High-temperature structural phase transitions in neighborite: a high-resolution neutron powder diffraction investigation

Abstract

The nature of the apparently continuous structural phase transition at 1,049 K in the perovskite-structured, MgSiO₃ isomorph, neighborite (NaMgF₃), from the orthorhombic (Pbnm) hettotype phase to the cubic ($Pm\overline{3} m$) aristotype structure, has been re-investigated using high-resolution, time-of-flight neutron powder diffraction. Using data collected at 1 K intervals close to the nominal phase transition temperature, the temperature dependence of the intensities of superlattice reflections at the M point $\left( {\frac{2\pi }{a}\left[ {\frac{1}{2},\frac{1}{2},0} \right]} \right)$ and the R point $\left( {\frac{2\pi }{a}\left[ {\frac{1}{2},\frac{1}{2},\frac{1}{2}} \right]} \right)$ of the pseudocubic Brillouin zone indicate the existence of a new intermediate tetragonal phase in space group P4/mbm, with a narrow phase field extending from ~1,046.5 to ~1,048.5 K, at ambient pressure. Group theoretical analysis shows that the structural transitions identified in this study, Pbnm–P4/mbm, and P4/mbm–$Pm\overline{3} m$, are permitted to be second order. The observation of the tetragonal phase resolves the longstanding issue of why the high-temperature phase transition, previously identified as Pbnm–$Pm\overline{3} m$, and which would be expected to be first order under Landau theory, is in fact found to be continuous. Analysis of the pseudocubic shear strain shows it to vary with a critical exponent of 0.5 implying that the phase transition from Pbnm to P4/mbm is tricritical in character. The large librational modes that exist in the MgF₆ octahedron at high temperature, and the use of Gaussian probability density functions to describe atomic displacements, result in apparent bond shortening in the Mg–F distances, making mode amplitude determination an unreliable method for determination of the critical exponent from internal coordinates. Crystal structures are reported for the three phases of NaMgF₃ at 1,033 K (Pbnm), 1,047 K (P4/mbm) and 1,049 K ($Pm\overline{3} m$).