Structure and properties of Li2–2xMg2+x(MoO4)3 crystals activated by copper ions

Article abstract of DOI:10.1134/S0022476616030069

By the low-gradient ?zochralski method, both undoped and copper ion activated Li_2–2xMg_2+x(MoO₄)₃ crystals are grown. The charge state and structural position of impurity copper ions are determined using EPR. Investigations of the luminescence properties reveal that luminescence with a maximum at λ = 520 nm is observed for Li_2–2xMg_2+x(MoO₄)₃ crystals. A decrease in the temperature increases the intensity of this luminescence. It is found that the doping of Li_2–2xMg_2+x(MoO₄)₃ crystals with copper ions also increases the luminescence intensity with a maximum at λ = 520 nm. It is supposed that cation vacancies, which provide the charge compensation when copper ions substitute for lithium ions, are responsible for the luminescence.

Full text of DOI:10.1134/S0022476616030069

Journal of Structural Chemistry. Vol. 57, No. 3, pp. 459-463, 2016.
Original Russian Text © 2016 A. A. Ryadun, V. A. Trifonov, V. A. Nadolinny, A. A. Pavlyuk, M. I. Rakhmanova.
STRUCTURE AND PROPERTIES OF Li Mg (MoO )
2–2x 2+x 4 3
CRYSTALS ACTIVATED BY COPPER IONS
A. A. Ryadun, V. A. Trifonov, V. A. Nadolinny*,
A. A. Pavlyuk, and M. I. Rakhmanova
UDC 535.37:543.429.22:539.1.07
By the low-gradient Čzochralski method, both undoped and copper ion activated Li2–2xMg2+x(MoO4)3
crystals are grown. The charge state and structural position of impurity copper ions are determined using
EPR. Investigations of the luminescence properties reveal that luminescence with a maximum at
λ = 520 nm is observed for Li2–2xMg2+x (MoO4)3 crystals. A decrease in the temperature increases the
intensity of this luminescence. It is found that the doping of Li2–2xMg2+x(MoO4)3 crystals with copper ions
also increases the luminescence intensity with a maximum at λ = 520 nm. It is supposed that cation
vacancies, which provide the charge compensation when copper ions substitute for lithium ions, are
responsible for the luminescence.
DOI: 10.1134/S0022476616030069
Keywords: EPR of transition metal ions, luminescence, scintillators, crystals of double molybdate.
The application of scintillation materials in different fields of science and technique stimulates the researchers to
search for novel materials with desired characteristics which meet the up-to-date requirements: large luminescence light
output in a given wavelength range, short decay times, radiation resistance, and others.
Hexavalent molybdenum oxides hold a special place among the promising scintillation materials because these
compounds enable the solution of the problem of recording the neutrinoless double β decay (0ν2β) [1]. A considerable
100
interest in this problem stems from the determination of the neutrino mass, and Mo is one of the most promising nuclei for
2β decay experiments due to its high transition energy Q2β = 3035 keV [2]. The studied crystals of simple molybdates
MMoO4 (M = Pb, Mg, Ca, Cd, Zn) [3-6] have some disadvantages (natural radioactive impurities, high effective atomic
number, low light output, etc.).
Recently, the crystals of double molybdate Li2–2xZn2+x(MoO4)3 have been studied in detail [7-11]. The authors of [7]
managed to grow only centimeter-size Li2–2xZn2+x(MoO4)3 crystals by the top-seeded solution growth (TSSG) technique,
which is primarily associated with the selection of growth conditions. In [8-11], by the Čzochralski method, under the
conditions of low temperature gradients (ΔT/Δх < 1°/cm), large (up to 100 mm) optically homogenous crystals of
Li2–2xZn2+x(MoO4)3 were grown, both undoped and doped with transition metal ions. The charge states and structural
positions of impurity ions were determined and the spin-Hamiltonian parameters were calculated using EPR. In addition, the
low-temperature luminescence with a maximum near 560 nm was proved to be due to cation vacancies which provide the
charge compensation of impurity transition metal ions.
Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia;
*spectr@niic.nsc.ru. Translated from Zhurnal Strukturnoi Khimii, Vol. 57, No. 3, pp. 488-491, March-April, 2016. Original
article submitted April 7, 2015.
0022-4766/16/5703-0459 © 2016 by Pleiades Publishing, Ltd.
459

