Ca Vacancies Research Articles

EFFECTS OF Eu-DOPING SITE ON STRUCTURAL AND PHOTOLUMINESCENT PROPERTIES OF CaTiO3 PARTICLES

In this paper, Eu3+ -doped CaTiO3 (CTO) nanoparticles were prepared by a sol-gel method. Three different charge compensation mechanisms were realized by substituting Eu ion at different sites. (i) Ca site substitution with Ca vacancy compensation (CTO-A); (ii) Ti site substitution with O vacancy compensation (CTO-B); (iii) simultaneous substitution at both Ca and Ti sites with self-compensation (CTO-AB). Strongly related to the site substitution, the photoluminescent (PL) intensities were increasing in the sequence of CTO-B > CTO-AB > CTO-A. The absorbance spectra analysis of pure CTO nanoparticles showed absorption in the UV light region, while the absorption of Eu -doped CTO samples exhibited the blue shift owing to the presence of Eu3+ . Moreover, the photocatalytic activities of samples for decomposing dyes were reverse to the sequence of PL intensities. The PL properties and photocatalytic activity of the Eu3+ -doped CTO nanoparticles were thus discussed in detail from the viewpoints of the microstructures and charge transfer-related processes.

Dissolution mechanisms of .BETA.-tricalcium phosphate doped with monovalent metal ions

The effects on solubility of the substitution of monovalent metal ions (Li+, Na+, and K+ ions) at the Ca sites in β-tricalcium phosphate (β-TCP) were investigated by immersion of β-TCP powder doped with metal ions in saline, and the dissolution mechanisms of these samples were inferred from the results of dissolution tests and from observations of the crystal structures. β-TCP powder doped with Li+, Na+, and K+ ions (Li-TCP, Na-TCP, and K-TCP) had a lower solubility than pure β-TCP powder, and the solubility decreased with an increase in the amount of metal ions. This decrease in solubility can be attributed to an improvement in crystal structure stabilization upon the substitution of monovalent metal ions at the Ca(4) sites and vacancies in β-TCP, and the resultant occupancy of Ca2+ and metal ions at all Ca sites in the β-TCP structure. However, the dissolution behavior of Li-TCP differed from that of Na-TCP and K-TCP because the atomic arrangement between the Li(4) sites and oxygen was less stable than that between the Na(4) or K(4) sites and oxygen. In addition, the solubility of β-TCP doped with divalent metal ions was lower than that of β-TCP doped with monovalent metal ions because the structural stability of β-TCP doped with divalent metal ions was higher than that of β-TCP doped with monovalent metal ions. These results show that the solubility and dissolution mechanisms of β-TCP doped with metal ions were significantly influenced by the crystal structure stabilization and atomic arrangement of doped β-TCP.

Structural, electrical, luminescent, and magnetic properties of Ba 0.77Ca 0.23TiO 3:Eu ceramics

The structural, electrical, luminescent, and magnetic properties of Eu 3+-doped Ba 0.77Ca 0.23TiO 3 (BCT23:Eu) ceramics were investigated in this paper. Three different incorporation mechanisms of Eu 3+ were realized in BCT23 by considering different charge compensation mechanisms: (i) Ca site substitution with Ca vacancy compensation; (ii) Ti site substitution with O vacancy compensation; (iii) simultaneous substitution at both Ca and Ti sites with self-compensation. Besides the evident ferroelectric properties, the BCT23:Eu ceramics showed the photoluminescence and paramagnetic properties, implying that they can be considered as the multifunctional materials. Both the electric and luminescent properties were sensitive to the Eu doping concentrations and positions, due to the variations of the microstructures and different charge compensation mechanisms.

(Sr1–xCax)(11+16y–25z)/2(Si1–yAly)16(N1–zOz)25 (x ≈ 0.24, y ≈ 0.18, z ≈ 0.19) – A Novel Sialon with a Highly Condensed Silicate Framework

