The phase transitions in double tungstate in extremely low-dimensional and low-symmetry compounds with cooperative Jahn-Teller effect

The rare earth double tungstates are of special interest because of manifestation of cooperative Jahn-Teller effect (CJTE) for low-dimensional and low-symmetry (monoclinic) crystallographic structure. The structural phase transition (SPT) as a result of the CJTE is unique one among large numbers of various SPT in solids. In the alkali-dysprosium double tungstates ADy(WO4)(2) (A= K, Rb, Cs) in spite of monoclinic symmetry the SPT of CJTE type are realized because the presence of Dy3+ ions with closely spaced energy levels. In alkali-dysprosium double tungstates the magnetically ordered structures possessing a number of features connected with low-dimensional interactions of Dy3+ ions were observed. Studies of specific heat in magnetic field and of magnetic susceptibility led to conclusion that the magnetic phase transitions (MPT) from paramagnetic to anti ferromagnetic state take place at subkelvin temperature region. Both MPT and SPT are very sensitive to magnetic field. In case of SPT this confirms its CJTE nature and allows to determine the type of the elastic ordering (SPT from para- to antiferroelastic states are observed). The external magnetic field induced the transitions from antiferro- to ferromagnetic states. The alkali-dysprosium double tungstates belong to the more general class of magneto-elastics with interrelation between magnetic and elastic ordering.