C. Bezou et al., INVESTIGATION OF THE CRYSTAL-STRUCTURE OF GAMMA-CASO4, CASO4-CENTER-DOT-0.5 H2O, AND CASO4-CENTER-DOT-0.6 H2O BY POWDER DIFFRACTION METHODS, Journal of solid state chemistry, 117(1), 1995, pp. 165-176
The crystal structures of soluble anhydrite, gamma-CaSO4, and the two
calcium sulfate subhydrates, CaSO4 . 0.5 H2O and CaSO4 . 0.6 H2O, were
investigated by powder diffraction methods; using synchrotron X ray r
adiation and neutron powder diffraction, and the positions of the wate
r molecules in the structure were determined. gamma-CaSO4 is orthorhom
bic, space group C222 (N degrees 21) with a = 12.0777(7), b = 6.9723(4
), c = 6.3040(2) a (from the neutron diffraction data), and Z = 6. The
structure contains two different chains of CaO8 coordination polyhedr
a (mean Ca-O = 2.460 Angstrom) held together by SO42- ions. CaSO4 . 0.
5 H2O (SH1) is monoclinic, space group I121 (No. 5) with a = 12.0317(4
), b = 6.9269(2), c = 12.6712(3) Angstrom, beta = 90.27(1)degrees (fro
m the neutron diffraction data), and Z = 12. The structure contains ch
ains of alternating CaO8 and CaO9 coordination polyhedra held together
by the SO42- ions. The average Ca-O distances in the CaO8 and in the
CaO9 coordination polyhedra are 2.469 and 2.500 Angstrom, respectively
. A monoclinic structure is proposed for CaSO4 . 0.6 H2O (SH2), space
group I121 (No. 5) with a = 11.9845(4), b = 6.9292(2), c = 12.7505(3)
Angstrom, beta = 90 degrees (from the neutron diffraction data), and Z
= 12. The structure contains chains of CaO8 and chains of CaO9 coordi
nation polyhedra, both held together by the SO42- ions. The average Ca
-O distances in the CaO8 and in the CaO9 coordination polyhedra are 2.
538 and 2.494 Angstrom, respectively. All of the three structures have
channels where the water molecules associated with the CaO9 polyhedra
are located in the case of subhydrates. In the case of the hemihydrat
e, the water molecules form hydrogen bonds with oxygen atoms of the su
lfate tetrahedra of 2.85-3.03 Angstrom. In the case of CaSO4 . 0.6 H2O
, two adjacent water molecules in the channels are linked by a strong
hydrogen bond (O-H = 1.9 Angstrom). (C) 1995 Academic Press, Inc.