Sintering, microstructure and microwave dielectric properties of rare earth orthophosphates, RePO4 (Re = La, Ce, Nd, Sm, Tb, Dy, Y, Yb)

Various rare earth orthophosphates, such as monazite and xenotime RePO4 (Re = La, Ce, Nd, Sm, Tb, Dy, Y, Yb) were prepared by a conventional solid-state reaction method. The crystal structure and microstructure of the sintered ceramics were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), respectively. The dielectric properties were measured in the microwave region using a network analyzer. It was found that the monazite RePO4 could be sintered near 1400 °C, although the xenotime RePO4, which had a smaller Re3+ ionic radius, could be sintered above 1600 °C. The permittivity (ɛr) of the monazite structures was higher than that of the xenotime structures. This difference was explained by the differences in ionic polarizability and bond strength. Both monazite and xenotime rare earth orthophosphates, however, exhibited a high quality factor (Q × f), where Q = 1/tan δ and f is the measuring frequency, of greater than 60,000 GHz. The temperature coefficient of resonant frequency (τf) was a negative value, ranging between −17 and −56 ppm/°C.