Investigations on the mixed Jahn-Teller system (Tb_x, Dy_1-x)VO₄. I. Phase diagram, specific heat and spectroscopic measurements

Abstract

At low temperatures TbVO, and DyVOlexhibit a structural phase transition from the same high-temperature structure to different low-temperature structures. Owing to symmetry, different crystallographic domains are possible. Investigations of single crystals of the mixed system (Tb,,Dy, -,)VO, with a polarising microscope gave information about domain distribution and distortion type. Specific heat measurements, spectroscopic and birefringence data yielded the transition temperatures. The phase diagram of the mixed system may be interpreted as a superposition of the phase diagram of diluted TbVOj and that of diluted D y V 0 4 . At high concentrations of one of the rare-earth ions the crystals distort like the respective pure substances, whereas at medium concentrations x = 0.4 a superposition of the distortion types is observed. The measurements are interpreted in the frame of a mean-field approximation, which is modified by the phenomenological compressible Ising model and by the inclusion of local strains of random distribution.' Sample I1 (see 5 5.3.2).

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…”