HR: 0830h
AN: T11C-0419    [PDF]
TI: Monochromatic Powder Diffraction Coupled With $^{19}$F and $^{23}$Na NMR: A Study of K$_{(x)}$Na$_{(1-x)}$MgF$_{3}$ Perovskite
AU: * Martin, C D
EM: ph_d_@hotmail.com
AF: Geosciences Department, State University of New York at Stony Brook, Stony Brook, NY 11794-2100 United States
AU: Chaudhuri, S
EM: sachaudh@ic.sunysb.edu
AF: Chemistry Department, State University of New York at Stony Brook, Stony Brook, NY 11794-3400 United States
AU: Grey, C P
EM: clare.grey@sunysb.edu
AF: Chemistry Department, State University of New York at Stony Brook, Stony Brook, NY 11794-3400 United States
AU: Parise, J B
EM: jparise@notes.cc.sunysb.edu
AF: Geosciences Department, State University of New York at Stony Brook, Stony Brook, NY 11794-2100 United States
AB: Structural phase transitions from orthorhombic (O, Pbnm) to tetragonal (T, P4/mbm) to cubic (C, Pm3m) and an increase in unit cell volume are confirmed as potassium is substituted for Na in Neighborite (NaMgF$_{3}$). Substitution of the 19% larger potassium ion for sodium in the A-site of the perovskite structure decreases tilting of MgF$_{6}$ octahedra. Transition from O to the T and C phases, as determined by powder XRD, occurs at $\sim$37 and 47 mole % substitution, respectively. Superlattice reflections, expected if ordering were to occur in the A-sites, are not observed. The superlattice reflections, uniquely defining the symmetry of the O- and T-phases, broaden between each perovskite endmember, possibly indicating a conflicting coherence length between small domains of variously tilted octahedra. $^{19}$F MAS NMR reveals multiple environments for fluorine that depend on the number and type of cations in the 1$^{st}$ A-cation coordination shell, F(Na)$_{y}$(K)$_{5-y}$. At intermediate substitution levels (30 - 60%), all five of these possible fluorine coordination environments are found, but the concentration of the environments with y = 2 are much lower than expected based on random substitution of K for Na on the A cation site. This data suggests that upon substitution of 15% K into NaMgF$_{3}$, potassium appears to segregate into domains resembling KMgF$_{3}$ but smaller than the coherence length of X-rays. The $^{23}$Na NMR is very sensitive to the O/T to C transition. For x $>$ or = 0.5, a sharp resonance is observed due to sodium atoms in a cubic environment, while broader resonances are observed for x $<$ 0.5.
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Tectonophysics [T]
MN: 2003 Fall Meeting