A single-crystal X-ray study indicates that the perovskite form of CaGeO 3 is orthorhombic (Pbnm), and isotypic with GdFeO 3 , although it was previously reported as a cubic form. The crystal structure is close to that of Sm-AlO 3 . Although the unit cell of CaGeO 3 is pseudocubic, the structure itself is very distorted through the tilting and disortion of polyhedra. The oxygen polyhedra are less tilted and less distorted than those of other GdFeO 3 -type perovskites. The structural deformation of the GdFeO 3 -type perovskite is determined primarily by the size-ratio of two kinds of cation occupying A and B sites. Some structural characteristics such as O(2)-O(2)-O(2) and A-O(1)-B angles and bond-length disortions exhibit systematic relationships as a function of the observed tolerance factor which is newly defined here. A strong correlation between the Goldschmidt tolerance factor and the observed tolerance factor has made possible some predictions for GdFeO 3 -type perovskites.--Modified journal asbstract.

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