ИСТИНА

In a temperature range 5 – 300 K the specific heat C(T) on a new mixed valence cobalt oxides REBaCo4O7 (RE = Dy, Ho, Er, Tm, Yb, Lu) was investigated. The ratio Co2+:Co3+ in these compouns is equal 3:1. In a crystal structure two types of Co-O layers alternate along the c-direction. One of these layers contain a corner-sharing tetrahedra Co2+O4 and Co2+ ions with spin only value S = 3/2 form a kagome-like network in the layer. Second type of Co-O layers contain Co3+O4 tetrahedra which have no connection in the layer and organize the links by corner-sharing tetrahedra between the Co2+-O and Co3+-O layers along the c-direction. The first order structural phase transitions from hexagonal P63mc to orthorhombic Cmc21 phase were revealed by X-ray diffraction in REBaCo4O7. This transition was indicated by a peak-like anomaly in C(T) curves at TS ~ 160 K, 178 K, 224 K, and 280 K for RE = Lu, Yb, Tm, and Er correspondingly. The magnetic phase transitions were found in REBaCo4O7 by the magnetic susceptibility measurements. The changes of slope on the C(T) curves were found at corresponding temperatures: TN ~ 50 K, 74, K, 98 K and 98 K for RE = Lu, Yb, Tm, and Er, correspondingly. At T < 50 K, the C(T) curve for LuBaCo4O7 is lower then those in other compounds and at T < 15 K this curve can be described by βT 3 law. In other compounds the specific heat at low temperatures contains the contributions from electronic and magnetic subsystems. From the estimations in a range 5 – 15 K the following Debye temperatures were found: θD ~ 200 K, 219 K, 237 K, 248 K, 258 K, and 286 K for RE = Tm, Lu, Dy, Yb, Er and Ho, correspondingly.