ISSN 10637834, Physics of the Solid State, 2012, Vol. 54, No. 10, pp. 2052–2058. © Pleiades Publishing, Ltd., 2012. Original Russian Text © L.G. Gilinskaya, R.I. Mashkovtsev, 2012, published in Fizika Tverdogo Tela, 2012, Vol. 54, No. 10, pp. 1925–1930.
IMPURITY CENTERS
Paramagnetic Center Cd+(2S1/2) in Natural Carbonate Apatites Ca5(PO4)3 – n(CO3)n(F,OH) L. G. Gilinskaya* and R. I. Mashkovtsev Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090 Russia * email:
[email protected] Received January 23, 2012; in final form, March 26, 2012
Abstract—The stable paramagnetic center Cd+(2S1/2) in natural sedimentary calcium phosphates containing a cadmium impurity has been studied using electron paramagnetic resonance. The parameters of the spec trum have been determined, and the influence of different radiation types on the paramagnetic center has been investigated. DOI: 10.1134/S1063783412100137
Ions in the 2S1/2 state are characterized by a large hyperfine interaction (HFI) constant A, which increases (in GHz) from the 1s1 ions (from 1.4 for H0) to the 4s1 ions (in the range 1.4 [1]–2.9 [2] for Zn+; 3.364 [3]–4.465 [4] for Cu0; 7.71 [5]–11.15 [6] for Ga2+), to the 5s1 ions (1.686 [7]–1.9 [8] for Ag0; 11.948 [9]–13.26 [10] for Cd+; 9.362 [5]–14.7 [11] for In2+; 9.538 [12]–23.51 [13] for Sn3+), and, especially, to the 6s1 ions (35.41 [14] for Hg+; 15.68 [15]–52.8 [16] for Pb3+; 71.98 [5]–108 [17] for Tl2+), without reaching, however, the value corresponding to the free ion (except for hydrogen). The stable centers with ns1 electrons are radiationinduced or photoinduced cen ters. They are formed from isomorphic impurity ions in cation positions of the crystals that act as electron traps (Cd2+ Cd+, Zn2+ Zn+) or hole traps 2+ 3+ 2+ 3+ Pb , Sn Sn ). (Pb In analyzing the published results of the investiga tions of similarly charged ions in the 2S1/2 state in dif ferent structures, attention has been drawn to the fol lowing feature. The range of fluctuations of the quan tity A(ΔA) for the ns1 ions upon the transition from one structure to another one correlates primarily not with the character of the bond (the degree of ionicity– covalency), as tried to explain earlier [17, 18], but with the position of the ion in the Periodic Table, i.e., with the presence of an unpaired electron in the outer or inner shell of this ion. In particular, Group I and II ions have an ns1 electron in the outer shell: Cu0— 3d104s1, Zn+—3d104s1, Ag0—4d105s1, Cd+—4d105s1, Au0—5d106s1, Hg+—5d106s1, whereas Group III and IV ions have an ns1 electron in the inner shell: Ga2+— 4s1p0, In2+—5s15p0, Sn3+—5s15p0, Tl2+—6s16p0, Pb3+—6s16p0. The changes in the parameter A for ions with “outer” ns1 electrons (Cu0, Zn+, Ag0, Cd+, Au0,
Hg+) upon the transition from one structure to another one are less significant (megahertz) than those in the case of ions with “inner” ns1 electrons (Ga2+, In2+, Sn3+, Tl2+, Pb3+), where they have already reached several tens of gigahertz, as can be seen from the experimental data presented above. Furthermore, the parameter A for Group I ions is closer to the value of A for the free ion as compared to Group II ions, where the scatter is considerably more significant. These ions have been investigated predominantly in synthetic crystals doped with different ions (with the possibility of their enrichment), i.e., potential pre centers with ns1 electrons. However, the number of studied matrices is restricted by fluoritetype crystals and alkali halide crystals. Among the studied natural crystals are calcite [19, 20], apatite [21, 22], quartz [23], and beryl [24]. The most studied are the Pb3+ ions. This paper reports on the results of the electron paramagnetic resonance (EPR) investigation of the roomtemperaturestable Cd+ center in the natural calcium phosphate, i.e., carbonate apatite of the sedi mentary origin: Ca5(PO4)3 – n(CO3)n(F,OH) (hexago nal crystal system, space group P63/m). The unit cell of the apatite contains two formula units: Ca10(PO4)6(F,OH,Cl)2. The structure is based on phosphoric acid tetrahedra, which are joined together through the calcium ions Ca2+(1) with the formation of hollow cylinders (“channels”), so that the calcium ions Ca2+(2) are arranged on the cylinder walls, while the F–, OH–, Cl– ions are located along the 63 axis (Fig. 1a). The Ca2+(1) ions form continuous chains parallel to the c axis, in which each calcium ion is bonded to nine oxygen ions (coordination com plexes CaO9, symmetry C3) (Fig. 1b). These chains are
