Structural phase diagram of perovskite A(0.7)A'0.3MnO3 (A=La, Pr; A'=Ca, Sr, Ba): A new Imma allotype

The structural phase diagram of A(1-x)A'xMnO3 (A = La, Pr, Y, ...; A' = Ca, Sr, Ba, ...) as a function of the average A-site ionic radius [r(A)] was determined from neutron powder diffraction data. In addition to the well-known Pnma and R (3) over bar c phases, a new orthorhombic phase, with space group Imma, was found for large [r(A)] at low temperatures. Refined structural parameters are reported for La0.7Ba0.3MnO3 at 1.6 K (Imma) and at 293 K (R (3) over bar c). The Imma --> R (3) over bar c phase transition was studied as a function of temperature for the same La0.7Ba0.3MnO3 sample, and determined to be first order, (C) 1996 Academic Press, Inc.