Spontaneous strain at the structural phase transition in NaNO3 |
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Authors: | Richard J. Reeder Simon A. T. Redfern Ekhard Salje |
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Affiliation: | 1. Department of Earth Sciences, University of Cambridge, Downing Street, CB2 3EQ, Cambridge, England
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Abstract: | The temperature dependence of the hexagonal c unit cell parameter of high-purity NaNO3 shows an anomaly at 553 K corresponding to the orientational ordering transition. The a unit cell parameter is barely influenced by the transition. The single component spontaneous strain for this zone boundary instability is large (55×10–3 at 295 K), and couples quadratically with the order parameter. The critical exponent is found to have the value 0.22 ± 0.01, which differs from that expected in the classical case. Below ca 450 K, crossover to tricritical behaviour is observed (=1/4). The temperature evolution of the macroscopic order parameter as revealed by the temperature dependence of the spontaneous strain follows a tricritical behaviour between 70 K and 450 K. At temperatures below 70 K order parameter saturation is observed. Combined with recent data from specific heat measurements, the critical exponents suggest that the three-dimensional, three-states Potts model may describe the transition.Precursor spontaneous strain above Tc is consistent with local ordering and may result from fluctuations associated with an antiordered NO3 group pair configuration. |
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