Solid solutions of InMCu 1-xMg xO 4 (M=Ga 3, Al 3) have been synthesized to understand the influence of the trigonal bipyramidal (TBP) crystallographic M 3/Cu 2 site of the YbFe 2O 4-type structure through powder X-ray and neutron diffraction, dielectric and magnetism measurements. The addition of Mg 2 in the InGaCuO 4 lattice results in a decrease of the dielectric constant, but the dielectric loss is not affected until the concentration of Mg 2 is above x=0.50. The shift in the diffraction peaks, and subsequently the change in the lattice parameters, cannot be exclusively explained by the change in ionic radii. These results show the first observation of the reduction in axial bond lengths for TBP Cu 2 in a YbFe 2O 4-type crystal lattice. © 2012 Elsevier Inc. © 2012 Elsevier Inc. All rights reserved.