Anomalous Ca Content Dependence of Dielectric Properties of Charge-Ordered Pr_1−xCa_xMnO₃ as a Signature of Charge-Ordered Phase Modulation

Low-temperature dielectric properties of charge/orbital-ordered manganite, Pr_1−xCa_xMnO₃ for 0.40 ≤ x ≤ 0.50, was investigated systematically as a function of Ca content, x. The Ca content dependence of dielectric permittivity and dissipation factor exhibited distinct maxima near x~0.45. The overall dielectric response of charge-ordered Pr_1−xCa_xMnO₃ was dominated by dielectric polarization induced by polaron hopping and exhibited thermally activated relaxation behaviour. The thermally activated dielectric relaxation behaviour over the investigated temperature range was further analysed with the help of two models: the small polaron hopping model and the Mott three-dimensional variable range hopping model. The estimated polaron transport parameters also displayed non-monotonic variation with x and exhibited a broad minima between x = 0.425 and 0.45. Considering the previous work reported so far, the charge order pattern of Pr_1−xCa_xMnO₃ below x = 0.425 was most likely to be of Zener-polaron type, while near x = 0.50 was checker-board type and for in-between compositions; neither pure checker-board type nor pure Zener-polaron type can be considered a ground state. The observed results suggest that a modulation of the checkerboard-type charge/orbital ordering pattern in Pr_1−xCa_xMnO₃ possibly takes place in the Ca content range of investigation, 0.40 ≤ x ≤ 0.50.