Spectral and quantum-mechanical study of electronic structure of ultradispersed calcium hydroxyapatite
The regularities of electronic structure of calcium hydroxyapatite samples of different genealogy were investigated by X-ray, IR spectroscopy methods and by quantum-mechanical calculations - Xa and LMTO (methods of the band theory). The evolution of valence band and charge states of atoms is describ...
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Інститут хімії поверхні ім. О.О. Чуйка НАН України
2002
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nasplib_isofts_kiev_ua-123456789-1263522025-02-09T14:09:20Z Spectral and quantum-mechanical study of electronic structure of ultradispersed calcium hydroxyapatite Shpak, A.P. Karbovskii, V.L. Vakhney, A.G. Kluyenko, L.P. Surface properties of inorganic materials The regularities of electronic structure of calcium hydroxyapatite samples of different genealogy were investigated by X-ray, IR spectroscopy methods and by quantum-mechanical calculations - Xa and LMTO (methods of the band theory). The evolution of valence band and charge states of atoms is described. It was established that distribution of p- and d-electronic states of calcium in samples of calcium hydroxyapatite of different origin is practically identical. Beside a high ionic component, covalent and hydrogen bonds are present in calcium hydroxyapatite. The indirect metal-metal interaction is observed in a metal sublattice. It was shown that a sublattice of the phosphatic tetrahedra determines the shape and main features of a valence band of calcium hydroxyapatite. The lack of a part of calcium ions in the nonstoichiometric samples results in weakening of P-O bond within a mirror plane. An influence of the crystal-ultradispersed state transition on the shaping of X-ray emission spectra was first investigated. Considerable localisation of metal d-states is observed in the ultradispersed hydroxyapatite samples. 2002 Article Spectral and quantum-mechanical study of electronic structure of ultradispersed calcium hydroxyapatite / A.P. Shpak, V.L. Karbovskii, A.G. Vakhney, L.P. Kluyenko // Поверхность. — 2002. — Вип. 7-8. — С. 57-68. — Бібліогр.: 12 назв. — англ. XXXX-0106 https://nasplib.isofts.kiev.ua/handle/123456789/126352 en Поверхность application/pdf Інститут хімії поверхні ім. О.О. Чуйка НАН України |
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Surface properties of inorganic materials Surface properties of inorganic materials |
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Surface properties of inorganic materials Surface properties of inorganic materials Shpak, A.P. Karbovskii, V.L. Vakhney, A.G. Kluyenko, L.P. Spectral and quantum-mechanical study of electronic structure of ultradispersed calcium hydroxyapatite Поверхность |
| description |
The regularities of electronic structure of calcium hydroxyapatite samples of different genealogy were investigated by X-ray, IR spectroscopy methods and by quantum-mechanical calculations - Xa and LMTO (methods of the band theory). The evolution of valence band and charge states of atoms is described. It was established that distribution of p- and d-electronic states of calcium in samples of calcium hydroxyapatite of different origin is practically identical. Beside a high ionic component, covalent and hydrogen bonds are present in calcium hydroxyapatite. The indirect metal-metal interaction is observed in a metal sublattice. It was shown that a sublattice of the phosphatic tetrahedra determines the shape and main features of a valence band of calcium hydroxyapatite. The lack of a part of calcium ions in the nonstoichiometric samples results in weakening of P-O bond within a mirror plane. An influence of the crystal-ultradispersed state transition on the shaping of X-ray emission spectra was first investigated. Considerable localisation of metal d-states is observed in the ultradispersed hydroxyapatite samples. |
| format |
Article |
| author |
Shpak, A.P. Karbovskii, V.L. Vakhney, A.G. Kluyenko, L.P. |
| author_facet |
Shpak, A.P. Karbovskii, V.L. Vakhney, A.G. Kluyenko, L.P. |
| author_sort |
Shpak, A.P. |
| title |
Spectral and quantum-mechanical study of electronic structure of ultradispersed calcium hydroxyapatite |
| title_short |
Spectral and quantum-mechanical study of electronic structure of ultradispersed calcium hydroxyapatite |
| title_full |
Spectral and quantum-mechanical study of electronic structure of ultradispersed calcium hydroxyapatite |
| title_fullStr |
Spectral and quantum-mechanical study of electronic structure of ultradispersed calcium hydroxyapatite |
