The crystal structure, magnetic, electrical and thermal properties of the anion-deficient − − + + + − 2 3 x / 2 2 3 x 3 1 x La A Mn O (A = Ca, Sr, Ba) ortomanganites have been experimentally studied depending on the temperature and magnetic field. It is found that the mean grain size of the stoichiometric solid solutions amounts approximately to 10 μm, while the grain size for anion-deficient solid solutions is approximately 5μm. It is established that the aniondeficient samples : for the Ca2+ are an O'-orthorhombic perovskites in the region of the 0 ≤ x ≤ 0.125 and an Oorthorhombic ones in the 0.175 ≤ x ≤ 0.30 [1] while for the Sr2+ [2] and Ba2+ [3] - an O'-orthorhombic perovskites in the region of the 0 ≤ x ≤ 0.125 and a rhombohedric ones in the 0.175 ≤ x < 0.30. As a doping level increases along with the oxygen vacancies number the samples in the ground state undergo a number of the magnetic transitions from the antiferromagnet A-type (x = 0) to the inhomogeneous magnetic (x > 0) state. It is assumed that the samples with the large amount of oxygen vacancies are a cluster spin glasses (0.175 < x ≤ 0.30) and temperature of the magnetic moment freezing is ~ 40 K. The mean size of ferromagnetic clusters (50 nm) in the spin glass state is estimated. It is shown that oxygen vacancies make a substantial contribution to the formation of magnetic properties of manganites.igureFig. Temperature dependence of magnetic susceptibility (bottom panel), heat capacity (top panel) and reduced heat capacity (middle panel) for the aniondeficient La0.825Sr0.175MnO2.91 sample. All the anion-deficient samples are semiconductors and show considerable magnetoresistance over a wide temperature range below temperature where magnetic ordering appears. Fitting of the electrical resistivity to T-1 and T-1/4 is realized. One can conclude that the transition to paramagnetic state is not an ordinary thermodynamic second-order phase transition, because the latter implies not a smooth C(T) dependence but a sharp anomaly near TMO (Fig). The most important feature of our observation is the linear dependence of the reduced molar heat capacity C/T(T) in the low-temperature region T < Tf and the presence of a smooth peak at T >> Tf, which determines a strongly degenerate ground state of the anion-deficient samples. Magnetic phase diagrams for the anion-deficient − − + + + − 2 3 x / 2 2 3 x 3 1 x La A Mn O (A = Ca, Sr, Ba) samples have been established by the magnetic, electrical and thermal measurements. The most probable mechanism of formation of the magnetic phase state in the anion-deficient manganites is considered. The magnetic properties of the anion-deficient samples may be interpreted on the base of the superexchange Mn3+(VI)-O2—Mn3+(V) interaction and chemical phase separation models. It is assumed that in the absence of orbital ordering, a decrease in the magnetic ion coordination number leads to sign reversal in indirect superexchange interactions Mn3+ – O2- – Mn3+ [4]. This work has been partly supported by Belarussian Republic Fund Fundamental Research (F06R-078), Scholarship of the President of Republic of Belarus and the State Committee for Scientific Research (Poland) (Grant : KBN 1 P03B 038 27).