Zeitschrift fur Kristallographie, Bd. 127, S. 145-159 (1968) The crystal structure of zinc orthoborate, Zn3(B03)2 By S. GARCIA-BLANCOand J. FAYOS lnstituto de Quimica Fisica Rocasolano, Madrid (Received September 13, 1967) Auszug Zna(BOa)2 hat die Gitterkonstanten a = 23,406, b = 5,048, e = 8,381 A und p = 97032'. Die Elementarzelle del' Raumgruppe Ie enthiilt 8 Formeleinheiten. Die Struktur wurde mit Hilfe von BUERGERS Minimumfunktion und von Fouriersynthesen in (010)- und (001)-Projektionen bestimmt und durch drei- dimensionale Differential-Fouriersynthesen verfeinert. Fiir 802 beobachtete Interferenzen ergab sich R = 0,132 als Endwert. Die Sauerstoffionen in den Ecken von (Boa)a- -Dreiecken gehoren auch (Zn04)6--Tetraedern an. Es gibt regelmiiJ3ige Tetraeder, die gemeinsame Ecken haben, und deformierte mit gemeinsamen Kanten. Diese Tetraeder bilden Bander. Abstract Zinc orthoborate crystallizes in the monoclinic system, space group Ie, with eight molecules in a cell of dimensions a = 23.406, b = 5.048, C = 8.381 A and p = 97032'. The structure has been determined by BUERGER'S minimum func- tion and Fourier synthesis in projections (010) and (001), and refined by three- dimensional differential Fourier synthesis, yielding a final R value of 0.132 for 802 observed reflections. The structure of Zna(BOa)2 consists of (Boa)a- triangles which share a common vertex with (Zn04)6- tetrahedra. There are two kinds of tetrahedra: regular ones, sharing common vertices, and distorted ones sharing common edges. These tetrahedra form ribbons of interconnected units. Introduction The binary system ZnO-B20a has been studied by several authors. INGERSON, MOREY and TUTTLE (1948) reported two compounds: ZnO . B20a and 5ZnO. 2B20a. TOROPOV and KONOVALOV (1949) reported three compounds: 3 ZnO . B20a, ZnO . B20a and ZnO . 3 B20a. HARRISON and HUMMEL (1956) found that two compounds exist, 5ZnO .2B20a and ZnO. B20a, both of them exhibiting low- and high-temperature polymorphic forms. BAUER (1963) reported the Z. Kristallogr. Bd. 127, 1-4 10