sIEBERT4) Dedicated to Prof. Dr. A. SEEQER on the occasion of his 60th birthday Single crystals of the systems (Tb,, Tml-,)AsO, and (Tb,, Tml-,)VO, are investigated by means of specific heat measurements in the temperature region 1 to 40 K. Whereas for the arsenate a cooperative Jahn-Teller distortion is observed over the whole concentration range x, in the vanadate the phase transition is missing for 0.1 5 x 0.4. This can be understood in mean field theory taking into account the effect of local strains which are assumed to have a random (Gaussian) distribution. Einkristalle der Systeme (Tb,, Tml -,)AsOl und (Tb,, Tml-,)VO, werden mit Hilfe von Spezifische-Warme-Messungen im Temperaturbereich 1 bis 40 K untersucht. Wkhrend fur das Arsenat bei allen Konzentrationen x eine kooperative Jahn-Teller-Verzerrung beobachtet wird, fehlt im Vanadat der Phasenubergang fur 0,l 5 x 5 0,5. Dies kann im Rahmen einer Molekularfeldtheorie verstanden werden, wenn man den EinfluB lokaler Vorverzerrungen berucksichtigt, deren Verteilung statistisch (Gaul3sch) angenommen wird. l ) Engesserstr. 7, POB 6980, D-7500 Karlsruhe 1, FRG.
The polarized absorption spectraof Ho3+ in HoVOr, HoAsOr, Hop04 and(Hoo,l,Yo,s)(OH)a werestudiedin therangefmm5OOOto12000cm-'. Nearly all crystal-field components were found and their crystal quantum numbers P. Y and S could be assigned.
Measurements of the linear birefringence in the distorted phase of (Tb, Tm)VO, and (Tb, Yb)VO, crystals allow to separately establish the contributions of the individual rare-earth ions and the anion complex for the first time. By this, the dissimilar wavelength dependence of the birefringence in pure Tb and Tm vanadate can be understood. A mean field theory including random-strain effects is used to treat the distorted phase and to elucidate the absence of the Jahn-Teller phase transition a t low Tb concentrations.Nessungen der linearen Doppelbrechung in der verzerrten Phase von (Tb, Tm)VO, und (Tb, Yb)VO, machen es moglich, die Beitrage der einzelnen Seltenen-Erd-Ionen und des Anionen-Komplexes zum erstenmal getrennt zu bestimmen. Hiermit kann die unterschiedliche Wellenliingen-Abhangigkeit der Doppelbrechung von reinem Tb-und Tm-Vanadat erklart werden. Es wird eice Molekularfeldtheorie, bei der statistische Vorverzerrungen beriicksichtigt werden, benutzt, urn die verzerrte Phase zu beschreiben und um das Fehlen des Jahn-Teller-Phasenibergangs bei kleinen Tb-Konzentrationen zu deuten. 1) Engesserstr. 7, POB 6980, D-7500 Karlsruhe 1, FRG. 2) Present address: Bruker Analyt. MeOtechnik, D-7512 Rheinstetten, FRG.
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