The Polar Phase of NaNbO₃: A Combined Study by Powder Diffraction, Solid-State NMR, and First-Principles Calculations

Abstract

A polar phase of NaNbO(3) has been successfully synthesized using sol-gel techniques. Detailed characterization of this phase has been undertaken using high-resolution powder diffraction (X-ray and neutron) and (23)Na multiple-quantum (MQ) MAS NMR, supported by second harmonic generation measurements and density functional theory calculations. Samples of NaNbO(3) were also synthesized using conventional solid-state methods and were observed to routinely comprise of a mixture of two different polymorphs of NaNbO(3), namely, the well-known orthorhombic phase (space group Pbcm) and the current polar phase, the relative quantities of which vary considerably depending upon precise reaction conditions. Our studies show that each of these two polymorphs of NaNbO(3) contains two crystallographically distinct Na sites. This is consistent with assignment of the polar phase to the orthorhombic space group P2(1)ma, although peak broadenings in the diffraction data suggest a subtle monoclinic distortion. Using carefully monitored molten salt techniques, it was possible to eradicate the polar polymorph and synthesize the pure Pbcm phase.

Electronic and Optical Properties of Sodium Niobate: A Density Functional Theory Study

“…In addition, benchmark calculations have been carried out for the high-temperature simple cubic perovskite phase (Pm3m, SG 221, Z � 1). All four phases have been initially set up using experimental data reported by Jiang et al [28] for the Pm3m phase, by Cheon et al [5] for the Pm phase, by Johnston et al [6] for the Pbcm phase, and by Darlington and Megaw [4] for the R3c phase, respectively. For all four phases, we performed a full geometry optimisation for several unit-cell volumes centred around the experimentally reported ones.…”

“…Darlington et al [4] and Cheon et al [5] reported on a possible admixture of a monoclinic phase (Pm) into the room-temperature orthorhombic AFE P phase (Pbcm) based on X-ray diffraction and neutron powder diffraction measurements. ere are also reports about a room-temperature phase transition into a FE phase (P2 1 ma) induced by an applied electric field [6,7], by nanoparticle growth [8] or by growth as a strained thin film [9], respectively. A full list of recent experimental data can be found in [2,[10][11][12].…”

Unusual Phase Behavior in the Piezoelectric Perovskite System, LixNa1–xNbO3

“…4 , 5 , 6 Although itself antiferroelectric (orthorhombic, Phase P, space group Pbcm) at ambient temperature, NaNbO 3 becomes ferroelectric (rhombohedral, Phase N, space group R3c, Glazer tilt system 7 a -a -a -) below 173 K. Depending on sample synthesis conditions, a ferroelectric polymorph, Phase Q (orthorhombic, space group P2 1 ma, Glazer tilt system a -b + a -) can co-exist with Phase P at room temperature. 8,9 Moreover, the orthorhombic ferroelectric phase of NaNbO 3 can be made via chemical doping; indeed doping as little as 2% of either potassium or lithium, to form the solid solutions K x Na 1-x NbO 3 or Li x Na 1-x NbO 3 , induces both a change in the octahedral tilt system of Phase P and a polarization to produce Phase Q. 10 The composition-dependent phase diagram of KNN has recently been re-analysed and is now fairly well understood, 11,12 but that of LNN has received much less attention.…”

From fluorite to pyrochlore: Characterisation of local and average structure of neodymium zirconate, Nd2Zr2O7

“…NaNbO 3 also exhibits complex polymorphism related from the synthetic route used. A polar polymorph may be synthesised when a sol-gel method is used and the ilmenite polymorph of NaNbO 3 can only be prepared via hydrothermal synthesis [18,19]; by adjusting the hydrothermal reaction conditions, a perovskite polymorph may be obtained [19]. NaNbO 3 also displays crystallite size related polymorphism [20].…”