Chemical and structural properties of a whitlockite-like phosphate, Ca9FeD(PO4)(7)статья
Статья опубликована в высокорейтинговом журнале
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Дата последнего поиска статьи во внешних источниках: 18 июля 2013 г.
Аннотация:A new phosphate, Ca9FeD(PO4)(7), with a whitlockite-like structure was synthesized by treating Ca9Fe(PO4)(7) with D-2 at 820 K. Mossbauer spectroscopy showed that the oxidation state of Fe in Ca9FeD(PO4)(7) is +2. Ca9FeD(PO4)(7) was paramagnetic in a temperature range from 5 to 300 K with an effective magnetic moment of 4.97 mu(B) per Fe2+ ion. The infrared spectrum of Ca9FeD(PO4)(7) exhibited a broad absorption band due to an O-D stretching vibration at 2225 cm(-1) in addition to bands assigned to PO4 groups. Ca9FeD(PO4)(7) was stable under an Ar atmosphere up to 840 K. Above this temperature, it decomposed to a beta-Ca-3(PO4)(2)-like phase, Ca9.333Fe1.167(PO4)(7), as well as beta-Ca2P2O7 with releasing D2O and keeping the oxidation state of +2 for Fe. Ca9FeD(PO4)(7) liberated D2O in air above ca. 720 K to produce Ca9Fe(PO4)(7). Second-harmonic-generation signals, I-2w/I-2w(SiO2), in Ca9FeD(PO4)(7) (ca. 0.15) were by a factor of ca. 17 less than ca. 2.5 in Ca9Fe(PO4)(7). Rietveld analysis of time-of-flight neutron diffraction data for Ca9FeD(PO4)(7) on the basis of space group R3c gave lattice parameters a = 10.369 21(9) Angstrom and c = 37.1289(3) Angstrom, making it possible to locate D atoms at two positions and propose a mechanism of reducing Ca9Fe(PO4)(7). Its reduction accompanies changes in orientation of parts of P1O(4) tetrahedra, formation of O11-D1(...)O21 and O11'-D2(...)O34 bonds, and static disordering of Ca2+ ions at the Ca3 site. Structure parameters of Ca9.333Fe1.167(PO4)(7) in a product of thermal decomposition of Ca9FeD(PO4)(7) under Ar were also refined with its X-ray powder diffraction data: space group R3c; a = 10.3576(2) Angstrom and c = 37.1429(5) Angstrom; Z = 6. Ca2+ and Fe2+ ions were found to split into two pieces along the c-axis to occupy a 6a site.