Abstract
X‐ray diffractional and electrochemical studies of the reduction of a spinel‐related manganese dioxide,
, were carried out in lithium nonaqueous cells. The reduction of this oxide proceeded topotactically in three steps; a reduction in two phases 
was characterized by a constant open‐circuit voltage (OCV) of 4.110(5)V for 
in 
, a reduction in one phase 
characterized by an S‐shaped OCV curve [mid‐point, 3.94(1)V] for 
, and a reduction in two phases of cubic 
and tetragonal 
characterized by an L‐shapedOCV curve at 2.957(5)V for 
. An observed voltage separation of ca. 1V, which begins at a critical composition of 
, was considered from the structural data of 
to be a difference in a solid‐state redox reaction of a
, i.e., the 
vs. 
redox reactions, and an effect of deformation of a 
from 
to 
(Jahn‐Teller distortion) upon an electrode potential of
was discussed.