Investigations on the polymorphism of the cesium dodecahalogeno-closo-dodecaborates Cs2[B12X12] (X = Cl-I)

Thermoanalytic DSC and temperature-dependent X-ray diffraction investigations on the cesium dodecahalogeno-closo-dodecaborates Cs-2[B12X12] (X = Cl-I) have revealed solid-solid phase transitions from their trigonal room-temperature a-forms (e.g. a-Cs-2[B12Cl12]: a = 959.67(3) pm, c = 4564.2(2) pm, Z = 6, space group R (3) over bar) into cubic high-temperature modifications. The isotypic title compounds crystallize in the space group Pm (3) over barn (e.g. beta-Cs-2[B12Cl12]: a = 1051.98(6) pm, Z = 2) with a W3O-type defect structure. The statistic occupation of six possible positions with only four Cs+ cations results in a cation-deficient A(2)B arrangement for Cs-2[B12X12]. Upon cooling the beta-phase, a third polymorph was observed, which also crystallizes in the cubic system, but now in the space group Ia3c1 (e.g. y-Cs-2[B12Cl12]: alpha = 2102.2(3) pm, Z = 16), and has to be regarded as a phase with only a partially disordered cation substructure. In this crystal structure the [B12X12](2-) anions exhibit a NaTl-type arrangement, in which the Cs' cations occupy suitable interstices. The phase transitions of the differently halogenated cesium salts follow no specific trend as the transition from the trigonal alpha- to the cubic beta-form occurs at 178 degrees C for the chlorinated, at 270 degrees C for the iodinated and at 325 degrees C for the brominated examples. On further heating however, beta-Cs-2[B2I12] starts to decompose at 945 degrees C first, followed by beta-Cs-2[B12Br12] and beta-Cs-2[B12Cl12] at 959 degrees C and 983 degrees C, respectively.