Anisotropic thermal expansions of select layered ternary transition metal borides: MoAlB, Cr2AlB2, Mn2AlB2, and Fe2AlB2

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The coefficients of thermal expansion (CTE) of the ternary borides, MoAlB, Cr2AlB2, Mn2AlB2, and Fe2AlB2, and the binary boride, CrB, were calculated from in situ high-temperature powder X-ray diffraction patterns. The order—from low to high—of the average linear thermal expansions was MoAlB (7.3 × 10−6 K−1), CrB (8.6 × 10−6 K−1), Fe2AlB2 (10.3 × 10−6 K−1), Cr2AlB2 (10.5 × 10−6 K−1), and Mn2AlB2 (14.0 × 10−6 K−1). Despite their structural and chemical similarities, the CTE anisotropies of these phases were different and could be grouped into two groups. In the first group, MoAlB and Fe2AlB2, the highest CTE values were along the stacking direction and the lowest were parallel to the B–B chains. In the second group, Cr2AlB2, Mn2AlB2, and CrB, the lowest CTE was along the stacking direction and the highest was normal to the chains. The thermal expansions parallel to the B–B chains were significantly lower (32% to 103%) than those perpendicular to the chains in all the ternaries except for MoAlB. In general, the relative CTEs parallel to the B–B chains in both the M2AlB2 ternaries and corresponding binary monoborides suggest that strong covalent character of the B–B bonds are at least partially responsible for the large thermal expansion anisotropies observed.