Redetermination of the crystal structure of ammonium chromate (NH4)2CrO4

Abstract

(NH₄)₂CrO₄ crystallises in the monoclinic system, space group C2/m, with the unit-cell dimensions a= 12·300, b= 6·294, c= 7·664 Å, and β= 115·6°. Its structure, determined by three-dimensional Patterson and Fourier methods and refined by least-squares techniques, contains isolated almost regular Cr^VIO₄^2– tetrahedra with an average Cr–O distance of 1·662(6)Å and ammonium ions occupying eight-co-ordinate positions. The oxygen co-ordination around each ammonium ion may be described as symmetrically bicapped trigonal prismatic, with six distances <3·1 Å(average 2·934(10)Å) plus two slightly longer contacts. The structure reported here is markedly different from that originally suggested by another author, but is the same as that found recently for the analogous molybdate which crystallises with the Iow-temperature K₂MoO₄ structure type.