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Luminescence and nonlinear optical properties of bariumfluoromolybdate (Ba2MoO3F4)
~)
Solid State Communications,Vol. 86, No. 4, PP. 239-241,1993. Printed in Great Britain.
0038-1098/93 $6.00+.00 Pergamon Press Ltd
LUMINESCENCE AND NONLINEAR OPTICAL PROPERTIES OF B A R I U M F L U O R O M O L Y B D A T E (Ba2MoO3F4)
Debye Research
M. Wiegel and G. Blasse Institute, University Utrecht, POB80.O00, (Received 1 December
3508 TA Utrecht,
The Netherlands.
1992 by D. Van Dyck)
The luminescence and nonlinear optical properties of Ba2MoO3F 4 are reported and discussed. The fluoromolybdate luminesces at low temperatures only. The luminescence properties are very similar to those of molybdate groups in the MgWO 4 lattice, ga2MoO3F 4 has an effective nonlinear optical tensor coefficient which is eight times larger than that of ~-quartz.
i. INTRODUCTION
2.2. Instrumentation.
The fluorotungstate Ba2WO3F 4 (space group: la) [1,2] contains chains of distorted WO4F 2 octahedra with two F- and two 02. ions, both in cis position, which coordinate to one W 6+ ion only. The individual octahedra share oxygen ions, ~iving a zig-zag chain with an O-W-O angle of 145 v [1,2]. Torardi and Brixner [2] reported that Ba2WO3F 4 has an effective tensor coefficient, which is five times larger than that of ~-quartz. Moreover, this compound shows an efficient luminescence at 500 nm under IF4 and X-ray excitation at room temperature and below [2,3]. The fluoromolybdate Ba2MoO3F 4 is isostructural with Ba2WO3F 4 [i]. Going from the tungstate to the molybdate, the nonlinear optical properties as well as the luminescence properties are expected to change. Molybdateabsorption and emission bands are at longer wavelengths than those of tungstates. As a result the luminescence efficiency and the quenching temperature will be lower for molybdates than for tungstates [4]. A shift of the first absorption band will also effect the nonlinear optical properties. According to Lines [5], the nonlinear optical respons will be largest in materials with a relatively small optical bandgap. Going from the fluorotungstate to the fluoromolybdate, the optical bandgap is expected to decrease and hence the nonlinear optical response is expected to increase. In this work the luminescence and nonlinear optical properties of bariumfluoromolybdate are investigated and discussed with respect to those of related tungstates and molybdates.
For the measurement of the effective d-tensor coefficient of polycrystalline BazMoO3F 4 (d.z~) a Philips Second Harmonic Analyser (SHA) [6] was used. The fluoromolybdate sample was measured versus a potassium lithium niobate standard [7]. The luminescence and decay measurements were performed as described elsewhere [4], using a Spex fluorolog spectrometer equipped with an Oxford liquid helium cryostat, and a Molectron UV-14 N2-1aser.
3. RESULTS AND DISCUSSION.
3.1. Luminescence
properties.
At room temperature Ba2MoO3F 4 does not show any luminescence. The diffuse reflection spectrum (see Figure i.) shows an optical
,"~.....................................
....J \ .
2. EXPERIMENTAL 4O0
500
6O0
70O
--~avelength (nmI- ~
2.1. Preparation Samples of Ba2MoO3F 4 were prepared by milling stoichiometric quantities of MoO 3 (Philips) and BaF 2 (Merck, Optipur). The mixtures were fired for 4 days at 600 °C in a sealed platinum tube. The samples were checked by X-ray powder diffraction and found to be single phase.
Figure I: Emission and excitation spectra of the luminescence of BazMoO3F 4 at 4.2 K. • gives the spectral radiant power per constant wavelength interval in arbitrary units, and qr gives the relative quantum output. The broken line represents the diffuse reflection spectrum at room temperature.
