  The structural phase transformation in the La2/3Ba1/3MnO3 perovskite
manganite was studied by ultrasound, neutron diffraction and electron
microscopy techniques. Minima of the velocity and maxima of the decrement of
the ultrasound point out the structural transition temperature Ts=200 K. The
hysteretic temperature behavior and giant alteration of the ultrasound velocity
and decrement present an evidence of martensitic character of the phase
transition at a macroscopic level. The martensitic transformation in
La2/3Ba1/3MnO3 is tightly connected with electronic transitions in both
coexisting high symmetry R3c and low symmetry Imma crystal phases. Neutron
diffraction data show that the electronic structure of the compound exhibits
anomalous behavior in the vicinity of Ts: temperature dependences of the
Mn-O-Mn bond angles and Mn-O bond lengths have singularities. Parameters of the
coexisting crystalline phases studied have different character of temperature
correlation with parameters of the magnetic subsystem of the compound.
