On the structural phase transition in rare earth elpasolites

Agfa Gevaert AG D-8000 München 70 FRG
Zeitschrift für Physik B Condensed Matter 08/1984; 55(3):219-226. DOI: 10.1007/BF01329014


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.

1 Follower
1 Read
  • [Show abstract] [Hide abstract]
    ABSTRACT: Group-theoretical normal-mode analysis of the antifluorite A2BX6 structure in space group P4/mnc (D4h6) identifies two effective modes with Eg symmetry. Either mode may condense independently of the other at the phase transition to space group P21/n (C2h5). One mode produces a displacement of the A+ ions, the other a rotation of the BX62- octahedra. Lattice distortion thereby results from the coupling between order parameter (Q) and elastic strain (e). Linear and quadratic symmetry invariant coupling in terms of free energy results in e13∝Q and, independently, e22-e11 ∝Q2. The linear coupling produces a discontinuous change in soft-mode frequency at Tc, whereas the change in specific heat is caused primarily by the quadratic coupling. Measurements on K2TeBr6 reveal that one Eg mode condenses at the second-order phase transition to P21/n at 400 K and follows a Landau critical power law with e13 ∝(400K-T)1/2, leaving e22-e11=0 for 359<T<400 K. The symmetry-conserving softening of the octahedral rotation Eg mode takes place at 359 K, with e22-e11 ∝(359K-T). The incompatible symmetry of the Eg and the A1g mode derived from the higher-temperature Fm3m (Oh5) to P4/mnc phase transition results in a sharp change in the tetragonal c-a dimension at the tetragonal-to-monoclinic transition as further octahedral rotation becomes blocked. Evidence for the two Eg-mode displacement fields is found in the literature for all elpasolite A2B′B′′X6 and cryolite A3BX6 structures determined in space group P21/n. Formation of an additional phase by the antifluorites K2SeBr6 and K2SnCl6 in space group C2/c (C2h6) on cooling from P4/mnc to P21/n results from two octahedral rotations due to the Eg-mode basis vectors having order parameters of equal magnitude. The orthorhombic pseudosymmetry in C2/c is caused only by the e12 strain component. The coupling found in K2TeBr6 is shown to apply generally to related structure types.
    Physical review. B, Condensed matter 11/1984; 30(11). DOI:10.1103/PhysRevB.30.6540 · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In cubic K2SnCl6 the dispersion curve für q q; [110] of [10] polarized acoustic phonons has been measured at three temperatures near the phase transition temperature Tc1. The acoustic branch shows no temperature dependence in the low wave vector region and stiffens slightly near the X-point of the fcc-Brillouin zone as the temperature approaches the phase transition Tc1= 262 K from above. The results support a previously developed model on acoustic anomalies in this compound.
    Solid State Communications 12/1984; 52(10):829–831. DOI:10.1016/0038-1098(84)90249-7 · 1.90 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Sound waves propagating in intermetallic 4f- or 5f-compounds can couple to localized f-electrons (magnetoelastic interaction) or to 5d-, 4f- and 5f-band states (deformation potential coupling). In this review we concentrate on electron-phonon coupling in systems with unstable magnetic ions (intermediate valence compounds, Kondo lattices, heavy fermion uranium compounds). We discuss magnetoelastic phenomena for CeAl2, CeB6, CeAl3, deformation potential coupling for CeAl3, UPt3, intermediate valence compounds, magnetoacoustic quantum oscillations for several rare earth compounds and ultrasonic attenuation in UPt3.
    Journal of Magnetism and Magnetic Materials 10/1985; 52(1-4-52):70-78. DOI:10.1016/0304-8853(85)90228-8 · 1.97 Impact Factor
Show more