Crystal structure and vibrational spectrum of the NaCaM92F7 pyrochlore

The crystalline structure of NaCaMg2F7 was determined using single crystal X-ray diffraction. This compound crystallizes in the cubic pyrochlore structure, i.e., space group Fd (3) over barm, lattice parameter: a = 10.2610(5) Angstrom and Z = 8. All atoms occupy special crystalline sites, but Na and Ca are randomly distributed in the anti-cristobalite sub-lattice of the pyrochlore structure. The vibrational spectrum was determined by polarized Raman scattering and infrared reflectance. The number of observed Raman and infrared active phonons is larger than that predicted by the factor group analysis of the pyrochlore structure. The anomalous vibrational spectrum is discussed in terms of a disorder-induced symmetry lowering mechanism. (C) 2004 Elsevier Inc. All rights reserved.