239
240
B ARIUMFLUOROMOLYBDATE (Ba2MoO3F4)
absorption edge at about 340 nm. At 4.2 K a broad emission band with a maximum at 655 nm is observed (see Figure i.). The quantum efficiency of the luminescence is approximately 80 %. The maximum of the first excitation band is at - 340 nm (see Figure i.). The Stokes shift is estimated to be 15,000 cm -I. At 130 K the emission intensity has dropped to 50 % of its value at 4.2 K, and at - 200 K it has practically disappeared. The luminescence properties of Ba2WO3F 4 are very similar to those of MgW04, which has a structural analogy with bariumfluorotungstate [3]. The luminescence of both tungstates is ascribed to the WOz(cis)-grou p . Since the molybdate MgMoO 4 is not isostructural with MgWO4, the luminescence properties of Ba2MoO3F 4 are compared to those of molybdate groups in the MgWO 4 lattice [8]. The positions of the first absorption band and the emission band are found to be very similar. More recently the luminescence properties of the MoO2(cis)-grou p on a silica surface and in K2MoO2F.H20 have been studied by Hazenkamp et al. [9,10]. The luminescence properties are comparable to those of BazMoO3F 4. We assume, therefore, that the emitting state of the fluoromolybdate is located in the MoO2(cis)-grou p . Charge transfer transitions from F- to Mo e+ are expected at much • 2higher energy than those from O [3]. Whereas the luminescence of bariumfluorotungstate is still efficient at room temperature [2,3], the luminescence of bariumfluoromolybdate is already quenched at low
Vol. 86, No. 4
temperatures. Furthermore the emission band of the molybdate compound is shifted t.o the red. The lower quenching temperature and quantum efficiency of bariumfluoromolybdate can. be related to its lower energy levels [4]. At 4.2 K the decay curve is, apart from a nonexponential part immediatly after the pulse, exponential, yielding a lifetime of 2 ms. Since the quantum efficiency is - 80 % at 4.2 K, the radiative decay time is probably slightly longer. The decay time of the emission of the fluoromolybdate is longer than that of the fluorotungstate emission ( - 2 ms versus 220 ~s at 4.2 K). This can be ascribed to the weaker spin-orbit coupling [Ii]. The nonexponential part of the decay curve decreases if the temperature is raised. The rather complicated splitting of the emitting triplet state [12] may account for this behaviour. Other explanations, for example, emission from the singlet state as well as the triplet state(s) due to a relatively slow filling of the latter, cannot be ruled out either. The temperature dependence of the decay time of the luminescence of bariumfluoromolybdate is shown in Figure 2. It resembles that of Ba2WO3F 4. Such behaviour has been observed also for other molybdates with MoO2(cis)-groups [9,10]. Moreover the decay times have about the same order of magnitude (some ms) [9,10]. In view of the results mentioned above and of the other luminescence data, the luminescence of bariumfluoromolybdate is ascribed to the MoO2(cis)-grou p. 3.2. Nonlinear optical properties.
2200
Ba2MoO3F 4 has an effective nonlinear optical tensor coefficient, which is 20 % of that of a potassium lithium niobate standard [7]. This is eight times larger than that of ~-quartz and thus larger than the signal measured for Ba2WO3F 4 [2], which can be explained by the fact, that the optical bandgap of BazMoO3F 4 is smaller (- 3.5 eV) than that of Ba2WO3F 4 (- 4.0 eV). Since the dominating (phase-matchable) tensor coefficient d33 of the potassium lithium niobate powder standard is (27 ± 4) pm/V [13], td.f~l of Ba2MoO3F 4 is estimated to be 5 pm/V. With the help of a geometric bond parameter model [14], which calculates bulk tensor coefficients by means of geometric factors and microscopic nonlinear bond polarizabilities, some nonlinear optical tensor coefficients of BazMoO3F 4 were calculated. The following estimated average values of the nonlinear bond
1800
I
1400
Q
lO00 0 0
600
polarizabilities 200 I
20
I
o
i
40 Temperature
?
60 (K)
)
Figure 2: The temperature dependence of the decay time of the luminescence of BazMoO3F 4 (~ext~ 337 nm and ~.m= 655 run).
