Praseodymium compounds are chemical compounds containing the lanthanide element praseodymium (Pr). The chemistry of praseodymium is dominated by the +3 oxidation state, and many Pr compounds are ionic and characteristically green. The +4 oxidation state is also well established, particularly in oxides, fluorides and coordination compounds. Molecular compounds containing Pr are known, while praseodymium is currently the only lanthanide for which the formal +5 oxidation state has been experimentally established.
Praseodymium forms a wide variety of inorganic, coordination and organometallic compounds, including oxides, halides, chalcogenides, hydrides, borides, carbides, silicides and pnictides. Its oxide chemistry is especially complex because of the coexistence of Pr and Pr, while its higher oxidation states and low-valent organometallic chemistry distinguish it from most other lanthanides. Praseodymium compounds are also used or studied in optical materials, ceramic pigments, catalysts, magnetic intermetallics and high-k dielectric materials.
Properties of praseodymium compounds
Selected crystallographic properties of praseodymium compounds are listed below. Lattice parameters refer to the reported crystalline modification and are given in nanometres.
Formula Appearance Crystal system Space group No. a (nm) b (nm) c (nm) Density (g/cm3) Ref. β- Green Hexagonal Pc1 165 0.70795 — 0.72380 6.28 Blue-green Hexagonal P6/m 176 0.7423 — 0.4272 4.03 Green Hexagonal P6/m 176 0.79248 — 0.43886 5.28 Green Orthorhombic Cmcm 63 0.43281 1.4003 0.9988 5.8 A- — Hexagonal Pm1 164 0.3857 — 0.6016 — Dark brown Cubic Fmm 225 0.5393 — — — PrS — Cubic Fmm 225 0.5743 — — 6.065 PrAs — Cubic Fmm 225 0.6009 — — 6.607 PrSb — Cubic Fmm 225 0.6366 — — 6.762 — Tetragonal P4 75 0.89680 — 1.8119 — — Tetragonal P4/mbm 127 0.7235 — 0.4116 — — Cubic Pmm 221 0.413 — — — — Tetragonal I4/mmm 139 0.3855 — 0.6434 5.728 — Cubic I3d 220 0.86072 — — 6.621 Light green Hexagonal P6/m 176 0.6456 — 0.3769 —
Halides
Trihalides
Praseodymium forms the four trihalides , , and , all containing Pr. They can be prepared by direct reaction of praseodymium metal with the corresponding halogen. Their structures change across the series as the halide ion becomes larger: is nine-coordinate and adopts the tysonite-type structure, and have the hexagonal structure type, and adopts the orthorhombic type.
Praseodymium(III) fluoride, , is a green crystalline solid and is the thermodynamically stable binary fluoride of Pr. It can be precipitated from aqueous praseodymium(III) salts with fluoride ions; reaction of praseodymium(III) nitrate with sodium fluoride, for example, has been used to prepare hollow nanoparticles by microwave-assisted hydrothermal synthesis.
Praseodymium(III) chloride, , is a blue-green anhydrous solid that readily forms hydrated salts, most notably the light-green heptahydrate. Anhydrous material can be purified by vacuum sublimation, while rapid heating of hydrated may cause partial hydrolysis.F. T. Edelmann, P. Poremba, in: Synthetic Methods of Organometallic and Inorganic Chemistry, W. A. Herrmann (ed.), Vol. 6, Georg Thieme Verlag, Stuttgart, 1997. Like other early-lanthanide trichlorides, is a hard Lewis acid. It forms ternary chlorides with alkali-metal chlorides; reaction with potassium chloride gives , whose optical and magnetic properties have been investigated spectroscopically.
Praseodymium(III) bromide, , adopts the hexagonal structure type. The Pr ions are nine-coordinate in a tricapped trigonal-prismatic environment. Neutron-diffraction measurements give two sets of Pr–Br distances, approximately 3.05 and 3.13 Å, and have also been used to determine the crystal-field splitting of Pr in both and .
Praseodymium(III) iodide, , can be prepared either by heating praseodymium with iodine under an inert atmosphere or by reaction of the metal with mercury(II) iodide. It is a hygroscopic green solid with the orthorhombic structure type, space group Cmcm. Reported lattice parameters are a = 0.43281(6) nm, b = 1.4003(6) nm and c = 0.9988(3) nm. Although that study principally concerns , it also reports crystallographic data for and praseodymium oxyiodide.
Praseodymium(IV) fluoride
Praseodymium(IV) fluoride, , is one of the few simple binary halides containing tetravalent praseodymium. It can be prepared by fluorination of a mixture of and sodium fluoride to give , followed by extraction of the sodium fluoride component with liquid hydrogen fluoride. forms pale yellow crystals with the structure type. The +4 oxidation state is strongly oxidising: the tetrafluoride decomposes on heating to and fluorine and reacts with water rather than forming a stable aqueous Pr species.
Other complex Pr fluorides are more stable than the binary tetrafluoride because the high oxidation state is stabilized within extended fluoride lattices. Hexafluoropraseodymate(IV) salts such as are therefore important intermediates in the preparation and chemistry of .
Reduced halides
Praseodymium also forms several electron-rich halides with halogen-to-metal ratios below three. The best known is bronze-coloured praseodymium diiodide, . Despite its formal stoichiometry, it is not best described as an ionic Pr salt. Its short Pr–I distances, metallic conductivity and electronic structure instead support the formulation : praseodymium remains essentially trivalent and the additional electron occupies a metal-derived conduction band.
At least five structural modifications of are known. Four are related to familiar layered structure types, including - and -type arrangements, while modification V has a cubic structure containing cubane-like clusters. In the latter phase the praseodymium atoms form tetrahedral groups and the observed distortions have been attributed to Pr–Pr bonding. The cubic modification crystallizes in space group F3m with a ≈ 1.236 nm.
