Rubidium Tetrachlorocadmate: X-Ray Diffraction Measurements and an Electronic Structure Study

The structure of Rb₂CdCl₄single crystal at room temperature has been determined from X-ray diffraction of the MoKαline (λ=0.7107 Å). After refinement through blocked least-squares methods, the reliability factorRin the final cycle is 3.07%. The following results have been obtained: tetragonal system, space groupI4/mmm,a=b=5.195(1) Å,c=16.130(1) Å;F(000)=380;Dm=3.243 g/cm³;Z=2. The structure can be viewed as made of layers of CdCl₆octahedra chains (Cd–Cl(1)=2.597(1) Å and Cd–Cl(2)=2.572(1) Å) separated by double slabs of rubidium atoms perpendicular to thecdirection. First-principles density functional theory calculations have been carried out to determine the electronic density distribution. The calculated equilibrium structure is in satisfactory agreement with the experimental data. Electronic density maps have been drawn from ab initio wavefunctions calculated both at the experimental and theoretical equilibrium geometries. Analysis of the calculated atomic populations confirms the highly ionic character of the electronic charge distribution in the crystal.     

Influence of the growth method upon the phase transition of RbCdCl $\mathsf{_{3}}$

Structural properties of two RbCdCl₃ samples grown either from the melt or from aqueous solution are studied via X-ray diffraction over a closed temperature cycle between 20 C and 300 C. During cooling step (300 C), the crystal grown from the melt undergoes a phase transition at 110 C that drives it from the cubic structure into a tetragonal structure that still persists at 20 C. It undergoes exactly the reverse phase transition at the same temperature during the heating ( C) step that immediately follows. The other crystal grows from aqueous solution at 20 C in an orthorhombic structure (i.e. not tetragonal as that of the crystal grown from the melt and cooled down to this temperature). During the heating ( C) step, it undergoes a direct orthorhombic-cubic phase transition at 240 C (without passing through the tetragonal phase) whereas, during subsequent cooling (300 C), it does not exhibit the corresponding reverse phase transition but rather exhibits exactly the same cubic-tetragonal phase transition at 110 C as the crystal grown from the melt. However, for both crystals, this tetragonal phase observed at room temperature is unstable and slowly converts into an orthorhombic phase over the course of time. Complementary Differential Scanning Calorimetry (D.S.C.) and Thermo Gravimetric Analysis (T.G.A.) measurements have been carried out over the range ( ) C in order to interpret diffraction experiments.Received: 19 May 2004, Published online: 30 September 2004PACS: 61.10.Nz X-ray diffraction - 64.70.Kb Solid-solid transitions - 65.40.Ba Heat capacity     

Plastic and glassy crystal states of caffeine

The present paper focuses on the high temperature form I of caffeine and on its low temperature metastable form. Structural, dynamic, and kinetic information has been obtained by X-ray, dielectric, and calorimetric investigations. This study shows the following features: (1) The high temperature phase (I) of caffeine is in a state of dynamically orientationally disordered crystalline state (so-called "plastic, or rotator, phase"). (2) This high-symmetry hexagonal phase can be maintained at low temperature in a metastable situation. (3) Under deep undercooling of form I a glass transition occurs in the disordered crystalline state near room temperature. It is associated with the orientational freezing in of the molecular motions. Otherwise stated, the metastable state I enters into a nonergodic unstable state, so-called "glassy crystal" state. These findings rationalize the difficulties seen with caffeine in pharmaceutical science.