Anomalous dielectric behaviour at the monoclinic to tetragonal phase transition in La(Nb0.9V0.1)O4

La(Nb0.9V0.1)O4 has been shown by in-situ Raman spectra and X-ray diffraction to undergo a ferroelastic phase transition from monoclinic fergusonite to tetragonal scheelite (TM-T) at 350 oC, accompanied by 3.55% spontaneous strain and an abrupt change in thermal expansion coefficient (αL) from +15.5 ppm/oC to +11.4 ppm/oC. Assuming a linear relation between polarizability and temperature, an anomalous decrease in relative permittivity (εr) at TM-T is predicted from the Clausius–Mosotti relation and Shannon’s additive rule. Such an anomalous decrease in εr at a phase transition has not previously been observed in ferroic and linear dielectrics and may aid in the design of novel microwave dielectric composites.