Electronic phase separation in the rare-earth manganates (La_{1 <i>x</i>}Ln_<i>x</i>)_0.7Ca_0.3MnO₃(Ln Nd, Gd and Y)

Electron transport and magnetic properties of three series of manganates of the formula (La1−xLnx)0.7Ca0.3MnO3 with Ln = Nd, Gd and Y, wherein only the average A-site cation radius ⟨rA ⟩ and associated disorder vary, without affecting the Mn4+/Mn3+ ratio, have been investigated in an effort to understand the nature of phase separation. All three series of manganates show saturation magnetization characteristic of ferromagnetism, with the ferromagnetic Tc decreasing with increasing x up to a critical value of x, xc (xc = 0.6, 0.3, 0.2 respectively for Nd, Gd, Y). For x > xc, the magnetic moments are considerably smaller, showing a small increase around TM, the value of TM decreasing slightly with increase in x or decrease in ⟨rA ⟩. The ferromagnetic compositions (x ≤ xc) show insulator–metal transitions, while the compositions with x > xc are insulating. The magnetic and electrical resistivity behaviour of these manganates is consistent with the occurrence of phase separation in the compositions around xc, corresponding to a critical average radius of the A-site cation, ⟨rAc ⟩, of 1.18 Å. Both Tc and TIM increase linearly when ⟨rA ⟩ > ⟨rAc ⟩ or x ≤ xc, as expected of a homogeneous ferromagnetic phase. Both Tc and TM decrease linearly with the A-site cation size disorder as measured by the variance σ2. Thus, an increase in σ2 favours the insulating AFM state. Percolative conduction is observed in the compositions with ⟨rA ⟩ > ⟨rAc ⟩. Electron transport properties in the insulating regime for x > xc conform to the variable-range hopping mechanism. More interestingly, when x > xc, the real part of dielectric constant (ε') reaches a high value (104 –106) at ordinary temperatures dropping to a very small (∼ 500) value below a certain temperature, the value of which decreases with decreasing frequency.