| title_full_unstemmed |
Spectral and quantum-mechanical study of electronic structure of ultradispersed calcium hydroxyapatite |
| title_sort |
spectral and quantum-mechanical study of electronic structure of ultradispersed calcium hydroxyapatite |
| publisher |
Інститут хімії поверхні ім. О.О. Чуйка НАН України |
| publishDate |
2002 |
| topic_facet |
Surface properties of inorganic materials |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/126352 |
| citation_txt |
Spectral and quantum-mechanical study of electronic structure of ultradispersed calcium hydroxyapatite / A.P. Shpak, V.L. Karbovskii, A.G. Vakhney, L.P. Kluyenko // Поверхность. — 2002. — Вип. 7-8. — С. 57-68. — Бібліогр.: 12 назв. — англ. |
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Поверхность |
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2025-11-26T15:21:53Z |
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2025-11-26T15:21:53Z |
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| fulltext |
57
Surface properties of inorganic materials
SPECTRAL AND QUANTUM-MECHANICAL STUDY OF
ELECTRONIC STRUCTURE OF ULTRADISPERSED
CALCIUM HYDROXYAPATITE
A.P. Shpak, V.L. Karbovskii, A.G. Vakhney, and L.P. Kluyenko
Institute of Metal Physics, National Academy of Sciences
Vernadsky Blvd. 36, 03680 Kyiv-142, UKRAINE
Abstract
The regularities of electronic structure of calcium hydroxyapatite samples of different
genealogy were investigated by X-ray, IR spectroscopy methods and by quantum-mechanical
calculations - Xa and LMTO (methods of the band theory). The evolution of valence band and
charge states of atoms is described. It was established that distribution of p- and d-electronic
states of calcium in samples of calcium hydroxyapatite of different origin is practically
identical. Beside a high ionic component, covalent and hydrogen bonds are present in calcium
hydroxyapatite. The indirect metal-metal interaction is observed in a metal sublattice. It was
shown that a sublattice of the phosphatic tetrahedra determines the shape and main features of
a valence band of calcium hydroxyapatite. The lack of a part of calcium ions in the
nonstoichiometric samples results in weakening of P-O bond within a mirror plane. An
influence of the crystal-ultradispersed state transition on the shaping of X-ray emission
spectra was first investigated. Considerable localisation of metal d-states is observed in the
ultradispersed hydroxyapatite samples.
Introduction
Calcium hydroxyapatite Ca5(PO4)3OH (HAP) attracts attention of the researchers
because of its use in quantum electronics, instrument making, medicine, biology and
household chemistry.
HAP belongs to the group of compounds with the structure relating basically to
hexagonal space group P63/m and chemical structure M5(ZO4)3X (for example, M=Ca, Sr, Pb;
Z=P, As, V, Si, Cr; X=OH, F, Cl). The crystallographic data are given in [1]. There are two
non-equivalent crystallographic positions for Ca, namely - Ca(1) (Ca in a column) at z=0 and
3/4, and Ca(2) (Ca screw axis) at z=l/4 and 3/4. Each Ca(1) ion is surrounded by 9 atoms of
oxygen from 6 PO4-groups, and Ca(2) ion - with 7 atoms of oxygen from 5 PO4-groups and
1 OH group. Thus, the apatite structure can be expressed as [Ca(1)]4[Ca(2)]6(PO4)6(OH)2.
The HAP crystalline structure practically remains unchangeable within a wide range
of structure deviations from stoichiometry (Ca/P=1.66), and the OH- ions are stable even at
rather high temperatures (under 1350°C). The property of “friability” of crystal structure
allows one to occur the isomorphous substitutions in wide range of element concentrations
that can be useful in an electronic structure investigation. It is known that Sr+2, Ba+2, Pb+2 and
other mono-, bi- and trivalent metals can substitute calcium.
Remarkable feature of the biological HAP is a fine crystallinity. It is known that
58
dimensions of ultradispersed crystals of a native bone do not exceed 500´80´80 Å. Such
crystal has a specific surface area of more than 400 m2/g. This fact is extremely interesting for
X-ray spectral investigations since the ratio of surface/volume atomic concentration increases
considerably. In such crystals more than 25% of atoms are in the surface positions.
So, the alteration of physical-chemical properties of non-stoichiometric samples in
many cases is determined by their electronic structure. The main aim of these researches was
a systematization of the HAP electronic structure data obtained from samples of various
origin and preparation procedures with use of XPS, X-ray and IR-spectroscopy, and quantum-
mechanical calculations as well.
