The title compound has been synthesized by the hydroxide route. The crystal structure has been investigated at room temperature from high-resolution neutron powder diffraction. (NPD) data. It crystallizes in a cubic spinel structure, space group Fd (3) over barm, Z = 8, with a = 8.3940(2) angstrom at 295 K. The crystallographic formula is (Ni0.202(1)Mn0.798(1))(8a)(Ni0.790(1)Mn1.210(1))(16d)O-4 where 8a and 16d stand for the tetrahedral and octahedral sites of the spinel structure, respectively. There is a significant inversion degree of the spinel structure, lambda = 0.80. In fact, the variable parameter for the oxygen position, u = 0.2636(4), is far from that expected (u = 0.25) for normal spinels. From a bond-valence study, it seems that the valence distribution in NiMn2O4 spinal is not as trivial as expected (Ni2+ and Mn3+), but clearly the tetrahedral Mn ions are divalent whereas the octahedral Mn and Ni are slightly oxidized from the expected +3 and +2 values, respectively. The mixed valence observed at the octahedral sites provides the charge carriers that, by a hopping mechanism between Mn3+/Mn4+ adjacent sites, leads to a significant conductivity, up to 0.85 S cm(-1) at 800 degrees C in air. (C) 2012 Elsevier Ltd. All rights reserved.