Structural phase transitions in the LaMnOs

Crystal Structure Phase Transitions in the PerovskitesA(Cul,Mn,)MnaOle(A = Na+, Cae+, Y8+, La8+) X-Ray diffraction and neutronographical studies on the crystal structure of the perovskitelike compounds A(Cu1 -zMnz)3Mn40,z have been undertaken. It has been shown t h a t the samples with 60% of Mn3+ in the octahedral sites have a cubic unit cell, while the samples with higher Mn3+ composition may have a monoclinic or rhombohedra1 unit cell. The temperature increase raises the symmetry to the cubic one. The concentretion ranges with different symmetry type have been determine&. The lowering of the symmctry of thr u n i t cell is due to the cooperative Jahn-Teller effect.

Neutron diffraction and magnetic methods are used to investigate the perovskite‐like compounds Ca(Cu1−xMnx)3Mn4O12 (0.09 ≦ x ≦ 1) within the temperature range 4.2 to 400 K. Magnetic structure and temperature dependence of magnetization are determined. It is shown that with substituting the Cu2+ ions by the Mn3+ ones the ferrimagnetic collinear structure transforms into a non‐collinear ferromagnetic one. For x > 0.69 a short‐range order transition occurs. According to the data of paramagnetic susceptibility the CaMn3Mn4012 compound is antiferromagnetic. A model for exchange interactions is proposed.

Magnetic properties of nonstoichiometric lanthanum manganite LaMnOj + A as a function of the Mn4+ concentration (from 0 to 27%) and temperature (from 77 to 300 K) are investigated. The Mn4+ ions concentration depends on the degree of oxidation 1. It MaHraHme jIaHTaHa @a3o~b1ii nepexoa @eppoMarHeTmK-napaMarHeTHK ~~O M C X O A H T

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Interstitial solid solutions with the perovskite -like structurc wort: synt hesi;.ecl by solid phase reactions a t high prcssure and temperature. The S J nthesis conditions of single phased samples, their stability on heating in air werP detcrinined. The iiiagiietic and electrical properties were measured. ExpcrirnentalPolycrystalline samples were synthesized under 5 GPa in the temperature range from 700 to 1400 "C. The heater was made of graphite and t o avoid interaction with it the samples were contained in nickel or tantalum capsules. X-ray studies were performed with a DRON-3 diffractometer with Fe-Ka-radiation. The magnetization of the samples was measured on a vibrational magnetometer, the dynamical susceptibility -by a mutualinductance bridge and the electrical conductivity by the four-probe method.