растворимости ионов щелочных и редкоземельных элементов в структуре гидроксиапатита на основе количественной кристаллоэнергетической теории изоморфизма В.С.Урусова. Установлено, что в системах с меньшими по размеру ионами редкоземельных элементов (Sm, Gd, Tb, Ho) теоретически рассчитанные пределы растворимости по схеме 2Ca2+ М+ + М3+ не достигаются вследствие перехода к замещению по схеме Ca2+ + OH- М3+ + O2-.
Анализ экспериментальных и расчетных данных в изученных системах в сопоставлении с литературными данными позволяет заключить, что имеет место конкуренция в замещении по схемам: 2Ca2+ М+ + М3+ и Ca2+ + OH- М3+ + O2-. Этим и обусловлено продолжение изменения параметров ячеек при переходе в гетерогенную область и меньшие пределы замещения по схеме 2Ca2+ М+ + М3+ в системах с малыми ионами р.з.э.
Ключевые слова: изоморфное замещение, гидроксиапатит кальция, щелочные элементы, редкоземельные элементы.
Annotation
Kanyuka Yu.V. Isomorphic substitution of calcium in the structure of hydroxyapatite by alkaline and rare-earth elements.-Manuscript.
Candidate Thesis in speciality 02.00.01. – inorganic chemistry. – Donetsk National Technical University, Donetsk, 2001.
Candidate thesis is devoted to study of regularities in isomorphic substitution of calcium in the structure of calcium hydroxyapatite by alkaline and rare-earth elements.
Solid hydroxyapatite-based solutions in a diversity of compositions – Ca(5 )MIxMIIIx(PO4)3OH, where MIII = La, Pr, Nd, Sm, Gd, Tb, and Ho, MI = Li, Na and K _have been obtained using solid-phase synthesis at 1100 0C.
In the system studied, the limits of substitution according to the Scheme 2Ca2+ М+ + М3+ were determined using X-ray phase analysis. The solubility regions are changed in the range of x from 0,1 to 1,0. For the systems with the same rare-earth element (La), the regions of homogeneity are unchanged (x 1) and vary for systems with different rare-earth elements. A decrease of ionic radius in going from La to Ho gives rise to substantial narrowing of solubility limits from х to x 0,1, i.e. ionic radius of rare-earth elements is the determining factor, although their radii change in a lesser extent than those of alkaline metals.
In the systems under study, as evidenced from NMR studies and IR- and EXAFS spectroscopy data, some portion of OH_ groups leaves the structure of hydroxyapatite, the rest changes their orientation. An examination of experimental data and theoretical calculations for the systems studied and comparison with the literature data suggest that substitutions according to the Scheme 2Ca2+ М+ + М3+ and Ca2+ + OH- М3+ + O2_ are in competition.
It is this factor which is responsible for further changes in parameters of lattice cell in the regions of heterogeneity and, for small-sized ions of the rear-earth elements, lower substitution limits in substitution according to the Scheme 2Ca2+ М+ + М3+.
Key words: isomorphic substitution, calcium hydroxyapatite, alkaline elements, rare-earth elements.