X-ray photoemission spectroscopic (XPS) studies and temperature dependent synchrotron X-ray diffraction (SXRD) investigation have been conducted on polycrystalline samples of Sm_0.5Y_0.5Fe_0.58Mn_0.42O₃ [SYFM (58-42)]. Room temperature XPS measurements revealed the mixed valance of the Fe and Mn ions along with the traces of oxygen vacancies in the material. The room temperature synchrotron X-ray diffraction (SXRD) of specimen shows the monophasic nature of the prepared composition, without the presence of any magnetic or non-magnetic impurity phases. SXRD measurements at variable temperatures between 298 and 873 K, indicates the absence of magnetostriction in the specimen at the magnetic transition temperatures T_SR1 ≈ 348 K, T_SR2 ≈ 300 K and T_N ≈ 361 K. At much higher temperatures above T_N, an anisotropic rapid positive thermal expansion (RPTE) to negative thermal expansion (NTE) of the α lattice parameter of the unit cell is further revealed for T > 573 K. The anomalous variation of phonons, are suggested to give rise to anomalous thermal variation in α lattice parameter of the specimen.