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The rhombohedral CaMn7O12 manganite is an important material which shows magnetoelectrioc coupling with very high values of the electric polarization [1]. These outstanding properties motivated many experimental studies and also theoretical analysis. The mechanism which leads to these extraordinary properties has not been explained up to now. A fundamental information needed for such studies is the crystal structure and the magnetic ordering. CaMn7O12 has a complex structure with a magnetic moments modulation below TN=90K [1,2], a modulation of the atomic positions below TC=250K [2] and also orbital ordering. The magnetic modulation propagation vector qm is related with the atomic positions modulation vector qp by the relation qp=2qm [2]. This 2:1 relation is valid across a large range of temperatures and show the importance of spin-lattice coupling. The crystal and magnetic structure of CaMn7O12 was studied by neutron powder diffraction at the instrument DMC at SINQ [3]. The magnetic and atomic position modulations are described by using the superspace group formalism. This approach is especially important for description of both modulations with the same model [2]. The resulting magnetic ordering model obtained in [3] is more precise as compared with earlier works [1,2]. The present results [3] differ from those published by other authors [1]. The important difference is that in the present studies the angle, Phi, between Mn3+ and Mn4+ magnetic moments located in the same (001) planes (Phi = 0.99(2)Pi), i.e. the moments are antiparallel, whereas Johnson et al. [1] determined this angle as Phi=0.84(4) Pi. This angle is an important parameter of the model Hamiltonians describing the electronic and magnetic properties of CaMn7O12.
abstracts
"Aluminophosphate framework structures have been widely studied because of their many technological applications. The most significant application of aluminophosphate type framework catalysts is in the methanol-to-olefin (MTO) conversion process [1], catalysed by SAPO-34 (the silicoaluminophosphate form of the chabazite (CHA) zeolite framework with silicon substituted into its structure). The effectiveness of SAPO-34 in the MTO process is due to both the shape selective properties of the framework and the concentration and strengths of the acid sites created by silicon substitution [2]. Another aluminophosphate framework MTO catalyst is SAPO-18 (zeolite framework type (AEI)), which has a very closely related structure to SAPO-34 and can form intergrowths with it. It has been suggested that the level of intergrowth can affect the efficiency of the MTO process [3], however, assessment of the level of intergrowth has remained difficult. We present a consistent model of the crystal structure of SAPO-18/34 family members which can accurately determine the level of intergrowth. The model utilises two types of stacking fault: Displacement and Growth which have significantly different effects on the diffraction pattern. A series of powder diffraction patterns is calculated using the Discus software package. Changes in the level of intergrowth and stacking fault type strongly affect the calculated pattern. A series of patterns has been calculated to illustrate this. The structure of an intergrown SAPO-34 sample with 4.8% Si content is modelled and refined using Displacement stacking faults. An example of ""defect-free"" AlPO-18 (0% Si content) is then presented. Refinement of the model shows that even this contains a small amount of stacking faults. Finally, a simple method for defect level estimation is proposed based on FWHM (Full Width at Half Maximum) ratios for selected Bragg reflections."
