The widespread pressure-induced metallization of semiconductors discovered in the 1960s and the associated polymorphism is an archetypal high-pressure problem. After decades of experimental and theoretical investigations, the structure types and bonding of the high-pressure polymorphs of III-V semiconductors have been largely regarded a settled matter1-3. Considering the structural details revealed by the recently characterized oS24 phase of gallium phosphide4, in which a phosphorus dimer is shown to be the shortest bond of the structure, and the new results here introduced, we suggest that the nature of bonding and ordering of these polymorphs ought to be reconsidered.
While probing GaP in a pressure range intermediate between the calculated stability of SC16 (∼16 GPa4,5) and the synthesis pressure of oS24 (∼40 GPa4), we discovered a new phase. The polymorph was synthesized between 22 and 30 GPa at high temperature in a laser-heated DAC. Its structure was subsequently studied via microdiffraction techniques and first-principles density functional theory (DFT) calculations. The new phase adopts a structure not previously found in this class of materials, with monoclinic space group C2/m and 16 atoms in the unit cell, hereafter referred to as mS16. The results of our DFT calculations support the high-pressure stability of mS16, in close competition with oS24, as well as the structural characterization resulting from the diffraction study, and shed light on the evolution of the electronic localization and bonding of mS16 under compression. mS16 shows strong structural similarities with oS24, they can both be described in terms of buckled NaCl-like layers parallel to the ab plane, both show short P-P dimers and have distorted coordination environments. Only half of the P atoms form dimers in mS16 while two third of them form monatomic bonds in the oS24 phase4.
Our findings show that the polymorphism of GaP includes complex, relatively low-symmetry phases in which different bond types coexist. Contrary to early suggestions, the two atomic species are chemically strongly differentiated in the metallic phase. Gradual changes in the nature of bonding with pressure explain the occurrence of different polymorphs in a rather narrow pressure range.
Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. HPCAT operations are supported by DOE-NNSA’s Office of Experimental Sciences. GeoSoilEnviroCARS is supported by the NSF-Earth Sciences (EAR-1634415) and DOE-GeoSciences (DE-FG02-94ER14466). The Advanced Photon Source is a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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3Mujica, A., Rubio, A., Munoz, A. & Needs, R. Rev Mod Phys 75, 863 (2003).
4Lavina, B. et al. Inorg Chem 57, 2432–2437 (2018).
5Mujica, A. & Needs, R. J. Phys Rev B 55, 9659–9670 (1997).