The Synthesis, Properties, and Applications of Columbite Niobates (M 2+ Nb 2 O 6 ): A Critical Review

The binary niobate ceramics, with the formula M 2+ Nb 2 O 6 where M 2+ =Ca, Mg, or a transition metal (TM), have the orthorhombic columbite structure. The best‐known members of this group are zinc niobate (ZnNb 2 O 6 ) and magnesium niobate (MgNb 2 O 6 ), but Ca, Co, Ni, Mn, Cu, Cd, and Fe 2+ cations can also be included in the columbite structure. The TM columbite niobates have been found to sinter at temperatures of 1100°–1200°C, much lower than the complex perovskites, and this can be lowered even more when Cu 2+ is used. The best columbite niobates have Q × f values similar to those of BaZn 0.33 Nb 0.67 O 3 , and all have ɛ r between 17 and 25 and negative τ f values of <−80 ppm/°C. There is a growing interest in the columbites as microwave dielectric ceramics, due to their lower processing temperatures, less complicated processing due to the simple chemistry of the binary compounds, and the lower cost of niobium compared with tantalum, and with incorporation of Cu 2+ they are approaching low‐temperature cofired ceramics (LTCC) temperatures. They have also been investigated combined with other dielectric ceramics (to compensate for the negative τ f values), and with additives to lower sintering for LTCC. Furthermore, MgNb 2 O 6 is in wide use as a precursor to synthesize single phase PMN (Pb(Mg 1/3 Nb 2/3 )O 3 ) in the “columbite” process, and NiNb 2 O 6 is being increasingly investigated as a catalyst for splitting water and organic compounds. CoNb 2 O 6 and other columbites have interesting magnetic properties, and CaNb 2 O 6 and CdNb 2 O 6 have useful optical properties. This review covers the various means of synthesis of these ceramics, and the effects of processing upon structural, physical, electronic, and optical properties. This review will concentrate on the dielectric properties and applications, as this is the greatest area of interest, but will also cover other properties and applications of these ceramics. All available reported microwave dielectric data for columbites is compiled, compared, and assessed.