PHYSICAL REVIEW B 91, 104519 (2015) Superconductivity and role of pnictogen and Fe substitution in 112-LaPd x Pn 2 ( Pn = Sb, Bi) Reiner Retzlaff, 1 Alexander Buckow, 1 Philipp Komissinskiy, 1 Soumya Ray, 1 Stefan Schmidt, 2 Holger M¨ uhlig, 2 Frank Schmidl, 2 Paul Seidel, 2 Jose Kurian, 1 and Lambert Alff 1, * 1 Institute of Materials Science, Technische Universit¨ at Darmstadt, Alarich-Weiss-Strasse 2, 64287 Darmstadt, Germany 2 Institut f ¨ ur Festk¨ orperphysik, Friedrich-Schiller-Universit¨ at Jena, Helmholtzweg 5, 07743 Jena, Germany (Received 8 January 2015; revised manuscript received 6 March 2015; published 26 March 2015) We report on the epitaxial growth of As-free and phase-pure thin films of the 112-pnictide compounds LaPd x Pn 2 (Pn = Sb, Bi) grown on (100) MgO substrates by molecular beam epitaxy. X-ray diffraction, reflection high-energy electron diffraction, and x-ray photoelectron spectroscopy confirm the HfCuSi 2 structure of the material with a peculiar pnictogen square net layer. The superconducting transition temperature T c varies little with Pd concentration. LaPd x Sb 2 has a higher T c (3.2 K) by about 20% compared with LaPd x Bi 2 (2.7 K). Fe substitution of Pd leads to a rapid decay of superconductivity, suggesting that these superconductors are conventional type II. DOI: 10.1103/PhysRevB.91.104519 PACS number(s): 74.70.Dd, 74.70.Xa, 74.78.−w, 81.15.Hi I. INTRODUCTION The research on Fe-based pnictides with ZrCuSiAs struc- ture [1] has led to the discovery of materials with superconduct- ing transition temperatures T c as high as 56 K [2–4]. In these compounds the pnictogen is arsenic or phosphor. It is desirable to search for As-free superconducting pnictides not only be- cause of the highly toxic nature of this element but also because of fundamental interest in understanding the role of the pnic- togen ion by investigating, e.g., Bi- and Sb-based compounds. There are predictions that the substitution of As by Sb could even lead to novel high-temperature superconductors [5,6]. While the vast majority of research has focused on the 1111-, 122-, 111-, and 11-type pnictides and chalcogenides, recently, high-transition temperatures of up to 45 K have also been reported in the 112-type iron pnictides Ca 1−x La x FeAs 2 , with x between 0.1 and 0.2[7–11]. One modification of the 112 structure belongs to the same symmetry group P 4/nmm (No. 129) as the 1111- or ZrCuSiAs-type materials. However, the atomic arrangement contains a peculiar pnictogen square net layer that was identified for the first time for the compound HfCuSi 2 in 1975 [12,13]. Sometimes, this structure type is also referred to as ZrCuSi 2 (which is easily confused with ZrCuSiAs). Recently, in this modification of the 112 type, superconductivity has been discovered at about 4 K in RENi x Bi 2 , with RE = La, Ce, Nd, and Y [14]. One intriguing property of these compounds is the mixed-valence state of the pnictogen. In the pnictogen square net layer, the ions have a formal valence of −1, while in the more three-dimensional metal-pnictogen layer (corresponding to the Fe-As layer, e.g., in LaFeAsO) the pnictide has a formal valence of −3. For most rare-earth elements it is difficult to achieve phase-pure bulk materials with a large superconducting volume fraction in RENi x Pn 2 [14]. In the case of CeNi 0.8 Bi 2 , samples suitable for neutron scattering were achieved, revealing that the antiferromagnetic ordering of Ce 3+ does not interact with the superconducting condensate. The missing coupling of spin fluctuations to the superconducting charge carriers has been interpreted as the reason for the low critical temperature * alff@oxide.tu-darmstadt.de in this compound [15]. Recently, it has been argued along the same lines that the isostructural compound HfCuGe 2 is a nonmagnetic analog of the 1111 iron pnictides and therefore has a reduced T c of 0.6 K [16]. Kodama et al. have suggested that superconductivity occurs in the pnictogen square net layer instead of the transition-metal-pnictogen layer [15]. Recently, the Pd-based compounds LaPd 1−x Bi 2 and CePd 1−x Bi 2 have been grown as single crystals, with a bulk T c of 2.1 K for LaPd 1−x Bi 2 , whereas CePd 1−x Bi 2 was nonsuperconducting with antiferromagnetic ordering at 6K[17]. Note that the previously mentioned high T c values above 34 K have been achieved in a second modification of the 112 type which is distorted from the space group P 4/nmm to the monoclinic space group P 2 1 (No. 4) [7–9]. Nevertheless, the key structural features of the quasi-two- dimensional square net layer where As bonds are arranged in a zigzag way and the typical Fe-pnictide layer common to all pnictide superconductors are the same for both modifica- tions. We stress that the reported high T c values still await confirmation. Some of the difficulties in preparing bulk samples of 112 type can be overcome by using a thin-film approach [18,19]. It has been reported for CeNi x Bi 2 and LaNi x Bi 2 that phase-pure epitaxial thin films with T c similar to or even higher than those of the bulk can be prepared by molecular beam epitaxy (MBE) [20,21]. For antimonides, superconducting transitions have been reported for LaNi x Sb 2 at 1.0 K, for LaCu x Sb 2 at 0.9 K, and for LaPd x Sb 2 at 2.7 K [22]. The 112 compounds tend to have a relatively large amount of metal vacancies (indicated by x< 1) due to the formal valence state of −1, which is partially avoided by vacancy formation. With respect to the role of Sb, it is interesting to note that slight Sb doping into Ca 1−x La x FeAs 0.99 Sb 0.01 with x = 0.15 has induced a higher T c [9]. Recently, several studies of the Pd-based 122 compounds APd 2 As 2 (A = Ca, Sr, Ba, La) and SrPd 2 Ge 2 have shown that they are conventional type-II nodeless (but anisotropic) s -wave electron-phonon superconductors with a maximum T c of 1.4 K [23–25]. Since it is difficult to stabilize the Fe-based 112 bismuthides and antimonides [26], we have investigated epitaxial thin films of LaPd x Sb 2 and LaPd x Bi 2 to study the role of the pnictogen ion. In addition, we have 1098-0121/2015/91(10)/104519(7) 104519-1 ©2015 American Physical Society