The a-(3 transformation of berlinite (AIP04) at 586°C was studied by X-ray diffraction. Atomic displacements were obtained from results of least-squares refinement of data taken between room temperature and 600 "C using reflections whosc intensity is unaffected by Dauphine twinning. The results suggest a rotational motion of the PO4 and ,410, tetrahedra around the two-fold axis together with a translational motion along the same axis as the transition is approached from below. The vibrational amplitudes of the atoms increase with temperature and have exceeded half of the separation between Dauphine twin-related configurations at 500 "C. The final (3-phase configuration is not achieved by this twinning due to the mismatch of the two configurational potential minima in the a direction. Analysis of the intensity vs. temperature data favours a single minimum model for the (3 phase configuration over a n order-disorder model. The (3-AIPO, structure consists of alternate PO4 and AlO, tetrahedra sharing all four corners with P-0 and Al-0 distances 1.505 and 1.694 A respectively. The results are correlated with those obtained from temperature dependent studies by Raman scattering and by EPR on Fe3+-doped AIP04.La transformation a-(3 de la berlinite (AIPO,) a 586 "C a etk ktudike par diffraction X. Les dCplacements atomiques ont ete obtenus au moyen d'ajustements par moindres carres de donnees prises entre la temperature ambiante et 600 "C en utilisant les reflexions dont I'intensitk n'est pas affectke par le maclage Dauphine. Les resultats suggerent un mouvement de rotation des tetraedres PO, et A10, autour de I'axe d'ordre deux, en meme temps qu'un mouvement de translation de long de ce m&me axe quand on approche de la transition par temperatures croissantes. Les amplitudes de vibration des atomes augmentent avec la temperature et ont depasse la moitie de la separation entre les deux macles Dauphine a 500 "C. La configuration finale de la phase (3 n'est pas obtenue par ce maclage, dfi au non ajustement des deux minima de potentiel de configuration dans la direction a. L'analyse des donnees des intensitks en fonction de la temperature favorise un modele 5 minimum unique pour la configuration de la phase (3 par rapport a un modele ordre-dksordre. La structure (3-A1P04 consiste en une alternance des tktraedres PO, et AIO, ayant leurs quatre sommets en commun avec des distances respectives P-0 et A1-0 de 1.505 et 1.694 A. Une correlation est etablie entre ces resultats et ceux obtenus par des etudes de la dependance en temperature de la diffusion Raman et de la RPE sur AIP0, dope par Fe3+.[Traduit par le journal]Can.
Attempts to synthesize-BaRe03 resulted in formation of a slightly nonstoichiometric compound with the ideal formula Ba3Re209. It was found to have the space group R h with hexagonal unit cell parameters a = 5.7699 ( 5 ) A and c = 20.799 (2) A, Z = 3. A total of 2816 reflections were measured on an automatic diffractometer, and the structure was refined by full-matrix least squares on 308 unique reflections to a final R value of 0.027 and R, of 0.036. It consists of a nine-layer repeat sequence of hexagonal and cubic close-packed layers of composition BaO,. The stacking of these layers along the c axis leads to the formation of two types of octahedral sites, M(1) and M(2), for the Re atoms. An M(2) octahedron shares opposite faces with two M( 1) octahedra. Site population analysis has shown that about 8% of the M(2) sites are occupied and that simultaneous occupation of two octahedral sites sharing a face is avoided, resulting in the Re deficient compound Ba3Rei,9,09. All the Ba atoms are 12-coordinated and both Reo, octahedra show a distortion corresponding to an elongation along the trigonal axis. IntroductionA large number of ternary oxides AMO,, where A is a large cation and M is a transition-metal cation, crystallize in the
The Raman spectra of small single crystals of the cristobalite form of aluminum phosphate (AlPO4) have been obtained in both the a and P phase. Particular emphasis has been placed on the behavior of the phonons near the phase-transition temperature. The transition is first order, but the latent heat and discontinuity in lattice parameters are small enough to permit cycling of the sample through the transition without damage. A soft mode is observed which has energy 102 cm ' at ambient temperatures. The observed spectra confirm the assignments of space groups C222 (D 2) and I4m2 (D2") for the a and P phases, respectively. Phonon comparisons are made with isomorphic BPO, and SiO&.
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