Agradecimentos: The work is being financed by FAPEMIG (MG-Brazil) Grants No. APQ-01577-09, APQ-02253-12, 2010-EXA023, and 2012-EXA011, CNPq (Brazil) Grants 308355/2009-1, 482549/2010-6, and PICT 1043. F. A. Garcia would like to acknowledge FAPESP Grant No. 2011/22261-5 for the financial support during the work and Eric Andrade for fruitful discussions on the basics of frustrated systems Ver menos

Abstract: We report a structural-magnetic investigation by x-ray absorption spectroscopy (XAS), neutron diffraction, dc susceptibility (X-dc), and electron spin resonance (ESR) of the 12R-type perovskite BaTi1/2Mn1/2O3. Our structural analysis by neutron diffraction supports the existence of structural trimers with chemically disordered occupancy of Mn4+ and Ti4+ ions, with the valence of the Mn ions confirmed by the XAS measurements. The magnetic properties are explored by combining dc-susceptibility and X-band (9.4 GHz) electron spin resonance, both in the temperature interval of 2 <= T <= 1000 K. A scenario is presented under which the magnetism is explained by considering magnetic dimers and trimers, with exchange constants J(a)/k(B) = 200 (2) K and J(b)/k(B) = 130 (10) K, and orphan spins. Thus, BaTi1/2Mn1/2O3 is proposed as a rare case of an intrinsically disordered S = 3/2 spin gap system with a frustrated ground state Ver menos