Introduction

As a highly representative III–V compound semiconductor, gallium phosphide (GaP) has attracted immense interest, which crystalizes in either zincblende or wurtzite structures at ambient pressure1,2. Zincblende GaP possesses extensive practical applications in the fields of light-emitting diodes, near-infrared optics, photonic crystals, and photocatalysis owing to its noncentrosymmetric crystal structure, large second-order nonlinearity, nonzero piezoelectric coefficient and high refractive index across the visible and near-infrared regime3,4,5,6,7. Nanostructured counterparts with various morphologies also display fascinating properties and have become promising materials for efficient second harmonic generation, ultrafast all-optical modulators, and integrated nonlinear photonics waveguides8,9,10,11. Moreover, nanostructured GaP can be further fabricated and applied in metasurface-based nanophotonics12,13,14,15. In contrast to the zincblende structure, which has an indirect bandgap of 2.26 eV, hexagonal wurtzite-type GaP nanowires host a direct bandgap and can be used as green light emitter candidates1,16.

Pressure, as a thermodynamic variable, is an effective and powerful way to modulate the crystal lattice as well as electronic states, thereby documenting the structure-property relationship of materials17,18,19,20. Recently, pressure-tuned crystal structure and physical properties have been reported among various layered optoelectronic and photovoltaic semiconductors, such as MoS2, ReS2, and SnSe21,22,23,24. Due to the excellent optical functionalities of three-dimensional GaP semiconductors with wide bandgaps mentioned above, their structural and electronic properties under pressure have been thoroughly studied in the past decades. It is now widely accepted that GaP undergoes a cubic-to-orthorhombic transition (F-43m to Cmcm) at a pressure slightly above 20 GPa, concomitant with the simultaneous appearance of metallization25,26,27,28. Moreover, the decompressed sample is mainly amorphous, indicating the irreversibility of the structural transition28,29,30. Compared with the systematic investigation of structural evolution under pressure, the study of electrical transport is limited28,31,32. Apart from some electrical transport studies conducted above the liquid nitrogen temperature, the low-temperature electrical properties of GaP under pressure are yet unexplored to date.

Here, we investigated the structure-property relationship of bulk GaP crystals by combining optical micrographic, Raman spectroscopic, and electrical transport measurements under high pressure. We show that the cubic-orthorhombic phase transition not only leads to the occurrence of piezochromic transition and metallization but also gives rise to the emergence of superconductivity. In accordance with the irreversible structural transition, the evolution of electronic properties is also irreversible. The superconductivity can be retained toward ambient pressure and displays broadening features due to the presence of amorphization.

Materials and methods

Synthesis and characterization of GaP at ambient pressure

GaP single crystals were synthesized by the Czochralski method33. Room-temperature X-ray diffraction (XRD) patterns of single crystals were obtained with Cu Kα radiation (λ = 1.5406 Å) using a Rigaku X-ray diffractometer (Miniflex-600). The atomic proportion of the crystal was confirmed by energy dispersive X-ray spectroscopy (EDXS, Helios Nanolab 600i, FEI) via area-scanning mode.

Raman spectra, synchrotron X-ray diffraction, electrical transport, and ac magnetic susceptibility measurements under high pressure

High-pressure experimental details were similar to those in ref. 34. The culet of the diamond was 300 μm. Raman spectra were recorded on bulk crystal GaP surfaces by using a Renishaw spectrometer with a pseudo backscattering configuration (λ = 532 nm). The laser power was kept below 2.5 mW to avoid sample damage and heating effects. The back-scattered signal was collected in an unpolarized geometry through a 20× objective and 1800 g/mm grating. A powder angle-dispersive synchrotron X-ray diffraction experiment was carried out using the beamline BL15U1 at the Shanghai Synchrotron Radiation Facility (λ = 0.6199 Å). Silicone oil was used as the pressure-transmitting medium (PTM). The ac magnetic susceptibility was measured using a magnetic inductance technique as reported previously35. The excitation current was 1 mA with a frequency of 1997 Hz. Systematic transport measurements with PTM NaCl were further performed in a four-probe configuration. The applied magnetic field was perpendicular to the surface of the flake. The pressure values for all of the above experiments were determined via the ruby fluorescence method at room temperature36.

