莱河矿的超结构和低温穆斯堡尔谱的对比研究

A CORRELATED STUDY ON SUPERLATTICE STRUCTURE AND 4.2K MOSSBAUER SPECTRUM IN LAIHUNITE

本文进行了莱河矿３Ｃ超结构和４．２Ｋ穆斯堡尔谱的对比研究。研究表明．莱河矿的非等效位置Ｍ２Ａ＋Ｍ２Ｃ，Ｍ２Ｂ，Ｍ１Ｂ，Ｍ１Ａ和ＶＢ能够分别与Ｋａｎ等（１９８５）测定的莱河矿穆斯堡尔谱的吸收双峰Ａ，Ｂ，Ｃ，Ｄ和Ｅ对应，不仅解决了超结构位置和吸收双峰之间的对应，而且圆满解释了吸收双峰Ａ：Ｂ和Ｃ：Ｄ的强度比为２：１的关系。另外，还利用莱河矿超结构的资料讨论了莱河矿的反铁磁性内部作用。

correlated study on super lattice structure and 4.2K Mossbauer spectrum, and the anti-ferromagnetic interaction in laihunite is carried out.The 4.2K Mossbauer spectrum of laihunite contains five kinds of doublet A,B,C,D,E (Kan et al. 1985). Kan et al. present a possible explanation that M1 site and M2 site can be split into two nonequivalent groups (M11, and M12, M21 and M22)as the distortion of crystal structure of laihunite at low temperature and A'B,C,Dand E doublet correspond to the absorptions of M21, M22, M11, M12 and M2. respectively. But then it can not explained why there exists an intensity ratio of A:B or C:D approximating 2:1. In the present paper a new explanation is present based on a 3c- axis super lattice structure of laihunite determined by Shen et al. (1986).namely, A,B,C,D and E boblets correspond to the absorptions of M2A+M2C, M2B.M1B, M1A and VB sites respectively. The elplanations are based on the following reasons.(1) In the 3c-axis super lattice structure of laihunite, because the c axis of the superlattice structure is three times larger than the c axis of unit cell, then, M1site is divided into M1A, M1B, VA and VB site, and M2 site into M2A, M2B and M2C site.(2) Because there are seven Fe ions and three vacant in the neighbour of M2A or M2C site, but no vacant in neighbour of M2B (see Fig. 2), then, M2A site is very similar to M2C site, and there are enough reasons to assume that M2A site and M2C site belong to a same group so that the occupancy ratio of M2A+M2C site and M2B site is equal to 2:1 which is in a good agreement with intensity ratio of doublet A:B (=2.26:1).(3) The occupancy of M1B site is equal to 1, but 0.5 in M1A site, then, the occupancy ratio of M1B site and M1A site is equal 2:1 which is also very similar to the intensity ratio of C:D (=2: 1.18).(4) Because the occupancy of VB site is equal to 0.33, therefore, it is suggested that the weak doublet E corresponds to absorption of VB.The evidence for anti-ferromagnetic interaction of Fe ions in laihunite mainly are as follows.(1) Ferric doublet disappears at below 100K, but ferrous doublet at below 50-70K. The disappear temperature for F2+ doublet is similar to Neel temperature of Fe2SiO4, but that for Fe3+ doublet is higher than that of Fe2SiO4. It is obvious that the anti-ferromagnetic interaction of Fe3+ is stronger than that of Fe2+.(2) The power neutron diffraction of laihunite shows a broad and purely magnetic 010 reflection at 80K.The strong anti-ferromagnetic interaction of Fe3+ appeared in laihunite is caused by the decoupled planes of ferric ions on M2 sites (Kan, et al. 1985). Although this explanation is basically correct, but has some roughness. So a detailed explanation will be present in this paper.The anti-ferromagnetic interaction between Fe ions depend on the exchange interaction between the ions in nonequivalent site and near neighbour ions.Fe3+ ions in M2A or M2B or M2C site do not have a symmetry center sharing octahedral edge or corner with three vacants in M2A or M2C site, and do not share them with vacant in M2B site, The net exchange of distance between Fe ions in M2C, M2A and M2B site are 11.1082 A, 10.583A and 7.943 A respectively.In addition, Fe2+ ions in M1A or VB site have a symmetry center, and the net exchange of distance between Fe ions is equal to zero. Although this is a symmetry center in M1B site, but sharing octahedral edge with one vacant, so the net exchange of distance between Fe ions does not equal to zero (=3.605 A ).Because the net exchange of distance between Fe ions in M2A, M2B and M2C site is larger than that in M1A, M1B and VB site, then, the anti-ferromagnetic interaction between Fe3+ ions in M2 sites is stronger than that between Fe2+ ions in M1 sites.