Page 20 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: FYK Revised Pages
Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19

228 Actinide Elements

Methods of growing importance for speciation and Thephysicochemicalpropertiesoftheactinidehydrides
complexation studies of actinides are the synchrotron- are as varied as any in the entire periodic table. Thorium
based X-ray absorption near-edge structure spectroscopy forms a “normal” dihydride like those of Zr and Hf, but
(XANES) and the extended X-ray absorption fine struc- also forms Th 4 H 15 , a unique superconductor. The hydrides
ture spectroscopy (EXAFS). of protactinium and uranium have cubic structures which
have no counterparts in the periodic table. The transura-
nium element hydrides are more lanthanide like with wide
VI. ACTINIDE COMPOUNDS cubic solid solution ranges. Hexagonal phases appear with
AND COMPLEXES regularity.

2. Oxides
A. Binary Compounds
The actinide oxides have received intensive scrutiny be-
1. Hydrides
cause their refractory nature makes them suitable for use
Representative actinide hydride compounds are repre- as ceramic fuel elements in nuclear reactors. UO 2 melts at
sented in Table X. Actinide metals react readily with hy- 3150 K, and ThO 2 has the highest melting point of any ox-
drogen when heated. The temperature needed for reaction ide, about 3465 K. The actinide oxides are complicated by
depends on the state of the metal, the amount of surface deviations from stoichiometry, polymorphism, and inter-
oxidation on the metal, and the purity and pressure of the mediate phases. The sesquioxides are basic, the dioxides
hydrogen used. The actinide hydrides are not very ther- are much less basic, and UO 3 is an acid in solid state
mally stable and are very air and moisture sensitive. The reactions. The reactivity of these oxides depends greatly
thermal instability of these compounds has been used to on their thermal history. If ignited, they are much more in-
obtain finely divided metal via thermal decomposition of ert. Table XI contains some representative data on actinide
the corresponding hydride. oxides.

TABLE X Actinide Hydrides
Lattice parameters
Space M-H Bond Density
Compound Color Symmetry a group a ( ˚ A) c ( ˚ A) length ( ˚ A) (g cm −3 )
AcH 2 Black fcc Fm3m 5.670 2.46 8.35
ThH 1.93 Black bct 5.73 4.99 2.39 9.50
ThH 2 Black bct 4.10 5.03 2.39 9.20
Th 4 H 15 Black bcc I ¯ 43d 9.11 2.29, 2.46 8.29
α-PaH 3 Black Cubic Pm3n 4.150
β-PaH 3 Black Cubic Pm3n 6.648 2.32 10.57
α-UH 3 Black Cubic Pm3n 4.160 2.32 11.12
β-UH 3 Black Cubic Pm3n 6.644
NpH 2 Black fcc Fm3m 5.348 2.32 10.41
NpH 2.36 Black fcc Fm3m 5.346
NpH 2.42 Black fcc Fm3m 5.348
NpH 3 Black Hexagonal P6 3 /mmc 3.777 6.720 9.64
PuH 2 Black fcc Fm3m 5.3594 2.32 10.40
PuH 2.5 Black fcc Fm3m 5.34
PuH 3 Black Hexagonal P6 3 /mmc 3.779 6.771 2.18–2.41 9.61
AmH 2 Black fcc Fm3m 5.348 2.316 10.6
AmH 2.67 Black fcc Fm3m 5.338
AmH 3 Black Hexagonal P6 3 /mmc 3.764 6.763 9.76
CmH 2(+x) Black fcc Fm3m 5.322 2.314 10.7
Black Hexagonal 3.77 6.73
CmH 3
BkH 2(+x) Black fcc Fm3m 5.25
BkH 3(−x) Black Trigonal 6.454 6.663
CfH 2+x Black Cubic 5.285
a bct, body-centered tetragonal; fcc, face-centered cubic.