615 Acta Cryst. (1973). B29, 615 Crystal Structure of Calcium Chloride Tetrahydrate BY ULF THEWALT* AND CHARLES E. BUGG Institute of Dental Research and Department of Biochemistry, University of Alabama h~ Birmingham, Birmingham, Alabama 35233, U.S.A. (Received 16 October 1972; accepted 28 October 1972) The crystal structure of calcium chloride tetrahydrate was determined by use of three-dimensional X-ray diffractometer data. Crystals of CaCI2.4H20 are triclinic, space group P], with a= 6.593 (2), b=6.364(5), c=8.557(3) /~, ~=97-77(5), fl=93.52(4), and y=110.56 (3) °. The structure was refined by least-squares calculations to R=0.085. The two chloride ions and four water molecules form a trigonal prismatic coordination polyhedron about the calcium ion. Introduction We determined the crystal structure of calcium chloride tetrahydrate as part of an investigation of calcium ion coordination properties. Here we describe the struc- ture, which displays an interesting pattern of calcium coordination. Experimental We obtained crystals of calcium chloride as a by- product from an attempt to prepare a geraniol-calcium chloride addition compound. A large volume of ethyl acetate was poured slowly into a test tube that con- tained a hot slurry of calcium chloride in geraniol. Over a period of several days, clear, well developed, triclinic crystals of CaC12.4H20 grew on the walls of the test tube. It is likely that the water contained in these crystals came from the undried samples of ethyl acetate and calcium chloride. We later checked the purity of the calcium chloride and found that when fused, it released a considerable amount of water. Since the crystals are hygroscopic, they were stored under mineral oil. A crystal with the approximate dimensions 0.4x 0.3 x 0.2 mm was transferred, while still heavily coated with mineral oil, into a Lindemann glass capillary, which was then sealed with wax. The crystal was mounted on a Picker FACS-1 diffractometer with its a axis slightly inclined to the ~0axis of the diffractom- eter. Approximate cell parameters for use in collec- tion of intensity data were calculated by a least-squares analysis of the angular settings for nine medium-angle reflexions (Cu K~, 2= 1.5418 A). Intensity data were collected with the diffractometer, by use of a scintillation counter, nickel-filtered copper radiation, and a 0-20 scanning technique. Measure- ments were made for each of the 1104 independent reflexions with 20< 128 °. Three standard reflexions, which were monitored periodically, showed no de- crease in intensity during the collection of data. * Permanent address: Institut ffir Anorganische Chemie der Technischen Universitgtt, 33 Braunschweig, Germany (BRD). Immediately after data collection, accurate values for the cell parameters were determined by a least- squares analysis of 20 values for 12 high-angle reflex- ions (Cu Kcq, 2=1.54051 A); these cell parameters were not significantly different from those obtained prior to the collection of intensity data. Crystal data are listed in Table 1. Table 1. Crystal data Stoichiometry Z Space group a= 6.593 (2) ]k b=6"364 (5) c = 8.557 (3) Q (calculated)= 1.837 g. cm -3 Q (observed)= 1-85 p = 148.3 cm -x CaCI2.4H20 2 PT = 97.77 (5) ° fl= 93.52 (4) 7 ----110.56 (3) The reported standard deviations are five times greater than those obtained from the least-squares analysis. The density was measured by flotation in a mixture of benzene and ethylene dibromide. The intensities were assigned variances, 0"2(I), ac- cording to the statistics of the scan and background counts plus a correctional term (0.03S) 2, S being the scan counts. The intensities and their variances were corrected for Lorentz and polarization factors. Correc- tions for the crystal absorption were applied by using the program ORABS (Wehe, Busing & Levy, 1962), no corrections were made for absorption by the mineral oil and capillary. The data were scaled by means of a Wilson (1942) plot. Coordinates for the calcium and chloride ions and for the oxygen atoms were readily obtained by direct methods with the use of the computer program MULTAN (Germain, Main & Woolfson, 1971). The trial structure was refined by use of a modified version of the full-matrix least-squares program ORFLS (Bus- ing, Martin & Levy, 1962). The quantity minimized was ~w(F 2- F2/k2) 2, where k is a scale factor and the weight w is equal to 1/02(F02). Scattering factors were from the International Tables for X-ray Crystallog- A C 29B - 16