inorganic papers Acta Cryst. (2004). E60, i127±i130 doi: 10.1107/S1600536804023232 Olga V. Yakubovich et al.  [Pb 2 (B 5 O 9 )Br] i127 Acta Crystallographica Section E Structure Reports Online ISSN 1600-5368 Reinvestigation of the Pb 2 [B 5 O 9 ]Br structure based on single-crystal data Olga V. Yakubovich, a * Natalya N. Mochenova, a Olga. V. Dimitrova a and Werner Massa b * a Department of Geology, Moscow Lomonosov State University, Vorob'evy Gory, 119899 Moscow, Russia, and b Department of Chemistry and Materials Science Center, Philipps- University, D-35032 Marburg, Germany Correspondence e-mail: yakubol@geol.msu.ru, massa@chemie.uni-marburg.de Key indicators Single-crystal X-ray study T = 293 K Mean (O±B) = 0.019 A Ê R factor = 0.034 wR factor = 0.080 Data-to-parameter ratio = 29.2 For details of how these key indicators were automatically derived from the article, see http://journals.iucr.org/e. # 2004 International Union of Crystallography Printed in Great Britain ± all rights reserved The structure of the title pentaborate, [Pb 2 (B 5 O 9 )Br] (dilead pentaborate bromide), known from powder data, has been re®ned based on single-crystal data (R = 0.034) and is discussed in terms of intercrossing anionic and cationic frameworks. Comment The mineral family of hilgardite includes three polymorphs with the common formula Ca 2 [B 5 O 9 ]ClH 2 O (triclinic phases hilgardite-1A and parahilgardite, and monoclinic hilgardite- 4M), tyretskite Ca 2 [B 5 O 9 ]OHH 2 O (Strunz & Nickel, 2001), that seems to be isotypic with hilgardite-1A (Ghose, 1985), and kurgantaite CaSr[B 5 O 9 ]ClH 2 O (Pekov et al. , 2001). Sr occupies one of two independent Ca positions of hilgardite-1A in the crystal structure of kurgantaite (Ferro, Pushcharovskii et al., 2000). Several synthetic compounds of composition M 2+ 2 [B 5 O 9 ]X (M = Ca, Sr, Ba, Eu, Pb and X = Cl, Br) have been obtained during attempts to prepare boracite-type structures (Fouassier et al., 1971; Peters & Baglio, 1970). The crystal structure investigation of the calcium (Ca 2 [B 5 O 9 ]Br; Lloyd et al., 1973) and europium (Eu 2 [B 5 O 9 ]Cl; Machida et al., 1981) members of this group revealed their orthorhombic symmetry (space group Pnn2) and a close relationship to the hilgardite structure type. Zeolite-like construction and piezoelectric properties have been noted earlier for hilgardites (Ghose & Wan, 1979; Ghose, 1982; Wan & Ghose, 1983). According to Ghose & Wan (1979), hilgardite was successfully tested for second harmonic generation and gives piezoelectric signals comparable to quartz. Blue emission at about 430 and 435 nm was established for Eu 2 [B 5 O 9 ]Br and Eu 2 [B 5 O 9 ]Cl (Machida et al., 1981), noting that their Eu 2+ activated alkaline earth analogues are ef®cient phosphors. The crystal structure of an orthorhombic synthetic Ba variety in the hilgardite structure group, Ba 2 [B 5 O 9 ]Cl0.5H 2 O, was reported by Ferro, Merlino et al. (2000) and two more Pb- Received 26 August 2004 Accepted 20 September 2004 Online 9 October 2004 Figure 1 The basic [B 5 O 12 ] fragment of the anionic framework.