New type of possible high-pressure polymorphism in NiAs minerals in planetary cores

The nickel arsenide (B8(1)) and related crystal structures are among the most important crystallographic arrangements assumed by Fe and Ni compounds with light elements such as Si, O, S, and P, expected to be present in planetary cores. Despite the simple structure, some of these materials like troilite (FeS) exhibit complex phase diagrams and rich polymorphism, involving significant changes in interatomic bonding and physical properties. NiP (oP16) represents one of the two principal structure distortions found in the nickel arsenide family and is characterized by P-P bonding interactions that lead to the formation of P-2 dimers. In the current study, the single-crystal synchrotron X-ray diffraction technique, aided by first principles density functional theory (DFT) calculations, has been applied to examine the compression behavior of NiP up to 30 GPa. Two new reversible displacive phase transitions leading to orthorhombic high-pressure phases with Pearson symbols oP40 and oC24 were found to occur at approximately 8.5 and 25.0 GPa, respectively. The oP40 phase has the primitive Pnma space group with unit cell a = 4.7729(5) , b = 16.6619(12) , and c = 5.8071(8) at 16.3(1) GPa and is a superstructure of the ambient oP16 phase with multiplicity of 2.5. The oC24 phase has the acentric Cmc2(1) space group with unit cell a = 9.695(6) , b = 5.7101(9) , and c = 4.7438(6) at 28.5(1) GPa and is a superstructure of the oP16 phase with multiplicity of 1.5. DFT calculations fully support the observed sequence of phase transitions. The two new phases constitute logical next stages of P sublattice polymerization, in which the dilution of the P-3 units, introduced in the first high-pressure phase, decreases, leading to compositions of Ni-20(P-3)(4)(P-2)(4) and Ni-12(P-3)(4), and provide important clues to understanding of phase relations and transformation pathways in the NiAs family.