Β -TmAlB4 was synthesized, and the magnetic properties and specific heat investigated in comparison with those of the α -type structure. Recent developments in rare earth aluminoboride compounds with two-dimensional boron layers have attracted interest due to the heavy fermion superconductivity in Β -YbAlB4, multiple anomalies manifesting below the Ńel temperatures in α -TmAlB4 attributed to intrinsic building defects, and field stable state in Tm2 AlB6. Strikingly, despite similarities in the metal-metal nearest neighbor distances and structural motifs, the strength of the magnetic interactions (temperature scale) in Β -TmAlB4 was discovered to be approximately 60% stronger that of the alpha phase (i.e., TN=9.2 versus 5.8 K). An anomaly in the specific heat with extreme field stability was also found with a temperature scale significantly higher compared to that of the α -type structure. This anomaly is conjectured to be a precursor to the antiferromagnetic transition and may be a particular feature of these "tiling" compounds with analogy to AlB2 -type structure.
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