A comprehensive review on phase diagrams, crystal
structures and thermodynamic properties of ternary chlorides formed in the
systems ACl/LnCl3 (A=Na,
K, Rb, Cs) is presented. It continues an earlier review with the same contents
on the lanthanides from La to Gd . In both papers the author's own
studies, published since 1985, together with original papers from other scientists
are treated. With the three larger cations compounds of the composition A3LnCl6,
and beginning with holmium Cs3Ln2Cl9
are formed. With sodium the compounds Na3Ln5Cl18
(Ln=La to Sm) and NaLnCl4
(Ln=Eu to Lu) also exist. The stability
of a ternary chloride in a system ACl/LnCl3 is given
by the 'free enthalpy of synreaction', the formation of a compound
from its neighbour compounds in its system. This must be negative. A surprising result is that the highest
– melting compounds in the systems, A3LnCl6,
are formed from ACl and A2LnCl5
with a loss of lattice energy, U. They
exist as high-temperature compounds due to a sufficiently high gain in entropy
at temperatures where the entropy term TΔS compensates the endothermic ΔH.
Ternary chlorides are stable if they can be formed from each pair of the other compounds in a system with a gain in energy. Especially, the energy of formation from the two adjacent compounds, the energy of synproportionation must be negative. At 0 K this condition is fulfilled, if (lattice) enthalpy is won. At higher temperature also theT·ΔS-term of the Gibbs-Helmholtz-equation can be of significance: a loss in ΔH must be compensated by a gain inT·ΔS, if a (high-temperature) compound shall exist.
The pseudobinary systems NaCl—LnCl3 (Ln=Tm, Yb, Lu) were investigated by DTA and X-ray diffraction. Two types of ternary chlorides exist: congruently melting compounds Na3LnCl6 with the cryolite-structure, incongruently melting compounds NaLnCl4 with the NaErCl4-Ln (Ln=Tm) or the NaLnCl4-structure (Ln=Yb, Lu). All these structure types contain [LnCl6]-octahedra.By solution calorimetry and e.m.f. measurements in galvanic cells for solid electrolytes could be proved that all compounds are formed from NaCl and LnCl3 by gain in lattice enthalpy.
Solid state reactions and reconstructive phase transitions exhibit more or less a large hysteresis between reaction temperatures taken from DTA-heating and -cooling curves. For ternary lanthanide chlorides the equilibrium temperatures could be obtained bye.m.f.-measurements in galvanic chlorine cells for solid electrolytes usingA+-ion (A=alkaline metal) conducting diaphragms. By quenching, high-temperature phases can often be transformed to metastable room-temperature phases. In this case the equilibrium state must be established by annealing at sufficiently high temperature, or it must be tried to synthesize the compound in its stability range from suitable precursor systems.
The phase diagrams of ACl/MoCl3 (A=Na, K, Rb, Cs) were elucidated by DTA measurements in sealed quartz ampoules in the range of 0–40 mol% MoCl3. The samples were prepared from alkali metal chlorides and the compounds A3MoCl6 or A3Mo2Cl9. The 3∶1 compounds withA=Na, Rb, Cs were obtained by sintering mixtures of 3ACl+MoCl3; the enneachlorides A3Mo2Cl9 withA=K, Rb, Cs were precipitated from solutions of MoCl3·3H2O and ACl in formic acid. Congruently melting compounds A3MoCl6 exist in all four systems, incongruently melting enneachlorides A3Mo2Cl9 in systems withA=K, Rb, Cs. Still unknown structures were determined by analog-indexing powder patterns according to known structure families.
Especially Cs3MoCl6 is isotypic with the recently found Cs3CrCl6 structure. Additionally, the unit cell parameters were determined for the compounds A3MoCl5·H2O (A=K, Rb, Cs) analogous to Cs2TiCl5·H2O, whose structure was determined by single crystal measurements.
The ternary system CsCl−NaCl−LaCl3 was investigated by means of differential thermal analysis and X-ray powder diffraction analysis. There exists one congruently
melting compound, Cs2NaLaCl6, crystallizing with the cubic elpasolite structure. No quasi-binary section exists for the whole system, however three binaries
range from the ternary compound Cs2NaLaCl6 to NaCl, CsLa2Cl7 and Cs3LaCl6 resp., dividing the system in three areas of composition: one triangle, Cs3LaCl6−Cs2NaLaCl6−CsLa2Cl7, containing additionally a compound Cs2LaCl5 below 510°C, and the two areas CsCl−NaCl−Cs2NaLaCl6−Cs3LaCl6 and Cs2NaLaCl6−NaCl−LaCl3−CsLa2Cl7, containing a mixed crystal range between LaCl3 and Na3La5Cl18. These areas could be further divided in five triangles, so that the whole system contains six Alkemade triangles.