Pr4B10O21: A new composition of rare-earth borates by high-pressure/high-temperature synthesis

High-pressure chemistry led to the synthesis of the rare-earth borate Pr4B10O21 using a Walker-type multianvil apparatus at 3.5 GPa and 1050 °C. The tetra-praseodymium(III)-decaborate crystallizes monoclinicly with four formula units in the space group P21/n and lattice parameters of a=710.2(2), b=1948.8(4), c=951.6(2) pm, and β=93.27(3)°. The boron–oxygen network consists of [BO4]5− tetrahedra and [BO3]3− groups; however, the [BO4]5− groups represent the major part (80%) due to the high-pressure conditions during the synthesis. The praseodymium ions are coordinated by 10 and 12 oxygen atoms. Along with a detailed description of the crystal structure, temperature programmed X-ray powder diffraction data are shown, demonstrating the metastable character of this compound.