High Temperature Phase Stability in Li0.12Na0.88NbO3: A Combined powder X-Ray and Neutron Diffraction Study

TL;DR

This study investigates the high-temperature phase stability of Li0.12Na0.88NbO3 using combined powder X-ray and neutron diffraction, revealing multiple phase transitions and coexistence of phases from 300 K to 1100 K.

Contribution

It provides detailed experimental evidence of phase coexistence and transitions in Li0.12Na0.88NbO3, highlighting the effects of temperature and chemical pressure on its structural phases.

Findings

Phase coexistence of orthorhombic paraelectric and ferroelectric phases between 525 K and 675 K

Orthorhombic to tetragonal phase transition around 925 K

Successive phase transformations are continuous and not strongly first order

Abstract

Lithium doped sodium niobate is an ecofriendly piezoelectric material that exhibits a variety of structural phase transitions with composition and temperature. We have investigated the phase stabilities of an important composition Li0.12Na0.88NbO3 (LNN12) using a combination of powder x-ray and neutron diffraction techniques in the temperature range 300 - 1100 K. Detailed Rietveld analyses of thermo-diffractograms show a variety of structural phase transitions ranging from non-polar antiferrodistortive to ferroelectric in nature. In the temperature range of 525 K to 675 K, unambiguous experimental evidence is shown for phase coexistence of orthorhombic paraelectric O1 phase (space group Cmcm) and orthorhombic ferroelectric O2 phase (space group Pmc21). The bp primitive lattice parameter of the ferroelectric orthorhombic phase (O2 phase) decreases, while the ap and cp primitive lattice parameters show normal increase with increase in temperature. Above 675 K, in the O1 phase, all lattice parameters come close to each other and increase continuously with increase of temperature, and around 925 K, ap parameter approaches bp parameter and thus the sample undergoes an orthorhombic to tetragonal phase transition. Further as temperature increases, the cp lattice parameter decreases, and finally approaches to ap parameter, and the sample transform into the cubic phase. The continuous change in the lattice parameters reveals that the successive phase transformations from orthorhombic O1 to high temperature tetragonal phase and finally to the cubic phase are not of a strong first order type in nature. We argue that application of chemical pressure as a result of Li substitution in NaNbO3 matrix, favours the freezing of zone centre phonons over the zone boundary phonons that are known to freeze in pure NaNbO3 as function of temperature.