Abstract

Studies of the new type of alkaline earth copper(II) borates, represented by SrCu2(BO3)2, have been performed by high temperature techniques. In the parent structure of SrCu2(BO3)2, the strontium content can be partially substituted by calcium or barium. A series of such phases with the composition Sr1-xMxCu2(BO3)2 where M = Ca or Ba and 0 ≤ x < 0.4 have been prepared. X-ray powder photographs verify that these phases are isostructural to SrCu2(BO3)2. The crystal structure of Sr1-xMxCu2(BO3)2 can be regarded as consisting of slightly puckered layers with the composition [CuBO3]- stacked along [001], with eight coordinated alkaline earth atoms located between the layers. Estimated bond valence sums for all atoms indicate normal bond length distributions in the structure. The structure possesses a pseudo-mirror symmetry but the deviations (<0.2 Å) from this symmetry (space group I4/mmm) are statistically highly significant. The substitution of strontium for smaller calcium or larger barium atoms has a pronounced effect on the interlayer separation. Measured magnetic susceptibilities of SrCu2(BO3)2 indicate a transition to an antiferromagnetic state below 14 K.

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