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 image (No. 176), with a=9.3142(5) Å, c=6.7010(4) Å, image. 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 image and image symmetry, and represent either partially ordered Gd–Ho distributions or crystal strain induced during quenching.