2052
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3–
(a)
linked with each other through the PO 4 tetrahedra. In the unit cell, the fraction of the calcium ions Ca(1) amounts to 40%. The fraction of the second coordina tion position of calcium ions, i.e., the calcium ions Ca(2) in the CaO6F(OH,Cl) complexes around the 63 axis with symmetry C1h, amounts to 60% (Figs. 1b, 1c) [25]. In natural samples of apatite, all structural posi tions are characterized by a great diversity of isomor phic substitutions that determine its varieties, in par ticular, carboniatehydroxylapatite (calcium carbonate phosphate fluoride) with substitution of carbon for part of phosphorus, which has been studied in this work. Among the eight natural isotopes of cadmium, two isotopes, namely, 111Cd and 113Cd, have negative nuclear magnetic moments μ = –0.5922 and –0.6195; the nuclear spins of both isotopes are I = 1/2. The nat ural abundance of these isotopes is as follows: 111Cd— 12.86% and 113Cd—12.34%. The electronic configu ration is Cd+ [Kr]4d105s1, the ground state is 2S1/2. The paramagnetism is determined by the unpaired 5s1 electron. In view of the great practical importance (phos phor, laser materials, biological object, raw materials for fertilizers, pharmaceutical drugs, plasticizers, implants, etc.), this mineral (both its natural form and synthetic analogues) has been investigated in all aspects over more than a hundred years. Ninetyfive percent of the phosphorus in the Earth’s crust is con centrated in apatites. This element is the main raw material for the preparation of phosphorus com pounds that are necessary for the organic life on the Earth. It is for this purpose that the group of sedimen tary apatites of marine origin studied in our work has been intended. The fundamental difficulty encoun tered in studying these minerals lies in the fact that, in this genetic group, there are no naturally occurring single crystals with sizes suitable for the EPR analysis. For these investigations, there are only available poly crystalline samples in which the presence of cadmium in trace amounts has been revealed from the analytical data. Cadmium in sedimentary apatites is a permanent impurity, which is explained by its genesis (namely, biophility). Cadmium is absorbed from the sea water by phytoplankton at the stage of the sedimentation; this element participates in the formation of metal loenzymes in these organisms, i.e., specific enzymes, or biocatalysts, which repeatedly accelerate the bio chemical processes [27]. Previously, we reported that, in the structure of the natural apatite, there occurs stabilization of ions in the 2S 0 1 3+ (6s1) [22], which 1/2 state: H (1s ) [21] and Pb reflect its structural and crystal chemical features asso ciated with the genesis. Atomic hydrogen is an indicator that, in the struc ture, there are sedimentary differences and bioapatites PHYSICS OF THE SOLID STATE
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Ca2+(1)
Ca2+(2) F−
Ca2+
3
PO 4
(b)
C3 c 63
2
2
2
3'
Ca(2) Ca(1) 3'
1
1 1
3'
F−
Ca(2)
O 2−
P5+
Ca2+ 63 ||c
(c)
z = 3/4 3 3 3
3'
3'
z2
3'
2
z = 1/4
1
3
3 3'
3
3'
Ca2+(2)
3'
z1
O 2−
P5+
OH −
Fig. 1. Crystal structure of the apatite: (a) general view (according to Lazarenko [25]), projection onto the (0001) plane; (b) scheme of the positions of the calcium cations 3–
Ca2+(1) and Ca2+(2), anions PO 4 , and halogen F– (1, 2, 3, and 3' are the oxygen atoms in the CaO9 coordination complexes); and (c) scheme of the positions of the OH– groups on the 63 axis and the neighboring atoms Ca2+(2) and O2– in the “channel” [26]; z1 and z2 are the z axes of 2–
the magnetic complexes of PO 3 2012
centers of two types.