Abstract(Sr1–xCax)(11+16y–25z)/2(Si1–yAly)16(N1–zOz)25 (x ≈ 0.24, y ≈ 0.18, z ≈ 0.19) was obtained by high‐temperature synthesis (1650 °C – 1700 °C) using CaSiAlN3, Si3N4, AlN, SiO2, and Sr2N as starting materials. The crystal structure [Imm2 (no. 44), a = 20.6973(6), b = 10.7292(4), c = 4.8881(2) Å, Z = 2, R1 = 0.0326, wR2 = 0.0735] was determined from single‐crystal X‐ray data using dual space methods and confirmed for the bulk material by Rietveld refinement on X‐ray powder diffraction data. The structural results are corroborated by lattice energy calculations (MAPLE). The compound is the first representative of a novel silicate framework. The anionic part of the structure is built up from highly‐condensed dreier ring layers extending parallel (100), which are interconnected by common N and O atoms. In the resulting voids of this framework, there are three different cation sites, which are coordinated by six, eight, and twelve nitrogen and oxygen atoms, respectively. The largest one is fully occupied by Sr atoms, whereas the other two sites host Sr and Ca atoms or vacancies, respectively.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Direct observations of Ca ordering in Ca0.33CoO2 thin films with different superstructures

Ca-ion superstructures such as 3a×3a (hexagonal) and 2a×3a (orthorhombic) in layered Ca0.33CoO2 crystalline film, which were prepared by the reactive solid-phase epitaxy and the subsequent ion-exchange treatment, were directly distinguished by using spherical-aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The fluctuation of Ca intensities in the HAADF-STEM image indicates the existence of many Ca vacancies in the 2a×3a orthorhombic superstructure, which is consistent with the partial occupation of Ca (x=0.33) in the cation sites of x=0.5.

Effect of Ca deficiencies on the photoluminescence of CaTiO 3:Pr 3+

The effect of different Ca deficiencies on structural and luminescent properties of the Ca 1− x− y Pr x TiO 3 powders was investigated in this study. Ca deficiencies affect both the fluorescence intensity at 613 nm and afterglow properties of the phosphors. An increase in the fluorescence intensity at 613 nm was observed when x = 0.25 mol% and y increases from 0 to 1.5 x. Similar result was found for x = 0.15 and 0.35 mol%. Increased afterglow was observed for samples having low y values after irradiated by 375 nm light. To explain the experimental results, the formation of trapping centers related to Pr 3+ defects when they are not compensated by Ca vacancies was proposed based on the research works of [P. Boutinaud, E. Pinel, M. Dubois, A.P. Vink, R. Mahiou, J. Lumin. 111 (2005) 69] and [W. Jia, D. Jia, T. Rodriguez, D.R. Evans, R.S. Meltzer, W.M. Yen, J. Lumin. 119/120 (2006) 13].

Computer simulation study of defect formation and migration energy in calcium fluoride

A theoretical study of the defect formation (vacancy, interstitial and Frenkel defects) energy, and vacancy migration energy in calcium fluoride is presented in this paper. Calculations were conducted using the ab initio method with the plane-wave pseudopotential and molecular-dynamics simulation to determine the energetic and defect properties of the CaF2 crystal. For cation and anion vacancies, the relaxed vacancy formation energies were calculated to be 13.75 and 8.34eV, respectively. It was also found that only the octahedral positions are stable sites for Ca and F interstitial atoms. For cation the second nearest neighbor Frenkel defects were found to be stable within the simulation time (5ps) and the defect formation energy was estimated to be 9.3eV, while the third nearest neighbor Frenkel defects for anion were stable and the formation energy was only 2.49eV. The results are in agreement with Ure’s experimental results. The migration energy of cation vacancy in the direction of 〈110〉 is lower than that of 〈100〉 (3.93eV versus 4.62eV), suggesting 〈110〉 is the most likely path for Ca vacancies to migrate, while most anion vacancies may diffuse in the 〈100〉 directions due to a very low (0.33eV) anion migration energy in the direction. The data are useful for better understanding the formation of nano-scale void superlattice in calcium fluoride under electron beam irradiation.

