Comparative analysis of NdCaCoO4 phase formation from cryogel and from solid state precursors

Abstract

In order to identify the reasons of the complicated synthesis of NdCaCoO4 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 NdCaCoO4 from the cryogel precursor and from the mixture of oxides and CaCO3 is observed at 1000–1100 °C. The complicated phase formation from the cryogel precursor is associated with the low reactivity of (Nd,Ca)CoO3 intermediate of the precursor thermolysis. Precipitation by NaOH/Na2CO3 instead of KOH/K2CO3 results in the reduction of NdCaCoO4 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 NaxCoO2 phase at 600–700 °C. The enhanced reactivity of (Nd,Ca)CoO3 in this case could be attributed to the enhanced ion mobility due to the simultaneous formation of NaxCoO2, CoO and NdCaCoO4 (Hedvall effect).