  The influence of the A-site ionic radii (<rA>) on the magnetic structure has
been investigated in La0.5Ca0.5-xSrxMnO3 compounds (0 .GE. x .LE. 0.5) using
neutron diffraction, magnetization, and resistivity techniques. All compounds
in the composition range x .LE. 0.3 crystallize in the orthorhombic structure
(Space Group Pnma). No further structural transition is observed as temperature
is lowered below 300K. The compound x = 0.4, is a mixture of two orthorhombic
phases crystallizing in Pnma and Fmmm space group. The x = 0.5 compound has a
tetragonal structure in the space group, I4/mcm. The charge ordered (CO) state
with CE-type antiferromagnetic order remains stable for x .LE. 0.3. Above x =
0.3, the CE-type antiferromagnetic state is suppressed. In x = 0.4 compound,
A-type antiferromagnetic ordering is found at temperatures below 200K. Orbital
ordering accompanying the spin ordering is found in all the samples with x .LE.
0.4. The system becomes ferromagnetic at x = 0.5 and no signature of orbital
ordering is observed. As a function of <rA>, the charge ordered state is stable
up to <rA> ~ 1.24A, and is suppressed thereafter. The magnetic structure
undergoes a transformation from CE-type antiferromagnetic state to a
ferromagnetic state with an intermediate A-type antiferromagnetic state.
