Lanthanide compounds polymorphism

Many different lanthanides and early transition metals form A3C compounds by this type of reaction, as described for the face-centred cubic iron polymorph. The limiting factor appears to be the size of the radius of A. A critical radius of 1.35 A seems to be required to form the face-centred cubic lattice with twelve coordinate ions at room temperature. 

The hydrate Th(HCOO)4,0.66H2O can exist at room temperature in two polymorphic forms, L and The structure of the L modification is well known it is isomorphous with A-Th(HCOO)4. An analysis of the JT-ray diffraction data of single crystals and powders of the M form of Th(HCOO)4,-O.66H2O has shown that at room temperature the crystals are orthorhombic, with space group P2i2i2i, and the unit-cell parameters area =6.761 0.003, b — 10.491 0.005, c = 12.323 0.005 A. The preparation of pluto-nium(m) formate by the reaction of plutonium(m) hydroxide with formic acid has been reported. Characterization of the new compound has been accomplished by analysis, t.g.a., A"-ray powder diffraction, and i.r. spectroscopy it was found to be formally analogous to the well-known lanthanide(ui) formates in all its properties. 

The existence of changes in the crystal structure, probably was not so earth-shaking since the R2O3 compounds were known to have three polymorphic forms in the lanthanide series. But the anomalous densities (or atomic volumes) for europium and ytterbium was an exciting development, since it showed that these two metals were divalent while the remaining lanthanide elements were trivalent. This discovery was one of the first examples verifying Hund s rule (that half-filled and completely filled shells are stable electronic states), which involved a solid-state property other than a magnetic or an optical property measurement. 

Compounds with composition RjOj 2Si02 are known for all binary rate earth silicate systems. Seven polymorphic forms have been observed at ambient pressure (fig. 32). All are of the type RjSijO, with the exception of R4(Si30ioXSi04). which is stable only for the lanthanides of medium size (Felsche, 1972a). 

The variable approximation parameter values (0d/ ei/ e2/ e3/ arid as functions of the molar volumes Vrf3 calculated from the X-ray data (ICDD Database, 1993) form fairly accurate linear dependences (see Figure 7) for two series of RF3 (R = La-Pm, and R = Sm-Lu). The separation into two series is caused by the formation of different polymorphs by these compounds. Under standard conditions, lanthanide trifluorides from lanthanum to promethium inclusive have LaFs-type structures, in which the CN of R is 6. All the other RF3 are characterized p-YF3-type crystal structures with the CN = 9. 

Considerable amounts of IR spectroscopic and thermoanalytical data are available for the Rb compounds, especially the RbR(S04)2 4H2O series (Gmelin, 1981b). Lipis et al. (1970, 1971) have studied both the tetra- and monohydrate series, using deuterated as well as normal compounds and Iskhakova et al. (1979) have reported the spectra of RbR(S04)2 4H2O for Gd and Tb. Thermal behavior of the tetrahydrate series will be discussed in sections 4.4.5 and 4.4.6 in connection with the isostructural Cs and NH4 compounds. For the RbR(S04)2 compounds, investigations have been made of their IR spectra (Remizov et al., 1969 Lipis et al., 1971 Couchot et al., 1971) and thermoanalytical properties (Prokofev et al., 1979 Korytnaya et al., 1980). DTA results indicate the existence of high-temperature polymorphs for several lanthanides (Prokofev et al., 1979). 

---