2054
GILINSKAYA, MASHKOVTSEV (a) DPPH
Fe3+
–
(c) DPPH evenCd+
VO2+ −
CO 3 3− CO 3 − CO 2
0.5 mT
F − −O − −F − 111
Cd+
Gain × 2 F − −O − −F −
113Cd+
(b) P(OR)2
2–
trontype paramagnetic center: PO 3 + e– PO 3 (S = 1/2, I = 1/2) with a characteristic splitting of the spectral lines of the 31P nucleus (100%). The method detected two types of centers with different EPR parameters that depend on the shift of O3,3' and O1 (or O2) with the intensity ratio of 2 : 1, which corre sponds to the crystal structural parameters. Moreover, the quality of the actinide ion substituting for Ca2+ (U4+ or Th4+) leads to different parameters of the 2– PO 3 center and characteristic features, namely, the appearance of a superhyperfine structure of the nucleus of the 1H atom of the OH– group (Fig. 1c) for one of the centers with Th. This means that, in the structure of the apatite, there are four detected types of 2– PO 3 center [28, 29]. The correlation of the presence 2–
2−
PO 3 ( U ) P(OR)3 200
300 400 Magnetic field, mT
500
Fig. 2. Experimental EPR spectra measured in the range 100–500 mT: (a) initial (unirradiated) natural apatite and (b) natural apatite irradiated with thermal neutrons. Frag ments of the spectra in the range of ~470 mT are recorded with a larger (×2) magnification. (c) Line position of the center of the even isotope evenCd+ (g = 1.9998). DPPH is the reference signal of diphenylpicrylhydrazyl (the posi tion is indicated by the arrow). The H1 lines of the 111Cd+ and 113Cd+ centers are marked in a magnetic field of ~470 mT.
of acid (HPO4)2– groups that meet the specific condi tions of formation (growth), namely, the values of pH < 6 and the atomic ratios Ca/P = 1.50, 1.52, and 1.63, which are less than those for the stoichiometric composition (pH = 7, Ca/P = 1.67). The Pb3+ ions are found in apatites of different genetic groups. Their presence in the apatite correlates with the paramagnetic center of the electron type 2–
PO 3 , which is formed as a result of significant struc 3–
–
PO 3 tural distortions of the tetrahedra PO 4 because of the shift of one of the oxygen atoms of the tetrahedron, namely, O3,3'(2) or O1(1) = O2(1) (Figs. 1b, 1c), due to the fact that the actinide ions (U, Th) occupy the Ca(2) positions of the apatite structure and participate in the organization of the nearest environ ment. The subsequent capture of an electron by the formed fragment leads to the formation of an elec
of the Pb3+ center in the apatite with the PO 3 (Th) center and with the content of Pb and Th impurities in the samples allowed us to consider the detected lead as a thorium lead [22] and the observed picture as a man ifestation of the paradefect. In this work, we have studied sedimentary apatites with different cadmium contents (0.0012–0.0109 wt %) from several deposits of the world. The samples were preliminarily investigated in detail using the Xray powder diffraction and chemical analyses. The con tent of trace impurities (Cd, Zn, V, U, etc.) in these samples was determined. The EPR spectra were mea sured on a Radiopan SE/X 2544 radiospectrometer (λ = 3.2 cm, fmod = 100 kHz) with a double cavity at room temperature. The intensity of the observed spec trum was increased using different radiation types: X rays (tube MC 61–0.4 × 12Cu, 45 kV, 40 mA), γrays (60Co, 3 Mrad), electrons (2.5 MeV, pulsed linear accelerator ILU6, Institute of Nuclear Physics, Sibe rian Branch of the Russian Academy of Sciences, Novosibirsk, Russia), and thermal neutrons (0.5 MeV, research reactor WWRK, Tomsk Polytechnic Uni versity, Russia). Figure 2a presents the EPR spectrum measured in the polycrystalline apatite samples under investiga tion. In this spectrum, the following features manifest themselves: the previously identified complexes of the vanadyl ion VO2+ [30]; paramagnetic centers of car – – bonate radicals in the region of ge, such as CO 2 , CO 3 , 3–
and CO 3 [31, 32]; the EPR line of the structural ions Fe3+ (g = 4.23); the line of F– –O– –F– complexes; and the unidentified clearly pronounced doublet of isotropic symmetrical lines in the region of magnetic fields of 471.4 and 475.1 mT. The distance between the lines of the doublet is 3.7 mT, and the line widths are ΔH = 1.0 and 1.2 mT. A detailed analysis of the spectrum and data obtained for the studied samples revealed the follow ing observations: (1) the intensity of the lines of the
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Intensity, arb. units
2 111Cd+
1
113
Cd+
H2 H1 H1
0
H2
×60
−1 −2 −3
even
Cd+
320
340 450 500 Magnetic field, mT
550
600
(b) 3 Intensity, arb. units
2 111
