Some of the RTX (R – rare earth, T – transition metal, X – p-metal element) including SmNiAl and SmPdAl, compounds crystallizing in the hexagonal ZrNiAl-type of structure exhibit a discontinuity in the temperature or composition dependence of the lattice parameters (a and c). This phenomenon is connected with some “forbidden” c/a ratio values. Single crystals of SmNiAl and SmPdAl have been studied and found to undergo a second order structure phase transition in the vicinity of 500 K, characterized by a change of the temperature dependence of the c/a ratio. The transition is seen also in the change of the temperature dependence of the magnetic susceptibility near to 500 K. The second order type of the transition induces appearance of c/a falling into the forbidden range of values. The enhanced thermal movement at high temperatures causes possibility of the forbidden values. The low temperature specific heat, resistivity and magnetization data point to antiferromagnetism in SmPdAl and SmNiAl with TN = 15.3 and 20.3 K, respectively. Various features of the specific magnetism of the Sm3+ ion, like strongly reduced magnetic moments, magnetocrystalline anisotropy, exotic (non-Curie-Weiss) temperature dependence of the paramagnetic susceptibility are presented and discussed.

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