Some optical and transport properties of a new subclass of ceramic tungstates and molybdates

UV–vis–NIR and electrical investigation of AgY1−x(Gd,Nd)x(WO4)2 and MRE2W2O10 (M=Cd, Co, Mn; RE=Y, Nd, Sm–Er) tungstates as well as Cd1−3xGd2x□xMoO4 molybdates proved them to be in the insulating state. The calculated value of the direct allowed energy gap Eg>2.4 eV. Only for CoPr2W2O10, MnPr2W2O10 and Cd1−3xGd2x□xMoO4 solid solutions when 0.0238≤x≤0.0839, thermally activated conduction of the Arrhenius-type was observed. The I–V measurements at higher temperatures showed lower carrier emission for AgY1−x(Gd,Nd)x(WO4)2 solid solutions and higher ones for the remaining samples. Colossal increase in electron emission of MPr2W2O10 (M=Co, Mn) and in Cd1−3xGd2x□xMoO4 for 0.0098 ≤x≤0.0839 at 400 K is interpreted in the framework of the Poole–Frenkel effect and a small polaron mechanism, respectively.