Growth and spectroscopic properties of Ca9Nd(VO4)7 single crystal

Optical Materials 60 (2016) 387e393

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Growth and spectroscopic properties of Ca9Nd(VO4)7 single crystal M.P. Demesh a, *, A.S. Yasukevich a, N.V. Kuleshov a, M.B. Kosmyna b, P.V. Mateychenko b, B.P. Nazarenko b, A.N. Shekhovtsov b, A.A. Kornienko c, E.B. Dunina c, V.A. Orlovich d, I.A. Khodasevich d, W. Paszkowicz e, A. Behrooz e a

Center for Optical Materials and Technologies, Belarusian National Technical University, Nezalezhnasti Ave. 65, 220013, Minsk, Belarus Institute for Single Crystals, NAS of Ukraine, Lenin Ave. 60, 61001, Kharkov, Ukraine c Vitebsk State Technological University, Moskovskaya Ave. 72, 210035, Vitebsk, Belarus d B.I. Stepanov Institute of Physics, NAS of Belarus, Nezalezhnasti Ave. 68, 220072, Minsk, Belarus e Institute of Physics, Polish Academy of Sciences, Lotnikow Al. 32/46, PL-02668, Warsaw, Poland b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 May 2016 Received in revised form 3 August 2016 Accepted 22 August 2016

Ca9Nd(VO4)7 single crystal was grown by the Czochralski method in inert atmosphere. The crystal structure and chemical composition were studied. Polarized absorption and luminescence spectra were investigated in details. It was found that the Ca9Nd(VO4)7 crystals belongs to self-activated laser materials with a weak concentration quenching of luminescence. Judd-Ofelt analysis was performed. The emission cross-sections spectra for 4F3/2 / 4I9/2, 4I11/2, 4I13/2 transitions were determined. For the first time Raman spectra of the Ca9Nd(VO4)7 single crystal were recorded and interpreted. © 2016 Elsevier B.V. All rights reserved.

Keywords: Double vanadates Neodymium Crystal structure Absorption Luminescence Raman spectra

1. Introduction Self-activated laser crystals, especially Nd containing materials, draw a considerable attention for development of microchip lasers and potential applications in the field of integrated optics [1,2]. Generally, self-activated materials can store a considerable amount of energy per unit volume, which is important for Q-switched lasers and amplifiers. The main drawback of such materials is decreasing of the life-time of 4F3/2 level of Nd ions due to concentration quenching of luminescence. The cross-relaxation deactivation of 4F3/2 level (4F3/2þ4I9/2 / 2 4I15/2) is the main mechanism of strong luminescence self-quenching of Nd:YAG [3], Nd-doped glasses [4], Nd:LaF3 [5] and others. When resonant conditions for such energy transfer are not satisfied, phonon-assisted energy transfer may be realized [2]. In Ref. [2] a criterion was proposed for selection of Nd - doped materials with weak luminescence quenching, namely, DE1 < 470 cm1 (DE1 is the Stark splitting of the 4 I9/2 level). This paper is devoted to the investigation of spectroscopic * Corresponding author. E-mail address: [email protected] (M.P. Demesh). http://dx.doi.org/10.1016/j.optmat.2016.08.014 0925-3467/© 2016 Elsevier B.V. All rights reserved.

properties of a new laser application promising self-activated Ca9Nd(VO4)7 (CNVO) single crystal which is close to this criterion, DE1 z 485 cm1, and the quantum yield is of about 16%. Also this crystal belongs to noncentrosymmetrical binary orthovanadates Ca9RE(VO4)7 (the family of “whitlockite” minerals where RE ¼ rareearth elements or yttrium), which demonstrate the second harmonic generation more effectively as compared with KDP and Ca3(VO4)2 crystals [6]. Recently, authors [7e13] have grown a number of pure and Nd, Yb doped crystals of calcium vanadates from “whitlockite” family, and studied their spectral and other physical properties. Laser operation for Nd3þ:Ca9La(VO4)7 crystal was obtained at flashlamp pumping in free running mode [14].

