B5Ba2ClO9, orthorhombic, Pnn2 (No. 34), a = 11.6358(9) A, b = 11.583(1) A, c = 6.6823(7) A, V = 900.6 A, Z = 4, Rgt(F) = 0.020, wRref(F) = 0.054, T = 293 K. Source of material The dibarium pentaborate chloride was synthesized using the method described for Sr2B5O9Cl [1]. Single crystals with dimensions up to 2 × 1 × 1 mm were grown from BaCl2 flux in a temperature range between 1273 K and 1173 K. Discussion Dibarium pentaborate chloride is isostructural to Ca2B5O9Br [2,3] and Eu2B5O9X (with X = Cl, Br) [4,5]. The structure consists of Ba cations, Cl and B5O9 anions. The pentaborate group is formed by a series of three corner-linked [BO4] tetrahedra, additionally two neighboring tetrahedra each are bridged by one planar [BO3] group ([BO3]:[BO4]=2:3). Using the description for polyanions in anhydrous borates introduced in [6] and modified in [7], the borate polyanions (“fundamental building block”) in Ba2B5O9Cl can be depicted as 2D3T: – . Sharing common oxygen atoms, the main structure unit is composed of the pentaborate groups to form wave-like, infinite chains of [BO4] tetrahedra parallel to the c axis. The chains are interlinked along the a and b axis via [BO3] groups of the pentaborate units. The result is a three dimensional framework of [B5O9] 3– parts with large channels, in which the Cl anions are placed in maximum distance. The Cl is separated from the anionic [B5O9] 3– structure part by Ba cations, which coordinate the chlorine in form of a distorted tetrahedra. The [ClBa4] tetrahedra are linked to another four ones to form the cationic part [ClBa2] 3+ of the structure. Both symmetrically independent Ba cations are surrounded by seven oxygen and two chlorine atoms. The interaction between the Ba cations and the next neighborhood seems to be spatially limited because of the special arrangement of the [B5O9] 3– and [ClBa2] 3+ framework. Z. Kristallogr. NCS 217 (2002) 1 © by Oldenbourg Wissenschaftsverlag, Munchen Crystal: colourless prism, size 0.24 × 0.24 × 0.24 mm Wavelength: Mo K radiation (0.71073 A) : 90.17 cm Diffractometer, scan mode: Enraf-Nonius MACH3, /2 2 max: 61.04° N(hkl)measured, N(hkl)unique: 8152, 2740 Criterion for Iobs, N(hkl)gt: Iobs > 2 (Iobs), 2636 N(param)refined: 156 Programs: MolEN [8], SHELXL-97 [9], ATOMS [10] Table 1. Data collection and handling.