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The lanthanite crystal structure consists of layers of 10-fold coordinated REE-oxygen (O) polyhedra and carbonate (CO 3 2−) groups connected by hydrogen bonds to interlayer water molecules, forming a highly hydrated structure.
At low temperatures, La 2 O 3 has an A-M 2 O 3 hexagonal crystal structure. The La 3+ metal atoms are surrounded by a 7 coordinate group of O 2− atoms, the oxygen ions are in an octahedral shape around the metal atom and there is one oxygen ion above one of the octahedral faces. [4]
3 adopts a hexagonal 7-coordinate structure that changes to the 6-coordinate structure of scandium oxide (Sc 2 O 3) and yttrium oxide (Y 2 O 3) at high temperature. When it reacts with water, lanthanum hydroxide is formed: [37] a lot of heat is evolved in the reaction and a hissing sound is heard.
The lighter/larger lanthanides adopt a hexagonal 7-coordinate structure while the heavier/smaller ones adopt a cubic 6-coordinate "C-M 2 O 3" structure. [50] All of the sesquioxides are basic, and absorb water and carbon dioxide from air to form carbonates, hydroxides and hydroxycarbonates. [57] They dissolve in acids to form salts. [18]
Similar to the ideal hcp structure, the perfect dhcp structure should have a lattice parameter ratio of = In the real dhcp structures of 5 lanthanides (including β-Ce) / variates between 1.596 (Pm) and 1.6128 (Nd). For the four known actinides dhcp lattices the corresponding number vary between 1.620 (Bk) and 1.625 (Cf).
Lanthanide metals react exothermically with hydrogen to form LnH 2, dihydrides. [1] With the exception of Eu and Yb, which resemble the Ba and Ca hydrides (non-conducting, transparent salt-like compounds),they form black pyrophoric, conducting compounds [6] where the metal sub-lattice is face centred cubic and the H atoms occupy tetrahedral sites. [1]
Differences in chemical composition and crystal structure distinguish the various species. Within a mineral species there may be variation in physical properties or minor amounts of impurities that are recognized by mineralogists or wider society as a mineral variety. Mineral variety names are listed after the valid minerals for each letter.
Neodymium commonly exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at about 863 °C. [14] Neodymium, like most of the lanthanides, is paramagnetic at room temperature. It becomes an antiferromagnet upon cooling below 20 K (−253.2 °C). [15]