2. Crystal growth and Ca9Nd(VO4)7 single crystals structure The dried initial components CaCО3 (99.99%), Nd2O3 (99.99%), and V2O5 (99.95%) were taken in the stoichiometric ratio. A solid state synthesis was carried out to produce Ca9Nd(VO4)7 compound according to the chemical reaction 18CaCO3 þ 7V2O5 þ Nd2O3 / 2Ca9Nd(VO4)7 þ 18CO2[.

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The crystal growth was carried out by the Czochralski method in argon or nitrogen atmosphere from Ir crucibles by means of “Kristall 3М” setup with inductive heating and an automated diameter control system of growing crystal along the crystallographic axis [001]. The pulling speed was changed within 1e5 mm/ h and the rotational speed was varied in the range of 5e25 rpm. When using both active and passive after-heating, the radial temperature gradient on the melt surface did not exceed 0.5  C/mm, the axial temperature gradient at the melt-argon interface was 75  C/cm. The crystal-melt interface was slightly convex. Under these conditions, Ca9Nd(VO4)7 single crystals were grown. The chemical composition of crystals was examined by using a scanning electron microscope JEOL JSM-820 equipped with energy dispersive X-ray analyzer. X-ray powder diffraction (XRD) patterns were recorded for 2q ranging from 5 to 159 using a laboratory X’PERT diffractometer (Panalytical) equipped with a Ge monochromator and a strip detector using Cu Ka radiation. The crystal structure was refined by the powder diffraction method. The XRD powder patterns were analyzed using FullProf software [15]. 3. Experimental details The absorption and luminescence spectra of the CNVO were recorded in polarized light at room temperature for different ori! entations of the electric field strength E with respect to the crys! ! tallographic c-axis ( E jjc (p-polarization) and E ⊥c (spolarization)). The absorption spectra in polarized light were recorded by a CARY 5000 spectrophotometer with spectral bandwidth (SBW) of 0.8 nm. Stationary luminescence spectra were recorded under a laser diode excitation at 802 nm. The luminescence emission was collected and focused on the input split of a monochromator MDR 23 (SBW 0.5 nm). The signal was detected with a photodetector on the base of Hamamatsu G5851 photodiode associated with a SR830 Standford lock-in amplifier. The decay lifetime of the upper manifold (4F3/2) was measured under pulsed excitation at 810 nm by means of an optical parametric oscillator LOTIS TII LT-2214, which was pumped by third harmonic of a flashlamp-pumped Q-switched Nd:YAG laser LOTIS TII LS-2137. Pump pulse duration was about 20 ns The emission was collected by wide aperture lens onto entrance slit of monochromator MDR-12 and detected by a fast Hamamatsu photodetector G5851 and oscilloscope with the bandwidth of 500 MHz. Raman spectra were measured using a laboratory Raman spectrometer based on a monochromator MS3504I (Solar TII). The monochromator was equipped with a liquid-nitrogen-cooled CCD detector Spec-10:256 (Roper Scientific). The SBW of the spectrometer was about 1.5 cm1. The same experimental conditions as well as identical scattering geometry of the 90 laser beam-tocollecting optics arrangement and a 442 nm notch-filter (Semrock) were used for measuring the polarized spectra with a dichroic polarizer (PF-40). Scattered light was excited by the CW vertically polarized radiation of a Stabilite 2017 Ar-Ion laser (Spectra-Physics) at the 488 nm wavelength. The laser power at the sample was about 10 mW. 4. Experimental results Ca9RE(VO4)7 (RE ¼ rare-earth elements and Y) binary orthovanadates form structures with mixed ionic-covalent bonding, belong to the type of the natural mineral «whitlockite» and are isostructural to calcium orthovanadate Ca3(VO4)2. The structure of Ca3(VO4)2 contains isolated (VO4)3- tetrahedrons (see Fig. 1). The calcium cations of this structure occupy 5 non-equivalent crystallographic positions M(1)-M(5). The coordination numbers are 7, 8, 8

