Enhanced Superconductivity in Close Proximity to the Structural Phase Transition of Sr1-xBaxNi2P2

TL;DR

This study investigates how structural phase transitions in Sr1-xBaxNi2P2 influence its superconducting properties, revealing a peak Tc near the phase boundary and linking P-P dimer breaking to structural changes.

Contribution

It provides new insights into the relationship between structural phase transitions and superconductivity in Sr1-xBaxNi2P2, highlighting the impact of P-P dimer breaking.

Findings

Structural phase transition at x=0.5 from orthorhombic to tetragonal

Maximum Tc of 2.85 K at x=0.4

P-P dimers break during the phase transition

Abstract

The structural evolution and superconductivity of a 122-type solid solution Sr1-xBaxNi2P2 were studied. We found that an orthorhombic-tetragonal structural phase transition takes place at x = 0.5, and is characterized by the P-P dimers breaking. The superconducting transition temperature exhibited its highest value of 2.85 K at x = 0.4.

Full text

The transition-metal compounds AT2X2 exhibit a variety of crystal structures, and the most common is the tetragonal ThCr2Si2-type (space group $I4/mmm$, $D_{4h}^{17}$, No. 139). [1] This structure can further be classified into two types: the “collapsed” tetragonal that is characterized by the formation of molecule-like X-X dimers along the crystallographic $c$-axis, and the “uncollapsed” tetragonal in which the X-X dimers are broken. Interestingly, some compounds exhibit structural phase transitions from the uncollapsed to collapsed tetragonal phases when hydrostatic pressure or chemical doping is applied. Typical examples include CaFe2As2 under pressure, [2, 3] CaFe2(As1-xPx)2, [4] and Ca(Fe1-xRhx)2As2, [5] in which the formation of X-X bonds results in the loss of the Fe magnetic moment and the disappearance of superconductivity. Other notable examples are Sr1-xCaxCo2P2,[6] LaCo2(Ge1-xPx)2,[7] and SrCo2(Ge1-xPx)2,[8] in which the ferromagnetic quantum critical point is induced by the X-X dimer breaking. [8]

A rare but intriguing structure among AT2X2 appears in SrNi2P2. This compound crystallizes in an orthorhombic structure with the space group $Immm$ ($D_{2h}^{25}$, No. 71) [9]. In this structure, the unit cell is made up of 12 phosphorus atoms, of which four phosphorus atoms form dimers while the remaining eight do not. This results in a superstructure along the crystallographic $b$-axis, and hence causes orthorhombic distortion, as shown in Fig. 1(a). On the other hand, BaNi2P2 crystallizes in an uncollapsed tetragonal structure ($I4/mmm$), [9] in which no P-P dimers are formed, as shown in Fig. 1(b). Both compounds exhibit superconductivity, and the superconducting transition temperatures $T_{\rm c}$ are 1.4 K in SrNi2P2[10] and 2.5 K in BaNi2P2[11, 12]. This raises questions regarding which composition causes the P-P dimer breaking to occur in the solid solution Sr1-xBaxNi2P2, and how $T_{\rm c}$ varies as a function of the composition $x$.

In this paper, we present the results of our synthesis of the SrNi2P2–BaNi2P2 solid solution along with the crystallographic and superconducting properties of the samples. Our observations suggest that there was interplay between the structural transitions and superconductivity in Sr1-xBaxNi2P2.

Polycrystalline samples of Sr1-xBaxNi2P2 with nominal Ba content of 0.00 $\leq$ $x$ $\leq$ 1.00 were synthesized by means of a solid-state reaction. The stoichiometric amounts of the starting materials Ba, Sr, Ni, and P were mixed, ground, placed in an alumina crucible, and then sealed in an evacuated quartz tube. The ampoule was heated at 300 ∘C for 5 h, at 700 ∘C for 3 h, and at 1000 ∘C for 24 h, followed by natural cooling in the furnace. Powder X-ray diffraction (XRD) studies confirmed that the resulting samples were a single phase of Sr1-xBaxNi2P2. The lattice parameters and interlayer P-P distances were estimated by the Rietveld refinement using the RIETAN-FP program. [13] The magnetization $M$ was measured using the Magnetic Property Measurement System (MPMS) by Quantum Design at temperatures above 1.8 K.

