Synchrotron x-ray diffraction and Raman scattering investigations of (LixNa1−x)NbO3 solid solutions: Evidence of the rhombohedral phase

Abstract

The $\left({\mathrm{Li}}_x{\mathrm{Na}}_{1-x}\right){\mathrm{NbO}}_3$ perovskite solid solutions are investigated using synchrotron x-ray diffraction and Raman spectroscopy in order to clarify the structural changes in the concentration range $0<~x<~0.145$ at room temperature. The orthorhombic antiferroelectric P phase (space group Pbma, $Z=8)$ exists up to $x=0.02\mathrm{}$ and in the concentration interval $0.02<~x<~0.03$ we have confirmed a previously reported transition to the orthorhombic ferroelectric Q phase (space group ${P2\mathrm{}}_{1}ma,$ $Z=4).\mathrm{}$ It is found that the Q phase is stable up to $x=0.145\mathrm{}$ but in the narrow concentration interval at $x=0.12\mathrm{}$ the dominating (86.2%) rhombohedral N phase (space group $R3c,$ $Z=2)$ coexists with the minor (13.8%) Q phase at room temperature. The crystal structure of this rhombohedral N phase has been refined using Rietveld method. Detailed comparison with similar low-temperature phase of pure ${\mathrm{NaNbO}}_3$ showed that the ${\mathrm{NbO}}_6$ octahedra are slightly distorted in the solid solution with $x=0.12\mathrm{}$ while they are rather regular in ${\mathrm{NaNbO}}_3.$ Raman spectra showed drastic changes associated with the $P-Q$ transition and revealed coexistence of the P and Q phases in the interval $0.02<~x<~\mathrm{0.03.}$ Abrupt transformation of the Raman response and remarkable narrowing of the Nb-O stretching vibrations related to the existence of the rhombohedral N phase at $x=0.12\mathrm{}$ were observed.