Orbital Ordering in the Perovskite Manganates

Examples of Charge and Spin Ordering in Oxides. Fe304 has a spinel structure, represented as Fe- [Fe-, Fe ]04 in which one third of the cations are tetrahedrally coordinated (A-sites) and two thirds of the cations are octahedrally coordinated (B-sites). It is ferrimagnetic below 858 K (7n), with the spins 

It is surprising that ferromagnetism is not encountered in the electron-doped regime (jc 0.5). Effects of magnetic and electric 

Case Studies. To understand typical scenarios of charge-ordered manganates, it is instructive to examine the properties of two manganates with different sizes of the A-site cations. For this purpose, we choose NdasSra MnO with a weighted average radius of the A-site cations, (xa), of 1.24 A and Pro.6-Cao.4Mn03 with an (xa) of 1.17 A (Shannon radii are used here). 

Paired JT distorted stripes or bistripes are believed to be present in Lao,3.iCao.67MnO,i. A supercell based on such ordering has been proposed, but a detailed structural investigation has shown that the diffraction data can be explained satisfactorily without bistripes. Apparently, owing to orbital disordering, a mixture of paired and unpaired stripes seems to occur in Lao.iiCao.sMnO. . Because such charge stripes generally are observed by electron microscopy, it is not clear whether these stmctures truly represent the bulk composition. 

sMn03 ((7a) = 1.19 A) reveals an intriguing sequence of phase transitions. On cooling, this manganate develops an incipient CO state below 220 K. The formation of this state is accompanied by an increase in electrical resistivity and the opening up of a gap in the density of states near f. The orthorhombic distortion also increases, as a consequence of cooperative JT distortion of the lattice and short-range 

Charge, Spin, and Orbital Ordering in the Perovskite Manganates, Ln( AAAMn03 (Ln = Rare Earth, A = Ca or Sr) 

Chemistry Physics of Materials Unit and the CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O, Bangalore 560 064, India 

Evidence for charge ordering can be observed in crystal structures at low temperatures. Thus, in Lao.jCao.sMnC, the Mn4+ environment is nearly isotropic, with all the Mn-0 distances being nearly equal ( 1.92 A). In the Mn3+C 6 octahedra, one of the Mn—O bonds is much longer ( 2.07 A) than the other bonds ( 1.92 A), consistent with orbital ordering. It must be noted that the Mn-O distances in the ab plane of the manganates are much longer than in the c-direction, 

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