Structural Disorder, Specific Heat, and Magnetic Transitions in Cu₂FeBO₅

Cu2FeBO5 ludwigite single crystals have been grown from a solution-melt by spontaneous crystallization. Using the X-ray diffraction method, the crystal structure has been resolved in detail. Cations in sites M2, M3, and M4 have turned out to be structurally disordered. It has been found that oxygen atoms are disordered in one of five nonequivalent sites (O4). As can be seen from Mossbauer spectroscopy data, Fe3+ ions occupy four nonequivalent sites with different distortions of coordination octahedra. In the temperature range 40 K <= T <= 300 K, the spectra represent a superposition of quadrupole doublets. Static susceptibility measurements have revealed two magnetic features at T-1 = 35 K and T-2 = 20 K and spin-glass effects. Specific heat measurements in the interval 4-300 K have not discovered magnetic-transition-related anomalies.