SYNTHETIC, STRUCTURAL, AND MAGNETIC STUDIES OF STRONTIUM COPPER(II) BORATES WITH THE COMPOSITION SR1-XMXCU2(BO3)2, M=BA OR CA

Studies of the new type of alkaline earth copper(II) borates, represen ted 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 w ith the composition Sr1-xMxCu2(BO3)2 where M = Ca or Ba and 0 less-tha n-or-equal-to x < 0.4 have been prepared. X-ray powder photographs ver ify that these phases are isostructural to SrCu2(BO3)2. The crystal st ructure of Sr1-xMxCu2(BO3)2 can be regarded as consisting of slightly puckered layers with the composition [CuBO3]- stacked along [001], wit h eight coordinated alkaline earth atoms located between the layers. E stimated bond valence sums for all atoms indicate normal bond length d istributions in the structure. The structure possesses a pseudo-mirror symmetry but the deviations (<0.2 angstrom) from this symmetry (space group I4/mmm) are statistically highly significant. The substitution of strontium for smaller calcium or larger barium atoms has a pronounc ed effect on the interlayer separation. Measured magnetic susceptibili ties of SrCu2(BO3)2 indicate a transition to an antiferromagnetic stat e below 14 K. (C) 1994 Academic Press, Inc.