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

The nickel arsenide (B81) 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 dia- grams and rich polymorphism, involving significant chan- ges in interatomic bonding and physical properties. NiP (oP16) represents one of the two principal structure dis- tortions found in the nickel arsenide family and is char- acterized by P-P bonding interactions that lead to the formation of P2 dimers. In the current study, the single- crystal synchrotron X-ray diffraction technique, aided by first principles density functional theory (DFT) calcula- tions, has been applied to examine the compression behavior of NiP up to 30 GPa. Two new reversible dis- placive 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) A ˚ , b = 16.6619(12) A ˚