Introduction

Superconductivity in unconventional high-temperature (high-Tc) superconductors is related to an antiferromagnetic (AFM) ground state in an undoped layered ‘parent’ compound1,2,3,4,5. Therefore, the layered AFM compounds with higher Neel temperature (TN) are believed to be good candidates for the parent compounds of new high-Tc superconductors6. The Mn-pnicitides of Mn-1111 and Mn-122 compounds, such as LaMnPO7,8,9, BaMn2P210,11,12, BaMn2As213,14,15,16,17,18,19,20,21,22 and BaMn2Bi223,24,25, are such kinds of compounds, having AFM ground state similar to the parent compounds of cuprates26 and the same crystal structure as Fe-pnictides2,3. For such similarities, it is reasonably expected that the BaMn2Bi2 might be a parent compound of Mn-based superconductors.

Previous studies demonstrated that the AFM order in the parent compounds of cuprates and FeAs-based superconductors can be efficiently suppressed by either external pressure or chemical doping, driving the samples into a superconducting state2,4,27,28,29,30,31,32,33,34,35,36,37,38,39,40. Neutron studies show that BaMn2Bi2 has an alternative magnetic structure from the parent compounds of cuprates and FeAs-122 superconductors, adopting G-type AFM with TN = 387 K in the tetragonal phase24. Doping potassium (K) for Ba sites has turned the sample from an insulating/semiconducting state to a metallic state23, but it has no significant effect on its AFM order. In this work, we perform high-pressure investigations on the Ba0.61K0.39Mn2Bi2 by in-situ high pressure electrical transport and X-ray diffraction (XRD) measurements in a diamond anvil cell (DAC). We find no evidence of superconductivity in the pressurized Ba0.61K0.39Mn2Bi2 up to 35.6 GPa, however, a pressure-induced tetragonal (T)-to-orthorhombic (OR) phase transition is observed. Our in-situ high pressure ac susceptibility measurements and theoretical calculations reveal that the Ba0.61K0.39Mn2Bi2 has robust antiferromagnetism in both of the T phase and OR phase and is a strong Hund's AFM metal with a hybridization between localized spin electrons and itinerant electrons.

Results

Figure 1(a) and 1(b) show temperature (T) dependent of resistance (R) of the Ba0.61K0.39Mn2Bi2 under pressure up 35.6 GPa. It is found that the R-T curves are pressure dependent, i.e. the R-T curve moves up with increasing pressure up to 19.9 GPa, while it changes its trend with further increasing pressure ranging from 19.9 to 35.6 GPa. Plot of pressure dependence of resistance measured at different temperatures illustrates this conspicuous feature (Fig. 1c). This feature may be associated with a first-order structure phase transition. Zooming in the R-T curves for lower temperature range, we found no sign of pressure-induced superconductivity down to 1.5 K under pressure up to 35.6 GPa (Fig. 1d and 1e). To confirm the experimental results obtained in Ba0.61K0.39Mn2Bi2, we loaded the sample with the actual composition of Ba0.68K0.32Mn2Bi2 from Sefat's Group into a DAC in a glove-box and performed experiments in the same manner. We observed the same high pressure behavior as found in Ba0.61K0.39Mn2Bi2.

Figure 1
figure 1

Electrical resistance as a function of temperature and pressure for Ba0.61K0.39Mn2Bi2.

(a) and (b) Temperature dependent resistance of Ba0.61K0.39Mn2Bi2 at different pressures. (c) Pressure dependence of resistance measured at different temperatures, displaying a dome-like feature centered at ~20–23 GPa. (d) and (e) Blown-up R-T curves at different pressures, showing no evidence of superconductivity.

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Structure is crucial in tailoring the superconductivity. At ambient pressure, BaMn2Bi2 adopts a body-centered ThCr2Si2 tetragonal structure in the space group I4/mmm. Hole-doping via substitution of Ba with K in the form of Ba1-xKxMn2Bi2 does not alter its crystal structure23. While, applying pressure on the Ba0.61K0.39Mn2Bi2 in this study found a T-OR phase transition at 19.8 GPa, as shown in Fig. 2(a), very similar to the case seen in the pressurized LaMnPO8. The OR phase in Ba0.61K0.39Mn2Bi2 persists up to 28.8 GPa. The evolutions of its lattice parameters and volume with pressure in the two phases are displayed in Fig. 2(b) and 2(c). The pressure induced T-OR phase transition determined by high-pressure XRD measurements is consistent with our resistance data. As shown in Fig. 1c, pressure dependence of resistance displays a dome-like feature. The resistance increases with pressure, reaches a maximum at ~20 GPa and then decreases with further increasing pressure. Notably, the change in resistance against pressure follows the similar trend at different temperatures down to 4 K, indicating that either the room-temperature T phase or the room-temperature OR phase can be maintained down to 4 K.