Results and discussion

The structural and physical properties of the GaP single crystal at ambient pressure were characterized via XRD, EDS, and Raman spectroscopic measurements, as shown in Fig. S1. Bulk GaP has a cubic structure (space group F-43m) and contains two atoms in a primitive cell. The as-grown GaP single crystal was yellowish, and its surface was oriented along (h00). The EDS measurement yielded a composition of GaP0.97. Two Raman peaks were observed at 367 and 405 cm−1 from the room-temperature Raman spectrum in the range of 300–500 cm−1, which could be assigned to the transverse optical (TO) and longitudinal optical (LO) modes, respectively37. All results indicate the good quality of our samples.

We first investigated the effect of pressure on the optical properties of GaP crystals at room temperature. As shown in Fig. 1a, the optical micrographs recorded at various pressures indicate that GaP changes from transparent yellowish to opaque black with increasing pressure above 22.0 GPa and remains opaque black upon further decompression from 31.1 to 0.4 GPa. This color change at approximately 22.0 GPa demonstrates a piezochromic transition with irreversibility after a compression-decompression cycle. This phenomenon is strongly correlated with the structural phase transition25,26,38. The relatively small color change of GaP below 22.0 GPa is compatible with the pressure variation of Eg of −14.9 meV GPa−1 extracted from the early high-pressure transmission experiments39. At 17 GPa, the indirect bandgap of ~2.0 eV is equal to 620 nm in the orange region (590 to 625 nm) according to the formula Eg = 1240/λ (eV)39.

Fig. 1: Room-temperature optical micrographs and Raman spectra of GaP crystal under high pressure.
figure 1

a Optical micrographs of the GaP single crystal in a DAC under compression and decompression (denoted by “D”). b Representative Raman spectra of GaP single crystals at room temperature (λ = 532 nm) under compression and decompression (denoted by “D”). TO and LO denote the transverse and longitude Raman modes, respectively. c Frequency of the Raman mode as a function of pressure upon compression. d Typical powder synchrotron X-ray diffraction patterns at room-temperature (λ = 0.6199 Å) under compression and decompression (denoted by “D”). The asterisks (*) denote the appearance of new peaks from the high-pressure phase (orthorhombic, space group Cmcm).

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To further investigate the pressure evolution of the lattice vibrational property of the GaP crystal, we performed Raman spectroscopic measurements at room temperature. Figure 1b displays the representative Raman spectra of GaP under high pressure. At 0.5 GPa, there exists two Raman modes, TO and LO, which are similar to those at ambient pressure (see Fig. S1d). With increasing pressure, both the TO and LO modes are continuously shifted toward higher wavenumbers and show a progressive reduction in intensity. Then, these two modes disappear above 25.1 GPa, where the Raman spectrum remains featureless under decompression from 49.6 to 0.6 GPa. Moreover, the high-pressure X-ray diffraction experiment shows a cubic-to-orthorhombic transition at 27.1 GPa and mainly amorphous features upon decompression to 0.2 GPa, indicating the irreversibility of the structural transition, as shown in Fig. 1d. This is consistent with the early experimental literatures25,26,27. The extracted lattice parameters and volume as a function of pressure are shown in Fig. S2. Therefore, the disappearance of Raman modes upon compression could be related to the structural phase transition. Additionally, the preservation of featureless Raman could be due to the presence of amorphization in the depressurized sample28,30. Fig. 1c shows the pressure-dependent Raman modes. For the TO and LO modes, the slopes extracted by linear fitting are 3.3 and 2.9 cm−1 GPa−1, respectively.

