Structural instability and poorly defined phase transitions in rare-earth dodecaborides RB12 (R=Ho-Lu) at intermediate temperatures

The single-crystal structures of YbB12, TmB12, and LuB12 were studied in the temperature range of 88-293 K, those of HoB12 and ErB12- in the range of 88-500 K using high-resolution X-ray diffraction data in order to correlate the structure changes with the changes in the physical properties of the dodecaborides under study. The lattice deformations caused by the cooperative Jahn - Teller effect were studied. A method is proposed for approximating the temperature dependences of atomic displacement parameters (ADPs) from lattice sites using the extended Debye or Einstein models. The break points in the temperature dependences of ADPs in combination with nonmonotonic changes in the lattice parameters near the critical temperatures T-c T-c indicate phase transformations revealed from diffraction data. The lattice instability and the rearrangement of the phonon spectrum near T-c are accompanied by the observed changes in the physical characteristics. A sensitive structural diagnostic tool was created to detect implicit phase transitions, quantum critical points and quantum instabilities of various nature that lead to the appearance of anomalies in the physical properties of crystals.