ground state. Along this line, the disappearance of the superconductivity is due to the structural origin, ascribed to the emergence of the AFM order. If this is the case, one can naturally explain why the phase diagram of the Al-42622(As,P) system differs from that of BaFe2(As,P)2 or LaFe(As,P)O, since the latter two systems possess the regular FeAs4 tetrahedron in the As end members (x = 0), and the P substitution results in deforming the tetrahedron away from the regular shape. So far, regular tetrahedron is believed to favor high-Tc. The present results suggest that in the stoichiometric compounds, regular tetrahedron also favors the AFM long-range order. This is naturally understandable, since the stronger antiferromagnetic/structural (orbital) interaction should yield higher Tc, once it contributes for the formation of Cooper pairs. In Summary, Non-superconducting region is found to exist in the phase diagram in solid solutions between (Ca4Al2O6)(Fe2As2) and (Ca4Al2O6)(Fe2P2) superconductors. Superconductivity strongly suppressed for a wide composition range of 0.50 < x ≤ 0.95 in (Ca4Al2O6)(Fe2(As1-xPx)2). It demonstrates again the importance of the α for superconductivity in the iron-pnictides. The non-superconducting and low Tc samples have resistive anomalies at the temperature range from 46 to 97 K depending on x. We propose that the development of AFM order below Ta is a possible reason for vanishing of the superconductivity. The Al-42622(As,P) will provide a new platform to understand the mechanism of superconductivity in iron-based superconductors.

ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Specially Promoted Research (20001004) from The Ministry of Education, Culture, Sports, Science and Technology (MEXT).

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