New aspect of low-temperature phase transitions of La-cuprates

The change in the microstructure on aging at 12 K below a low-temperature structural phase transition in La_1.5Sr_0.1Nd_0.4CuO₄ has been examined by transmission electron microscopy. From a series of 100 forbidden dark field images obtained in in situ observation, spotty-shape regions with a Pccn/LTT tilt of the CuO₆ octahedron were found to be nucleated and grow inside a LTO domain, accompanying the annihilation of some spotty-shape regions. In addition, there exists a tweed texture in fundamental dark field images, which is due to the macroscopic strain involved in the transition. Because a map of the tweed texture is different from that of the spotty-shape region, the tilt of the octahedron is not always coupled to a proper spontaneous strain.