Lanthanum trifluoride, structure

The group of ion-selective electrodes with fixed ion-exchange sites includes systems with various membrane structures. The membranes are either homogeneous (single crystals, pressed pellets, sintered materials) or heterogeneous, set in an inactive skeleton of various polymeric materials. Important electrode materials include silver halides, silver and divalent metal chalcogenides, lanthanum trifluoride and various glassy materials. Here, the latter will be surveyed only briefly, for the sake of completeness. 

The trifluorides of lanthanum and ceriiun through samarium adopt the tysonite type of structure with elevenfold coordination of the metal ion (Wells 1975). The [RFn] unit. 

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. 

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