THERMODYNAMIC PROPERTIES OF THE LANTHANIDE(III) HALIDES
2. Polymorphism 1. LnF 3
Four crystallographic modifications have been reported for the lanthanide trifluorides (Greis and Haschke, 1982; Meyer and Wickleder, 2000). The trifluorides of La, Ce, Pr and Nd have a hexagonal/trigonal structure from room temperature up to the melting point. Greis and Cader (1985) reported that these compounds undergo aλ-type second order transition before melt- ing, which was easier to detect when the samples were heated at low heating rates. As a result
150 R.J.M. KONINGS AND A. KOVÁCS
Fig. 1. The polymorphism in the lanthanide tri- fluorides.
the transition temperatures are not well defined. Greis and Cader (1985) argued that at low temperatures a highly ordered superstructure exits (space groupP3c1) whereas the anion- disordered structure (space groupP63cm) dominates at higher temperatures. The trifluorides of Sm to Lu have an orthorhombic structure (space group Pnma) at room temperature. Tran- sitions from the orthorhombic to a hexagonal structure at elevated temperature have been reported for all compounds in the series Sm–Lu: the LaF3type hexagonal structure for the tri- fluorides of Sm to Ho and the YF3hexagonal structure for the trifluorides of Er to Lu. Stankus et al. (1999) concluded that the transition involves a deformation mechanism and may occur over a broad temperature range. Pastor and Robinson (1974) and Sobolev et al. (1976a, 1976b) discussed the variation in transition temperatures in view of hydroxide, oxygen and alkaline earth impurities and argued that the transitions in TbF3, DyF3and HoF3do not occur and that the reported claims are due to impurities. The polymorphism as a function of atomic number is shown in fig. 1, which is essentially identical to the figure of Greis and Haschke (1982) in an earlier chapter of this Handbook series.
The reported transition and melting temperatures are summarised in table A.1 of Appen- dix A. As follows from the discussed above, there is quite some variation in the transition temperatures, but there is generally good agreement for the melting temperatures of the lan- thanide trifluorides. The selected values (considering corrections to ITS-90) are summarised in table 1. Figure 1 shows that a minimum in the melting curve of the lanthanide trifluo- rides occurs at ErF3: the melting point steadily decreases with increasing atomic number for the light hexagonal/trigonal and orthorhombic but slightly increases for the heavy hexagonal trifluorides. Since data for PmF3are lacking, its melting point has been interpolated.
2.2. LnCl3
The lanthanide trichlorides of La to Eu have a hexagonal crystal structure (space group P63/m) at room temperature. For GdCl3the hexagonal as well as the orthorhombic struc- tures have been reported at room temperature. There is some uncertainty which of these is the
THERMODYNAMIC PROPERTIES OF THE LANTHANIDE(III) HALIDES 151 Table 1
The selected transition and melting temperatures for the lanthanide trihalides, in K
F Cl Br I
Ttrs Tfus Ttrs Tfus Ttrs Tfus Ttrs Tfus
La 1766±3 1133±5 1060±3 1045±3
Ce 1703±3 1090±2 1005±2 1033±2
Pr 1670±3 1060±2 965±3 1011±2
Nd 1649±3 1032±2 955±2 859±3 1059±2
Pm 1605±15 994±15 930±15 900±10 1090±10
Sm 743±3 1571±3 950±5 913±5 943±10 1123±5
Eu 973±3 1549±3 894±3 978±10 decomp.
Gd 1347±3 1501±3 872±10 875±2 1043±5 1013±5 1204±3
Tb 1446±3 783±5 855±3 1102±3 1080±5 1229±3
Dy 1426±3 611±5 924±3 1152±3 1101±5 1251±3
Ho 1416±3 820±20 993±3 1192±3 1150±15 1267±5
Er 1388±3 1413±3 1049±5 1196±5 1195±15 1288±3
Tm 1325±3 1431±3 1095±3 1228±3 1240±15 1294±5
Yb 1267±3 1435±3 1138±5 decomp. 1280±15 decomp.
Lu 1230±3 1455±3 1198±5 1298±5 1320±15 1323±5
thermodynamically stable form. Because Sommers and Westrum Jr. (1977) were not able to transform the hexagonal form to the orthorhombic by annealing (367 K, 7 days) or cooling (liquid nitrogen, 7 h), they considered the hexagonal structure to be the thermodynamically stable form. This was confirmed by Raman spectroscopic studies by Daniel et al. (1989).
TbCl3has an orthorhombic structure (space group Cmcm). The trichlorides of Dy to Lu have a monoclinic crystal structure (space groupC2/m).
