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Ultrasonic velocity and attenuation data are presented at T < 1 K for pure KTaO 3 and for KTaO 3 :X where X = Na and Li. Evidence is obtained for coherent tunnelling of Na ions within a correlation volume containing many impurities through barriers with distributed height and asymmetry parameters. Tunnelling is correlated with dipole-glass formation. No evidence of tunnelling is found in KTaO 3 and KTaO 3 :Li.
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Doussineau et al. (1989) studied this question.
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