Experimental
Samples preparation
HAP Samples in the area of homogeneity (1.58 < Ca/P < 1.72) were synthesized from
water solutions of calcium nitrate and ammonium phosphate (“wet” chemical synthesis) with
subsequent annealing at various temperatures in the range of 110-1300°C. The chemical
composition and the structure of samples obtained were determined by chemical and X-ray
phase analysis methods.
The synthesis of samples in anhydrous annealing atmosphere resulted in the formation
of oxyapatites Ca10(PO4)6O□ -samples of pink colour (“pink” samples, □- vacancy). All
“pink” samples had a lack of calcium (Ca/P=1.61-1.63). The amorphous calcium phosphate
(ACP) and ultradispersed samples were produced by the method described in [1].
X-ray emission spectral techniques
The investigations were carried out by a secondary fluorescent method, which
guaranteed an invariability of phase and chemical structure of samples during the experiment.
The electronic spectrometer DEE-15, modernized spectrograph DRS-2M, X-ray spectrometer
SARF-1, and also X-ray spectrometer-monochromator RSM-500 were used.
The X-ray spectra were obtained by the standard method described in [2, 3]. All
spectra are shown after addition and averaging of several scans results. Because of optical
luminescence of samples the time of exposition of calcium K-spectra has reached to about
250 hours in the investigated compounds.
Techniques of vibration spectra obtaining
A powder of the substance studied is mixed up with a potassium bromide (with a
weight part of the researched substance of 1-3%) to obtain IR-spectra on spectrometer
Specord M-80. Then the tablets with diameter of 3 mm and thickness of ~1mm were pressed.
The tablet in the holder was located in cuvette section of IR-spectrometer with transparency
geometry. The measurements were carried out in the range of KBr transparency
(4000-400 cm-1).
Techniques of electronic spectra registration
The absorption of samples was measured in UV range with the use of electronic
spectrometer Specord M-40. Since the research of samples in UV and visible regions of
spectrum in the transparency geometry is not practically possible, the diffuse reflectance
method with the use of focusing sphere was applied. The spectra were recorded in the range
from 50000 cm-1 up to 30000 cm-1.
Quantum-mechanical calculations
The experience shows that the best agreement of theoretical and experimental data in
researching metal phosphates is achieved with use of LMTO or Xa-SW methods.
Quantum-mechanical calculations of the HAP electronic structure were carried out by the
LMTO method with the use of data about the atoms position in an unit cell [1]. Also the
[PO4]3- cluster calculations were carried out by Xa-SW method.
The self-consistent calculation of electronic structure, total and partial densities of
59
states were carried out by a linear method of MT-orbitals (LMTO) with taking combined
corrections into account [4-6]. The procedure of the self-coordination included charge density
of core states. The Bart-Hedin approximation [7] was used for the exchange-correlation part
of potential. The detailed description of calculation method one can find elsewhere [8, 9],
therefore we point only some details out here. The calculations were carried out in so-called
scalar-relativistic approximation, i.e. all relativistic effects (dependence of mass on speed, the
Darvin term) except for spin-orbital interaction were considered. In the expansion of wave
function s, p, d-harmonics for both metals and non-metals were taken into account. The
integration in Brillouin zone was carried out by the improved tetrahedron method [10] on a
grid of 650 points.
Results and discussion
Researches of vibration and electronic spectra
All samples were investigated in the IR range of wavelengths from 2.5 up to
25 microns. The optical absorption in HAP is the result of the vibrations of phosphate and
hydroxyl ions. The IR absorption spectrum of stoichiometric HAP is given in Fig. 1. It is
known that free PO4
3--tetrahedron has active vibrational modes such as n1=980 cm-1,
n2=363 cm-1 and n3=1082 cm-1, n4=515 cm-1. Since the symmetry is usually lowered in a
crystal, the spectra are split into peaks with more complex structure. The symmetry of
phosphate group (6/m) results in nine IR-active vibrational modes in HAP structure. Six of
these (n1, n2a, n3a, n3b, n4a, n4b) are vibrations belonging to E1u notion, in which the movements
of atoms are symmetric to the mirror plane. Other three vibrations (n2b, n3c, n4c) belong to the
symmetry Au, movements of these atoms are antisymmetric with respect to the mirror plane.