Fig. 1. Structure of the Li2–2xMg2+x(MoO4)3
compound; view along the a axis, M1, M2,
M3 are the positions of lithium and
magnesium ions in the structure.
It is interesting to study the growth and properties of other compounds isostructural to Li2–2xZn2+x(MoO4)3. This
series includes Li2–2xMg2+x(MoO4)3: orthorhombic crystal symmetry, space group Pnma; the range of variation in the
Li2–2xMg2+x(MoO4)3 composition is 0 ≤ x ≤ 0.30. Lithium and magnesium cations statistically occupy M1, M2, and M3
positions in the crystal structure (Fig. 1) [12, 13]. The melting point is 1030°C (incongruent) [14].
In this work, optically homogenous Li2–2xMg2+x(MoO4)3 crystals doped with copper ions were grown by the low-
gradient Čzochralski method. The luminescence properties of previously grown undoped crystals [15] and crystals doped
with copper ions at temperatures of 77 K and 300 K were examined and the electronic state and structural position of
activating ions were studied.
EXPERIMENTAL
2+
Undoped and Cu doped double lithium-magnesium molybdate Li2–2xMg2+x(MoO4)3 crystals were grown by the
Čzochralski method under conditions of low temperature gradients (ΔT/Δx < 1°/cm) on 4×4×20 mm seeds oriented along the
[010] direction. The draw rate was 2 mm/day, the rotation speed was 30-40 rpm. A mechanical mixture of high purity grade
Li2CO3, MgO, MoO3 reagents was used as a load in amounts necessary to obtain the melt–solution with the 1:2 molar ratio of
2+
Li2–2xMg2+x(MoO4)3 to Li2MoO4. To grow a Cu doped Li2–2xMg2+x(MoO4)3 crystal, high purity grade CuO was added in the
load in the 1 at.% amount. The growth was carried out in a platinum crucible with a diameter of 70 mm and a height of
120 mm on a device with a weight control of the process (NKh-620N laboratory unit).
The electronic state and positions of copper ions in the structure of Li2–2xMg2+x(MoO4)3 crystals were studied by
EPR. The EPR spectra were recorded on a Varian E-109 spectrometer equipped with an analog-to-digital signal converter and
the original software for spectrum collection and processing, at temperatures of 77 K and 300 K. The absorbance spectra
were measured on the spectrometer in a range 200-800 nm. The luminescence spectra were obtained on a DFS-24
spectrometer.
RESULTS AND DISCUSSION
Undoped Li Mg (MoO ) crystals. The study of the absorption spectra of undoped Li2–2xMg2+x(MoO4)3 crystals
2–2x 2+x 4 3
(Fig. 2) and Li2–2xMg2+x(MoO4)3 crystals activated by copper ions (Fig. 3) shows that the crystals are transparent in a range up
to 300 nm. The investigation of the luminescence properties of Li2–2xMg2+x(MoO4)3 crystals reveals that under band-to-band
excitation (300 nm) at room temperature, a broad luminescence band with a maximum at the wavelength near 520 nm is
observed (Fig. 4a).
460

2+
Fig. 2. Li2–2xMg2+x(MoO4)3 crystal.
Fig. 3. Li2–2xMg2+x(MoO4)3:Cu
(1 at.%) crystal.
At a temperature of 77 K, the luminescence spectrum also represents a broad single line with the same maximum at
λmax = 520 nm, but as the temperature lowers from 300 K to 77 K the intensity of luminescence eight times increases (Fig. 4c).
2+
Li Mg (MoO ) :Cu . The EPR spectrum of the studied crystal doped with copper ions (Fig. 5) represents two
2–2x
2+x
4 3
line quartets with an anisotropic g factor. Line quartets in the EPR spectra are due to HFS from the atoms with a nuclear
moment I = 3/2 and a natural abundance of ∼100%. In the crystal under consideration, these impurity ions are copper ions
63
65
having two isotopes Cu and Cu with I = 3/2 and a natural abundance of 69.17% and 30.83% respectively. The HFS
constants for the isotopes have similar values and, with the EPR spectrum line width ΔH = 20 G, are not resolved in the EPR
spectra. An analysis of the angular dependence of the EPR spectrum shows that there are two magnetically non-equivalent
positions in the crystal structure. The simulation of the angular dependence of the spectrum by the software developed in [16]
is in good agreement with the experimental dependence at the spin-Hamiltonian parameters g|| = 2.398, g⊥ = 2.061,
9
A|| = 102 G, А⊥ = 6 G (Fig. 6). The copper ions have the 3d ground state with the electron spin S = 1/2, with the directions of
the principal g|| and A|| values coinciding and corresponding to the oxygen–oxygen direction in the oxygen octahedron of M3
magnesium ions, and their values are typical of the octahedral oxygen environment of copper ions. Based on the above, it can
be supposed that copper ions substitute for the magnesium ions in two possible structural positions (M3), for which the
principal values of the g factor g|| and the HFS tensor A|| lie in the a plane and make an angle of 30° with the c axis.
As it was noticed in [17], for structurally identical Li2–2xZn2+x(MoO4)3 crystals, an increase in the concentration of
impurity ions, in particular, copper ions, increased the luminescence intensity with a maximum at 560 nm. The doping of
Li2–2xMg2+x(MoO4)3 crystals with copper ions also increases the intensity of a broad luminescence band with a maximum at
Fig. 4. Luminescence spectrum of the
Li2–2xMg2+x(MoO4)3 crystal: undoped,
T = 300 K (a); undoped, T = 77 K (b);
Fig. 5. EPR spectrum of the Li2–2xMg2+x(MoO4)3:
2+
Cu crystal.
2+ 2+
Cu doped, T = 300 K (c); Cu doped,
T = 77 K (d).
461