and 8 for the M(1)-M(3), M(5) positions, respectively. The M(4) position is half - occupied. The formation of the compounds Ca9RE(VO4)7 is accompanied with heterovalent substitution of calcium by a rare-earth element according to the scheme: 3Ca2þ ¼ 2RE3þþ▫;, where ▫ is a vacancy. So, the position of Ca(4) in the regular structure of such compounds becomes completely vacant. In this structure a rare-earth cation may occupy 3 crystallographic positions, but the degree of their occupancy depends on the RE3þ ionic radius [16]. The CNVO crystallizes into the rhombohedral symmetry, R3c 6 ) space group. Such a space group implies that as an optical (C3v medium, the CNVO crystal is uniaxial one with the optical axis along the crystallographic c-axis. The local symmetry groups of possible sites which may be occupied by the Nd ions are as following: C1 and C3 [1]. The refined lattice parameters of the CNVO single crystal are as following: a ¼ 10.86693(4) Å, c ¼ 38.1327(2) Å and the cell volume V ¼ 3899.79(5) Å3. These values differ from the values determined for polycrystalline sample in reference [17]. The grown CNVO single crystals, like Ca9RE(VO4)7 (RE ¼ Y, La, Gd) single crystals, contained opaque regions distributed in whole crystal volume. It may caused by inclusions of several micrometer sizes with a distinct compositional contrast [14,18,19]. According to the element analysis data, these inclusions were enriched with a rare-earth element. Mechanism of inclusion formation is under discussion and can be connected either with redistribution of cations and cationic vacancies during high-temperature phase transition or minimization of internal stresses at heterovalent substitution of calcium by a rare earth cation [20]. It was shown in Ref. [18] that the concentration of inclusions increases with the rise of ionic radius difference of calcium and rare earth cations. The chemical composition of a grown CNVO crystal was examined along the growth direction. It was established that the vanadium and calcium concentrations are constant along the crystal length in the frames of measurement errors. At the same time we observed the neodymium deficit of ~20% at the top part of CNVO crystal (see Fig. 2). The specimens for investigation were cut from the region where the concentrations of Ca, V and Nd ions are close to the stoichiometric ratio (see Fig. 2a). It is important to emphasize that the powder pattern of CNVO shows that there are no impurity phases in the investigated specimens. The number density N0 of the Nd3þ ions (Nd3þ concentration) was calculated by using the experimentally determined volumetric density of CNVO (3,3 g/cm3) and the mass content of Nd ions (10%) and obtained to be 14$1020 cm3. The absorption cross section spectra for the CNVO crystal for p and s-polarizations in the spectral range of 350e950 nm are given in Fig. 3. These spectra were corrected for the Fresnel losses of the sample. The spectral lines interpretation in the absorption spectra was performed on the base of Ref [21]. The sufficient difference between s and p polarizations in the absorption spectra was not observed. In comparison with well structured and narrow absorption lines of Nd:YVO [22] the ones here are sufficiently broadened. We connect this with the structure disordering of neodymium ions environments in the crystal. The full width at half maximum (FWHM) for spectral band associated with 4I9/2 / 4F5/2þ2H9/2 (lpeak ¼ 810 nm) transition is 9.5 nm for both polarizations. Peak absorption coefficient at the wavelength of 810 nm for sepolarization reaches to 87 cm1 (the path length in the crystal for 80% absorption is about 0.18 mm), so the CNVO crystal can be a gain medium for the microchip lasers. The value of DE1 easily can be obtained by measuring the distance between the first and the last components of the band associated with 4I9/2 / 2P1/2 transition [2] which is located near 435 nm. For both polarizations DE1 485 cm1 (see the inset in Fig. 3). The Judd-Ofelt [23,24] theory was applied to determine

M.P. Demesh et al. / Optical Materials 60 (2016) 387e393

389

Fig. 1. The structure unit cell of Ca9Nd(VO4)7 crystal.

Fig. 2. The mass content of neodymium (a), vanadium (b) and calcium (c) along the crystal length. The solid line corresponds to the stoichiometric ratio. The marked area indicates the crystal region where the specimens were cut.

manifold-manifold radiative transition probabilities and branching ratios of 4F3/2 / 4IJ0 luminescence channels (J0 ¼ 9/2, 11/2, 13/2, 15/ 2). We used for the Judd-Ofelt calculations the formulae based on [25]. The calculated oscillator strengths of the electric-dipole transition can be expressed as

ed fcalc ðJJ 0 Þ

" 2 # n2 þ 2 ¼ 9n 3hlð2J þ 1Þ  D  E2 X  Ut  4f n ½SLJ U ðtÞ 4f n ½S0 L0 J 0  ; 8p2 mc

t¼2;4;6

Fig. 3. Polarized absorption cross-section spectra of Ca9Nd(VO4)7 at room temperature.