The hydrogen bond is reflected in IR absorption of OH- groups. The absorption bands
observed at 631 and 3573 cm-1 (is not given) corresponds to libration and stretching modes.
OH-stretching band, which becomes apparent as a weak peak, shows weakness of hydrogen
bonds of OH- groups. The only atoms capable to form such bonds in HAP structure are those
of oxygen from neighbouring OH and PO4-groups. In HAP, the O-O distance between the
neighbouring OH-groups is 3.44 Å and too large to form of hydrogen bond, while the distance
between oxygen atom of phosphate group and that of OH-group is 3.05 Å. Hence, the
hydrogen bond between -OH and PO4
3- can exist as O-H¼OPO3 and thus it is directed along
hexagonal axis of crystal that can be an important factor for a crystal growth.
The bands in the area of 1400 cm-1 should be attributed to stretching and librational
vibrations of OH group. In crystalline HAP this band has the large intensity and besides an its
splitting occurs because of the influence of internal crystal field. The insignificant intensity of
this band in ACP testifies to extreme difficulty of the preparation of OH-groups of free ACP
samples. However, taking into account the forms of spectra (Fig. 1) and practically identical
positions of peaks, it is necessary to note that in amorphous phosphate the environment of
phosphorus remains practically the same. Thus, it is possible to approve that samples ACP are
characterized by the near order and can have a polycluster structure.
As the result of calcium atoms absence in points of lattice of non-stoichiometric HAP
samples there is a further downturn of PO4-tetrahedron symmetry, that entails decrease or
increase of phosphorus-oxygen distance for the corresponding points reflected in the change
of the position of the corresponding bands of IR spectra (Table 1). So the deviation HAP
structure from stoichiometric one results in a drop of line position close to 473 cm-1. These
vibrations characterize a displacement of O-2 ions within the mirror symmetry plane. Thus, the
absence of the certain share of calcium ions results in weakening of some P-O bonds within
the mirror symmetry plane. As it was mentioned above, the line of absorption about 630 cm-1
is attributed to the librational vibrations OH- ions. These vibrations are vibrations of
60
OH-groups in the perpendicular direction of a crystal axis what corresponds to the assumption
of hydrogen atom swinging relatively to the oxygen atom or the centre of system weights.
Fig. 1. IR absorption spectra of HAP and ACP.
Table 1. Spectral positions of IR absorption bands of the various HAP samples.
Sample Bands, cm-1
HAP, Са/Р=1.66 965 473 1098 1045 602 570 3573 632
HAP, Са/Р=1.58
synthesized at 180°С
965
470
1100
1060
603
567
3563
630
at 700°С 962 470 1094 1039 600 563 3563 630
at 1300°С 966 470 1087 1040 602 565 - 630
Са10(РО4)6О� (“pink”, N2) 963 465 1095 1047 602 567 - -
Са3(РО4)2 957 442 1100 1055 607 565 - -
Volcanic HAP
(The peninsula Kamchatka)
470
1090
1040
600
572
3530
-
Bone mineral 965 465 1100 1040 610 565 3510 625
Ultradispersed HAP
(ashes of bone)
965
477
1094
1052
603
566
3510
633
Note: an error of the measurements is ±3 cm-1
The intensity of the observed band is determined by hydroxyl amount in researched
samples and it correlated well with the results of NMR studies [10]. For all samples the band
position coincides within the limits of accuracy of experiment, except for a mineral of the
61
bone, where the nature of displacement is not precisely established. In IR-spectra absorption
of “pink” samples there are no bands of librational vibrations of OH- groups but there are
bands close to 440 and 460 cm-1, characteristic of TCP. The reason of occurrence of these
phenomena can be connected with an absence of oxygen ions in the corresponding points of
lattice. The shortage of oxygen ions can also result in formation of additional levels in the
energy-band structure (so-called “colour centres“) in ionic crystals. The lines of optical
absorption in such “pink” samples are really established, they are about 2.4 eV. The similar
lines of absorption close to 2400 cm-1 are established in reflection spectra of ultradispersed
HAP samples of the human bones. The increase of intensity of optical reflection in a
yellow-red part of spectrum causes the corresponding bone colour.