Fig. 6. Angular dependence of the EPR spectra of
2+
Li2–2xMg2+x(MoO4)3:Cu at sample rotation about
the +c crystallographic axis. The experimental
dependence is denoted by dots, the modeling results
are denoted by solid lines.
520 nm (Fig. 4). The Introduction of copper ions into the structure of the Li2–2xMg2+x(MoO4)3 crystal increases the number of
cation vacancies to provide the charge compensation. It is supposed that the observed luminescence with a maximum at
520 nm is caused by cation vacancies whose number increases when the crystal is doped with copper ions, which, in turn,
increases the luminescence intensity.
CONCLUSIONS
Large undoped and copper-ion doped Li2–2xMg2+x(MoO4)3 crystals were grown by the low-gradient Čzochralski
method. The study allowed us to determine the charge state and structural position of impurity copper ions. It was found that
for an undoped Li2–2xMg2+x(MoO4)3 crystal, the luminescence with the maximum at a wavelength of 520 nm is observed
whose intensity increases with decreasing temperature. It was established that the doping of Li2–2xMg2+x(MoO4)3 crystals with
copper ions increases the luminescence intensity at 520 nm. It is supposed that cation vacancies providing charge
2+
compensation when Cu ions substitute for lithium and magnesium ions are responsible for the luminescence.
REFERENCES
1. G. Audi, A. H. Wapstra, and C. Thibault, Nucl. Phys., A729, 337-746 (2003).
2. E. V. D. van Loef, P. Dorenbos, C. van Eijk, et al., Appl. Phys. Lett., 79, No. 10, 1573-1575 (2001).
3. M. Kapusta, M. Balcerzyk, M. Moszynski, and J. Pawelke, Nucl. Instrum. Methods, А42, 610 (1999).
4. K. S. Shah, J. Glodo, M. Klugerman, et al., Nucl. Instrum. Methods, A505, 76 (2003).
5. R. Zhu, Proc. of the Int. Conf. on Inorganic Scintillators and Their Applications “SCINT 97”, Y. Zhiwen et al. (eds.),
Shanghai, China (1997), p. 73.
6. V. B. Mikhailik, H. Kraus, D. Wahl, and M. S. Mykhaylyk, Phys. Status Solidi, B242, 17 (2005).
7. L. Xue, Y. Wang, P. Lv, D. Chen, Z. Lin, J. Liang, F. Huang, and Z. Xie, Cryst. Growth Des., 9, 914 (2009).
8. S. F. Solodovnikov, Z. A. Solodovnikova, E. S. Zolotova, L. I. Yudanova, T. Yu. Kardash, A. A. Pavlyuk, and
V. A. Nadolinny, J. Solid State Chem., 182, 1935 (2009).
9. V. A. Nadolinny, A. A. Pavlyuk, S. F. Solodovnikov, Z. A. Solodovnikova, E. S. Zolotova, N. A. Nebogatikova,
V. F. Plyusnin, and A. A. Ryadun, J. Struct. Chem., 52, No. 4, 708 (2011).
10. V. A. Nadolinny, A. A. Pavlyuk, A. A. Ryadun, S. F. Solodovnikov, Z. A. Solodovnikova, E. S. Zolotova,
V. F. Plusnin, M. I. Rakhmanova, and E. G. Boguslavskii, Funct. Mater., 18, No. 3, 368 (2011).
462

11. A. A. Ryadun, V. A. Nadolinny, A. A. Pavlyuk, S. F. Solodovnikov, and E. G. Boguslavskii, J. Struct. Chem., 54,
No. 1, 59 (2013).
12. L. Sebastian, Y. Piffard, A. K. Shukla, F. Taulelle, and J. Gopalakrishnan, J. Mater. Chem., 13, 1797 (2003).
13. V. K. Trunov, Zh. Neorg. Khim., 16, No. 2, 553 (1971).
14. V. G. Penkova, P. V. Klevtsov, and V. K. Trunov, Zh. Neorg. Khim., 22, No. 6, 1713 (1977).
15. V. A. Trifonov, A. A. Pavlyuk, K. N. Gorbachenya, A. S. Yasyukevich, and N. V. Kuleshov, Inorg. Mater., 49, No. 5,
517 (2013).
16. N. V. Chernei and V. A. Nadolinny, Zavod. Lab., 72, No. 2, 20 (2006).
17. V. A. Nadolinny, N. V. Chernei, A. V. Sinitsyn, A. A. Pavlyuk, and S. F. Solodovnikov, J. Struct. Chem., 49, No. 5,
859 (2008).
463