Abstract:

We studied the crystal and magnetic structure of the La1-xCaxMnO3 compound for (0.11 <= x <= 0.175) using stoichiometric samples. For x < 0.13 the system's ground state is insulating canted antiferromagnetic. For 0.13 <= x <= 0.175 below the Jahn-Teller transition temperature (T-JT) the crystal structure undergoes a monoclinic distortion. The crystal structure can be described with P2(1)/c space group which permits two Mn sites. The unit-cell strain parameter s=2(a-c)/(a+c) increases for T < T-JT, taking the maximum value at the Curie point, and then decreases. Below T-M'M''approximate to 60 K s abruptly changes slope and finally approaches T=0 K with nearly zero slope. The change of s at T-M'M'' is connected to a characteristic feature in the magnetic measurements. As x increases towards the ferromagnetic metallic boundary, although s is reduced appreciably, the monoclinic structure is preserved. The monoclinic structure is discussed with relation to the orbital ordering, which can produce the ferromagnetic insulating ground state. We also studied samples that were prepared in air atmosphere. This category of samples shows ferromagnetic insulating behavior without following the particular variation of the s parameter. The crystal structure of these samples is related to the so-called O-* (c > a > b/root 2) structure.