The room-temper
ature cryst
al structures of the
n = 3 Ruddlesden-Popper ph
ases C
a4Mn
2FeO
9.75 and Sr
4Mn
2FeO
9.80 h
ave been refined from neutron
and X-r
ay powder diffr
actiond
at
a. Both
adopt sp
ace group
I4/
mmm with (
a,
c) = (C
a, 3.73683(1), 27.0860(1) Å), (Sr,3.83393(1), 27.8148(1) Å). In both compounds the c
ation site
at the center of the perovskiteblocks is preferenti
ally occupied by Fe (C
a, Mn:Fe = 0.424:0.576(4)),
and the
anion v
ac
ancies
are found
around this site. The occupied
anion sites show st
atic disorder in C
a4Mn
2FeO
9.75but not in Sr
4Mn
2FeO
9.80. Both compounds
are electric
al insul
ators which order
antiferrom
agnetic
ally
at
TN = 75 K (C
a) or 90 K (Sr). Susceptibility
and
M(
H) d
at
a suggest th
at not
all the Mn
and Fe c
ations t
ake p
art in the long-r
ange m
agnetic ordering,
and there is evidenceof
a spin gl
ass tr
ansition in both compounds
at ~11 K. The m
agnetic structure of C
a4Mn
2FeO
9.75 at 5 K h
as been determined by neutron diffr
action. No ordered moment w
as detectedon the Mn/Fe site
at the center of the perovskite blocks; 0.74(1)
ages/entities/mgr.gif">
B per tr
ansition met
alc
ation w
as me
asured
at the sites on the block edges. Possible c
auses of m
agnetic frustr
ationin this cryst
al structure
are considered. C
a4Mn
2FeO
9.75 h
as
a m
agnetotresist
ance of -4%
at137 K in
a 14 T field.