Hydrides
Binary Oxides

Ln₂O₃

• The most common lanthanide oxide (notable exceptions; CeO₂, Pr₆O₁₁, Tb₄O₇)
• Sesquioxides, Ln₂O₃, adopt three structure types
• Strongly Basic
• Absorb water / CO₂ from air Æ hydroxide / carbonate salts
• Pale colours
• Properties strongly resemble alkaline earth oxides

A-type

• light Ln
• unusual LnO₇ capped-octahedra

B-type

• middle Ln
• LnO₇ units of three types:
  • 2 x capped trigonal prisms
  • 1 x capped octahedron

• B-M₂O₃ structure is very complex
• Densest of M₂O₃ structure types Þ favoured by increased pressure

C-type

• heavy Ln
• LnO₆ units, but not octahedra
• (face & body - divacant cubic)

• C-type M₂O₃ is related to Fluorite (MO₂) with ¹/₄ of anions removed

Polymorphism

A (high T)

B (medium T)

C (low T)

See: Wells, Structural Inorganic Chemistry, p. 543-547 for structural details

Ln(OH)₃

• obtained by action of conc. NaOH on Ln₂O₃ under hydrothermal conditions
• 9-coordinate Ln with tricapped trigonal prismatic (ttp) geometry
• basicity increases with Z - correlates with decrease in r(Ln³⁺)

LnO₂

• CeO₂ (most stable)
• Fluorite (CaF₂ ) structure
• Pr₆ O₁₁ , Tb₄ O₇ (formed at high P_O2 and high temperature)
• Range of non-stoichiometric phases between Fluorite LnO₂ and C-type LnO_1.5

  ~ intermediate phases were the first known examples of shear structures

  See: Wells, Structural Inorganic Chemistry, p. 226-228 for an introduction to shear structures

LnO

• known for some Ln
• Preparation: Comproportionation Ln + Ln₂O₃ Æ 3LnO
• NaCl structure
• NdO, SmO lustrous golden yellow, conducting formulated as Ln³⁺(O^2-)(e^-)
• EuO (dark red), YbO (greyish-white) insulating genuine Ln²⁺O^2-
• EuO is ferromagnetic and an insulator when pure

  [C.N.R. Rao & J. Gopalkrishnan, New Directions in Solid State Chemistry, 2nd ed, CUP, 1996 p. 302-3]

Bibliography