Specific Heat Experiments at the Cooperative Jahn‐Teller Transition in the Mixed Crystal Systems (Tb_x, Tm_1−x)AsO₄ and (Tb_x, Tm_1−x)VO₄

Abstract

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.

Jahn-Teller effects in the mixed vanadate/phosphate crystalsTbV1−xPxHirano1, Wakabayashi2, Loong3 et al. 2003Phys. Rev. B

“…Analyses on the basis of the pseudospin formalism were performed previously on the data for the Jahn-Teller transitions in ͑Tb, Tm͒ VO 4 and ͑Tb, Dy͒ VO 4 . 11,12,17 In these analyses, the difference in the sizes of the rare-earth ions was assumed to give rise only to the local strain ␦ . The analysis performed in the present study is an extension through the use of an expanded Hamiltonian to calculate the changes in the wave functions due to the modified CF.…”

“…[5][6][7][8][9][10][11][12][13][14][15][16][17] In general, a reduction of the transition temperature T D was observed in these mixed compounds. A modified molecular-field model was developed to explain the experimental data.…”

“…In the studies on mixed compounds in which a fraction of the Tb ions is replaced by other rare-earth ions, the model has been extended to take into account randomness within the framework of the pseudospin formalism. [10][11][12]17 However, since doping with P ions is expected to create a CF that favors an easy magnetization along the c axis, the pseudospin formalism, which is based on the four low-lying states of TbVO 4 consisting of J z components in the basal plane, is not adequate to describe the Jahn-Teller effects in…”

Quadrupole ordering destruction and crossover in the Jahn–Teller compound TbVO4

“…In relation (5) the total heat capacity C H includes the lattice heat capacity C lat ¼ 12p 4 =5k B nðT=T D Þ 3 (the Debye temperature for the zircon lattice being T D ¼ 275 K [6], n ¼ 6) and the magnetic heat capacity C mag . The magnetic contribution to the entropy is given by the expression (per one RE ion)…”

Linear Birefringence Investigations of the Jahn‐Teller Distorted Phase in the Mixed Crystal Systems (Tbx, Tm1−x)VO4 and (Tbx, Yb1−x)VO4

“…The distortion of the crystals is treated on the basis of the local-strain model described in detail in [3].…”