SYNTHESIS, CRYSTAL-GROWTH AND STRUCTURE INVESTIGATIONS OF RARE-EARTH DISILICATES AND RARE-EARTH OXYAPATITES

The rare-earth disilicates RE(2)Si(2)O(7) (RE=La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Y) and the rare-earth oxyapatites RE(9.33)square(0.67)( SiO4)(6)O-2 (RE=La, Nd and Eu) were synthesized in solid-state reactio ns at temperatures from 1050 to 1500 degrees C. Single crystals of the disilicates Ho2Si2O7, Er2Si2O7 and Y2Si2O7 and of the oxyapatites Eu( 9.33)square(0.67)(SiO4)(6)O-2 and Dy(9.33)square(0.67)(SiO4)(6)O-2 wer e grown in a flux grown mode in the temperature interval 1260-960 degr ees C from a flux containing Bi2O3 and V2O5. Purity of single crystals and reaction products was investigated by X-ray single-crystal and po wder-diffraction techniques. X-Ray single-crystal diffraction analyses are reported on Ho2Si2O7 and EuVO4. Ho2Si2O7 type D is monoclinic P2( 1)/c with Z=4 and a=4.6868(5), b=10.8618(12), c=5.5872(5) Angstrom, be ta=95.993(7)degrees. EuVO4 is tetragonal. I4(1)/amd with Z=4 and a=7.2 480(9), c=6.3778(10) Angstrom. Neutron powder diffracton analyses are reported on Er2Si2O7, Ho2Si2O7 and Y2Si2O7 all of type D structure and on the oxyapatites La(9.33)square(0.67)(SiO4)(6)O-2 and Nd(9.33)squar e(0.67)(SiO4)(6)O-2. The type D structure is monoclinic with P2(1)/c a nd Z=4. Er2Si2O7 has a=4.6943(4), b=10.8097(10), c=5.5646(4) Angstrom, beta=96.037(4)degrees. Ho2Si2O7 has a=4.6929(5), b=10.8627(12), c=5.5 895(5) Angstrom, beta=96.017(4)degrees. Y2Si2O7 has a=4.6916(4), b=10. 8521(10), c=5.5872(5) Angstrom, b=96.040(3)degrees. The rare-earth oxy apatite structure is hexagonal with P6(3)/m and Z=1. LaB(9.33)square(0 .67)(SiO4)(6)O-2 has a=9.7259(10) and c=7.1899(3) Angstrom, and Nd(9.3 3)square(0.67)(SiO4)(6)O-2 has a=9.5731(8) and c=7.0336(2) Angstrom.