To explore the conductivity change of the sample, we performed a high-pressure transport experiment. Figure 2a shows representative temperature-dependent resistance R(T) curves. Due to the upper limit of the experimental setup (<107 Ω), the room-temperature resistance of the sample could only be detected with a gradual approach to 18.2 GPa. Clearly, at 18.2 GPa, GaP displays metallic behavior over the entire temperature range. The appearance of metallization is accompanied by a resistance decrease of several orders of magnitude, similar to the results of refs. 31,32. Metallic conductivity and zero resistance are simultaneously observed, signaling the concurrence of metallization and the superconducting transition in pressurized GaP. Further high-pressure ac magnetic susceptibility measurements show the Meissner effect above 20 GPa, indicating the bulk nature of superconductivity (see Fig. 2e). The zero-resistance state retains under compression to 49.7 GPa, as shown in Fig. 2b. The critical magnetic field of superconducting GaP is measured by varying the external magnetic field at 18.2 and 46.1 GPa. The superconducting transition is gradually shifted toward lower temperatures and is nearly suppressed above 2.0 T (see Fig. 2c, d). The temperature-magnetic field phase diagram is plotted in Fig. 2e. By fitting the data to the GL model, the upper critical field μ0Hc(0) is 2.9 and 2.15 T at 18.2 and 46.1 GPa, respectively. These are much lower than the corresponding Pauli limiting field of μ0HP(0) = 1.84Tc40.

Fig. 2: Electrical transport property of GaP under compression.
figure 2

a Selective resistance-temperature curves R(T) with four probes configuration. For clarity, the curves are offset after the first one. b Low-temperature R(T) curves below 7 K. c, d R(T) curves at 18.2 and 46.1 GPa with different applied magnetic fields. Temperature-dependent upper critical field μ0Hc at 18.2 and 46.1 Gpa. Tc value at specific magnetic field is defined by a 90% drop of the normal-state resistance. The black solid lines denote the fitting based on the GL model. e Real part of the ac magnetic susceptibility as a function of temperature under compression and decompression (denoted by “D”).

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Based on the irreversibility of the structural phase transition and the presence of amorphization in the depressurized GaP, we further investigated the electrical transport behavior when the pressure was gradually released from 49.7 GPa, as shown in Fig. 3. GaP does not recover to its original insulating state with decreasing pressure to 2.1 GPa. The superconducting transition width appears to be inert to pressures above 6.5 GPa but becomes much larger at 6.5 GPa, as reflected by the tail in the R(T) curve before reaching the zero-resistance state. Since the two probes were suddenly broken at 2.1 GPa, we did not observe zero resistance upon cooling to 2 K. Nevertheless, the metallic conductivity is maintained at this pressure, as shown in the inset of Fig. 3c. We measured the critical magnetic field of superconducting GaP at 20.5 GPa. The upper critical field μ0Hc(0) is 2.68 T, which is comparable to that of 18.2 GPa under compression (Fig. 2c). This result demonstrates that the superconductivity is preserved toward ambient pressure, indicating the irreversibility of the electrical transport properties after a compression-decompression cycle. According to the XRD results and early literature30, this result also indicates the presence of superconductivity embedded in the amorphous phase.

Fig. 3: Electrical transport property of GaP under decompression.
figure 3

a R(T) curves at different pressures toward 6.5 GPa. For clarity, the curves are offset. b Low-temperature R(T) curves below 7 K. c R(T) curve with two probes at 2.1 GPa due to the break of two probes. d Temperature-dependent upper critical field μ0Hc at 20.5 GPa. Tc value at specific magnetic field is defined by 90% drop of the normal-state resistance. The black solid line represents the GL fitting. Inset: R(T) curves at 20.5 GPa with different applied magnetic fields.

