Nickelphosphide from the Vicenice octahedrite: Rietveld crystal structure refinement of a synthetic analogue

Abstract

The original description of nickelphosphide from the Butler meteorite [(Ni_1.83Fe_1.21)_Σ3.04P_0.96 to (Ni_1.71Fe_1.28Co_0.01)_Σ3.00P_1.00] lacks detailed and accurate powder data – with only 11 observed reflections down to 1.37 Å. Consequently, no crystal structure information for this phase is available. Here we present the crystal structure for a synthetic analogue of the phase found in the Vicenice IID medium octahedrite (with a mean empirical formula of (Ni_1.69Fe_1.31)_Σ3.00P_1.00) and a hypothetical Ni₃P end-member. Unit-cell dimensions of the synthetic analogue of Ni_1.68Fe_1.33P_0.99 composition from individual peak profile fitting of XRD data collected with Cu radiation are a= 9.0168(2)Å, c= 4.4491(1)Å, V= 361.72(1)ų and those for Ni₃P composition are a= 8.9546(1)Å, c= 4.38714(8)Å, V= 351.783(8)ų. The space group of the mineral is I4¯ and Z= 8. The crystal structure refined from powder data by the Rietveld method is consistent with that refined and published for Fe₃P and Ni₃P synthetic compounds as well as Fe-dominant schreibersite from single-crystal data. However, because of the similarity of the scattering of iron and nickel, we were not able to refine directly the site occupancies in the synthetic analogue of the nickelphosphide from the Vicenice iron meteorite.