(a) Temperature dependence of heat capacity C_p(T) for Ba₂BiFeS₅ single crystal at zero field. The red solid curve represents the lattice contribution, fitted by a polynomial. Inset: enlarged part at low temperature region. The red solid line is the fit using the formula C p ( T ) / T = β T 2 . (b) Temperature dependence of C_m(T) (left label, green symbols) and S_m(T) curves (right label, orange solid line). The brown dashed line represents the theoretical value of S_m ( R ln 6 for S = 5/2 system).