Thermal characteristics of the phase transition near 100 K in BaFe2Al9

Novel intermetallic BaFe₂Al₉ has been of current interest due to indications of the charge-density wave (CDW) behavior associated with the structural phase transition. To further shed light on the nature of the phase transition, we carried out a detailed study by means of the specific-heat and thermoelectric measurements on single-crystalline specimens, mainly focusing on the signatures around the transition temperature TC100 K. For comparison, we investigated the isostructural compounds of BaCo₂Al₉ and SrCo₂Al₉, which showed no indications of the phase transition. While the measured physical quantities in both cobalt-based aluminides varied smoothly with temperature, those in BaFe₂Al₉ exhibited profound features with noticeable thermal hysteretic behavior in the vicinity of TC. Based on the observed sudden enhancement in the Seebeck coefficient below TC, we provide evidence for a decreased contribution from the n-type carriers undergoing the phase transition. The analysis of the thermal conductivity further indicated that both electronic and phononic origins were responsible for the drastic reduction in the observed thermal conductivity at TC. We thus highlighted that the strong interplay between electronic and structural degrees of freedom gave rise to the peculiar phase transition in BaFe₂Al₉. Moreover, we compared the observed signatures with those reported in various CDW systems, and revealed important aspects of the emergence of CDW behavior in BaFe₂Al₉.