No. 20001 Deposited with http://cryst.instr.yamaguchi-u.ac.jp/20001/

Title:　Disordered and Displacive Models for the Structure of the Normal Phase in {N(CH₃)₄}₂MnCl₄

Authors:　Naohiro KOSHIJI^* and Hiroyuki MASHIYAMA⁺

Affiliation:　Kurume National College of Technology, Kurume, Fukuoka 830-8555

⁺ Department of Physics, Faculty of Science, Yamaguchi University, Yamaguchi 753-8512

^* Corresponding author (e-mail: koshi@ges.kurume-nct.ac.jp)

Abstract:

The crystal structure analysis for the normal phase of {N(CH₄}₂MnCl₄ has been performed for two models: a displacive model and a disordered model. In the former model, the constituent tetrahedra of tetramethylammonium and tetrachloromanganate satisfy a mirror symmetry themselves imposed by the crystal symmetry. In the latter model, each tetrahedron occupies two equivalent positions related by the mirror symmetry with an equal occupation probability: split atom method. A special attention has been paid to their thermal parameters; the former model exhibits an abnormal increase of the parameters near the transition temperature indicating a critical phenomenon. From the significant test for the R factor, the latter model seems to be more appropriate than the former one. The disorderness of tetrahedra of tetramethylammonium is also confirmed by double peaks in Fourier synthesis map for carbon atoms. On the other hand, MnCl₄ can be well represented both by ths displacive and the disordered models.

Keywords: normal phase, structure analysis, displacive model, disordered model, split atom method

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