Hyperfine Interactions 128 (2000) 409–415 409 A M¨ ossbauer study of La 1-x Eu x B 6 compounds synthesized at high pressure and temperature Li-Ping Li a,b,∗ , Guang-She Li a , Wen-Hui Su a,b , Xu-Dong Zhao a and Xiaoyang Liu a a Department of Physics, Jilin University, Changchun 130023, PR China b Key Laboratory of Rare Earth Chemistry and Physics, Chinese Academy of Sciences, Changchun 130022, PR China Received 4 December 1999; accepted 11 January 2000 A series of hexaborides La1−xEuxB6 (x = 0.0–1.0) were synthesized under a pressure of 3.5 GPa and at a temperature of 1600 ◦ C using La2O3, Eu2O3 and amorphous boron as the starting materials. The products were characterized by X-ray Diffraction (XRD) and M¨ ossbauer spectroscopy. XRD data analysis shows that all samples crystallize in a cubic CsCl-type structure, and the cell volume increases with x. Room temperature 151 Eu M¨ ossbauer measurements reveal that Eu ions in all samples are in the divalent state, except for the x = 1.0 sample where a small amount of Eu 3+ ions was detected. The quadrupole splitting of the Eu 2+ ions is positive. Eu ions were reduced from trivalent to divalent during the high-pressure and -temperature processes. The isomer shifts of the Eu 2+ ions are all smaller than −12.5 mm/s, suggesting that there is no valence fluctuation in the samples. The hexaborides doped by divalent Eu are not metallic. 1. Introduction Boron atom having an electron configuration 1s 2 2s 2 2p 1 usually exhibits a special bonding state and a complex structure when forming compounds. For example, B 6 O synthesized by high temperature and pressure, has a rhombohedral structure, com- pletely fitting the geometrical requirements for forming icosahedred twins, the latter case has been described as a Machey packing of icosahedral B 12 units with long-range order [1]. In this compound, the valence state of B cannot be determined just from the formula of B 6 O, instead one should take into account the related crystal structure. Similar phenomena have also been observed in the metal borides. Except for a very small amount of alloys, such as Ti 3 B 4 , FeB etc., in which the borides can be consid- ered as negative trivalenced, the valence of boron ions in many metal borides cannot be described as an unusual oxidation state, especially for REB 6 (RE = rare earth el- ement). The structure of the rare earth hexaborides may be viewed as a CsCl-type lattice where the cesium ions were replaced by the rare earth ions and the chlorine by ∗ Corresponding author. E-mail: lp li@hotmail.com  J.C. Baltzer AG, Science Publishers