The effect of impurities on the R-point instability in KMnF3. I. An X-ray scattering study

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Abstract
The wavevector dependence of the critical fluctuations at the cubic-to-tetragonal structural phase transition in KMnF3 doped with 1% and 10% of KMgF3 has been studied using X-ray scattering techniques. It is found that the introduction of impurities lowers Tc and that the fluctuation-driven first-order transition becomes continuous when the impurity concentration exceeds a certain threshold. At temperatures far above Tc, the temperature dependence of the X-ray critical scattering cross section measured in the samples KMnF3, KMn0.99Mg0.01F3 and KMn0.9Mg0.1F3 is described by the same critical exponents nu and gamma . Contrary to expectation, the magnitude of the anomalous narrow component observed in the critical scattering in pure KMnF3 close to Tc is significantly reduced by the introduction of the impurities.