X-RAY-POWDER DIFFRACTION AND VIBRATIONAL SPECTROSCOPIC INVESTIGATION OF THE LAO(CL1-XBRX) SOLID-SOLUTIONS

The formation of the anion solid solutions in the LaO(Cl1-xBrx) (0 les s than or equal to(xBr)less than or equal to 1; Delta x(Br)=0.2) serie s was studied by the vibrational (FT-Raman) spectroscopic and X-ray po wder diffraction (XPD) techniques. The Raman spectra were successfully analyzed by using the factor group analysis. The crystal structures f or all members of the system were resolved from the X-ray diffraction data by using the Rietveld profile refinement method. The LaO(Cl1-xBrx ) system appears to retain the tetragonal structure throughout the who le series. However, there exists two regions of the tetragonal LaOX (X =Cl or Br) phases: the chloride- and bromide-rich regions between 0 le ss than or equal to x(Br)less than or equal to 0.4 and 0.4 less than o r equal to x(Br)less than or equal to 1, respectively. Only insignific ant formation of solid solutions could be observed since the lattice p arameter a and c values changed only slightly from those for the pure LaOCl and LaOBr in these two regions. Despite the absence of the solid solubility, no phase separation occurred as predicted by the Vegard's rule since the difference in the ionic radius of the Cl- and Br- ions is only about 8%. The FT-Raman studies yielded practically constant n umber and energies of the vibrations throughout the whole LaO(Cl1-xBrx ) series resulting in similar conclusions.