The hexagonal structure of the trichlorides of La to Eu persists up to the melting point, whereas GdCl3retains its hexagonal structure up to few degrees below to the melting point (Daniel et al., 1989). The high temperature behaviour of the orthorhomic and monoclinic lanthanide trichlorides is not so well known. The enthalpy increment studies for TbCl3, DyCl3 and HoCl3(Dworkin and Bredig, 1971) do not reveal any phase transformations up to the melting point (see below), but differential scanning calorimetric measurements of DyCl3and ErCl3revealed transitions at 611 K and 1025 K (Gaune-Escard et al., 1994), respectively, the nature of which was not explained. Büchel et al. (1995) claimed that in the case of ErCl3the DTA peak is due to the reaction with the silica crucible used by Gaune-Escard et al. (1994).
Morrison et al. (2000) studied the polymorphism of TbCl3by Raman spectroscopy and X- ray diffraction and demonstrated that a phase change occurs around 773 K and tentatively assigned it to a tetragonal structure (space groupP42/mnm), in agreement with the work of Gunselius et al. (1988).
The reported transition and melting temperatures are listed in table A.2 of Appendix A.
There is general agreement between the studies and the selected values (considering correc- tions to ITS-90) are summarised in table 1. The polymorphism as a function of atomic number is shown in fig. 2. The minimum in the melting curve of the lanthanide trichlorides occurs at TbCl3where a change in structure of the LnCl3occurs, as was the case for the trifluorides.
Data for PmCl3are lacking and its melting point has been interpolated in the LnCl3series.
152 R.J.M. KONINGS AND A. KOVÁCS
Fig. 2. The polymorphism in the lanthanide trichlorides.
2.3. LnBr3
The tribromides of La, Ce and Pr have a hexagonal structure (space groupP63/m), the tri- bromides of Nd, Pm, Sm and Eu have an orthorhombic structure (space group Cmcm), and the tribromides of Tb to Lu have a hexagonal/rhombohedral structure (space groupR3). For GdBr3several different crystal structures have been reported: hexagonal/rhombohedral, trig- onal and monoclinic, the former being the thermodynamically stable phase.
Phase transformations have not been reported for any of the rare earth tribromides, but only a few detailed studies of the high-temperature phase behaviour have been made. Brown et al.
(1968) did not find evidence for polymorphism in GdBr3by X-ray diffraction; Dworkin and Bredig (1971) none for LaBr3, CeBr3, PrBr3, NdBr3, GdBr3or HoBr3by drop-calorimetric measurements; and Cordfunke et al. (Cordfunke and Blacquère, 1997; Cordfunke et al., 1999) none for NdBr3and DyBr3by differential scanning calorimetric measurements.
The reported melting points for the tribromides are in good agreement as shown in table A.3 of Appendix A. The recommended melting points (considering corrections to ITS-90) are listed in table 1, and shown as a function of atomic number in fig. 3. The minimum in the melting curve of the lanthanide tribromides occurs at SmBr3or perhaps PmBr3. Unlike the fluoride, chloride and iodide series, no change in structure has been reported in the bromide series at this point. This observation may, however, be an indication that a high-temperature orthorhombic-hexagonal phase transformation occurs for EuBr3and eventually for SmBr3, and the stability fields in fig. 3 have been drawn schematically to reflect this. Because of these uncertainties the melting point of PmBr3is difficult to estimate accurately.
2.4. LnI3
The triiodides of La to Nd have an orthorhombic structure (space group Ccmm), the triiodides of Sm to Lu have a hexagonal/rhombohedral structure (space groupR3). High temperature transformations have been reported for the lanthanide triiodides from NdI3to DyI3. The or- thorhombic NdI3probably transforms into the rhombohedral structure (Dworkin and Bredig,
THERMODYNAMIC PROPERTIES OF THE LANTHANIDE(III) HALIDES 153
Fig. 3. The polymorphism in the lanthanide tri- bromides.
Fig. 4. The polymorphism in the lanthanide tri- iodides.
1971). The nature of the transformations in for the triiodides SmI3to DyI3is not defined and it is unclear whether this transformation also occurs in HoI3, ErI3, TmI3and LuI3as these compounds have hardly been studied. As the enthalpies of these transitions are small, it is most likely that they involve a rearrangement of the rhombohedral lattice. For the diagram in fig. 4 we have linearly extrapolated the transition temperatures beyond DyI3based on the straight line established by the LnI3transformation temperatures for Ln=Nd, Sm, Gd, Tb, and Er. This figure is, however, of a speculative nature.
The melting points of LaI3to NdI3have been studied by several authors and the results agree well and selected values (considering corrections to ITS-90) are given in table 1. For the heavy lanthanide triiodides only a limited number of studies has been made, but the reliability is good (see table A.4 of Appendix A). No experimental studies are known for PmI3, EuI3 and YbI3. The latter two compounds start to decompose below the melting point. The melting point of PmI3has been interpolated in the LnI3series.
154 R.J.M. KONINGS AND A. KOVÁCS