We report here an anomalous peak in the composition dependence of the dielectric permittivity of non-ferroelectric $(1-x)\mathrm{Na}\mathrm{Nb}{\mathrm{O}}_3\text{−}x\mathrm{Ca}\mathrm{Ti}{\mathrm{O}}_3$ ceramics for the composition range $0.10<x<0.20$. This is reminiscent of a similar phenomenon in the ferroelectric morphotropic phase boundary ceramics. Rietveld analysis of the powder x-ray diffraction data for various compositions reveals that this peak is linked with a change of crystal structure from one orthorhombic phase in the $Pbma$ space group for $0⩽x⩽0.10$ to another orthorhombic phase in the $Pbnm$ space group for $x⩾0.20$ through an intermediate long period modulated orthorhombic phase whose lattice parameter is $\sim 14$ times the lattice parameter of the $Pbnm$ phase of $\mathrm{Ca}\mathrm{Ti}{\mathrm{O}}_3$ in the [010] direction $(q\sim [\mathrm{0,1}∕\mathrm{14,0}])$.

• Received 1 December 2006

DOI:https://doi.org/10.1103/PhysRevB.77.052104