Cd+
1
113
Cd+
H2 H1 H1
0
H2
×10
−1 −2
evenCd+
−3 ~ ~
PHYSICS OF THE SOLID STATE
(a) 3
~ ~
doublet increases with an increase in the content of cadmium in the sample; (2) the lines of the doublet correlate with the single line at g = 1.9998 (Fig. 2c), and the ratio of their intensities is consistent with the ratio of the cadmium isotopes; (3) the preliminarily estimated hyperfine structure (HFS) constant is com parable to that for the Cd+ ions in the previously inves tigated structures; and (4) the samples contain an impurity of uranium (0.00990.0154 wt %), which serves as a source of natural irradiation that transforms the Cd2+ ions into the Cd+ paramagnetic state. On this basis, the observed doublet and single line in the region of ge were identified with the Cd+ center obtained under natural irradiation of Cd2+ isomorphic ions in the Ca2+ cation positions by the uranium con tained in the samples. In the field region of ~470 mT, one transition (H1) was detected for each of the iso topes 111Cd and 113Cd with the isotopic lines of approximately equal intensity (for 113Cd, the lines are somewhat weaker than the line for 111Cd, according to the natural abundance of the isotopes). In order to obtain the complete spectrum for the reliable identification (or, what is most important, for the observation of its weaker and broadened compo nents H2), the samples were irradiated with different ionizing sources and thermal neutrons. Xray irradia tion for 2 h leads only to a slight increase in the inten sity of the observed doublet, whereas γrays enhance the intensity of the doublet and change the picture in the region of ge, where the spectra of carbonate radi cals are superimposed on the line of the even isotopes. Irradiation with electrons leads to a similar effect, i.e., an increase in the doublet lines; however, the H2 com ponents of the spectrum are observed only after an increase in the irradiation dose to 10 Mrad. Figures 3a and 3b show the experimental EPR spectrum of the Cd+ center and the computersimulated spectrum, respectively. The performed analysis of the intensity of the spec trum as a function of the irradiation dose in the range from 0.5 to 10 Mrad demonstrated that the greatest change in the intensity is achieved under irradiation with a low dose (0.5 Mrad); an increase in the dose to 5 Mrad and, then, to 10 Mrad is accompanied by smaller changes. The center is stable at room temper ature. It should be noted that the observation of the doublet of spectral lines for 20 years in the samples irradiated in 1992 by γrays (3 Mrad) revealed a decrease in the intensity by ~30%. Irradiation of the samples with thermal neutrons results in significant changes in the observed spectra, as is seen from Fig. 2b. The spectra of the centers Cd+, VO2+, and carbonate radicals disappear. However, 2– instead there appear lines of the PO 3 centers, which, according to the scheme described above, are formed when the U4+ ions occupy the Ca2+(2) positions in the
2055
320
340 450 500 Magnetic field, mT
550
600
Fig. 3. (a) Experimental and (b) computersimulated EPR spectra of the centers evenCd+, 111Cd+, and 113Cd+ in the apatite irradiated with electrons at a dose of 10 Mrad.
apatite structure. The intensity of the spectra of the F– –O– –F– complexes (O2– O–) located on the 63 axis increases significantly. Furthermore, there arises a new center represented by a single symmetrical line with g = 2.0001. The observed changes in the spectra of paramagnetic centers can be caused by a neutron flux. According to the performed investigations, it is known that the energy of thermal neutrons is relatively low (several hundred electronvolts); however, the energy of γquanta, which are emitted during the con version of the 113Cd isotope into the 111Cd isotope, reaches very high values (up to 10 MeV) [33], which are sufficient for the formation of defects in the struc tures under investigation. It has been found that the 113Cd isotope has the largest cross section of the cap ture of slow neutrons among all the other isotopes [34]. This isotope is the major absorber of thermal neutrons, and its conversion into the 114Cd isotope 2012
2056
GILINSKAYA, MASHKOVTSEV (a)
15 10
111
Cd+
0
H2
F=0
5 ν, GHz
mF
1
H1
0 −5 F = 1 0
−10 0
−1 100
15 10
200
300
400
113Cd+
F=0
5 ν, GHz
500 600 (b) H mF 2 0
1
H1
0 −5 F = 1 0
−10 0
−1 100
200 300 400 500 Magnetic field, mT
600
Fig. 4. Energy levels and transitions between these levels for the centers (a) 111Cd+ and (b) 113Cd+ in the apatite. For the even isotopes evenCd+, the energy levels are indi cated by dotted lines.