The active vibrational modes describing PO4
3- tetrahedron in a number of compounds
(Table 1) get small changes except for TCP where they are caused by an absence of hydroxyl
groups. It is possible to explain the significant changes observable for stretching band of
OH-group in a number of researched samples in such a way: during the transition from
stoichiometric HAP to samples of other origin the essential changes in structure are observed
only lengthways the hexagonal axis of a crystal. So, in HAP samples of a volcanic origin
natural temperature annealing results in the formation of (HPO4)2- ions in HAP structure,
initiating the weakening of hydrogen bond directed lengthways hexagonal axis of a crystal.
For ultradispersed HAP crystals of the bone and its ashes, because of their large specific
surface area, there is an interaction of superficial atoms with their environment resulting in
weakening of bonds of OH-groups (Table 1). According to the data of samples absorption in
UV region it was established that the width of the HAP forbidden band of stoichiometric
structure makes 5.59 eV. In biological HAP samples as well as in stoichiometric one the
width of a band has a significant energy extent (about 5.52 eV) what determines their
dielectric properties. The dependence of a width of the forbidden band for non-stoichiometric
HAP samples on contents of OH-groups is observed.
X-ray spectral and quantum-mechanical researches
As a rule, only experimental methods are not enough for the complete description of
an electronic structure of compound. So we apply a technique of comparison calculated and
experimental data. It is clear from the Fig. 2-4 that the calculation data have been shown to
have a good conformity to the experimental ones. The experimental features of an electronic
structure of investigated compound are considered on the basis of the analysis of X-ray
spectra combined in one energy scale by a standard procedure [2, 3]. The X-ray photoelectron
spectra of valence band are not given because of their small informational ability for the given
compounds. The matching is carried out in accordance with the binding energy of the core
levels (Table 2).
Table 2. The binding energy of the core electrons in HAP
Substance Са 2р1/2 Са 2р3/2 Р 2р О 1s
Са10(РО4)6(ОН)2 351.2 347.5 133.6 532.4
The space orientation of PO4-groups causes more complex form of the calcium HAP
valence bands in comparison with CaCO3, where CO3-group has a plane orientation [11].
It follows from the comparative analysis of X-ray emission spectra that in HAP
crystals strong polarized Coulomb interactions prevail. The displacement of inner emission
lines of calcium and phosphorus testifies to significant charge transfer from calcium to PO4
3-
ions, and inside of PO4
3-- from phosphorus to oxygen. The form of the curve of total density
62
of states (TDOS) also characterizes compound with a large part of ionic bond (Fig. 2).
Fig. 2. X-ray spectra and curve of total density of states (TDOS, band calculation),
combined in one energy scale, for the hydroxyapatite sample.
Two features determine basically a form of the main maximum of calcium K-spectra.
D feature is formed by d- and p- states of calcium and short-wave maximum F, related mainly
with display of p-states of oxygen and d-states of calcium. Also the р-states of phosphorus
and oxygen contribute to feature D of TDOS. E feature is basically formed due to
hybridization of p-, d-states of calcium and p-states of phosphorus and oxygen. The observed
splitting of the main maximum of a Ca K-band in hydroxyapatites in comparison with TCP
[11] is related, as it was mentioned above, with existence of two various structural calcium
positions.
As was established earlier in [11, 12], atomic effects play a significant role in the
formation of calcium La-spectra form of such compounds. So the presence of Ca d-states in
the bond is levelled by its significant localization.
The position of G feature in Ca La-spectrum corresponds to position of the main
maxima of K- and L-spectra in metal [12], and thus reflects metal-metal interaction having in
HAP, by absence of feature in calcium K-spectrum, mainly s-s character.
As the direct interaction between Ca atoms is excluded by a atomic structure [1], such
interaction occurs with a certain part of the influence of atoms of oxygen Ca-O-Ca. As it
shown from Fig. 3 and 4, the features corresponding to G are observed on calcium density
curve of both Ca1(s) crystallographic positions, Ca1(p), Ca2(s), Ca2(p) and oxygen O4(s) from
hydroxyl group and O3(p) from phosphate of group. The features in Ca1(s), Ca1(p) and O3(p)
have considerably smaller intensity in comparison with Ca2(s), Ca2(p) and O4(s), that can
63
testify to the much greater participation of Ca2 atoms in bond. The metal-metal interaction
with the participation of calcium atoms in Ca1 positions is realized with contribution of
p-density of oxygen from PO4-group, in contrast to an interaction, which is executed via
s-density.
Fig. 3. Densities of states for Ca10(PO4)6(OH)2.
