Room- and high-pressure crystal chemistry of CoAs and FeAs

Abstract

The MnP-type crystal structures of FeAs and CoAs have been studied at both room and high pressure. In the room-pressure study, refinements of both structures were carried out in space groups Pnam and Pna2~ using single-crystal integrated X-ray intensity data collected from a four-circle diffractometer. For CoAs, both space groups refined to a weighted residual, Rw = 0-054, whereas for FeAs, Rw = 0.030 for Pnam and 0-027 for Pna2~. The Hamilton significance test, applied to FeAs, shows a significant improvement in Rw for Pna2,. However, the Hamilton test is not a conclusive test for space-group identity. Comparison of interatomic distances, derived from Pnam and Pna2~ refinements for both CoAs and FeAs, shows no significant change in the structures. High-pressure X-ray data were collected from single crystals enclosed in a Merrill & Bassett-type diamond-anvil cell. Cell parameters and structures were refined at 2.80 (5), 3.70 (5), and 4.90 (5) GPa for CoAs, and at 2.25(5), 4.36 (5), 6-10(5), and 8.3(3) GPa for FeAs. Cell data were used to determine the elastic constants, isothermal bulk modulus and its pressure derivative (Kr and K~r, respectively), by least-squares fitting of the unit-cell volumes to a thirdorder Birch-Murnaghan equation of state. This gave Kr = 123 (6) GPa and K~r = 88(33) for CoAs. For FeAs, KT = 118 (4) GPa and K~r = 72 (1.8). Refinements in Pnam show that the structure remains an MnP-type throughout these pressure ranges. A precession photograph of CoAs at 7.8 GPa revealed previously sharp split and diffuse diffraction spots. Similar effects are known to occur in FeS when it undergoes a change from an MnP-type structure to a high-pressure phase above 6.8 GPa. Thus, these effects in CoAs may indicate the onset of a phase transition. Changes in both interatomic distances and atomic displacements from positions in an NiAs-type structure reveal some interesting aspects of the anisotropic compression these compounds undergo. These changes are correlated with high-pressure phase transitions that may occur in MnP-type structures.