The synthesis, crystal structures and magnetic properties of LnM(BO 3 ) 2 (Ln = Y, Ho-Lu; M = Sc, Cr) were investigated. The LnCr(BO 3 ) 2 compounds crystallize in the dolomite-type structure with space group R 3 , in which the Ln and Cr ions occupy two octahedral sites. From the result of structural analysis, it was found that there is an anti-site disorder between these two sites and its chemical formula is more exactly Ln 1-r Cr r [Cr 1-r Ln r ](BO 3 ) 2 . On the other hand, the LnSc(BO 3 ) 2 adopt the calcite-type structure with space group R 3 c. The Ln and Sc ions randomly occupy an octahedral site and the chemical formula is represented as (Ln 0.5 Sc 0.5 )BO 3 . From the magnetic susceptibility and specific heat measurements, we found that all the LnCr(BO 3 ) 2 show an antiferromagnetic transition at 6.1-8.1 K. This transition is mainly due to the ordering of Cr 3+ magnetic moments. Among the compounds with magnetic Ln 3+ ions, only YbCr(BO 3 ) 2 shows an antiferromagnetic ordering of Ln 3+ ion at 2.1 K.-3 -

LnM(BO3)2 (Ln: Y, Ho—Lu; M: Sc, Cr) is prepared by solid state reaction of Ln2O3, M2O3, and H3BO3 (30% excess for volatility loss compensation) in the ratio of 1:1:5.2 (repeated heating to 1100 °C, 6 h).

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