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Based on these transport results and the pressure evolution of the crystal structure28,30, we constructed a comprehensive phase diagram to elucidate the structure-property relationship in pressurized cubic GaP, as depicted in Fig. 4. This clearly demonstrates the structural origin of the unusual evolution of electrical properties. First, since the structural phase transition from the LP to HP phase leads to a unit-cell volume contraction of ~15.5% at a critical pressure Pc of ~18 GPa17, such a large volume variation can account for the immediate appearance of both metallization and superconductivity. Simultaneously, a piezochromic transition and featureless Raman spectra were observed around the structural phase transition. Note that the discrepancy in the critical transition pressure in the different experiments could be attributed to the pressure distribution due to the difference in the pressure medium41. Although the HP orthorhombic structure with the D2h point group could host Raman-active modes according to the group theoretical analysis, no Raman mode was observed in the high-pressure phase due to the metallic conductivity42. Second, a decrease in Tc with increasing pressure in the HP phase, namely, a negative pressure effect on the superconductivity, could be easily accounted for within Bardeen–Cooper–Schrieffer (BCS) theory, which is based on an electron-phonon pairing interaction43,44. We further calculated the electronic band structures and density of states (DOSs) at the Fermi level of the HP phase, as shown in Fig. S3. Notably, the overall band evolution remains relatively stable, while the DOS peak value decreases within the range of 0.25 eV. Thus, the decreased DOS along with lattice stiffening could be responsible for the observed decrease in Tc with increasing pressure in HP superconducting GaP within BCS theory. For the amorphous phase developed during the decompression process, however, the Tc evolution versus pressure shows a positive pressure effect. Considering the formation of a mainly amorphous structure during decompression, this is likely correlated with the metastable amorphous structure with disorder. Third, the irreversibility of the structural transition after a compression-decompression cycle is consistently reflected by the preserved metallic conductivity and the unrecovered Raman modes. Lastly, although the GaP decompressed from 49.7 GPa is reminiscent of as-grown amorphous GaP45, the decompressed amorphous GaP exhibited metallic conductivity, as shown in Fig. 3c. This result indicates that different amorphous states of GaP with fine local structure features could host different physical properties in nature. Apart from the same superconducting phenomenon in both the HP orthorhombic phase of GaP and its derivative ZnGeP234, they have some differences as follows: (1) GaP crystalizes in a cubic structure at ambient pressure. ZnGeP2 hosts a cubic phase as its first HP phase with cation disorder. (2) Similar amorphous behavior during decompression was observed for both GaP and ZnGeP2; this could be attributed to the connection with the pressure dependence of kinetic barriers for the phase transitions by using a configuration-coordinate model.29 However, the starting phases of amorphization are different, i.e., orthorhombic in GaP versus cubic in ZnGeP2. Therefore, the same amorphous superconductivity in GaP and ZnGeP2 is likely caused by their different parent lattices.

Fig. 4: Relationship between the structural and electronic properties in pressurized GaP bulk.
figure 4

The onset and zero-resistance transition temperature Tc’s are extracted from the R(T) curves. The blue and green symbols denote the data from the compression and decompression processes, respectively. The dashed lines below 6.5 GPa are guides to the eye. The colored areas indicate the different electronic state along with the different crystal structures, i.e., pristine low-pressure (LP) phase (cubic, S.G. F-43m, Z = 4), high-pressure (HP) phase (orthorhombic, S.G. Cmcm, Z = 2), and amorphous phase. The dashed gray area is correlated to the transformation between the HP and amorphous phase under decompression.

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Conclusion

In summary, we have investigated the relationship between the structural, optical, and electrical properties of bulk GaP single crystals at high pressures by combining Raman scattering, piezochromism, synchrotron X-ray diffraction, electrical transport, and ac magnetic susceptibility experiments. We show that pressure-engineered electronic properties are effectively correlated with the structural and optical evolutions. During compression, metallization and superconductivity simultaneously appear, which is accompanied by a piezochromic transition and attributed to the appearance of a structural transition from the cubic to orthorhombic phase. Upon further decompression, the superconductivity is preserved toward ambient pressure, consistent with the irreversibility of the cubic-orthorhombic phase transition and the presence of an amorphous phase in the depressurized sample. The newly observed two high-pressure superconducting phases, as well as the irreversible evolution of the structural and electrical properties provide a comprehensive understanding of the physical properties of semiconductor GaP under pressure.