High-pressure synthesis and single-crystal structure refinement of gadolinium holmium silicate hydroxyapatite Gd4.33Ho4.33(SiO4)6(OH)2

Single crystals of gadolinium holmium silicate hydroxyapatite Gd4.33Ho4.33(SiO4)6(OH)2 have been synthesized at 2.0 GPa and 1450 °C using a piston-cylinder-type high-pressure apparatus. The crystal symmetry by single-crystal X-ray diffraction analysis is hexagonal, space group P63/mP63/m (No. 176), with a=9.3142(5) Å, c=6.7010(4) Å, Z=1Z=1. Gadolinium and Ho are disordered over the two large cation positions, A(1) and A(2), and charge balance in this silicate apatite is maintained by cation vacancies in A  (1). Two other apatite-structure crystals investigated have P3¯ and ImmaImma symmetry, and represent either partially ordered Gd–Ho distributions or crystal strain induced during quenching.

Graphical abstractCrystal structure of double RE hydroxyapatite Gd4.33Ho4.33(SiO4)6(OH)2 along [001] direction. RE(2) atoms define the apatite channel and are, in turn, enclosed by a hexagon of RE(1).Figure optionsDownload full-size imageDownload as PowerPoint slide