Reduction and Re-oxidation Behavior of Calcium Iron Phosphate, Ca₉Fe(PO₄)₇

Abstract

Reduction (in H 2 ) and re-oxidation (in air) behavior of Ca 9 Fe(PO 4 ) 7 was studied by X-ray powder diffraction, Mo ¨ssbauer spectroscopy, thermogravimetry, hydrogen absorption, electrical conductivity, and magnetic-susceptibility measurements. The β-Ca 3 (PO 4 ) 2 -like framework of Ca 9 Fe(PO 4 ) 7 was stable in 100% H 2 up to 820 K. In the temperature range from 680 to 820 K, reversible redox reactions occurred without changing the stoichiometry of oxygen and phosphorus atoms and destroying the structure. The reduction process in Ca 9 Fe(PO 4 ) 7 included the change of the oxidation state of Fe atoms and incorporation of hydrogen atoms into the structure to form Ca 9 FeH x (PO 4 ) 7 (0 < x e 1). Above 820 K, treatment in 100% H 2 was accompanied by loss in mass, partial destruction of the structure, and appearance of FeP and Fe 2 P phases. Re-oxidation studies in air showed that samples partially lost phosphorus atoms during reduction above 820 K. Behavior of Ca 9 Fe(PO 4 ) 7 under a reduction atmosphere depended on H 2 concentration and temperature. At low H 2 concentration, the sample destruction started from ca. 870 K and proceeded with maintaining the oxidation state of Fe, releasing H 2 O, and forming a β-Ca 3 (PO 4 ) 2 -like phase and β-Ca 2 P 2 O 7 . In this case, iron phosphides appeared above ca. 1200 K.

Structural Confinement toward Giant Enhancement of Red Emission in Mn2+‐Based Phosphors

“…As an example discussed herein, β‐Ca 3 (PO 4 ) 2 ‐type mineral structure is capable of such a design principle toward new phosphor systems owing to the possibility for heterovalent substitution of cations and multivariate structural types. [2a,8] This phase provides five crystallographic cationic position with opportunities for heterovalent substitution and close interactions between cations . Moreover, a valid approach is to target an efficient energy transfer (ET) for color‐tunable phosphors, so that the ET process can be easily realized in the multiple sites of the β‐Ca 3 (PO 4 ) 2 ‐type compounds .…”

The structure, photoluminescence and influence of temperature on energy transfer in co-doped Ca9La(GeO4)0.75(PO4)6 red-emission phosphors

“…27,28 The different substitutions in the β-Ca 3 (PO 4 ) 2 can create many kinds of materials with a β-Ca 3 (PO 4 ) 2 -type structure. These materials have served as the basis for the crystallochemical design of compounds with ferroelectrics, catalysts, two-stage hydrogen oxidation, nonlinear optical and ion-conductive properties, [29][30][31][32] and have been confirmed to be efficient phosphors for application in white-LEDs, such as Ca 3 (PO 4 ) 2 :Eu 3+ , Dy 3+ , 33 Ca 9 Lu(PO 4 ) 7 :Eu 2+ ,Mn 2+ , 34 Ca 9 Gd(PO 4 ) 7 :Eu 2+ ,Mn 2+ (ref. 35) and Ca 9 Y(PO 4 ) 7 :Eu 2+ ,Ce 2+ .…”

Luminescence and microstructures of Eu3+-doped Ca9LiGd2/3(PO4)7

“…2 materials have served as the basis for the crystallochemical design of compounds with ferroelectrics, catalysts, two-stage hydrogen oxidation, nonlinear-optical, and ion-conductive properties. [15][16][17][18] b-Ca 3 (PO 4 ) 2 structure gives the possibilities for iso-and heterovalent substitutions of Ca 2+ by M + (monovalent), R 3+ (trivalent), and R 4+ (tetravalent) cations, 4, 19 for example, 3Ca 2+ →2R 3+ + (R 3+ = RE ions, Y, Fe, Al, denotes a vacancy) gives the formation of the cationic vacancies in solid solutions of Ca 3-x R 2x/3 (PO 4 ) 2 (0 £ x £ 3/7); and 3Ca 2+ + →R 3+ + 3M + or Ca 2+ + →2 M + (M = Li, Na, K) gives the occupancy of cationic vacancies in the solid solutions of Ca 3-x M x R x/3 (PO 4 ) 2 (0 £ x £ 3/7) or Ca 3-x M 2x (PO 4 ) 2 (0 £ x £ 1/7). 20, 21 In the RE doped compounds with a b-Ca 3 (PO 4 ) 2 structure, the nature of a monovalent ion has a strong influence on the luminescence properties.…”