occurs with the release of high energies [33]. Appar ently, the disappearance of Cd+ centers in the studied apatites irradiated by neutrons is associated with the change in the valence of cadmium ions to Cd2+. The unambiguous conclusion regarding the factors responsible for the significant structural changes in this complex matrix requires additional experiments. The EPR spectrum of Cd+ ions is described by the spin Hamiltonian
Ᏼ = βHgS + IAS,
(1)
where S = 1/2, I = 1/2, and the term describing the hyperfine interaction dominates over the Zeeman term. Because of the large hyperfine interaction con stant A, the positions of hyperfine lines in the spec
trum are determined for a system in which the electron and nuclear spins are coupled together F = S + I and (F, mF) are “good” quantum numbers. The spectral lines for Cd+ correspond to the transitions H1—(1, 0) (1, 1) and H2—(1, 1) (0, 0). Using the Breit–Rabi solution for equation (1) given in [35], the spectrum of the even isotopes (g = 1.9998 ± 0.0005), and the transition (F = 1, mF = 0) (F = 1, mF = 1), we determined the parameters A (in GHz) and the positions of the transition H2 for both isotopes. Then, we performed the refinement of the parameters according to the EPR–NMR program [36] for polycrystalline samples. The refined values of the parameters were found to be as follows: A(111Cd) = –12.986 ± 0.005 GHz and A(113Cd) = –13.583 ± 0.005 GHz for the fields H2(111Cd) = 422.98 mT and H2(113Cd) = 549.31 mT, respectively. Using the same program, we carried out the simulation of the spec trum (Fig. 3b) and calculated the energy levels of the Cd+ center in the apatite for both isotopes (Figs. 4a, 4b). Attention is drawn to the substantial difference in the positions of the transition H2(1, 1) (0, 0) for the isotopes despite the close values of the hyperfine structure constants. The intensities of the transitions H2 are significantly less than those of the transitions H1 and also differ for the two isotopes: the intensity of the transition H2 is less than the intensity of the transition H1 by a factor of 5 for the 111Cd isotope and by a factor of 6.6 for the 113Cd isotope. The overall picture of the energy levels for Cd+ ions in the apatite, which is shown in Fig. 4, corresponds to the picture of the energy levels for alkali halide crystals [9] and fluoritetype crystals [10]. The values of the parameter A for the Cd+ ion in the apatite correlate with those of other matrices and do not violate the pat tern noted above for the difference ΔA. The ratio of the hyperfine interaction parameters A for the two iso topes A(111Cd)/A(113Cd) = 0.9560 corresponds to the ratio of their magnetic moments μ (0.9559). The isotropic parameters of the EPR spectra do not provide a direct answer to the question regarding the position occupied by Cd+ ions in the apatite structure. However, the analysis performed in [37] for the corre lation bonds between the components of the structure and trace impurities showed that cadmium has a posi tive bond only with isomorphic impurities, CO2 and zinc. The isomorphic substitution of carbon in the apatite structure has long been proved. The correlation bonds of cadmium with organic substances, namely, carbohydrates and lipids, were investigated in detail in [27]. This allows us to assume the isomorphism Cd2+(Cd+) Ca2+, but in the higher symmetry posi 2+(1) (symmetry C ), based on the shape of the tion Ca 3 absorption (isotropic, symmetrical) line observed in the EPR spectrum of the polycrystalline sample. The position Ca2+(2) (symmetry C1h) does not correspond
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PARAMAGNETIC CENTER
to the observed spectrum, because, apart from the dif ference in symmetry, the substitution in this position should be accompanied by the splitting of the lines due to the nucleus of the neighboring ion F– (I = 1/2, 100%), as is seen from Fig. 1b. It seems likely that the formation of organometallic complexes with cadmium takes place in the structure. Cadmium is a hazardous toxicant that comes into human and animal organisms from food, drinking water, and other environmental sources. Plants grown in the fields fertilized with cadmium superphosphate absorb cadmium in particularly significant amounts in the case of acid soils. Among the organs most suscep tible to the toxic effects of cadmium are the kidneys and liver. The duration of excretion of the excess absorbed cadmium from the organism reaches ~20 years [38]. The amount of this dangerous element in fertilizers is strictly regulated (no more than 0.0005–0.0015 wt %), especially in the environmentally concerned countries of the Western Europe. The performed analysis of the apatite rock phosphates from different deposits of the world has revealed the presence of cadmium in consid erably larger amounts, or, more precisely, up to 0.0060–0.0109 wt % (Senegal, Florida, Seybinskoe (Russia)) [39]. On the other hand, there is a discrep ancy in one or one and a half orders of magnitude in the results of the analyses for cadmium phosphorites from the same deposits [40]. We believe that the per formed identification of the EPR spectrum of the Cd+ center in natural calcium phosphates can serve as an additional (to analytical methods) reliable tool for detecting the presence of cadmium (toxicant) in the structure of apatites, because the characteristic dou blet of the lines H1 in the spectrum of the odd isotopes Cd+ manifests itself even under Xray irradiation.