As it seems the interaction between calcium atoms in the Ca1-position either does not
exist or is weakly expressed. The presence of the feature on s-density of hydrogen curve in the
examined area can testify that the indirect interaction metal-metal occurs with participation of
hydrogen s-density. From above-stated it is possible to conclude that the interaction in
sublattice of metal exists mainly between atoms in Ca2 positions with participation of atoms
64
of oxygen from hydroxyl group, thus the channel of Ca2-O3-Ca1 interaction through atoms of
oxygen from PO4-groups is less expressed.
Kb1-emissional band of phosphorus reflects 3p-states of PO4
3- ion. One double
(coupled) and three single (uncoupled) bonds connect phosphorus with ions of oxygen that
corresponds to valence 5. Direct bond P-Ca in hydroxyapatite is not observed, and the bond
with Ca ions can be realized only through oxygen P-O-Ca. Common feature observed in
Ca K-, O K- and P K-spectra can testify to such a bond.
The maxima A and B of TDOS in the range of 12¸15 eV completely coincide with
long-wave features of Ca K-bands and are generated from that part of 2p electronic density of
oxygen, which occurs in chemical bond with phosphorus. The peaks in the area of bottom of
valence band in the range of 20¸25 eV are basically formed by s-states of oxygen (Fig. 3, 4).
Comparative researches of calculation data of the occupied part of valence band of
[PO4]3--cluster (Xα-RW cluster calculation and TDOS of HAP cell-band LMTO calculation)
revealed the common tendencies in formation of structure of a valence band.
The identical regularities of a cluster and crystal valence band features as in area of
bottom valence band and its middle part (Fig. 5) are practically observed. The identical
quantity of features on cluster and crystal TDOS curves, except for feature on 15 eV,
determined by presence of hydroxyl group, is also observed. The discrepancy of energy
positions of the features of valence band of a cluster and crystal in the area of its top can be
related to the peculiarities of a cluster calculation method.
Whereas the energy position of hybridized s-, p-states of hydroxyl cannot be
determined unequivocally on X-ray emission spectra data, the contributions to TDOS from
PO4 sublattice of calcium and hydroxyl groups were appreciated. It is shown (Fig. 5) that the
largest contribution to TDOS is made by PO4-groups.
The contribution of metal sublattice is much less, and contribution of sublattice groups
is levelled by an insignificant part, mainly in the range of energies up to 10 eV. Thus, it is
necessary to note that the hydroxyl electronic states, as it seems from the calculated data, form
exactly the top of valence band - feature at 2.5 eV reflecting, as it was mentioned above,
indirect interaction of metal ions.
Comparing the obtained data with the results of researches [11] (where for
ultradispersed samples a lot of features in low energy area O Kα-, P Kb- and Ca Kb5- bands
were found) it is possible to draw conclusion that apatite structure changes, related with
OH-groups, determine these features.
X-ray spectroscopy researches of samples with non-stoichiometric structure
A unique characteristic of HAP is non-stoichiometry of the composition. The
concentration ratio, Ca/P, is used as an index non-stoichiometry. Most HAP powders
synthesized by a “wet” chemical procedure are non-stoichiometric. We investigated this
peculiarity in details by calcium Kb-bands. As it is seen from Fig. 6 with increase of Ca/P
ratio from 1.58 value up to stoichiometric one and higher widening of A, B, C peaks, related
with the display of phosphorus 3p-state and 2p-state of oxygen, with conservation of absolute
values of their intensities in relation to the main peak is above-observed. In Ca K-band
spectrum of the sample with Ca/P ratio 1.58 the A, B and C peaks has more marked character,
that can testify to the greater part of ionic bond, and thus about more atom-like character of
hybridized orbitals.
The discrepancy of A, B, C peaks of D-35 sample and “pink” samples (Fig. 6), and the
researches carried out by NMR- and IR-spectroscopy methods can testify that the temporary
factor results in evolution of ultradispersed bone crystals of a dinosaur with formation of a
mineral containing combined water, instead of replacement hydroxyl by oxygen.
65
Fig.4. Partial densities of states of Ca10(PO4)6(OH)2
Fig. 5. Density of states [(PO4)3-]
cluster and HAP cell (band
calculation): 1 - contribution to
crystal TDOS is made by
PO4-tetrahedrons; 2 - contribution
to crystal TDOS is made by a
sublattice of metal; 3 - contribution
to TDOS is made by of hydroxyl
groups.
