Tb${}_2$Co(B${}_2$O${}_5$)${}_2$ and Tb${}_2$Cu(B${}_2$O${}_5$)${}_2$. Two new borates with gadolinite-type structures

Using high-pressure/high-temperature synthesis methods, it was possible to increase the number of known borates with a crystal structure derived from gadolinite. Syntheses of Tb${}_2$Co(B${}_2$O${}_5$)${}_2$ (8 GPa, 1000 °C) and Tb${}_2$Cu(B${}_2$O${}_5$)${}_2$ (11 GPa, 650 °C) were carried out in a Walker-type multianvil device. The crystal structures were determined by single-crystal X-ray diffraction methods and Rietveld refinements of the powder patterns. The new compounds were further characterized by their infrared spectra. The lanthanoid cobalt and copper borates Tb${}_2$Co(B${}_2$O${}_5$)${}_2$ and Tb${}_2$Cu(B${}_2$O${}_5$)${}_2$ crystallize in the monoclinic space group P2${}_1$/c (no. 14) with the unit cell parameters a = 4.5630(1), b = 7.2991(2), c = 9.4161(2) Å, and β = 91.14(1)° for the cobalt and a = 4.5757(1), b = 7.3078(2), c = 9.3771(3) Å, β = 90.73(1)° for the copper compound.