Intermediate compositions are also known in the bromide and iodide systems. and are isostructural layered compounds and are likewise better described as “free-electron” halides, , rather than conventional mixed-valence Pr/Pr salts. Both are semiconducting and show unusually high magnetic ordering temperatures for praseodymium halides: orders antiferromagnetically at 50(1) K and at 37(1) K. The structure of can be regarded as an intergrowth of structural motifs related to the corresponding trihalide and reduced-halide phases, with two crystallographically inequivalent Pr sites.
Chalcogenides
Oxides
Praseodymium forms an unusually complex series of oxides spanning compositions from praseodymium(III) oxide, , to praseodymium(IV) oxide, . Between these end members is a homologous series of mixed-valence oxides with the general composition PrnO2n-2. The most stable oxide in air at room temperature is praseodymium(III,IV) oxide, , in which both Pr and Pr are present.
Praseodymium(III) oxide, , is yellow-green and occurs in two principal polymorphs. The thermodynamically stable high-temperature form has the hexagonal A-type rare-earth sesquioxide structure, whereas a metastable cubic C-type form with the bixbyite structure can also be obtained. The hexagonal phase has lattice parameters of approximately a = 0.3859 nm and c = 0.6008 nm.
Praseodymium(IV) oxide, , crystallizes in the cubic fluorite structure, with Pr eight-coordinate to oxygen. It is less stable than under ambient conditions and requires strongly oxidizing conditions for bulk preparation; highly oxidized material can also be obtained by prolonged treatment of with hot water.
Praseodymium(III,IV) oxide, , has a defect-fluorite-derived structure and is black. It readily exchanges oxygen with the surrounding atmosphere, and heating or cooling at different oxygen partial pressures produces a sequence of intermediate oxide phases between and . This ability to reversibly incorporate and release lattice oxygen is responsible for much of the catalytic interest in praseodymium oxides.
The well-established members of the homologous oxide series include:
Formula Phase x in Average oxidation state of Pr θ 1.500 +3 ι 1.714 +3.429 ζ 1.778 +3.556 ε 1.800 +3.600 δ 1.818 +3.636 β 1.833 +3.667 α 2.000 +4
Other chalcogenides
Praseodymium forms monochalcogenides PrS, PrSe and PrTe. All three adopt the cubic sodium chloride structure at ambient pressure and are metallic. Although the 1:1 stoichiometry might suggest Pr, electronic-structure studies instead support predominantly trivalent praseodymium with two localized 4f electrons and an additional conduction electron. Among them, PrTe undergoes an experimentally observed pressure-induced transition from the sodium chloride structure to the caesium chloride structure near 9 GPa, accompanied by a volume collapse of about 11.5%.
Praseodymium(III) sulfide, , crystallizes at ambient conditions in the orthorhombic A-type rare-earth sesquisulfide structure, space group Pnma. Its lattice parameters are approximately a = 0.74822 nm, b = 0.40551 nm and c = 1.56074 nm, and the two crystallographically distinct Pr sites have coordination numbers of seven and eight. Single crystals can be grown by reaction of reduced praseodymium chlorides with sulfur in the presence of an alkali-halide flux.
The praseodymium–selenium system is particularly rich. In addition to and PrSe, several selenium-rich phases occur between and . has been reported in the cubic structure type, space group I3d, with a lattice parameter of approximately 0.891 nm.
Thermal decomposition of praseodymium diselenide, , proceeds stepwise through discrete intermediate phases with approximate compositions , and before reaching . itself is monoclinic and contains dimers in planar selenium layers, while has an incommensurately modulated structure arising from ordered selenium vacancies and charge-density modulation.
The praseodymium–tellurium system contains a similarly broad range of phases, including PrTe, , several intermediate compositions, and the polytelluride . Praseodymium ditelluride, , can be prepared by chemical vapour transport from praseodymium and tellurium using iodine as a transport agent. It crystallizes in a tetragonal superstructure, space group P4, with lattice parameters a = 0.89680 nm and c = 1.8119 nm. Its structure contains alternating praseodymium–tellurium slabs and planar tellurium layers in which Te atoms form several polyanionic motifs, including dumbbells and four-membered units.
Hydrides
Praseodymium forms a continuous series of hydrogen-rich phases conventionally described in terms of the dihydride and trihydride . At ordinary pressures, compositions between approximately and retain the cubic fluorite-derived metal sublattice: in the dihydride the hydrogen atoms occupy tetrahedral interstices, while additional hydrogen at higher H/Pr ratios enters octahedral interstices. The cubic lattice parameter decreases from about 0.5515 nm for to about 0.5486 nm near the trihydride composition. Measurements of hydrogen dissociation pressures between 250 and 450 °C show that the cubic hydride accommodates substantial additional hydrogen continuously over compositions , rather than undergoing the structural change found in several heavier rare-earth hydrides. Subsequent thermal studies likewise found no phase transition within the system under hydrogen pressures near atmospheric pressure.
At very high pressures, much more hydrogen-rich praseodymium hydrides can be formed. Laser heating of praseodymium in a hydrogen-rich environment above 100 GPa produces superhydride in both cubic F3m and hexagonal P6/mmc modifications. At 120 GPa the measured lattice parameters are a = 0.4967 nm for the cubic phase and a = 0.3588 nm, c = 0.5458 nm for the hexagonal phase. Electrical-resistance measurements showed a field-dependent drop below about 9 K, interpreted as a possible superconducting transition, although zero resistance was not reached because the samples contained mixed phases.
Borides