Microwave Dielectric Properties of Rare‐Earth Ortho‐Niobates with Ferroelasticity

In the search for more temperature‐stable materials, herein, we report on the microwave dielectric properties and trends of the rare‐earth niobate series (RENbO 4 ), with the main emphasis being placed on LaNbO 4 . LaNbO 4 has excellent microwave dielectric properties, especially an anomalous small positive temperature coefficient of resonant frequency of 9 ppm/°C. We present the phase transformation and ferroelastic domain configuration of RENbO 4 studied using in situ X‐ray diffraction and high‐resolution transmission electron microscopy. The structural distortion caused by the phase transformation is a dominant factor in the relative dielectric constant and temperature coefficient of the resonant frequency in RENbO 4 .