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2. V. F. Krutikov, N. I. Silkin, and V. G. Stepanov, Para magn. Rezon., No. 13, 79 (1978). 3. I. K. Amanis and J. G. Kliava, Phys. Status Solidi A 41, 385 (1977). 4. M. Narayana, V. Sivasankar, and S. Radhakrishna, Phys. Status Solidi B 105, 11 (1981). 5. A. Räuber and J. Schneider, Phys. Status Solidi B 18, 125 (1966). 6. P. G. Baranov and V. A. Khramtsov, Sov. Phys. Solid State 20 (6), 1080 (1978). 7. N. I. Mel’nikov, D. P. Peregood, and R. A. Zhitnikov, J. NonCryst. Solids 16, 195 (1974). 8. N. I. Mel’nikov, R. A. Zhitnikov, and P. G. Baranov, Sov. Phys. Solid State 13 (5), 1117 (1971). 9. P. G. Baranov, R. A. Zhitnikov, and N. I. Mel’nikov, Sov. Phys. Solid State 15 (12), 2353 (1973). 10. V. F. Krutikov, N. I. Silkin, and V. G. Stepanov, Para magn. Rezon., No. 10, 113 (1978). 11. P. G. Baranov and V. A. Khramtsov, Sov. Phys. Solid State 21 (5), 839 (1979). 12. A. Hausmann and P. A. Schreiber, Z. Phys. 245, 184 (1971). 13. N. I. Mel’nikov, R. A. Zhitnikov, and V. A. Khramtsov, Sov. Phys. Solid State 17 (11), 2129 (1975). 14. V. F. Krutikov, N. I. Silkin, and V. G. Stepanov, Sov. Phys. Solid State 14 (10), 2642 (1972). 15. K. Suto and M. Aoki, J. Phys. Soc. Jpn. 22, 1307 (1967). 16. V. F. Krutikov, N. I. Silkin, and V. G. Stepanov, Sov. Phys. Solid State 17 (11), 2201 (1975). 17. W. Frey, R Huss, H. Seidel, and E. Werkmany, Phys. Status Solidi B 68, 257 (1975). 18. B. Andlauer and J. Schneider, Phys. Rev. B: Solid State 8 (1), 1 (1973). 19. R. M. Mineeva and L. B. Bershov, Sov. Phys. Solid State 11 (3), 653 (1969). 20. F. F. Popescu and V. V. Grecu, Phys. Status Solidi B 68, 595 (1975).
ACKNOWLEDGMENTS We would like to thank Yu.N. Zanin for providing a collection of comprehensively analyzed natural phos phorite apatites for our investigation and also M.V. Korobeinikov (Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sci ences, Novosibirsk, Russia) and A.V. Travin (Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences, Novosi birsk, Russia) for their assistance in irradiating the samples.
21. L. G. Gilinskaya and M. V. Chaikina, Izv. Akad. Nauk SSSR, Neorg. Mater. 13, 577 (1977). 22. L. G. Gilinskaya, Phys. Solid State 35 (1), 35 (1993). 23. V. P. Solntsev and R. I. Mashkovtsev, Sov. Phys. Solid State 20 (3), 471 (1978). 24. R. I. Mashkovtsev, L. V. Kulik, and V. P. Solntsev, J. Struct. Chem. 51 (5), 869 (2010). 25. E. K. Lazarenko, Course of Mineralogy (Vysshaya Shkola, Moscow, 1963) [in Russian]. 26. R. A. Young, Trans. N.Y. Acad. Sci. 29 (Ser. II), 949 (1967).
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Translated by O. BorovikRomanova
SPELL: OK
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