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Influence of defects on ionic transport in LiTaO3 – A study using EXAFS and positron annihilation lifetime spectroscopy

DOI: 10.1016/j.ssi.2020.115355 DOI Help

Authors: B. Gadermaier (Graz University of Technology) , L. Resch (Graz University of Technology) , D. M. Pickup (University of Kent) , I. Hanghofer (Graz University of Technology) , I. Hanzu (Graz University of Technology) , P. Heitjans (Leibniz University Hannover) , W. Sprengel (Graz University of Technology) , R. Würschum (Graz University of Technology) , A. V. Chadwick (University of Kent) , H.m.r. Wilkening (Graz University of Technology; Alistore–ERI European Research Institute, CNRS FR3104)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Solid State Ionics , VOL 352

State: Published (Approved)
Published: September 2020
Diamond Proposal Number(s): 14239

Abstract: Defects of various types in crystalline and nanocrystalline materials govern a range of electrical, optical and mechanical properties. In particular, they are at the heart of translational ion dynamics in solid electrolytes. One of the most prominent examples revealing a drastic increase in ionic conductivity σDC by several orders of magnitude when going from an ordered crystalline matrix to a structurally disordered one is lithium tantalate. Here, structurally disordered, nanocrystalline LiTaO3 served as a model substance to shed light on the question to what extent the degree of disorder decreases upon annealing an originally defect-rich oxide. Disorder can be introduced by high-energy ball milling of LiTaO3 crystallites with diameters in the μm range. Broadband conductivity spectroscopy, EXAFS and positron annihilation lifetime spectroscopy were used to correlate ion transport properties with interatomic distances, bond distortions and positron lifetimes. It turned out that milling times of only 30 min are sufficient to generate a highly defective oxide. Upon annealing at temperatures of T = 200 °C the defects can almost be preserved. Annealing at 750 °C for 1 h is needed to induce healing of the defects. Although we observe a recovery of the original interatomic distances and an increase in activation energy Ea for ionic transport from 0.75 eV to 0.81 eV, the initial transport properties of the unmilled sample (0.97 eV) cannot be fully restored. Most interestingly, the change in Ea is accompanied by a change of the entropy-controlled Arrhenius pre-factor governing the temperature dependence of σDCT. Moreover, positron lifetimes remain high in the annealed samples. Hence, our results point to samples with fewer distortions but still rich in vacancy-type defects. Altogether, the combination of ball milling and annealing helps adjust ionic conductivities in LiTaO3 to vary over 4 to 5 orders of magnitude.

Journal Keywords: Lithium tantalate; EXAFS; Positron lifetime spectroscopy; Defects; Ionic conductivity

Subject Areas: Materials, Chemistry

Instruments: B18-Core EXAFS