On the structural phase transition in rare earth elpasolites

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Zeitschrift für Physik B Condensed Matter 08/1984; 55(3):219-226. DOI: 10.1007/BF01329014

ABSTRACT The structural phase transition in two representatives of the rare earth elpasolite fluorides, Rb2NaHoF6 and Rb2NaTmF6, are studied in detail. The symmetry of the soft mode is determined from Raman spectra, X-ray and neutron powder diffraction data to be
. From the temperature dependence of the elastic constants of both compounds, estimates of various phase transition parameters are given, e.g. strain soft mode coupling constant and soft mode frequency. The softening of the elastic constants forT>T

is explained by the strain coupling to the fluctuations of the soft mode coordinates. The energy of the soft phonon at the zone boundary is estimated from the data and compared with that of K2ReCl6. The phase transition mechanisms in rare earth elpasolites and hexahalometallates are discussed.

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    ABSTRACT: The elpasolite-type compounds Rb2KFeF6 and Rb2KYF6 undergo a structural phase transition at 170 and 398 K, respectively, from high-temperature Fm3m symmetry to a tetragonal or pseudo-tetragonal low-temperature phase. The transitions are studied by Raman scattering. Group to sub-group relations are proposed, leading to the “family tree” of the elpasolite series. The space group for the low-temperature phase of Rb2KYF6 is either P121/nl or P1121/n. It can be concluded that the transition is weakly of first order, although it is apparently continuous. It is probably of the “triggered” type and order-disorder in nature. In the cubic phase of Rb2KYF6, the orientational disorder of the YF6 octahedra may be coupled with a positional disorder of the Rb+ cations. On the basis of group-theoretical considerations and of experimental data, it can be assumed that P4/m, P2/m, P1121/n, and P1 space groups are candidates for the low-temperature phase of Rb2KFeF6; the strong first-order character of this transition prevents from any further hypothesis concerning the transition mechanism.Les composés de type elpasolite Rb2KFeF6 et Rb2KYF6 comportent des transitions de phase structurales respectivement à 170 et 398 K, conduisant de la variété de haute température Fm3m à une phase distordue de basse température, quadratique ou pseudo-quadratique. Ces transitions de phase ont été étudiées par diffusion Raman. Les relations de groupe à sous-groupe constituant les „filiations” dans la série des elpasolites ont été établies. Le groupe d'espace de la phase de basse température de Rb2KYF6 est P121/nl ou P1121/n. On peut en conclure que cette transition est faiblement du premier ordre, bien qu'elle soit apparemment continue, probablement de type „déclenché” et de caractère ordre-désordre. Une hypothèse est avancée selon laquelle, dans la phase cubique de Rb2KYF6, un désordre orientationnel des octaèdres YF6 est couplé à un désordre positionnel des cations Rb+. A partir de considérations basées sur la théorie des groupes et des résultats expérimentaux, il apparait que les groupes d'espace P4/m, P2/m, P1121/n et P1 constituent les candidats pour la phase de basse température de Rb2KFeF6; cette transition étant fortement du premier ordre, aucune discussion n′a pu ětre menée quant à son mécanisme.
    physica status solidi (a) 01/1986; 98(2):423-434. · 1.21 Impact Factor
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    ABSTRACT: The pattern of lattice distortions occurring in the vicinity of Yb3+ ions during the transition of the Rb2NaYF6: Yb3+ crystal from the cubic to tetragonal phase has been revealed using all the parameters of the empirically found crystal fields for paramagnetic centers of the Yb3+ ions with cubic and tetragonal symmetry. It has been shown that the YbF6 octahedra are rotated about the fourfold axis through an angle approximately equal to 1.2°. Moreover, the octahedra themselves are deformed so that the F- ions symmetrically located in the plane perpendicular to the axis of rotation come close to the impurity ion at a distance of 0.0004 nm. The fluoride ions located on the axis of rotation, conversely, move away from the Yb3+ ion at a distance of 0.0005 nm. Based on the obtained results, it has been concluded that the total condensate of order parameters of the studied phase transition involves not only the critical rotations of octahedral groups but also the noncritical displacements of atoms in the rotated octahedra.
    Physics of the Solid State 01/2013; 55(12):2558-2565. · 0.77 Impact Factor
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    ABSTRACT: The structural phase transition has been observed for the first time in the Rb2NaYF6 crystal and studied by EPR and optical spectroscopy. EPR spectra of Dy3+ and Yb3+ ions present as unintentional dopants in the nominally undoped crystal and forming tetragonal paramagnetic centers have been identified. A characteristic splitting of some optical lines has been observed in the temperature dependence of the Yb3+ optical spectra. It indicates the splitting of the cubic quartet energy levels of Yb3+ ions by the tetragonal crystal field. The empirical schemes of the energy levels for cubic and tetragonal paramagnetic centers of Yb3+ ions have been established and parameters of the corresponding crystal fields have been determined. The latter have been used for analyzing the crystal lattice distortions occurring in the vicinity of the Yb3+ ion during the phase transition. It has been established using the superposition model that the nearest octahedral environment of the Yb3+ ion is distorted as follows: the fluorine ions are rotated by the angle of 2.1° around the fourfold axis; the F− ions located symmetrically in the plane perpendicular to the rotation axis approach the dopant by 0.0014 nm, whereas the F− ions located on the rotation axis move away by 0.0028 nm. It has been concluded that the studied phase transition includes the critical rotations of the octahedral F groups and noncritical displacement of atoms in the rotated fluorine octahedra.
    Physical review. B, Condensed matter 03/2013; 87(11). · 3.66 Impact Factor