Figure 2
figure 2

X-ray diffraction patterns and Rietveld refinements results of Ba0.61K0.39Mn2Bi2 at different pressures and its lattice parameter and volume as a function of pressure.

(a) Representative XRD patterns for Ba0.61K0.39Mn2Bi2 at various pressures. (b) Pressure dependence of lattice constant a (red circle, a = b in the T phase), b (blue circle) and c (green square). (c) Volume as a function of pressure in the tetragonal (T) and orthorhombic (OR) phases. (d) and (e) Rietveld refinement results of the X-ray diffraction patterns in the tetragonal (T) phase (I4/mmm) at 1.7 GPa and the orthorhombic (OR) phase (Fmmm) 19.8 GPa, together with the corresponding crystal structures shown on the right. (f) Representative X-ray diffraction images at different pressures, showing structure evolution with pressure.

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Figure 2(d) and 2(e) present XRD patterns of the pressurized sample and corresponding Rietveld refinement results. It is seen that the XRD pattern obtained at 1.7 GPa can be well refined as the T phase in the I4/mmm space group, yielding the reliability factor of Rp = 4.65% and weighted factor of Rwp = 6.18%, respectively, as well as the fitting goodness χ2 = 1.242. The refinement of the XRD data collected at 19.8 GPa is in good agreement with OR phase in the Fmmm space group; the Rp = 4.77%, Rwp = 5.84% and χ2 = 1.421, respectively. The obtained R values in the two phases are comparable with the refinement for the ambient-pressure data of the same material23. Figure 2(f) shows the X-ray diffraction images for the Ba0.61K0.39Mn2Bi2 at different pressures. It is seen that the quality of the crystallinity is getting worse with increasing pressure. At 28.8 GPa, a halo-like ring is observed, together with corresponding broadening XRD pattern suggesting that part of the OR phase has transformed to an amorphous-like phase.

To trace the evolution of TN of Ba0.61K0.39Mn2Bi2 with pressure, we performed high-pressure alternating current (ac) susceptibility measurements for the sample in a house-built refrigerator41. Although there is no experimental data that confirm AFM ground state of Ba0.61K0.39Mn2Bi2 and allow to evaluate TN at ambient pressure, magnetization results for Ba1-xKxMn2As2 at lower doping level (x ≤ 0.1) show the existence of the AFM ground state above 400 K13,17,21. Moreover, the similar magnetically anisotropic phenomenon is observed in both of Ba0.9K0.1Mn2Bi2 and Ba0.68K0.32Mn2Bi223. Together with the result of the same temperature dependence of resistance in pressurized Ba0.61K0.39Mn2As2 and Ba0.68K0.32Mn2Bi2, we argue that the uncompressed Ba0.61K0.39Mn2Bi2 is in the AFM state below 300 K. In our ac susceptibility measurements, the direction of the applied magnet field is perpendicular to the ab plane of the single crystal. As shown in Fig. 3, the temperature dependence of the ac susceptibility is featureless at 3.1 GPa, except for a broad hump in higher temperature range. Further investigation indicates that this broad hump is not from the sample intrinsically, but originated from the trace impurity in the supporting plate of diamond anvils. The χ as a function of temperature at 3.1 GPa is consistent with the ambient-pressure result23. With increasing pressure up to 20.2 GPa, the temperature dependence of real part (χ) of the ac susceptibility remains unchanged. This demonstrates that the TN of the pressurized Ba0.61K0.39Mn2Bi2 is still higher than 300 K, otherwise it should be detected by our instrument, as done in pressurized LaMnPO9.

Figure 3
figure 3

Raw data of the temperature dependence of ac susceptibility of the Ba0.61K0.39Mn2Bi2 at different pressures.

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Discussion

It is known that, with pressure-induced volume shrinking, the band structure and density-of-states (DOS) may change correspondingly. To identify whether the pressure may induce a significant change in electronic structure, we carried out theoretical calculations on basis of our high-pressure XRD data for the T and OR phases of Ba1-xKxMn2Bi2. Our calculation results are summarized in Fig. 4. We find that the ground state of the pressurized Ba0.61K0.39Mn2Bi2 is a robust G-type AFM metal in the T phase with an ordered magnetic moment ~3.4 μB/Mn aligned parallel to the c axis. The band structure and DOS of the Ba0.61K0.39Mn2Bi2 in the paramagnetic (PM) and AFM states at 17.49 GPa are shown in Fig. 4(a) and Fig. 4(b), respectively. Both states are metallic. In the AFM state, the total energy per unit cell is lower than that in the PM state (Supplementary Information), indicating that the AFM state is the ground state. In the PM state, the 3d states of Mn dominate near Fermi level as shown in Fig. 4(a). However, in the AFM state, there is large weight redistribution for the 3d states of Mn. The 3d electron density has large concentration from −3 eV to −2 eV and from 0.5 eV to 2 eV, which indicate the large energy splitting caused by the AFM ordering, consistent with the large local spin moment formed through strong Hund's coupling42. In the OR phase, the band structure and DOS at 24.6 GPa in both PM and AFM states of the OR phase are almost unchanged (Fig. 4(c) and Fig. 4(d)). The ground state is still a G-type AFM state with a magnetic moment of ~3.3 μB/Mn. However, the direction of magnetic moment is parallel to the a-axis. Comparing with the band structure and DOS at 17.49 GPa, our results indicate that K-doped BaMn2Bi2 seems completely insensitive to pressure. The theoretical calculations are in good agreement with our high-pressure ac susceptibility results (Fig. 3).