E II and fll of
the Mo-O bonds
were used: (72.0 x 10 -28) and (12.0 × 10 -28) m 3 (relative to d36 of KH2PO4), respectively [15]. The contributions of the ionic Mo-F bonds to the nonlinear optical effect were neglected, since only more covalent bonds contribute to the nonlinear optical effect [13,14]. In this way we obtained values for d31 and d33 of; (-3.4 ± 50%) and (-2.2 i 50%) pm/V, respectively. These values agree with the estimated Ideffl value of 5 pm/V. Furthermore the calculations show that the major contributions to the d-coefficients (> 70%) come from the Mo-O bonds of the MoO2(cis )
Vol. 86, No. 4
BARIUMFLUOROMOLYBDATE (Ba2MoO3F4)
groups. This agrees nicely with the luminescence properties, of Ba2MoO3F 4, since the luminescence is probably also mainly due to the MoO2~cis)-group. In conclusion BazMoO3F 4 shows interesting
nonlinear optical properties, only a t low temperatures.
241 but
luminesces
ACKNOWLEDGEMENT / This work was supported Philips Research Laboratories, Eindhoven.
by
REFERENCES
I. G. Wingefeld and R. Hoppe, Z. anorg, allg. Chem., 518, 149 (1984). 2. C.C. Torardi and L.H. Brixner, Mat. Res. Bul., 20, 137 (1985). 3. G. Blasse, H.C.G. Verhaar, M.J.J. Lammers, G. Wingefeld, R. Hoppe, P. De Maayer, J. Lumin., 29, 497 (1984). 4. M.Wlegel and G. Blasse, J. Sol. State Chem., 99, 388 (1992). 5. M.E. Lines, J. Appl. Phys., 69, 6876 (1991). 6. J.P. Dougherty and S.K. Kurtz, J. Appl. Cryst., 9, 145 (1976). 7. M. Ouwerkerk, Adv. Materials, 3, 399 (1992). 8. F.A. Kr6ger, Phillps Res. Rep., 2, 177
(1948). 9. M.F. Hazenkamp and G. Blasse, Ber. Bunsengesell. Phys. Chem., in press. i0. M.F. Hazenkamp, E.H. Voogt and G. Blasse, J. Sol. State Chem., i01, 26 (1992). ii. G. Blasse, Struct. Bonding, 42, i (1980). 12. W. Barendswaard and J.H. van der Waals, Mol. Phys., 59, 337 (1986). 13. C.R. Jeggo and G.D. Boyd, J. AppI. Phys., 41, 2741 (1970). 14. J.G. Bergman and G.R. Crane, J. Solid State Chem., 12, 172 (1975). 15. C.R. Jeggo, J. Phys. C: Solid State Phys., 5, L133 (1972).
Solid State Communications,Vol. 86, No. 4, PP. 239-241,1993. Printed in Great Britain.
0038-1098/93 $6.00+.00 Pergamon Press Ltd
LUMINESCENCE AND NONLINEAR OPTICAL PROPERTIES OF B A R I U M F L U O R O M O L Y B D A T E (Ba2MoO3F4)
Debye Research
M. Wiegel and G. Blasse Institute, University Utrecht, POB80.O00, (Received 1 December
3508 TA Utrecht,
The Netherlands.
1992 by D. Van Dyck)
The luminescence and nonlinear optical properties of Ba2MoO3F 4 are reported and discussed. The fluoromolybdate luminesces at low temperatures only. The luminescence properties are very similar to those of molybdate groups in the MgWO 4 lattice, ga2MoO3F 4 has an effective nonlinear optical tensor coefficient which is eight times larger than that of ~-quartz.
i. INTRODUCTION
2.2. Instrumentation.
The fluorotungstate Ba2WO3F 4 (space group: la) [1,2] contains chains of distorted WO4F 2 octahedra with two F- and two 02. ions, both in cis position, which coordinate to one W 6+ ion only. The individual octahedra share oxygen ions, ~iving a zig-zag chain with an O-W-O angle of 145 v [1,2]. Torardi and Brixner [2] reported that Ba2WO3F 4 has an effective tensor coefficient, which is five times larger than that of ~-quartz. Moreover, this compound shows an efficient luminescence at 500 nm under IF4 and X-ray excitation at room temperature and below [2,3]. The fluoromolybdate Ba2MoO3F 4 is isostructural with Ba2WO3F 4 [i]. Going from the tungstate to the molybdate, the nonlinear optical properties as well as the luminescence properties are expected to change. Molybdateabsorption and emission bands are at longer wavelengths than those of tungstates. As a result the luminescence efficiency and the quenching temperature will be lower for molybdates than for tungstates [4]. A shift of the first absorption band will also effect the nonlinear optical properties. According to Lines [5], the nonlinear optical respons will be largest in materials with a relatively small optical bandgap. Going from the fluorotungstate to the fluoromolybdate, the optical bandgap is expected to decrease and hence the nonlinear optical response is expected to increase. In this work the luminescence and nonlinear optical properties of bariumfluoromolybdate are investigated and discussed with respect to those of related tungstates and molybdates.