(2)

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  where hU t i are double reduced matrix elements of the unit tensor operator U(t) of rank t [21], and the Ut are empirical intensity parameters. The experimental oscillator strength is obtained according to following expression

Z

mc2

ed fexp ð JJ 0 Þ ¼

2 pe2 N0 l

kJJ0 ðlÞdl

(3)

h i. Here kJJ’ ðlÞ ¼ 2ksJJ’ ðlÞ þ kpJJ’ ðlÞ 3 is the polarization averaged absorption coefficient. The values of experimental and calculated ed and f ed and the root mean absorption oscillator strengths fexp calc square deviation are given in Table 1. Table 2 lists the Judd-Ofelt parameters for CNVO crystal obtained in this work and compares them with ones for other Nddoped calcium vanadates. The value of the intensity parameter U2 is sensitive to the local symmetry of the Nd-ion sites and to the covalency of the bonds between rare earth ions and their ligands [27]. We believe that in our case parameter U2 has a high value due to disordering structure surrounding the Nd-ion. In the series of the vanadates crystals (see Table 2) the U2 e parameter indicates increasing of local disordering of Nd3þ ions from YVO4 to Ca10Li(VO4)7 crystal. The electric-dipole spontaneous transition probability Aed(J’J) between the upper J and lower J0 states, the luminescence branching ratios bJ0 J and the radiative lifetime trad are given by the formulas

Aed ðJJ 0 Þ ¼

(5)

J

X

11 Aed ð J JÞA 0

:

(6)

J

The results of the Judd-Ofelt analysis are collected in Table 3. Measured luminescence spectra were corrected to the spectral sensitivity of luminescence set-up, and used to find the branching ratios for s and p polarizations luminescence emission. The experimental branching ratios for J'/J transition were found from

Table 1 The absorption oscillator strengths. Excited states

l, nm

ed  106 fexp

fcalc106

4

887 809 751 686 633 587 524 471 432

4.62 12.93 11.79 0.91 0.23 78.42 14.85 3.91 0.91

4.8 12.95 11.82 1.01 0.28 78.45 14.41 2.59 1.4

F3/2 F3/2 þ 2H29/2 S3/2 þ 4F7/2 4 F9/2 2 H211/2 4 G5/2 þ 2G27/2 2 K13/2 þ 4G7/2 þ 4G9/2 2 K15/2 þ 2G9/2 þ 2G11/2 þ 2D3/2 2 P1/2 RMS deviation 4 4

U2, 1020 cm2

U4, 1020 cm2

U6, 1020 cm2

Refs

Ca10Li(VO4)7 Ca10K(VO4)7 Ca9Nd(VO4)7 YVO

38.1 25.9 18.68 5.88

1.3 16.6 8.1 4.08

12.5 9.5 6.38 5.11

[11] This work [26]

Table 3 The calculated transition probabilities and branching ratios in CNVO. Transition

Wavelength, nm

AED, s1

bcalc

trad, ms

4

884 1067 1348 z1850a

3495 3198 583 30

0,478 0,438 0,08 0,004

137

F3/2 F3/2 F3/2 4 F3/2 4 4

/ / / /

4

I9/2 I11/2 I13/2 4 I15/2 4 4

a The wavelength of 4F3/2 / 4I15/2 transition is evaluated on the data from Ref. [21].

Z

lIJ0 J ðlÞdl bJ 0 J ¼ P Z lIJ 0 J ðlÞdl J

(7)

where IJ’J (l) is the polarization averaged spectral density of luminescence power in arbitrary units. The experimental averaged values of bJ0 J over p and s polarizations with respect to calculated

 2  D  E X n n2 þ 2 Ut  4f n ½SL J U ðtÞ 4f n ½S0 L0  J 0 ; 3 9 3hð2J þ 1Þl t¼2;4;4

A ðJ 0 JÞ ; Aed ðJ 0 JÞ

trad ¼ @

Crystals

64p4 e2

bJ 0 J ¼ P ed

0

Table 2 Judd-Ofelt intensity parameters for Nd3þ-doped crystals.

