CRYSTAL DATA 143

1977) et poss6de gt temp&ature ambiante m6me structure que (CH3NH3)2HgCI 4. Les donn6es pour (CH3NH3)2HgBr 4 sont rapport6es dans le Tableau 1.

L'environment du mercure est t6tra6drique. La structure est constitu6e de groupements (HgBr4) 2- et (CH3NH3) ÷ isol6s; eile est isomorphe ~ ceile de [(CH3)4N]2HgBr 4 (Kamenar & Nagl, 1976). L'ion HgBr] - dans ce cas n'est pas parfaitement r+gulier, ceci est dfi fi la pr6sence de liaison hydrog6ne.

A basse temp&ature, vers 173 K, ce compos6 pr6sente une transition, probablement du type ordre-d6sordre pour les cations d'ammonium, comme le chlorure correspondant.

Nous proc6dons actuellement ~i la caract6risation de cette maille ~ basse temp6rature et fi l'&ude de cette transition.

R6f6rences BEN SALAH, A., BAATS, W., KALUS, R., FUESS, H. & DAOUD,

A. (1983). A paraitre. DAOUD, A. (1977). J. Appl. Cryst. 10, 133. KAMENAR, B. & NAGL, A. (1976). Acta Cryst. B32,

1414-1417.

MOROSIN, B. & LINGAFELTER, E. G. (1959). Acta Cryst. 12, 611-612.

J. Appl. Cryst. (1983). 16, 143

Crystal data for antimony molybdenum oxide Sb2MoO6.* By AHMED LAARIF, Ddpartement de Chimie, Facultd des Sciences, la Bouloie, 25030 Besan¢on CEDEX, France, ALAN HEWAT, Institut Laue-Langevin, Boulevard des Martyrs, 38 Grenoble, France and FRANfOIS THEOBALDt and HERV~ VIVIER, Micro-analyse des Surfaces, ENSMM, la Bouloie, 25030 Besan¢on CEDEX, France

(Received 2 July 1982; accepted 26 August 1982)

Abstract The powder data for antimony molybdenum oxide Sb2MoO 6 are reported. This compound is triclinic, P1, with a=7.481(4), b=7.504(3), c=10-120(1) /~, ~=70.43(3), /3 = 70.91(4), y = 83"34(7)°; U = 505-9(5) ,~3; Z = 4; Dx = 5.74 Mg m -3. The structure is of the fluorite type but it is distorted. Pseudo-monoclinic cells as reported by

*The full text of the paper has been deposited with the British Library Lending Division as Supplementary Publication No. SUP 38110 (8 pp.). Copies may be obtained through The Executive Secretary, International Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England.

tAuthor to whom requests should be addressed.

Parmentier, Courtois & Gleitzer [Bull. Soc. Chim. Ft. (1974), 1-2, 75-77] or that obtained with program TRACER [Lawton & Jacobson (1965). Mobil Oil Corporation Re- search Department; Lawton (1967). Mobil Oil Corporation Research Department] are also considered and discussed. Neutron diffraction gives further evidence of the triclinic lattice. The JCPDS Diffraction File No. for SbzMoO 6 is 33-1491.

The authors are grateful to Mr Sylvain Lecocq ofthe Centre de Diffractom6trie de l'Universit6 de Lyon, for measuring the cell parameters.

J. Appl. Cryst. (1983). 16, 143

Crystal data for synthetic K2Mg(CO3)2.* By BRUNO S~MONS, Mineralogisch-Petrographisches Institut und Museum der Universitiit Kiel, Olshausenstrasse 40-60, 2300 Kiel, Federal Republic of Germany and Smv K. SHARMA, Hawaii Institute of Geophysics, University of Hawaii, 2525 Correa Road, Honolulu, Hawaii 96822, USA

(Received 27 April 1982; accepted 27 September 1982)

Abstract Dipotassium magnesium carbonate, K2Mg(CO3)z, was crys- tallized at 573 K and 0.025 GPa in a cold-seal hydro-

*The full text of the paper has been deposited with the British Library Lending Division as Supplementary Publication No. SUP 38148 (7 pp.). Copies may be obtained through The Executive Secretary, International Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England.

thermal vessel with C O 2 as pressure-transmitting medium. Cell parameters [a = 5.1543(5); c = 17-307(2) ,~] were deter- mined by the powder diffraction method. V= 398-2(2) A 3, Z = 3, Dx = 2.784 g cm - 3, Cu Ks. Crystalline KzMg(CO3)/is isostructural either to buetschliite, K zCa(CO3)2, with space group R32/m or eitelite, NazMg(CO3)z, with space group R3. K2Mg(CO3) z may be present as a significant component in natural buetschliite as well as in eitelite, if not present as a natural mineral itself. The JCPDS Diffraction File No. for K2Mg(CO3) 2 is 33-1495.

0021-8898/83/010143-01501.50 © 1983 International Union of Crystallography