tailieunhanh - Neutron Scattering from Magnetic Materials part 4

Tham khảo tài liệu 'neutron scattering from magnetic materials part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 84 T. Chatterji Fig. 44. Crystal structures of Ruddlesden-Popper phases. determined by Ruddlesden and Popper in 1958 113 . The Rietveld profile refinements on neutron and both laboratory and synchrotron X-ray powder diffraction data produced a huge amount of literature on the structure analysis of Ruddlesden-Popper phases. As already mentioned in the previous section the structure is flexible with respect to the cation order disorder between the 12-coordinated perovskite P and 9-coordinated rock-salt R sites and also with respect to the octahedral distortion. To make the situation more complicated there exists the possibility of phase separation mentioned before. The possibility of electronic phase separation also exists. Despite these complications accurate structural parameters have been determined both as a function of hole doping x and also as a function of temperature. Also available are structural parameters as a function of both hydrostatic pressure and applied magnetic field. Of special interest are the Mn-O bond lengths. There is only one Mn site 4e 00z but there are three O sites in the structure O 1 2a 000 O 2 4e 00z and O 3 8g 01 z . As mentioned before there are two cation sites La Sr P 2b 002 and La Sr R 4e 00z . The essential result of the structural evolution at room temperature with x is that the JT distortion Ajt which is about for x decreases continuously and becomes very small AJT at x 114 . This result remains qualitatively the same at low temperature but AJT is slightly smaller 114 . The temperature variation of AJT for x shows a minimum close to TC 115 . Investigation of the crystal structure of the same compound 115 as the temperature is lowered through TC in a field of T reveals a significant magnetostriction. The equatorial Mn-O bond contracts and the apical Mn-O bond expands in a magnetic field. For Magnetic structures 85 LaSr2Mn2O7 x charge-orbital ordering 116 takes place at Too 225 K. The