| Research Abstract |
Kozoite-(Nd), Nd(CO_3)(OH), occurs in cavities and fissures of alkali olivine basalt exposed at Niikoba, Hizen-cho, Higashi Matsuura-gun, Saga Prefecture, Japan. The shape of aggregates of minute crystals of kozoite-(Nd) is usually platy, which corresponds to the crystal habit of lanthanite-(Nd), Nd_2(CO_3)_3・8H_2O. This occurrence mode suggests that kozoite-(Nd) is probably alteration products of lanthanite-(Nd) in the postmagmatic hydrothermal activity.
The alteration condition was examined with a chemical reagent of Nd_2(CO_3)_3・8H_2O [lanthanite-(Nd)] and sodium aqueous solutions under hydrothermal conditions. The powder of starting material, Nd_2(CO_3)_3・8H_2O, was sealed with sodium aqueous solution (0-0.5M NaHCO_3 or NaOH) in a Teflon vessel or a platinum capsule. The reaction containers were kept at 150,200,or 250℃. The reaction process was traced varying the reaction duration up to 7 days. The reaction products were filtered, washed with distilled water, and dried overnight at
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room temperature. They were characterized by XRD.
The starting material, Nd_2(CO_3)_3・8H_2O was decomposed within 30 minutes. A phase with the XRD pattern of Nd_2(CO_3)_3・2.5H_2O [ICDD #33-0932] was formed with the decomposition of Nd_2(CO_3)_3・8H_2O. Subsequently, kozoite-(Nd) and hydroxylbastnasite-(Nd), the polymorph of kozoite-(Nd), were crystallized. The higher reaction temperature and the higher pressure tend to promote the formation of hydroxylbastnashe-(Nd) instead of kozoite-(Nd). Actually, kozoite-(Nd) is the less dense polymorph in comparison to hydroxylbastnasite-(Nd). It is reasonable that kozoite-(Nd) with lower density crystallizes under lower pressure conditions. These trends indicate that kozoite-(Nd) may be formed relatively lower temperature and lower pressure. On the other hand, the yield of kozoite-(Nd) was reduced and hydroxylbastnasite-(Nd) was the dominant reaction product in the reaction runs using sodium solutions with higher concentration and in the runs with elongated reaction duration. These observations auggest that kozoite-(Nd) is a metastable phase in the present reaction system.
Rare earth carbonate hydroxides, RE(CO_3)OH, were hydrothermally synthesized from formic acid and hydroxide gels of La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Y. A hexagonal phase with the bastnasite-type structure was obtained for RE=La, Ce, Nd and Sm. An orthorhombic phase wilh the kozoite-type structure was obtained for RE=Nd and Sm. In addition to these two major phases, another orthorhombic modification of the kozoite-type structure was obtained for RE=Eu, Gd, Tb, Dy, Ho, Er, Tm and Y. The latter phase has a lower symmetry (space group : P2_12_12_1) than the true kozoite-type structure (Pnma). A new tetragonal phase (space group : P4_2/nmc) was found for RE=Tm and Yb.
The crystal structures of RE(CO_3)OH were refined for Pnma (RE=Nd and Sm), P2_12_12_1(RE=Eu, Gd, Tb, Dy, Ho, Er, Tm and Y) and P4_2/nmc phases (RE=Tm and Yb) utilizing single crystal intensity data. The distinct feature of the difference among the three structures are the coordination numbers of the RE^<3+> ions : 10 for the Pnma phase,9 for the P2_12_12_1 phsase, and 8 for the P4_2/nmc phase. A systematic comparison of two orthorhombic structures revealed the following dynamic variation in the coordination environment about the RE^<3+> ions with ionic radii. Although the interatomic RE-O distances tend to decrease with the lanthanide contraction, exceptions were observed for two of the RE-O distances. The mutual close proximity of CO_3^<2-> anions caused by the lanthanide contraction led to repulsion between the CO_3^<2-> anions, which caused a decreases in the symmetry of configuration of CO_3^<2-> anions around the central RE^<3+> ion and some oxygen atoms of CO_3^<2-> became away from the first coordination shell of the RE^<3+> ions dueto the elongation of the RE-O distances.
The crystal structure of tetragonal RE(CO_3)OH show a characteristic feature distinct from those of orthorhombic phases. It consists of ladders of 8-coordinated RE^<3+> cation with CO_3^<2-> and OH^- anions arranged in a form of double cross. The powder XRD pattern of the tetragonal RE(CO_3)OH was identical with that of a synthetic material previously reported as RE_6(OH)_4(CO_3)_7[1] indicating that the latter formula is not adequate. Less
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