Quadrupole Hyperfine Interaction and Magnetic Hyperfine Field in FeOCl and Its Intercalates

Abstract

FeOCl is a layered compound belonging to the orthorhombic space group P_mnm (D ¹³_2h ), with two formula units per unit cell. Adjacent layers in the (010) crystallographic plane, in which chlorine atoms face each other, are held together by van der Waals forces which are readily disrupted by the insertion of appropriate Lewis base molecules. This intercalation of “guest” species causes an expansion in the b-direction, leaving the remainder of the unit cell dimensions unchanged. Among base molecules which are readily intercalated are NH₃, pyridine and a wide variety of substituted pyridines, nitrogen containing heterocyclic molecules, phosphines and phosphites. The limiting stoichiometry ranges from 1:3 (“guest” to Fe atom ratio) for small intercalants to 1:6 for large “guest” molecules. Using both powder X-ray diffraction and temperature dependent ⁵⁷Fe Mössbauer effect spectroscopy, the lattice dynamics, hyperfine interactions, magnetic ordering temperature and magnetic hyperfine field (at 4.2 K) have been studied for the neat matrix and a number of base intercalates. The magnetic hyperfine field at the iron atom at 4.2 K is 441≠3 KOe, independent of the magnitude of the b-axis expansion. V_zz is negative for neat FeOCl and intercalates in which the “guest” to host ratio is ≤ 1:3, and positive when this ratio is ≥ 1:4.