Figure 4
figure 4

Band structures and density-of-states at 17.49 GPa and 24.6 GPa.

(a) and (b) Band structures and density-of-states for the tetragonal phase of Ba0.61K0.39Mn2Bi2 at 17.49 GPa in the paramagnetic and antiferromagnetic states, respectively. (c) and (d) Band structures and density-of-states for the orthorhombic phase of Ba0.61K0.39Mn2Bi2 at 24.6 GPa in the paramagnetic and antiferromagnetic states, respectively.

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Comparing with the evolution of AFM order with pressure in Fe-based pnictides43,44 and chalcogenides45,46 whose AFM order can be destroyed at modest pressure below ~10 GPa, the AFM order in the K-doped BaMn2Bi2 exhibits insensitive to pressure. These results demonstrates that the Mn's d5 electrons in the sample investigated are much more localized than Fe's d6 electrons in the Fe-based pnictides and Fe-based chalcogenides, which leads to the K-doped BaMn2Bi2 belongs to a strong Hund's AFM metal with a hybridization between localized spin electrons and itinerant electrons.

Our previous high studies on LaMnPO showed that the similar pressure as applied on the K-doped BaMn2Bi2 is sufficient to completely destroy the AFM order9. The different responses to pressure between these two Mn-containing compounds may be associated to the different anion radii between these two kinds of pnictides. The radius of P anion in LaMnPO is considerably smaller than that of Bi anion in BaMn2Bi2, which leads to the overlap and interaction between the d-orbital of Mn and the p-orbital of Bi much less than those between the d-orbital of Mn and the p-orbital of P in LaMnPO. As we know that the d-p interaction is the key to mediate the AFM order, as a result, the AFM order in BaMn2Bi2 is much more robust than LaMnPO in the same pressure regime.

In conclusion, in-situ high-pressure measurements of resistance, ac susceptibility and XRD as well as theoretical calculations find that the AFM order in Ba0.61K0.39Mn2Bi2 is robust under external pressure. No evidence for superconductivity is found under pressure up to 35.6 GPa, however, a pressure-induced structural transition from T phase to OR phase is observed at ~20 GPa. Theoretical calculations demonstrate that the values of magnetic moment on Mn in the T phase and OR phase are nearly identical, suggesting that the K-doped BaMn2Bi2 is a strong Hund's AFM metal with a hybridization of localized spin electrons and itinerant electrons. The robust AFM order in Ba0.61K0.39Mn2Bi2 essentially prevents the emergence of superconductivity.

Methods

High quality single crystals with nominal composition Ba0.4K0.6Mn2Bi2 were synthesized by the similar method as described in Ref. 23. High-pressure resistance measurements using the standard four-probe method were performed in a DAC made from Be-Cu alloy in a house-built refrigerator with closed cycle refrigeration41. Diamond anvils of 500 μm and 300 μm flats were used for this study. NaCl powders were employed as pressure medium for the high-pressure resistance measurements. High-pressure ac susceptibility measurements were conducted using home-made coils that were wound around a diamond anvil41,47. The nonmagnetic rhenium gasket with 200 μm and 100 μm diameter sample holes was used for different runs of high-pressure resistance and magnetic measurements. Structural information under pressure was obtained through the angle-dispersive powder XRD experiments, performed at beamline 4W2 at the Beijing Synchrotron Radiation Facility (BSRF). Diamonds with low birefringence were selected for the high-pressure XRD experiments. A monochromatic X-ray beam with a wavelength of 0.6199 Å was adopted for all measurements. To keep the sample in a quasi-hydrostatic pressure environment, silicon oil was used for the XRD measurements. Pressure was determined by ruby fluorescence method48. Since the sample is air sensitive, the samples either for high-pressure resistance or XRD measurements were loaded into the DAC in a glove-box.

Our calculations were performed using density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) code49. The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional50 and the projector-augmented-wave (PAW) approach51 are used. Throughout the work, the cutoff energy is set to be 400 eV. The positions of all the atoms are fully relaxed during the geometry optimizations with forces minimized to less than 0.01 eV/Å. On the basis of the equilibrium structure, 20 k points are used to compute the band structure. We have also performed GGA+U calculations,where U is the onset coulomb repulsive energy of Mn. With a modest U value (5 eV), we find that the result reported in the following for both band structures and magnetisms are not qualitatively modified.