An accuracy assessment of the refinement of partial metal disorder in solid solutions of Al(acac)3 and Cr(acac)3

Abstract

A series of solid solutions have been prepared by co-crystallization of Al(acac)₃ and Cr(acac)₃ and studied by X-ray diffraction. The end members and seven isotopic members of the series Al_1 − xCr_x(acac)₃, with x varying from 0.02 to 0.91, crystallize in the monoclinic space group P2₁/c with Z = 4. The unit cell parameters vary slightly with composition, and fall into the following ranges: a, 13.987–14.009; b, 7.534–7.553; c, 16.308–16.348 Å; β, 98.78–98.99°. The cell axes do not vary in a linear manner with the mole% Cr. The Al ∶ Cr molar ratio in each crystal studied by X-ray diffraction was determined by wavelength dispersive spectroscopy (WDS) and compared with the refinement of the Al ∶ Cr site occupancy, which was attempted by the following methods: (1) refinement based upon bulk sample analysis, (2) refinement of occupancy with common displacement parameters, (3) refinement of occupancy and displacement parameters, and (4) construction of a mixed atom type based on the results of all three methods. Results from methods 2 and 3 were found to agree with WDS data within experimental error for all crystals studied. Method 1 was the least successful, the refinement often being significantly problematic. A discussion of suitable methods for refinement of partial site occupancy in solid solutions for isomorphous structures is presented. The variance of crystal lattice parameters and the M–O and O–M–O structural parameters with Al ∶ Cr ratio is investigated.