0.37$106

(4)

ones are presented in Table 4. The calculated and experimentally obtained values are in a reasonable agreement. The luminescence spectra were used also for calculation the emission cross-sections spectra (EMCS) by the FüchtbauereLadenburg equation (see Fig. 4b, c) [28]:

saem ðlÞ ¼ b

l5

8pcn2 trad

Z h

3IJa0 J ðlÞ

; i 2IJs0 J ðlÞ þ IJp0 J ðlÞ ldl

(8)

a ðlÞ is the spectral density of luminescence power (a ¼ s, p) where IJ0J for J'/J transition, trad is the radiative lifetime, b is the branching ratio. The emission cross-sections spectra are shown in Fig. 4, they are characterized by relatively broad and smooth bands. Luminescence radiation of the transition 4F3/2 / 4I9/2 reabsorbs due to thermally populated sublevels of terminated 4I9/2 manifold. This potentially results in distortion of luminescence spectra and, consequently, of stimulated emission spectra calculated by the FüchtbauereLadenburg equation. This is especially true for crystals

Table 4 The calculated and experimentally obtained values of branching ratios for transitions from 4F3/2 level. Transition

Wavelength range, nm

bcalc

bmeas

4

850e1000 1020e1150 1280e1460

0,478 0,438 0,08

0463 0484 0,053

4

F3/2 / I9/2 F3/2 / 4I11/2 F3/2 / 4I13/2

4 4

M.P. Demesh et al. / Optical Materials 60 (2016) 387e393

391

Fig. 4. Polarized emission cross section spectra of Ca9Nd(VO4)7 at room temperature.

with high concentration of active centers. The stimulated emission cross-section spectra were calculated for 4F3/2 / 4I9/2 transition by the integral reciprocity method [29], which is free of reabsorption effect (see Fig. 4a):

saem ðlÞ ¼

8pcn2 trad

P i

Table 5 Emission cross sections of CNVO. Transition

lpeak, nm

20 speak cm2 em , 10

FWHM, nm

4

884 1067 1348

4.5 (s), 1.9 (p) 6.5 (s), 3.5 (p) 0.54 (s), 0.77 (p)

42 (s), 41 (p) 18 (s), 30 (p), 41 (s), 50 (p)

F3/2 / 4I9/2 4 F3/2 / 4I11/2 4 F3/2 / 4I13/2

3 expð  hc=kT lÞ Z saabs ðlÞ: l4 siabs ðlÞexpð  hc=kT lÞdl (9)

From the gain coefficient spectra (see Fig. 5) calculated as g a ðlÞ ¼ N0 ½bðsaem ðlÞ þ saabs ðlÞÞ  saabs ðlÞ, (the inversion parameter b ¼ Nex /N0 shows the ratio of the volumetric density of excited Nd ions Nex to the total Nd ions concentration N0), one can see that even for low inversion parameter (b ¼ 0.2), it is possible to reach laser oscillation within the range from 895 to about 935 nm. peak The wavelengths lpeak of peak emission cross section bands sem and their FWHM are listed in Table 5. For the common used laser channel of Nd3þ ions (4F3/2 / 4I11/2) all lpeak are located at 1067 nm for both polarizations and the largest stimulated emission cross section at this wavelength is about of 6.5$1020 cm2 for s e polarization. Luminescence lifetime measurement was performed for the emission channel 4F3/2 / 4I11/2 at 1067 nm. Luminescence decay curve is shown in Fig. 6. It revealed non exponential character of decay. We believe that this is due to energy transfer effects in the CNVO crystal. Mean luminescence lifetime tmeas was estimated to be 22 ms by

Fig. 5. Gain coefficient curves for 4F3/2 / 4I9/2 transition.

Fig. 6. Luminescence decay curve of Ca9Nd(VO4)7 crystal at 1067 nm.

