Publication date: 10th April 2024
This study investigates the LaxNa1-3xNbO3 perovskite solid solution formed between nominally A-site
occupied NaNbO3 and highly A-site deficient La1/3NbO3 end members. Specifically, this work aims to
create A-site vacancies, introduced within the solid solution as charge compensating defects
consequent of the trivalent La3+ substitution for Na+ in the ABO3 perovskite structure, and to correlate
their influence on the dielectric properties observed.
Conventional dielectric spectroscopy (DS) analysis reveals a noticeable suppression
of the permittivity’s temperature dependence, alongside an associated antiferroelectric-relaxor type
transition, as the A-site vacancy (VNa’) concentration increases. This antiferroelectric-relaxor transition
can be classified as an order-disorder type transition at La concentrations of 7.5mol% and above, and is
reflected in significant changes to long range polarisation correlations. Structural
investigations involving Rietveld analysis of laboratory X-ray diffraction data reveal a series of phase
transitions across the solid solution, with notable diffuse scattering offering insight into the disruption
of long-range ordering associated with the NaNbO3 end member as La/VNa’ concentrations increase –
something that may be responsible for the observed flattening in the dielectric response with
temperature. La doping beyond 15mol% results in a quasi-linear type permittivity response, which is retained for the rest of the solid solution - up to and including the La1/3NbO3 end member.
Impedance Spectroscopy (IS) measurements suggest good dielectric
behaviour (electronically insulating behaviour) can be retained for low doping concentrations of La
(0<x<0.15), complementary to low dielectric loss data from DS measurements. Concentrations above
this threshold are proposed to induce ionic conduction with a maximum conductivity of ~6.3x10-5 Scm -1 at
300°C for a composition for La0.25Na0.25NbO3 . This ionic conductivity correlates well with the high
dielectric loss observed for these materials by DS measurements.
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