L Journal of Alloys and Compounds 305 (2000) 49–57 www.elsevier.com / locate / jallcom OH–F disorder in non-centrosymmetric Zn (BO )(OH) F : ab initio 2 3 0.75 0.25 structure determination and NMR study; comparison with tridymite and fluoride borates a, b c a * G. Corbel , E. Suard , J. Emery , M. Leblanc a ´ Laboratoire des Fluorures, UPRES- A 6010, Universite du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France b Institut Laue-Langevin, Avenue des Martyrs, B.P . 156, 38042 Grenoble Cedex 9, France c ´ ´ Laboratoire de Physique de l’ Etat Condense, UPRES- A 6087, Universite du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France Received 17 May 1999; received in revised form 5 January 2000; accepted 7 January 2000 Abstract The crystal structure of Zn (BO )(OH) F is determined ab initio from X-ray and neutron powder data. The unit cell is 2 3 0.75 0.25 3 ˚ ˚ ˚ ˚ monoclinic, space group P2 (no. 4), Z52, with a56.8738(4) A, b54.9178(2) A, c55.7018(3) A, b 598.829(3) 8, V5190.46(3) A 1 32 (neutron powder data). Alternating up and down ZnO (OH,F) tetrahedra and triangular BO groups built corrugated layers. Their 3 3 connection by hydroxyl ions gives a 3D network similar to that of tridymite. The statistical distribution of fluorine and hydroxyl in infinite chains is evidenced by magical angle spinning-NMR spectroscopy.  2000 Elsevier Science S.A. All rights reserved. Keywords: Hydroxyfluoride borate; Ab initio structure determination; Non-centrosymmetry; Nuclear magnetic resonance 1. Introduction 2. Experimental According to Xia [1], the polar orientation of inorganic Powder of Zn (BO )(OH) F was prepared through 2 3 0.75 0.25 triangular groups in crystals leads to acentric structures and hydrothermal process from a mixture of Na B O ?10 H O, 2 4 7 2 consequently favours non linear optical properties. Numer- ZnF and H O in the molar ratio 1:1:55, heated in a 2 2 ous borates are investigated: KBe (BO )F (KBBF) [2], PTFE-lined Parr autoclave at 453 K for 24 h and cooled at 2 3 2 Sr Be (BO ) O (SBBO) [3], Ca Gd(BO ) O (GdCOB) about 6 K/h. The pH decreased from 9 to 8 during the 2 2 3 2 4 3 3 [4] and recently several fluoride carbonates reaction. The resulting product was washed with distilled K Ln (CO ) F (Ln5Pr, Nd, Sm, Eu, Gd) [5] were hot water in order to dissolve NaF, then with acetone and 4 2 3 3 4 evidenced. All these phases present parallel layers of finally dried at 373 K. A small amount of monoclinic 32 22 triangular BO or CO groups and the acentric character b-HBO was found as impurity and cannot be removed 3 3 2 results from the similar orientation of the triangles in the from the sample. The most intense peaks are indicated on layers. In this paper, we present a new acentric borate, the X-ray diffraction pattern (Fig. 1). obtained by hydrothermal synthesis in sub-critical con- The X-ray diffraction pattern of the sieved powder was 11 19 ditions. B and F MAS-NMR spectroscopic study is recorded on a Siemens D501 diffractometer. The neutron performed and structural comparisons with tridymite SiO diffraction pattern, recorded on the D1A high resolution 2 [6,7] and fluoride borates Be (BO )F [8], NaBe (BO )F powder diffractometer at the I.L.L., Grenoble (France), 2 3 2 3 2 [9] and KBe (BO )F [2] are discussed. was used for locating boron and hydrogen atoms. The 2 3 2 conditions of data collection are reported in Table 1. Structural calculations were performed with TREOR-90 [10], DICVOL-91 [11], FULLPROF [12], SHELX-76 [13] and SHELXS-86 [14] programs. The program DIAMOND [15] was *Corresponding author. Correspondence address: Department of used for the structure drawings. Chemistry, University of Cambridge, Lensfield Road, CB2 1EW Cam- An optical second-harmonic generation test was per- bridge, UK. E-mail address: gc260@cus.cam.ac.uk (G. Corbel) formed by the Kurtz–Perry method [16] on a powder 0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388(00)00713-1