Observation of low thermal expansion behavior and weak thermal anisotropy in M3A2C phases

M3A2X phases, named 321 phases, are an atypical series of MAX phases featuring in the MA-triangular-prism bilayers, with the A = As/P, exhibiting excellent elastic properties. This work systematically studies the thermal expansion properties of 321 phases. We found their average linear thermal expansion coefficients (TECs), αL = 5-6 μK–1, are the lowest among the reported values of MAX phases. The lowest average TEC was found in Nb3As2C (αa = 4.46(4) μK–1, αc = 5.09(4) μK–1, αL = 5.09(4) μK–1). The average TEC and anisotropy factor (αc/αa) of Nb3As2C and Nb3P2C were lower than the ones of the corresponding 211 phases. The best isotropy performance was found in Nb3P2C (αc/αa = 1.11). Moreover, our first-principles calculations demonstrate that the weaker chemical bonding between Nb-As/P than Nb-C induces thermal expansion in M3A2X phases. Furthermore, a relatively weaker anharmonic effect in 321 phases than in the 211 phases was revealed by the as-calculated average Grüneisen parameters, which account for the lower TECs in 321 phases. The low TECs and enhanced thermal isotropy make 321 phases outstanding among MAX phases, which could be sound candidates for varying-temperature structural-functional components.