Defect chemistry and dielectric properties of Yb3+:CaTiO3 perovskite

The defect chemistry of Yb3+:CaTiO3 solid solutions has been investigated both theoretically and experimentally. Three different incorporation mechanisms with similar solution energy were predicted for Yb3+ by atomistic simulation: (i) Ca site substitution with Ca vacancy compensation; (ii) Ti site substitution with O vacancy compensation; (iii) simultaneous substitution at both Ca and Ti sites with self-compensation. X-ray diffraction and scanning electron microscopy results strongly support the possibility to realize the above defect chemistries in CaTiO3 by changing the Ca∕Ti ratio to force Yb3+ on the Ca site (Ca∕Ti<1), on Ti site (Ca∕Ti>1), or on both sites (Ca∕Ti=1) according to the calculations. The temperature dependence of the relative dielectric constant (102–105Hz) of ceramics corresponding to predominant Yb substitution either at the Ca site or the Ti site is qualitatively similar to that of undoped CaTiO3. The Curie-Weiss temperature is shifted to more negative values in comparison to CaTiO3, suggesting that the compositions Ca1−3∕2xYbxTiO3 and CaYbxTi1−xO3 are further driven away from the ferroelectric instability. In contrast, the dielectric properties (102–105Hz) of ceramics corresponding to Ca1−x∕2YbxTi1−x∕2O3 are radically different. The relative dielectric constant is increased of about one order of magnitude (2200 at 30K), is almost independent of temperature, with a maximum variation of 20% in range of 20–300K, and shows frequency dispersion above 150K. The loss tangent at 20–300K is <5% for frequencies ⩾1kHz. The possible mechanism for the observed dielectric behavior is discussed.

Initial stages of hydration and Zn substitution/occupation on hydroxyapatite (0 0 0 1) surfaces

The initial stages of the hydration process have been simulated on a single-Ca(I) terminated hydroxyapatite (0 0 0 1) surface in step-by-step fashion using periodic slab density functional theory (DFT). Adsorption configurations and energetic properties have been described at different H 2O coverage. At low H 2O coverage, oxygen prefers to form Ca O bonds with surface Ca cations, but as coverage increases, H 2O tends to loosely float on the already-formed water layer. The height of the first layer H 2O relative to the surface is found to be 1.6 Å. The hydration process does not cause the decomposition of surface phosphate groups and hydroxyl channel, but does affect the energetics of subsequent Zn substitution and occupation on Ca(I) and Ca(II) sites. The Ca(II) vacancy site is found to be energetically more favorable for occupation due to less spatial constraint. This suggested mechanism of preferential occupation is different from previous attempts to explain the cation substitution site preference in bulk by ionic radius and electronegativity differences.

Non-ideality and defectivity of the akermanite-gehlenite solid solution: An X-ray diffraction and TEM study

This paper reports a structure analysis of the åkermanite-gehlenite solid solution. This solution is non-ideal with a negative excess volume in the entire compositional range. X-ray diffraction shows anomalous behavior of the cell parameters close to the gehlenite end-member (åk 08 ge 92 , åk 35 ge 65 ). This behavior is correlated with an excess of Si and deficiency of Al with respect to the Mg content, which implies a defective, non-stoichiometric structure with Ca vacancies. Electron microscopy images have confirmed an increase in the defectivity on the atomic scale for Al-rich compositions, and single-crystal structure refinements show a correlated decrease of the tetrahedral volume preferentially occupied by Si. The incommensurate modulation, characteristic of åkermanite, has been observed also in åk 95 ge 05 , and it is still visible as diffuse scattering in åk 75 ge 25 . (210) twinning has been observed in the entire compositional range.

Pathways to new magnetic semiconductors and half metals

This is a copy of the slides presented at the meeting but not formally written up for the volume. I will discuss theoretical and experimental results pointing to possibly new magnetic materials without resorting to the use of traditional magnetic elements. These ideas will involve the use of special surfaces and interfaces as well as defects. We show that a simple uninteresting oxide like CaO can at least theoretically be turned into a half metallic ferromagnet by introducing Ca vacancies. This mechanism opperates via a molecular analog to Hunds rule coupling and longer range exchange interactions. Polar surfaces and interfaces between polar mismatched layers can develop metallic and magnetic 2 dimensional properties. We also suggest that the use of atomic Hunds rule coupling in O and N which is as large as in the 3d transition metal compounds can be used to generate interesting ferromagnetic materials. Experimental evidence for the fasibility of such materials will also be presented.

