Alkaline-earth-rare-earth silicate and germanate apatites

Abstract

Summary.A number of new alkaline-earth-rare-earth silicate and germanate compounds with the apatite-type structure have been synthesized including a new calcium lanthanum silicate, Ca4La6(SiO4)6(OH)2 , which has a 9'63 -~, c 7.12, and c/a 0"740. This has a random distribution of Ca 2+ and La a+ over the (4f) and (6h) sites in the lattice. X-ray data are given for Ca4La6(SiOa)6(0H)2 together with cell dimensions for other synthetic silicate and germanate apatites. Silicate apatites with partially occupied or vacant halogen (2a) sites analogous to those of the pyromorphites have been prepared, and a classification for silicate apatites based on synthetic end-members is proposed.T HE large number of naturally occurring and synthetic compounds known that have the apatite-type structure (hexagonal, space group P63/m ) form a group of unique interest and importance. Mineral members occur as common minor constituents of many igenous, metamorphic, and sedimentary rocks, and occasionally form deposits of considerable economic importance. In the biosphere, the calcium phosphate apatites are important constituents of teeth, bones, and urinary calculi, while their synthesis throughout human life during the progressive mineralization of the artery walls affects the lives of us all.During the course of a study of synthetic compounds for possible use as low-dielectric-loss ceramic dielectrics stable under reducing conditions, a number of new apatite-type compounds were prepared and their electrical properties determined. The extensive ionic substitutions of simple or complex type that may be made without destroying the apatite-type structure, coupled with the ease with which most members may be synthesized, makes the apatites an attractive group in which to seek a range of controlled dielectric properties. In addition, the relationships of some naturally occurring members to each other have been clarified and this has enabled a classification of alkaline-earth-rare-earth silicate apatites based on synthetic end members to be proposed.

Rudimental research progress of rare-earth silicate oxyapatites: their identification as a new compound until discovery of their oxygen ion conductivity

“…38) The main analytical method they used was powder XRD by applying the space group P6 3 /m to the hydroxyapatites and fluorapatites. In addition, an important possibility was introduced, namely, formation of oxyapatite, because we found the description "Attempts to confirm the presence of the hydroxyl ion in the calcium lanthanum silicate apatite by infra-red absorption spectroscopy have so far been unsuccessful, and an oxyapatite formula Ca 4 It was interesting that the oxyapatite type was distinguished by the number of oxygen atoms.…”

Phase relationships in the quasi-ternary LaO1.5–SiO2–MgO system at 1773 K

“…The slight variation of the lattice parameters of the oxyapatite phase in the La 6 Mg 4 (SiO 4 ) 6 O and La 4 Mg 6 (SiO 4 ) 6 samples is related to the formation of a liquid phase as discussed in section 3.4. Considering the other phase relationship in the LaO 1.5 -SiO 2 -MgO system discussed below, we conclude that the solid solubility limit of MgO substitution for LaO 1.5 • system is about 12.5 mol%, which is smaller than the solubility limit of CaO, SrO or BaO in lanthanoid silicate oxyapatites [28]. In order to confirm the compositional width of the oxyapatite single-phase region around the La 9 the MgO solution limit in La 2 Si 2 O 7 would be very small because the lattice parameters of La 2 Si 2 O 7 were almost independent of the sample composition and were consistent with the reported data [20].…”

“…Previous reports in the fields of mineralogy and cement engineering suggest that alkali-earth ions substitute for lanthanoid ions in lanthanoid silicate oxyapatites [28,29] (x = 0, 50, 100) can be indexed to the oxyapatite structure as shown in figure 3. On the other hand, samples 11 (La 6 Mg 4 (SiO 4 ) 6 O) and 12 (La 4 Mg 6 (SiO 4 ) 6 ) were mixtures composed of the oxyapatite (monoclinic clinoenstatite structure, symmetry group P21/c) [17], and Mg 2 SiO 4 (orthorhombic forsterite structure, symmetry group Pnma) [18], as shown in figure 3.…”

X-ray structure refinement, vibrational spectroscopy, and ionic conductivity of Na0.11Ca9.64Sm0.23(PO4)6F2

“…The structure of apatite Ca 10 (PO 4 ) 6 A 2 A=F, OH, Cl apatite in the space group P6 3 /m allows a wide range of cation and anion substitutions [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. In fact, the two Ca positions have distinct stereochemistries (Ca(1), equipoint 4f, CaO 9 polyhedron, Ca(2), equipoint 6 h, CaO 6 (A polyhedron)) are able to accommodate a variety of univalent, divalent, and trivalent cations as substituents.…”

“…Actually, minor amounts appear to replace Ca on the smaller Ca(2) position. Moreover, lanthanum, which is a synthetic Ca 4 La 6 (SiO 4 ) 6 (OH) 2 , is reported to be randomly distributed on both Ca positions [10]. In NaY 9 (SiO 4 ) 6 O 2 , while Y is ordered in the 6 h position [Ca (2)], it is disordered with Na in 4f [Ca (1)] [12].…”