CALORIMETRIC AND RESISTOMETRIC STUDIES OF LA2CUO4-CRYSTALS - DISPLASIVE CHARACTER OF PHASE-TRANSITIONS IN 190-310-K(DELTA SINGLE)

Measurements of heat capacity, resistivity, and magnetic susceptibilit y for the La2CuO4+delta single crystals in the range 0.001 less-than-o r-equal-to delta less-than-or-equal-to 0.035 revealed that following f our kinds of phase transitions appear between 10 and 320 K: supercondu cting transition around 33 K, a transition around 200 K with small hea t anomaly, martensitic transition around at 250-270 K, and a second or der transition at 295 K. These transition temperatures strongly depend ed on the amount of excess oxygen content. The heat capacity anomalies for the sample with delta=0.035 in the range of 190 to 310 K were int erpreted as connected with cooperative displacements of ions involving no diffusion process of oxygen ion.