87Rb NMR study at the cubic to tetragonal phase transition in RbCaF 3

Abstract

2014 Le spectre de résonance magnétique nucléaire de 87Rb a été étudié dans un monocristal de RbCaF3 de part et d'autre de la transition de phase cubique-quadratique à 198 K. Dans la phase cubique (phase haute température) la dépendance en température de l'amplitude de la raie de résonance s'écarte de la loi de Curie ce qui peut être attribué à l'existence de domaines de symétrie quadratique. Dans la phase quadratique (phase basse température) on observe un déplacement de la raie centrale dû à une interaction quadripolaire au second ordre, duquel on peut déduire l'angle de rotation de CaF6 (paramètre d'ordre). La dépendance en température du paramètre d'ordre est décrite par une loi de puissance d'exposant 0,5 et 0,32 respectivement au-dessous et au-dessus de 150 K. L'amplitude de rotation a été comparée à celles déduites des études radiocristallographiques et de diffraction de neutrons sur poudre. Abstract. 2014 The 87Rb nuclear magnetic resonance spectrum, in perovskite single crystal of RbCaF3, is studied above and below the cubic-to-tetragonal phase transition occurring at 198 K. In the high-temperature cubic phase, the temperature dependence of the resonance line amplitude deviates from the Curie law and this can be attributed to the existence of tetragonal domains. In the low temperature tetragonal phase, a second-order quadrupole shift of the central line is observed, from which the CaF6 tilt angle (order parameter) is derived. The order parameter temperature dependence is described by a power law with a cross over from exponent 0.5 to exponent 0.32 at 150 K. The tilt angle 03A6 is compared to the values obtained from X-ray and neutron powder diffraction data.

A new model for the phase transitions due to octahedra rotations in the perovskite structure

“…experiment clearly indicates that an orderdisorder local behaviour occurs in SrTi03 near 7. [28] ; in the same way, 87Rb NMR study in RbCaF3 [29] seems to indicate a local symmetry lowering far above 7~. In this context, RbCaF~w ith its large tilt angles is one of the most suitable compound for testing the validity of this description ; to this purpose NMR spin-lattice relaxation time measurements are being carried out.…”

Static Model and Order–Disorder Model in RbCaF3. Electric Field Gradient Calculations in the Quadratic Phase

“…So, nine experimental lines could be expected (three per domain). Actually, only two lines were observed by Bulou et al [5]. These results are coherently explained by the fact that: 1. two tetragonal domains are equivalent (y = 0 in agreement with the symmetry deduced from X-rays), 2. strains related to the creation of domains were strong enough to wipe out satellite transitions…”

“…NMR experiments were performed by Bulou et al [5] on a RbCaF, crystal in the tetragonal phase using s7Rb nuclei as probes (nuclear spin 8. The static magnetic induction B, was applied along the (110) direction of the cubic phase.…”