Botta, C.; Kahlenberg, V.; Hejny, C.; Többens, D.M.; Bykov, M.; van Smaalen, S.: Structural investigations, high temperature behavior and phase transition of Na6Ca4(SO4)6F2. Mineralogy and Petrology 108 (2014), p. 487-501

Polycrystalline material of a sulfate apatite with chemical composition Na6Ca4(SO4)6F2 or (Na2Ca4)Na4(SO4)6F2 has been synthesized by solid state reactions. Basic crystallographic data are as follows: hexagonal symmetry, a=9.3976(1) Å, c=6.8956(1) Å, V=527.39(1) Å3, Z=1, space group P63/m. For structural investigations the Rietveld method was employed. Thermal expansion has been studied between 25 and 600 °C. High temperature (HT) powder diffraction data as well as thermal analysis indicate that the apatite-type compound undergoes a reconstructive phase transition in the range between 610 and 630 °C. Single-crystals of the HT-polymorph were directly grown from the melt. Structural investigations based on single-crystal diffraction data of the quenched crystals performed at −100 °C showed orthorhombic symmetry (space group Pna21) with a=12.7560(8) Å, b=8.6930(4) Å, c=9.8980(5) Å, V=1097.57(10) Å^3 and Z=2. Unit cell parameters for a quenched polycrystalline sample of the HT-form obtained at ambient conditions from a LeBail-fit are as follows: a=12.7875(1) Å, b=8.7255(1) Å, c=9.9261(1) Å, V=1107.53(2) Å3. The lattice parameters of both modifications are related by the following approximate relationships: aHT≈2cRT, bHT≈-(½aRT+bRT), cHT≈aRT. The HT-modification is isotypic with the corresponding potassium compound K6Ca4(SO4)6F2. The pronounced disorder of the sulphate group even at low temperatures has been studied by maximum entropy calculations. Despite the first-order character of the transformation clusters of sulfate groups surrounding the fluorine anions can be identified in both polymorphs. Each of the three next neighbor SO4-tetrahedra within a cluster is in turn surrounded by 8–9 M-cations (M: Na,Ca) defining cage-like units. However, in the apatite structure the corresponding three tricapped trigonal prisms are symmetry equivalent. Furthermore, the central fluorine atom of each cluster is coordinated by three next M-neighbors (FM3-triangles), whereas in the HT-polymorph a fourfold coordination is observed (FM4-tetrahedra).