Comparative analysis of NdCaCoO 4 phase formation from cryogel and from solid state precursors

In order to identify the reasons of the complicated synthesis of NdCaCoO 4 by the cryogel method, the phase formation processes during thermal processing of precursors were investigated by in-situ X-ray diffraction analysis, Rietveld refinement and energy dispersive X-ray analysis of reaction products and intermediates. Formation of NdCaCoO 4 from the cryogel precursor and from the mixture of oxides and CaCO 3 is observed at 1000–1100 °C. The complicated phase formation from the cryogel precursor is associated with the low reactivity of (Nd,Ca)CoO 3 intermediate of the precursor thermolysis. Precipitation by NaOH/Na 2 CO 3 instead of KOH/K 2 CO 3 results in the reduction of NdCaCoO 4 formation temperature to 800 °C. According to energy dispersive X-ray analysis, the coprecipitation of Nd, Ca and Co is accompanied by the capture of Na ions followed by the formation of Na x CoO 2 phase at 600–700 °C. The enhanced reactivity of (Nd,Ca)CoO 3 in this case could be attributed to the enhanced ion mobility due to the simultaneous formation of Na x CoO 2 , CoO and NdCaCoO 4 (Hedvall effect). Graphical Abstract