Structural environments of incompatible elements in silicate glass/melt systems: I. Zirconium at trace levels

Zr complexation in high pressure fluids and silicate melts and implications for the mobilization of HFSE in subduction zones

Raman spectra of ZrF4–KF molten mixtures have
been measured at compositions up to 66 mol% ZrF4 and at temperatures
up to 1000 °C. The data indicate that in mixtures rich in alkali
fluoride two kinds of chemical species predominate the melt structure; octahedral
ZrF62− (ν1(A1g)
570 cm−1 and ν5 (F2g) 248
cm−1) and pentagonal bipyramidal ZrF73−
(ν1(A1′) 535 cm−1
and ν9(E2′) 340 cm−1).
An equilibrium between the two species is established which depends on temperature
and composition. Spectral changes upon melting A2ZrF6
(A = Li or K) and A3ZrF7 (A = K
or Cs) polycrystalline compounds support the proposed two species equilibrium
scheme. At 33 mol% ZrF4 the predominant species present are ZrF62−
octahedra. With increasing ZrF4 mole fraction the strong ν1(A1g)
band at 570 cm−1 shifts continuously to higher wavenumbers
and new bands appear in the spectra. At the maximum composition studied of
66 mol% ZrF4 the spectra are characterized by two polarized (630
cm−1, strong and ≈500 cm−1, weak)
and two weak depolarized bands (245 and ≈180 cm−1).
The observed spectral trends with variation of composition are similar to
those found for ThCl4–CsCl molten mixtures and are interpreted
with an analogous model where the structure of the rich in ZrF4
melts is dominated by small size chains formed by “ZrF6”
octahedra bound by corners and/or edges.

Raman spectroscopic studies of molten ZrF4–KF mixturesand of A2ZrF6, A3ZrF7 (A = Li,K or Cs) compounds

A Fourier analysis of X-ray scattering by glasses in the system ZrF4-BaF2 is presented. The first peak in the radial distribution functions occurs in the range 0.209 to 0.211 nm; the average coordination numbers estimated for the zirconium atom are in the range 7.08 to 8.35, depending on the chemical composition. The second peak has been broken down into two components attributed to two kinds of Ba-F distances, 0.261 to 0.266 and 0.307 to 0.319 nm, with a corresponding coordination number of barium with respect to fluorine atoms ranging from 8.70 to 10.68. Interpretation of the cation-cation region is suggested; the average angle values of the Zr-F-Zr bridge bond, varying from 150° to 160° prevent two neighboring zirconium atoms from being bonded through two fluorine atoms. Comparative discussions with crystalline structures of fluorozirconates are presented.

X‐Ray Scattering Studies of Glasses in the System ZrF4‐BaF2

Differential thermal analysis and Raman spectroscopic studies have been performed of crystalline and vitreous BaZrF6, PbZrF6, and SrZrF6 compounds in order to examine the thermal properties and the F coordination of Zr. The differential thermal analyses clarified the definite phase transformation characteristics of crystalline BaZrF6, and SrZrF6 and also suggested that the low- and high-temperature modifications are present in PbZrF6 as well as in BaZrF6 and SrZrF6. The Raman spectra of crystalline BaZrF6, PbZrF6, and SrZrF6 were discussed in relation to the F coordination of Zr in fluorozirconate compounds. The crystallization behaviors and Raman spectra of vitreous BaZrF6, PbZrF6 and SrZrF6 revealed that Zr in the vitreous compounds is eight-coordinated by F although small amounts of seven-coordinated Zr may exist in vitreous BaZrF6.

Thermal properties and Raman spectra of crystalline and vitreous BaZrF6, PbZrF6, and SrZrF6.

Nach Drehkristall-, Weisenberg- (hk0–hk2) und Prazessionsaufnahmen (h0l, h1l, 0kl1, 1kl) sowie Zweikreisdiffraktometerdaten (334 hk0–hk2), MoKα, kristallisiert K2ZrF6 im Gegensatz zu fruheren Angaben nicht orthorhombisch, sondern monoklin mit a = 6,572, b = 11,442, c = 6,942 A, β = 90,3°, Z = 4, Raumgruppe C–C2/c, dro = 3,604, dpyk = 3,61 g cm−3, Parameter vgl. Text. Es ist R = 5,9 und R′ = 6,0%.



Der Madelunganteil der Gitterenergie, MAPLE sowie „Effektive Koordinationszahlen” (ECoN), werden berechnet und diskutiert.



Crystal Structure of K2ZrF6



Due to rotating-crystal, Weisenberg (hk0–hk2) and precession photographs (h0l, h1l, 0kl, 1kl) as well as two circle diffractometer data (334 hk0–hk2), MoKα, K2ZrF6 contrary to former statements doesn't crystallize orthorhombic but monoclinic, a = 6.572, b = 11.442, c = 6.942 A, β = 90.3°, Z = 4, space group C–C2/c, dro = 3.604, dpyk = 3.61 g · cm−3, for occupied positions and atomic parameters see text. It is R = 5.9 and R′ = 6.0%.



The Madelung part of lattice energy (MAPLE) as well as the Effective Coordination Numbers (ECoN) are calculated and discussed.

R. Hoppe

Papers

Die Kristallstruktur von K2ZrF6