Dielectric and Piezoelectric Properties of Ca 1-x(Li,Ce) x/2Bi 4Ti 4O 15 Ceramics

The influence of (Li, Ce) 4+ on the properties (piezoelectric, dielectric ferroelectric properties) of bismuth layer (Ca 1-x(Li, Ce) x/2Bi 4Ti 4O 15)(CLCBT) lead free piezoelectric ceramics used for high frequency and high temperature were investigated by means of conventional solid state method. The relationship between the additive amount of (Li, Ce) 4+ and the properties of CLCBT ceramics were obtained. The influences of additive amount of (Li, Ce) 4+ on the microstructure and material phase of CLCBT ceramies were studied by scanning electron microscope and X-ray diffraction. The mechanism for modifying the properties and microstructure of the CLCBT ceramics by (Li, Ce) 4+ doping was investigated. Results showed that the lead free bismuth layer CLCBT ceramics having good comprehensive properties was obtained when the (Li, Ce) 4+ additive amount was 0.075 mol, which the dielectric constant was 176.52, the dielectric loss was 0.00579, and the piezoelectric strain constant was 13 pC·N −1. This materials was suitable to be used for high frequency and high temperature piezoelectric device, and so on. (Li, Ce) 4+ doping could affect the properties and micro-structure of bismuth layer CLCBT ceramics by means of (Li, Ce) 4+ substituting for Ca 2+ in CLCBT, forming pyrochlore, densifying ceramics, stopping grain growing, forming Ca vacancy, and increasing specific resistance.

A computational investigation of stoichiometric and calcium-deficient oxy- and hydroxy-apatites

Computer modelling techniques have been employed to qualitatively and quantitatively investigate the dehydration of hydroxyapatite to oxyapatite and the defect chemistry of calcium-deficient hydroxyapatite, where a number of vacancy formation reactions are considered. The dehydration of hydroxyapatite into oxyhydroxyapatite is calculated to be endothermic by E = +83.2 kJ mol(-1) in agreement with experiment, where thermal treatment is necessary to drive this process. Calcium vacancies are preferentially charge-compensated by carbonate ions substituting for phosphate groups (E = -5.3 kJ mol(-1)), whereas charge-compensating reactions involving PO4 vacancies are highly endothermic (E > or =652 kJ mol(-1)). The exothermicity of the charge compensation of a Ca vacancy accompanied by a PO4/CO3 substitution agrees with their co-occurrence in natural bone tissue and tooth enamel. Our calculations of a range of defect structures predict (i) that calcium vacancies as well as substitutional sodium and potassium ions would occur together with carbonate impurities at phosphate sites, but that other charge compensations by replacement of the phosphate groups are unfavourable, and (ii) that the hydroxy ions in the channel are easily replaced by carbonate groups, but that the formation of water or oxygen defects in the channels is thermodynamically unfavourable. Calculated elastic constants are reported for the defect structures.

Magnetism without Magnetic Ions: Percolation, Exchange, and Formation Energies of Magnetism-Promoting Intrinsic Defects in CaO

We investigate theoretically the prospects of ferromagnetism being induced by cation vacancies in nonmagnetic oxides. A single Ca vacancy V(0)(Ca) has a magnetic moment due to its open-shell structure but the ferromagnetic interaction between two vacancies extends only to four neighbors or less. To achieve magnetic percolation on a fcc lattice with such an interaction range one needs a minimum of 4.9% vacancies, or a concentration 1.8 x 10(21) cm(-3). Total-energy calculations for CaO show, however, that due to the high vacancy formation energy even under the most favorable growth conditions one can not obtain more than 0.003% or 10(18) cm(-3) vacancies at equilibrium, showing that a nonequilibrium vacancy-enhancement factor of 10(3) is needed to achieve magnetism in such systems.

Substitution Model of Monovalent (Li, Na, and K), Divalent (Mg), and Trivalent (Al) Metal Ions for β‐Tricalcium Phosphate

The maximum substitution of monovalent, divalent, and trivalent metal ions for β‐tricalcium phosphate (β‐TCP) was investigated, and a substitution model of these metal ions for β‐TCP was proposed. Monovalent metal ions (MI) could replace Ca(4) site and vacancy (VCa(4)) in β‐TCP as 2MI=Ca2++VCa(4) and the maximum substitution was about 9.1 mol%. In the case of divalent metal ions (MII), Ca(4) and Ca(5) sites were replaced by divalent metal ions as MII=Ca2+, and the maximum substitution was about 13.6 mol%. Trivalent metal ions (MIII) could be substituted for Ca(4) site and vacancy as 3MIII=2Ca2++VCa(4), and the maximum substitution was about 9.1 mol%.

