Crystal chemistry of synthetic M (super 2+) Be (sub 2) P (sub 2) O (sub 8) (M (super 2+) =Ca,Sr, Pb,Ba) beryllophosphates

Four beryllophosphates with general formula M (super 2+) Be (sub 2) P (sub 2) O (sub 8) (M (super 2+) = Ca, Sr, Pb, Ba) have been synthesized under hydrothermal conditions at 200 degrees C. Their crystal structures were solved by direct methods and refined by full-matrix least-squares techniques on the basis of F (super 2) for all unique reflections to agreement indices R (sub 1) of 2.3, 4.8, 1.5, and 2.1%, respectively. CaBe (sub 2) P (sub 2) O (sub 8) is monoclinic, space group P2 (sub 1) /c, Z = 4, a 7.809(1), b 8.799(1), c 8.309(1) Aa, beta 90.51(1) degrees , V 570.98(2) A (super 3) }. SrBe (sub 2) P (sub 2) O (sub 8) is monoclinic, space group P2 (sub 1) /c, Z = 4, a 8.000(1), b 8.986(1), c 8.418(1) Aa, beta 90.22(1) degrees , V 605.10(6) A (super 3) . PbBe (sub 2) P (sub 2) O (sub 8) is monoclinic, space group P2 (sub 1) /c, Z = 4, a 8.088(1), b 9.019(1), c 8.391(1) Aa, beta 90.12(1) degrees , V 612.22(1) A (super 3) . BaBe (sub 2) P (sub 2) O (sub 8) is hexagonal, space group P6/mmm, Z = 1, a 5.028(1), c 7.466(1) Aa, V 162.51(1) A (super 3) . The three first compounds are isostructural and show a paracelsian-type structure composed of a framework of corner-sharing BeO (sub 4) and PO (sub 4) tetrahedra. These tetrahedra are assembled in four- and eight-membered rings showing the typical UUDD and DDUDUUDU patterns, respectively. The M (super 2+) (Ca, Sr, Pb) cation occurs in a distorted 7+3-coordinated polyhedron located in the eight-membered ring. CaBe (sub 2) P (sub 2) O (sub 8) and SrBe (sub 2) P (sub 2) O (sub 8) are isostructural with the minerals hurlbutite and strontiohurlbutite, respectively. The structure of BaBe (sub 2) P (sub 2) O (sub 8) consists of double layers of tetrahedra, which contain both Be and P in a 1:1 ratio. Inside the layers, the (Be,P)O (sub 4) tetrahedra form six-membered rings by sharing corners. The Ba atoms are located in very regular 12-coordinated polyhedra and connect two successive double layers. This Ba beryllophosphate is the synthetic counterpart of minjiangite, and shows similarities with the mineral dmisteinbergite, CaAl (sub 2) Si (sub 2) O (sub 8) , a hexagonal polymorph of anorthite. Other hydrothermal experiments were performed in order to establish the stability of these M (super 2+) Be (sub 2) P (sub 2) O (sub 8) compounds as a function of temperature and pH; the results show that synthetic hurlbutite can crystallize in acidic as well as in basic conditions. Furthermore, these experiments show that the four title compounds are stable at high temperatures and pressures (600 degrees C, 1 kbar).