Z

tmeas ¼ Z

tIðlÞdl IðlÞdl

:

(10)

The calculated radiative and experimentally determined lifetimes make it possible to estimate the quantum yield of luminescence which is about 16%. In Table 6 some self-activated Nd3þ crystals with weak concentration quenching are presented. As mentioned above the calcium orthovanadates are efficient crystals for the second harmonic conversion. Also the stimulated Raman frequency conversion of picosecond pulsed laser radiation has been performed by the Ca3(VO4)2 crystal [34]. Therefore the CNVO crystal can be of interest for researchers in the field of nonlinear optics. To our knowledge the Raman spectra of CNVO crystal were not investigated earlier. The geometry of pump and Raman scattered light beams is described by means of four symbols according to [35], namely, x(yz)y. The first and last symbols indicate directions along with light beams of pump and scattered light propagate. The second and third symbols indicate polarization orientations of pump and scattered light respectively. The polarized Raman spectra are shown in Fig. 7. The Raman

392

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Table 6 Some luminescence properties of self-activated Nd3þ crystals. Crystal

Nd3þ ion concentration, 1020 cm3

sem, 1020 cm2

trad, ms

tmeas, ms

Quantum yield

Reference

Na5Nd(WO4)2 K5Nd(MoO4)4 NdP5O14 KNdP4O12 Ca9Nd(VO4)7

26 23 39 41 14

e 7 18 15 6

220 215 300 275 137

80 70 110 10 22

36 33 37 36 16

[30] [31] [32] [33] This paper

and to the broadening of the Raman bands with the bandwidths of about 9e37 cm1. Nevertheless the broad linewidth of an intensive spontaneous Raman band is suitable for Raman conversion of picosecond pulsed laser radiation. The most intensive Raman lines located at 357 cm1 and 865 cm1 have the widths of about 19 and 18 cm1, respectively. Thus the CNVO crystal can be attractive for Raman conversion of picosecond pulsed laser radiation polarized parallel to the a or c axes. Moreover the pumping of CNVO crystal by p-polarized laser radiation allows to reduce the nonradiative losses (see, for example, Raman spectrum for c(ps)a excitation configuration). 5. Conclusions CNVO single crystal was grown by the Czochralski method, its structure and chemical composition were studied and analyzed. Spectroscopic investigations of CNVO crystal were carried out at Fig. 7. Polarized Raman spectra of Ca9Nd(VO4)7 crystal.

bands of the CNVO crystal are distributed in two region of wavenumbers 750e950 and 200e450 cm1 as in Raman spectra of the Ca3(VO4)2 [36]. The intensity distribution of the Raman bands depends essentially on the direction of laser radiation polarization and the configuration of crystal excitation. The most intense Raman lines are located at 357 cm1 (c(ps)a configuration), 865 cm1 (a(ps)c configuration). Two lines with approximately equal intensity are located at 356 cm1 and 865 cm1 for c(pp)a configuration. The deconvolution of the Raman spectra of CNVO crystal was performed to find the individual Stokes modes. For a narrow exciting laser line and homogeneous broadening of Raman bands the Lorentzian line shape may be expected [37]. The using a multiple Lorentzian function was not satisfactory in our case. The widely used approach with multiple Gaussian functions [38] was used later for the following reasons. An overlap of many modes can result from a disordering of the nonequivalent oxygen atoms with different local coordination spheres [39]. Also the disordering can lead to broadening of the Raman bands observed at room temperature. The Raman shifts n and linewidths Dn of the 34 vibrational modes were found in the polarized Raman spectra for three exciting-recording configurations. They are listed in Table 7. The Raman bands in the low-frequency region of 200e450 cm1 are principally assigned to the OeVeO bending modes and also to the Сa2þ cations displacements. These displacements perpendicular to the c axis for Ca3(VO4)2 crystals correspond to the Stokes modes of about 294, 220 cm1 [40]. They can refer to the Raman lines of about 304 and 213 cm1 in our case. The Raman bands in the highfrequency region of 750e950 cm1 are attributed to the VeO stretching modes. But the lines in the region of 880e910 cm1 can be attributed to vibrations of V(IV)eOe … Nd(III) chain. The chain appearance is due to Nd3þ ions implantation [41]. As a result, the VO4 tetrahedra are distorted. The structure disordering of the CNVO crystal leads to the overlap of many of the predicted Raman modes

Table 7 Stokes modes observed in polarized Raman spectra of Ca9Nd(VO4)7 crystal (cm1). Ca9Nd(VO4)7 c(ps)a