Ca-Fe3+-rich, Si-undersaturated buchite from Zelenky, North-Bohemian Brown Coal Basin, Czech Republic

Si-undersaturated Ca-Fe-rich buchite produced by fossil burning of a coal seam occurs in porcelanite deposit at Želenky in the North-Bohemian Brown Coal Basin, Czech Republic. Buchite is highly inhomogeneous, dense (ρ = 2.9–3.1 g/cm 3 ), strongly magnetic rock (10 −2 −10 −1 [SI]) composed of melilite, clinopyroxene (esseneite), spinel-group minerals, hematite, Ca 2 SiO 4 , calcite, aragonite, anorthite, anhydrite, perovskite, barite, two calcium ferrites - orthorhombic Ca 2 Fe 2 3+ O 5 (srebrodolskite) and CaFe 4 3+ O 7 , barium hexaferrite - BaFe 12 3+ O 19 , along with other several poorly identified phases. Clinopyroxene is rich in esseneite component (19–63 mol. %) and in a hypothetical end-member CaFe 3+ Fe 3+ SiO 6 (up to 25 mol. %) due to Fe 3+ substitution in tetrahedral sites (up to 0.26 apfu), and also in a hypothetical component Ca(Fe 2+ Mg) 0.5 Ti 0.5 AlSiO 6 (2–36 mol. %) due to incorporation of Ti (0.012–0.18 apfu), in M1 site. Melilite shows a wide compositional variability (mol. %): gehlenite 21–93, ferrigehlenite 0–34, akermanite 1–49, ferroakermanite 0–25, soda-melilite 0–15. Spinel-group minerals show solid solutions of Mf 12–80, Mt 1–62, Sp 2–68, Hc 0–21 (mol. %). Srebrodolskite, orthorhombic Ca 2 (Fe 3+ Al) 2 O 5 is widely substituted by Si and Ti. There are negative correlations among Fe + Al vs. Si + Ti ( R 2 = 0.998), and Ca (or R 2+ ) vs. total vacancy ( R 2 = 0.94), which are consistent with an exchange vector 2R 4+ □2R 3+ -1 Ca -1 , and suggest a hypothetical end-member Ca □ (SiTi) 2 O 5 . Calcium ferrite associated with srebrodolskite, ideally CaFe 4 3+ O 7 , shows extensive compositional variability: Ca 0.6–0.8 Fe 3+ 2.7–3.6 Al 0.1–0.6 Mg 0.2–0.6 Si 0.1–0.5 O 7 . First natural occurrence of barium hexaferrite, (Ba 0.6–1.1 Ca 0.1–0.2 )(Fe 3+ 10.0–11.7 Al 0.3–0.9 Ti 0.1–0.6 Mg 0.1–0.4 Mn 0.0–0.2 ) O 19 , ideally BaFe 3+ 12 O 19 , is reported. Variations of magnetic susceptibility with temperature were also investigated. Buchite originated by fusion of Tertiary clay and residual ash with overlying Quaternary sediments (loess loam with carbonate horizon) during fossil combustion of Miocene coal seam.

Positron Annihilation Lifetime Spectroscopy of Air‐Fired Ca(1−x)(La)xTiO3 Perovskites

In polycrystalline air‐fired perovskites of Ca(1−x)(La)xTiO3 stoichiometry (x=0.00–0.15), the positron annihilation lifetime increased monotonically with x, and showed an ∼40% increase of the lifetime at x∼0.1, at which value the lifetime increase saturated. These data, together with measured perovskite stoichiometry, indicate that equal numbers of Ca and Ti vacancies were the prime charge compensators. Thermal gravimetric data are consistent with charge compensation occurring via the cation vacancies in the air‐fired samples and Ti3+ in equivalent samples prepared under reducing conditions.

Features of EPR dosimetry with CaSO 4:Dy phosphor

The electron paramagnetic resonance(EPR) technique was used to study the paramagnetic centers induced by ionizing radiation in thermoluminescent CaSO 4:Dy. Two different radiation induced centers, detectable in the (1–10 7) Gy dose range and attributed to (SO3) − and Ca–vacancy center, (VCa) 2−, respectively, can be used for dosimetry purpose. The (VCa) 2− center is detectable only upon sample annealing at temperatures higher than about 180 °C. A reliable dose assessment can be performed in the (1–10 7) Gy dose range and likely over 10 7 Gy. The sensitivity decreases with thermal cycles, while the yield of both centers increases with accumulated dose (pumping effect), at a greater extent for (VCa) 2− center. (VCa) 2− center is stable with time, while (SO 3) − center shows a dose-dependent fading. Among the applications that can be imaged for this dosimeter, very high dose assessment capability is of particular importance.

