The structure of La SrMnO LSMO x

The structure of La1–SrMnO3 (LSMO-x) is far from cubic perovskite. According to Ref. [7], this structure has two types of distortions, which are caused by either the mismatch between the size of the cations and the size of the corresponding free space (type 1), or by the Jahn–Teller (JT) effect (type 2) [4,8]. The author of Ref. [7] suggested that the second type of distortions was caused by the fact that the Mn3+ smoothened in a cubic crystalline field is degenerate in the d-orbitals, i.e., this field splits the atomic d-level to two- and three-fold degenerate levels and t2. Since the first level is higher than the second one, the t2 level is fully occupied by four electrons of the d-level of Mn3+, while the level is only partially occupied. The lanthanum strontium manganite undergoes two structural transitions, however, the data on them greatly differs. According to the results obtained by the authors of Refs. [5,9,10], the high-temperature structural transition in LSMO-0.125 occurs at T=270K from one orthorhombic structure to another, accompanied by the appearance of cooperative distortions of the second type. With a further decrease in temperature, a transition back to the initial structure occurs at Т=150K with the suppression of these distortions. At the same time, it follows from the results of Refs. [6,11,12] that the first transition occurs from the orthorhombic phase (Pbnm) to the monoclinic one (P21/с); the second transition is from the monoclinic phase to the triclinic one (P1) [13]. For example, according to the data in [14], LSMO-0.125 in a high-temperature paramagnetic phase belongs to the orthorhombic space group Pnma and has the parameters a=5.5624(6) Å, b=7.7360(6) Å, c=5.5478(6) Å, while according to the results of [15], this phase in LSMO-0.2 belongs to the rhombohedral space group Rc and has the following unit cell parameters: a ≈ 5,5Å, c ≈ 1,34Å, γ=120º. La1–SrMnO3 crystals have become a model object for studying colossal magnetoresistance [16], which is often associated with the charge and phase separation and with the percolation of the nanoregions with metallic-type conductivity [17]. It was predicted that inhomogeneous states could occur even above the Curie temperature [17]. The presence of unusual magnetocapacitance properties provides opportunities for practical applications of this type of manganites. For example, the authors of Ref. [2] proposed, for achieving high values of permittivity and magnetocapacitance effect, to modulate the properties of charge inhomogeneities in doped manganites LSMO-x. Extremely high values of permittivity (up to 107 in the 0.1–1kHz frequency range) and magnetocapacitance effect (up to 105%) were detected even at room temperature. The authors of [2] suggest that this may be due to the strong interaction between charge, spin and lattice degrees of freedom, leading to charge and phase separation before the percolation threshold [17]. Despite extensive studies in this field, the microscopic origin of this behavior has not yet been found. The study carried out by the authors of Ref. [5] can be considered one of the more successful attempts; a full Т–х phase diagram was constructed for LSMO-x with the x concentrations from to 0.45 and in a wide temperature range (4.2–1050K), and detailed investigations of the electrical and magnetic properties of LSMO-x single crystals were examined in detail in the above-noted range of concentrations. Temperature studies of the properties of LSMO-0.07 and LSMO-0.125 [5] have revealed that these compounds undergo a variety of magnetic and structural transformations, which include the appearance of antiferromagnetic and ferromagnetic orderings, structural transitions between strongly and weakly distorted orthorhombic phases, transition to the rhombohedral phase and transition to polaron ordering. According to Ref. [6], the polaron phase is an ordered arrangement of the Mn3+ and Mn4+ ions in which one of the two alternating atomic layers of the (001) plane contains, as in pure LMO, only the Mn3+ ions, while the second one contains both types of ions, i.e., holes [5].