Charge/orbital ordering structure and microstructure of Pr1-xCaxMnO3 (x <=0.5) studied by low-temperature TEM

The crystal structure of perovskite-type manganese oxides, Pr1-xCaMnO3 (x l ess than or equal to 0.5) in the charge/orbital ordering state has been inv estigated by means of low-temperature transmission electron microscopy (TEM ). The superlattice reflection spots with wave vector a*[0,1/2,0] appear fo r 0.3 less than or equal to x less than or equal to 0.5 compounds below nea r the charge-ordering transition temperature (similar to 230 K). With furth er decreasing of temperature, the new structural modulation is generated ad ditionally with the superstructure for 0.36 less than or equal to x less th an or equal to 0.4 compounds. The superlattice reflections appear as sharpe n spots with wave vector a*[1/4,1/4,1/2] below similar to 100 K. Such modul ation structure can be interpreted in terms of ordering of d(3z2-r2)-type o f e(g) electrons, which are caused by a deviation of x from 1/2, along c-ax is. The relations between chemical composition and microstructures in charg e-ordering phase were discussed. (C) 2001 Elsevier Science B.V. All rights reserved.