66
Fig. 6. X-ray Ca Kb-bands of the
nonstoichiometric calcium apatites -
“pink” samples: “Pink” N1 - 900°C,
3 hrs; “Pink” N2 -1250°C, 5 hrs, and also
in the thigh bone of a Dinosauria
Saurolophus (D-35) (the sample was
given by the State Archaeological
Museum of National Academy of
Sciences of Ukraine, the age is about
35 million years, territory of Mongolia).
Fig. 7. Energy positions of gravity centre in O Ka-spectra of hydroxyapatites
obtained at various annealing temperatures.
Spectral researches of samples prepared at various annealing temperatures
The presence of OH- ions is confirmed by various methods in all samples in range of
annealing temperature up to 1200°C. In some samples with annealing temperature
1250-1300°C these ions are practically absent. The samples produced at temperatures
67
~1300°C are characterized by lack of calcium. It is known [1], that in HAP crystals with Ca
lack HPO4
2- ions can appear, and, at high temperatures, take place transformations:
HPO4
2- + OH- ® PO4
3- + H2O
As the result of this reaction the amount of OH- ions decreases sharply. The molecules
of water evaporate partially, however, their presence was fixed in NMR spectra [10] of
volcanic origin samples, and in samples with the annealing at 1300°C.
Taking into account that at temperature of annealing the main changes occur in
sublattice of hydroxyl, the analysis of K-spectra of oxygen was undertaken. The energy
positions of a centre of gravity (CG) of spectra of oxygen (Fig. 7) were exposed to the
analysis, since the form of all obtained spectra was practically the same. As it is seen from the
given figure of CG position has only one feature. Practically identical positions for samples
with 180, 800, 1000, and 1100°C annealing temperature testify about the identical charge
state of oxygen atoms in these samples. The decrease of CG position in a sample with
annealing temperature of 600°C testifies to increase of electronic density on oxygen atoms,
with can be related to the greater part of delocalization of calcium electrons.
Conclusions
The presence of several areas is characteristic for HAP electronic structure. The
structure of each area is formed by the contribution of s-, p- and d-electronic conditions of
calcium, phosphorus and oxygen ions. The structure of subvalent states is totally determined
by s-states of oxygen. The distribution of p- and d-electronic states of calcium in HAP
samples of a various origin practically coincides. The strong ion component of chemical bond
in HAP crystals ranks with hydrogen component. In sublattice of metal the indirect
metal-metal interaction existing between atoms in Ca(2) position with the presence of oxygen
atoms from hydroxyl group is observed thus the channel of interaction Ca2-O3-Ca1 through
atoms of oxygen from PO4
- groups is less expressed. It is shown that sublattice of phosphate
tetrahedron is the main in the forming TDOS calcium hydroxyapatite form and it's features.
On the basis of the analysis of electronic spectra it is established that all investigated samples
have significant extent of forbidden band. The quantity of OH- ions in HAP samples
influences on spectral position of the edge of optical absorption and width of the forbidden
band. The hydrogen bond in HAP exists as O-H¼OPO3 and thus is directed lengthways the
hexagonal axis of a crystal, that can be important factor at crystal growth. The absence of
certain amount of calcium ions in the points of lattice of non-stoichiometric samples results in
weakening of some P-O bonds within mirror pane. In HAP samples of a volcanic origin a
natural temperature annealing initiates the weakening of hydrogen bond. In general the
annealing temperature of HAP samples only at 800°C results in a certain delocalization of
calcium electronic states. Transition from crystal to ultradispersed HAP does not bring
considerable changes in X-ray emission metal K-band formed by nearest calcium
environment mainly. Apparently, calcium occupying volume that formed by PO4
3- tetrahedron
and having no salient directed bonds cannot occupy surface position and thus cannot change
coordination environment in case of ultradispersed crystal state. So hydroxyl group occupying
its surface place results in increasing of crystal surface chemical activity. The basic binding
energies in ultradispersed samples of biological origin are close to artificial crystals, in which
the lack of Ca is found, and the O-H binding energy in bones is lower, that is caused by an ion
exchange between liquid and solid components of bone tissue. Temporary degradation of
ultradispersed HAP crystals of native bone occurs through the channel of formation of the
combined water, instead of the HAP mineral formation.
68
References
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Sample
Sample
Bone mineral
Bone mineral
Bone mineral
Substance
Р 2р
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