Neutron structural characterization, inversion degree and transport properties of NiMn2O4 spinel prepared by the hydroxide route

A. SAGUA; GABRIELA M. LESCANO; J.A. ALONSO; R. MARTıNEZ-CORONADO; M.T. FERNANDEZ-DıAZ; E. MORAN

MATERIALS RESEARCH BULLETIN

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: London; Año: 2012 vol. 47 p. 1135 - 1338

0025-5408

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 Fd3m, Z = 8, with a = 8.3940(2) A¢ª at 295 K. The crystallographic formula is (Ni0.202(1)Mn0.798(1))8a(Ni0.790(1)Mn1.210(1))16dO4 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, l = 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 spinel 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 cm1 at 800 8C in air.