Based on the XRD data measured at room temperature, the structure at $x<$ 0.5 was confirmed to be the orthorhombic structure ($Immm$), whereas at $x>$ 0.5 the structure was confirmed to be the uncollapsed tetragonal ($I4/mmm$). The orthorhombic–tetragonal structural transition can be seen in the $x$ dependence of the $c$ parameter, which exhibited a jump at $x$ = 0.5, as shown in Fig. 2(a). On the other hand, the variation in the $a$ parameter was very small, as shown in Fig. 2(b). The orthorhombic phase at $x<$ 0.5 can be characterized by the two distinct P-P distances, P(1)-P(1) and P(2)-P(2), as shown in Fig. 2(c). The P(2)-P(2) distance was comparable to the P-P distances of 2.251–2.710 Å in the collapsed tetragonal phase of CaTM2P2 (TM = Fe, Co, Ni, Cu), [1] and thus indicates a covalent bond. A large P(1)-P(1) distance suggests the absence of a covalent bond between them. In contrast, in the tetragonal phase at $x>$ 0.5, the P-P distance was uniform and much longer than that of the covalent P-P bond, as shown in Fig. 2(c), and the covalent P-P bonds were completely broken.

Figure 3 shows the temperature dependence of the magnetization $M$ for Sr1-xBaxNi2P2 at a magnetic field of 30 Oe in the zero-field and field cooling conditions. Using this data, we estimated the superconducting transition temperature $T_{\rm c}$ and superconducting volume fraction as a function of the doping $x$, as summarized in Fig. 4. Bulk superconductivity with a substantial superconducting volume fraction was observed at $x$ $\geq$ 0.4 for Sr1-xBaxNi2P2. The superconducting transition temperature exhibited the highest value of $T_{\rm c}$ = 2.85 K at $x$ = 0.4. This $T_{\rm c}$ value was twice as high as the $T_{\rm c}$ = 1.4 K of the end member SrNi2P2. [10] $T_{\rm c}$ decreased monotonically with increasing $x$, and the end member BaNi2P2 exhibited $T_{\rm c}$ = 2.53 K, which is in good agreement with previous reports. [11, 12] In contrast, the superconductivity at 0.15 $\leq$ $x$ $<$ 0.4 is not bulk in nature. The superconducting volume fraction decreased with decreasing $x$ below $x$ = 0.4, and the $T_{\rm c}$ was almost independent of $x$. This behavior suggests the coexistence of superconducting and non-superconducting phases, and their phase boundary is located at $x$ $\simeq$ 0.4. However, the XRD data suggested the formation of a continuous solid solution in this doping range. The orthorhombic–tetragonal structural phase boundary was located at $x$ $\simeq$ 0.5, as shown in Fig. 2. To account for this discrepancy, another phase transition, presumably to the collapsed tetragonal structure, should be invoked below room temperature. Incidentally, SrNi2(P1-xAsx)2 is known to exhibit three structural phases, namely the orthorhombic ($Immm$), collapsed, and uncollapsed tetragonal ($I4/mmm$) structures, depending on the temperature and composition. [14]

In conclusion, we have demonstrated that the SrNi2P2-BaNi2P2 solid solution exhibits a structural phase transition that is characterized by the breaking of the P(2)-P(2) dimers of the orthorhombic structure. The superconducting transition temperature was enhanced up to $T_{\rm c}$ = 2.85 K when the composition was in the vicinity of the structural transition. This $T_{\rm c}$ was higher than those of the end members SrNi2P2 (1.4 K) and BaNi2P2 (2.5 K). Thus, the tuning of the X-X bonding state in AT2X2 with the ThCr2Si2-type structure is an effective means to develop superconductors with enhanced $T_{\rm c}$.

\acknowledgment

A part of this work was performed at the Advanced Science Research Center, Okayama University. This work was partially supported by Grants-in-Aid for Scientific Research (No. 26287082, 15H01047, 15H05886, and 16K05451) provided by the Japan Society for the Promotion of Science (JSPS) and the Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers from JSPS.