Interaction of Uranyl with Calcite in the Presence of EDTA

Adsorption of uranyl at the surface of calcite was investigated by using batch sorption experiments and synchrotron X-ray standing wave (XSW) measurements. Aqueous solutions containing 236U(VI) (4.5 x 10(-7) to 1.0 x 10(-4) M) and EDTA (5.0 x 10(-7) to 1.1 x 10(-4) M) were reacted for 90 s to 60 min with freshly cleaved calcite (104) surfaces and calcite powders. Surface exchange coefficients, sorption kinetics, and influence of powder surface area/solution volume (SA/V) ratio were investigated by alpha-counting of 236U. Powder sorption results at SA/V = 870 cm2/mL fit a Freundlich isotherm [log [U]surface (in monolayers) = log K + n log [U]aq (in moles/L)], where K = 1.9+/-0.5 and n = 0.9+/-0.1, consistent with uptake of U(VI) by a specific surface reaction where the availability of sorption sites is nonlimiting in the U concentration range measured. Measured U(VI) coverages along this isotherm, based on the calcite (104) surface Ca site density, ranged from 0.04% to 5.4% of a monolayer. Steady state surface coverages were obtained within 90 s. Sorption of U(VI) on calcite (104) single-crystal cleavage surfaces using identical solutions yielded higher coverages, because of increased step density induced by dissolution at the relatively low SA/V ratio (approximately 1) of these measurements. The crystallographic location of the sorbed U(VI) was examined with the synchrotron XSW technique. Measurements were performed at the Advanced Photon Source on fresh calcite (104) cleavage surfaces reacted for 90 s with U(VI) solutions. Coherent fractions for sorbed U ranged from 0.14 to 0.62, and the mean value of the U coherent position was 0.84+/-0.02. This position was independent of dissolved U(VI) concentration and corresponds to a distance between the U atom and the calcite (104) plane of 2.55+/-0.06 A. These results are consistent with U(VI) adsorption atthe calcite surface as an inner-sphere uranyl-carbonate surface complex bonded with the outer oxygen atom(s) of a single surface carbonate group. Steric considerations allow this observed U(VI) surface complex to occur both at step sites ((441)_ and (481)_) and on terrace areas adjacent to Ca vacancies.

EXAFS STUDIES ON STRUCTURAL DISORDER IN CALCIUM DEFICIENT HYDROXYAPATITE

The structural disorder in calcium-deficient hydroxyapatite (Ca-def HAp) was investigated by extended X-ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge structure spectroscopy (XANES). Ca-def HAp was prepared by hydrolysis of α-tricalcium phosphate. The Ca/P molar ratio in Ca-def HAp was 1.54. In order to synthesize Ca-def HAp with various Ca/P molar ratios, a hydrothermal treatment in Ca(NO3)2 aqueous solution for Ca-def HAp was carried out. The Ca/P molar ratio in the sample obtained by the hydrothermal treatment increased linearly with increasing reaction time. By hydrothermal treatment for 13 days, the Ca/P molar ration in the sample increased up to 1.65 which almost stoichiometric ratio. Ultraviolet-Visible (UV-VIS) absorption at 280nm appeared in Ca-def HAp and the peak intensities decreased with increasing Ca/P molar ratio in Ca-def HAps. The UV-VIS absorption, therefore, could be identified as intrinsic peak due to Ca vacancies in the HAp lattice. The fact also would suggest that Ca ions in the solution were taken into the vacancies in the lattice during the hydrothermal treatments. Intensity of background subtracted and normalized XANES spectra of Ca-def HAp was weaker than that of stoichiometric HAp The reason would be why the distortions are introduced by loosing of structure resulting from deficiency of Ca ion in HAp lattice. From EXAFS analysis of both Ca-def HAp and stoichiometric HAp, it was revealed that the coordination number of Ca ion in Ca-def HAp was lower than stoichiometric HAp. In addition, the Ca-O distance in Ca-def HAp became to be slightly longer than that of stoichiometric HAp Therefore, it could be concluded that the structural disorder resulting from deficiency of Ca ions will caused the extension of Ca-O distance in HAp structure.