On the incommensurate phase in BaMnF4: sample dependence of the order parameter

Abstract

2014 Nous présentons des résultats sur la dépendance en fonction des échantillons, du vecteur d'onde de la modulation, et de l'amplitude du paramètre d'ordre qui décrit la transition N (A21am) ~ incommensurable dans BaMnF4 : des mesures de diffraction de rayons X, de Cp (« DSC »), et de biréfringence, ont été effectuées sur un large éventail de cristaux synthétisés par les méthodes de Bridgman, de Czochralski, et hydrothermale. Nous proposons l'existence d'un point tricritique pouvant être atteint sous contrainte 03C3yz, et donnons une nouvelle interprétation de l'anomalie de Cp qui avait été observée en 1982 par Scott, Habbal et Hidaka. Abstract. 2014 We present results concerning sample dependence of not only the modulation wave vector, but also of the amplitude of the order parameter describing the N (A21am) ~ incommensurate phase transition in BaMnF4: X-ray diffraction, Cp (« DSC »), and birefringence measurements were performed on a wide set of crystals which were synthetized by Bridgman, Czochralski, and hydrothermal methods. The existence of a tricritical point which can be reached under a 03C3vz stress is proposed, and a new interpretation of the Cp anomaly observed in 1982 by Scott, Habbal and Hidaka, is given.

Linear birefringence studies of incommensurate systems: II. KFeF4

“…The largest precursor effect is obtained on the H sample, whereas on B and F the effect is reduced by about 1/3. This observation reminds of a similar experience made with samples of the incommensurate compound BaMnF, of different origins (St. -Gregoire, Kleemann, Schafer and Moret, 1988). Similarly as in the present case of KFeF,, hydrothermally grown BaMnF, exhibits particularly large precursor tails of its orthorhombic LB.…”

“…Hence, by virtue of extrinsic parameters like strains or lattice defects the X phase might easily be suppressed. Presently, there is only indirect evidence for an incommensurate phase extending between 395 and 410 K in samples originating from hydrothermal or Bridgman growth methods (St. -Gregoire et al, 1990). The (1/2, 1, 3) X-ray Bragg peak, which is to become allowed in phase I11 exhibits anomalous precursor intensity in that temperature range.…”

“…q = (ql, q2) is the two-component order parameter, which describes an incommensurately modulated rotation of the FeF, octahedra along the a direction as measured by the x coordinate. It has been argued (St. -Gregoire et al, 1990) that the very existence 156 U. BIANCHI, W. KLEEMANN AND P. SAINT-GREGOIRE of the X phase depends on a delicate balance between the Lifshitz and a fourth-order anisotropy term. Hence, by virtue of extrinsic parameters like strains or lattice defects the X phase might easily be suppressed.…”

Thermodynamics of the incommensurate state inRb2WO4:The Lifshitz point inLuk’yanchuk1, Jório2, Saint‐Grégoire3 2000Phys. Rev. BSelf Cite11030View full textAdd to dashboardCite

“…Note first that they cannot be ascribed to fluctuation effects that were widely studied last decades in relation with transitions to the modulated phases [14][15][16][17][18]. In such a case the first order character is attributed to the lack of a stable fixed point as in BaMnF 4 [19] and the discontinuity is expected to be small due to the smallness of the critical region. We propose that the observed discontinuities are caused by the coupling of the order parameter with elasticity of the crystal that is known [20][21][22] to be able to change the order of transition.…”

Defects as a cause of continuity of normal-incommensurate phase transitions

“…1 as example of continuous cases and Ref. 2 for rare examples of discontinuous ones). This is in contrast with other types of structural phase transitions that are generally discontinuous.…”