Assignment a(ps)c

c(pp)a

n

Dn

n

Dn

n

Dn

920vw 897vw

21 21

863w

32

920w 898w 881m 865vs 850s

21 25 17 18 17

919w 896w 879m 865s 850w

24 19 18 18 19

835w

22 830s

34

828s

30

795w

35

796m

33

761w

26

763w

30

454vw

20

447w

20 441w

22

434vw 424w

14 12 418w

37

412w

14

394w

17 387w

25

Other lattice vibrations

371s 356vs

18 17

d(E)

340vs

18

325m 313w

16 13

236vw

21

817vw

21

790w 760w 738vw

32 23 18

433w

409m

19

n(A1) V(IV)eOe … Nd(III) n(A1) n(B2)

d(B2)

28

357vs

19

338vs 333w 323vs

19 19 19

304w 252vw

20 18

213vw

18

379w 369m 356m 347w 339m

15 14 17 12 17

324w 311vw

13 9

R(E) T Ca2þ R(E) T Ca2þ

Vibrational modes: n, stretching; d, bending; T, translational; R, rotational. Abbreviations: vs, very strong; s, strong; m, middle; w, weak; vw, very weak.

M.P. Demesh et al. / Optical Materials 60 (2016) 387e393

room temperature. Polarized absorption spectra in the range 350e950 nm were studied in details. It was determined that this crystal belongs to self-activated laser materials with weak concentration quenching of luminescence. The FWHM of the 4I13/ 4 2 2 / F5/2þ H9/2 transition (lpeak ¼ 810 nm) is z 9.5 nm for p and s polarizations. The Judd-Ofelt analysis was performed and used to determine intensity parameters, transition probabilities, branching ratios and the radiative lifetime of the upper laser level 4F3/2. Calculated and experimentally obtained branching ratios are in a good agreement. The emission cross section spectra for 4F3/2 / 4I9/ 4 4 4 4 2, I11/2, I13/2 transitions were calculated. For the F3/2 / I11/2 transition the maximal value speak is located at 1067 nm and equals em to 6.5 1020 cm2 (s polarization) along with the FWHM of 30 nm (p polarization). The quantum yield of luminescence from the 4F3/2 level was obtained to be z 16%. According to our best knowledge the Raman spectra of CNVO single crystal were recorded and analyzed with a view to be a promising Raman medium for the first time. The most intense Raman lines are located at 357 cm1 (c(ps)a excitation configuration), 865 cm1 (a(ps)c). Two lines with approximately equal intensity are located at 356 cm1 and 865 cm1 for c(pp)a configuration. The structure disordering of the CNVO crystal leads to the broadening of the Raman bands with the FWHM of about 9e37 cm1. References [1] A.A. Kaminskii, Laser Crystals. Their Physics and Properties, second ed., Springer-Verlag, Berlin, 1990. [2] F. Auzel, Materials for ionic solid state lasers, in: B. Di Bartolo, G. Armagan (Eds.), Spectroscopy of Solid-state Laser-type Materials, Springer US, New York, 1987, pp. 293e341. €tte, P. Balmer, Appl. Phys. 1 (1973) 269e274. [3] H.G. Danielmeyer, M. Bla [4] J. Chrysochoos, J. Chem. Phys. 61 (1974) 4596e4599. [5] C.K. Asawa, M. Robinson, Phys. Rev. 141 (1966) 251e258. [6] J.S.O. Evans, J. Huang, A.W. Sleight, J. Solid State Chem. 157 (2001) 255e260. [7] X. Hu, X. Chen, N. Zhuang, R. Wang, J. Chen, J. Cryst. Growth 310 (2008) 5423e5427. [8] L. Li, G. Wang, Y. Huang, L. Zhang, Z. Lin, G. Wang, J. Cryst. Growth 314 (2011) 331e335. [9] M.B. Kosmyna, P.V. Mateychenko, B.P. Nazarenko, V.M. Puzikov, A.N. Shekhovtsov, W. Paszkowich, O. Ermakova, A.S. Yasukevich, N.V. Kuleshov, V.E. Kisel, A.E. Gulevich, M.P. Demesh, in: Proceeding of International Conference on Oxide Materials for Electronic Engineering (ОМЕE2012), Lviv, Ukraine, 2012, p. 19. [10] M.B. Kosmyna, B.P. Nazarenko, V.M. Puzikov, A.N. Shekhovtsov, Acta Phys. Pol. A 124 (2013) 305e313.

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