CaCl2 was mixed with 30 mL deionized water, next, sodium polystyrene sulfonate (PSS) (0.3 g) and the prepared strontium ferrites (m(CaCl2)/m(strontium ferrites) = 2:1) were added under ultrasonication to obtain an aqueous dispersion. Then, an aqueous solution of NaOH (14.2 mmol) in deionized water (10 mL) was added dropwise into the mixture under a vigorous agitation at room temperature and then the resultant mixture was kept with stirring for 30 min. Sequentially, the system was aged at 65 °C for 18 h. After filtered, the resulting precipitation was washed with saturated calcium hydroxide solution, and dried in a vacuum oven at 80 °C for 12 h. The dried sample was then calcined at 700 °C for 3 h at a heating rate of 2 °C/min [27].