Crystal optical and x-ray diffraction studies show that the high-temperature phase I of the perovskite-type layer compounds (C,I.12,,1NH,),CuC1, with n = 1, 2, 3 is Jahn-Teller distorted orthorhombic with space group Bbcm (Dif). As a consequence, the order of the CnH2n+ INH3 group leads to two configurations, realised in a monoclinic phase I1 with space group P2,/c (C&) and an orthorhombic phase I11 with Pbca (D::). The transition sequences found above 300 K depend on n: I +-T,, = 348 K+ I1 for n = 1; I + T,, = 364 K-+ I1 +-T& = 356 K, Te2 = 338K+ I11 forn = 2; and I +-T,, = 434K+ I11 forn = 3. The domain phenomena at T,, and T,, point to a subtle balance between the tilt and order configurations of phases I1 and 111. Birefringence and lattice constants indicate a very loose inter-layer contact in phase I.

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The h e a r optical birefringence of the iron-group difluorides (rutileatype) has been determined between 5 and 700 K. A magnetic contribution to the birefringehce occurs during the ordering of the antiferromagnetic compounds MI&',, FeF,, CoF,, and the weak ferromagnetia NiF,; the linear magnetic birefringence (LMB) can be well separated from the temperature dependent natural optical anisotropy. Both the magnetic short-range as well as the longrange order contribute to the LMB. The temperature derivative of the LMB follows the course of the magnetic specific heat to a very good degree. It is shown that this proportionality is to be expected if all spin pair correlation functions exhibit the same temperature behaviour. Whethcr this condition is valid for the lattice distortions is discussed using the existing structural data. Assuming the obMerved LMB is mainly due to spontaneous lattice distortions, the influence of the fluorine parameter seems to play an important role.An den Difluoriden der Eisenreihe wurde zwischen 5 und 700 K der Verlauf der linearen Doppelbrechung bestimmt. Der bei den Antiferromagneten MnF,, FeF,, CoF, sowie beim schwachen Perromagneten NiF, gefundene magnetische Beitrag zur Doppelbrechung (LMB) ist gut vom Temperaturverlauf der naturlichen optischen Anisotropie zu trennen. Magnetische Nah-und Fernordnung trngen zur LMB bei. Die gefundene magnetkche Doppelbrechung, nach der Temperatur differenziert, stimmt ausgezeichnet mit dem Verlauf der magnetischen spezifischen Wiirme iiberein. Es wird gezeigt, daB diese Proportionalitiit zu erwarten ist, wenn alle Spin-Paar-Korrelationen gleiches Temperaturverhalten aufweisen. Die Gtiltigkeit dieser Bedingung auch fiir die Gitterdeformationen wird an Hand vorliegender Strukturdaten gepriift.

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