For the measurement of the effective d-tensor coefficient of polycrystalline BazMoO3F 4 (d.z~) a Philips Second Harmonic Analyser (SHA) [6] was used. The fluoromolybdate sample was measured versus a potassium lithium niobate standard [7]. The luminescence and decay measurements were performed as described elsewhere [4], using a Spex fluorolog spectrometer equipped with an Oxford liquid helium cryostat, and a Molectron UV-14 N2-1aser.
3. RESULTS AND DISCUSSION.
3.1. Luminescence
properties.
At room temperature Ba2MoO3F 4 does not show any luminescence. The diffuse reflection spectrum (see Figure i.) shows an optical
,"~.....................................
....J \ .
2. EXPERIMENTAL 4O0
500
6O0
70O
--~avelength (nmI- ~
2.1. Preparation Samples of Ba2MoO3F 4 were prepared by milling stoichiometric quantities of MoO 3 (Philips) and BaF 2 (Merck, Optipur). The mixtures were fired for 4 days at 600 °C in a sealed platinum tube. The samples were checked by X-ray powder diffraction and found to be single phase.
Figure I: Emission and excitation spectra of the luminescence of BazMoO3F 4 at 4.2 K. • gives the spectral radiant power per constant wavelength interval in arbitrary units, and qr gives the relative quantum output. The broken line represents the diffuse reflection spectrum at room temperature.
239
240
B ARIUMFLUOROMOLYBDATE (Ba2MoO3F4)
absorption edge at about 340 nm. At 4.2 K a broad emission band with a maximum at 655 nm is observed (see Figure i.). The quantum efficiency of the luminescence is approximately 80 %. The maximum of the first excitation band is at - 340 nm (see Figure i.). The Stokes shift is estimated to be 15,000 cm -I. At 130 K the emission intensity has dropped to 50 % of its value at 4.2 K, and at - 200 K it has practically disappeared. The luminescence properties of Ba2WO3F 4 are very similar to those of MgW04, which has a structural analogy with bariumfluorotungstate [3]. The luminescence of both tungstates is ascribed to the WOz(cis)-grou p . Since the molybdate MgMoO 4 is not isostructural with MgWO4, the luminescence properties of Ba2MoO3F 4 are compared to those of molybdate groups in the MgWO 4 lattice [8]. The positions of the first absorption band and the emission band are found to be very similar. More recently the luminescence properties of the MoO2(cis)-grou p on a silica surface and in K2MoO2F.H20 have been studied by Hazenkamp et al. [9,10]. The luminescence properties are comparable to those of BazMoO3F 4. We assume, therefore, that the emitting state of the fluoromolybdate is located in the MoO2(cis)-grou p . Charge transfer transitions from F- to Mo e+ are expected at much • 2higher energy than those from O [3]. Whereas the luminescence of bariumfluorotungstate is still efficient at room temperature [2,3], the luminescence of bariumfluoromolybdate is already quenched at low
Vol. 86, No. 4
temperatures. Furthermore the emission band of the molybdate compound is shifted t.o the red. The lower quenching temperature and quantum efficiency of bariumfluoromolybdate can. be related to its lower energy levels [4]. At 4.2 K the decay curve is, apart from a nonexponential part immediatly after the pulse, exponential, yielding a lifetime of 2 ms. Since the quantum efficiency is - 80 % at 4.2 K, the radiative decay time is probably slightly longer. The decay time of the emission of the fluoromolybdate is longer than that of the fluorotungstate emission ( - 2 ms versus 220 ~s at 4.2 K). This can be ascribed to the weaker spin-orbit coupling [Ii]. The nonexponential part of the decay curve decreases if the temperature is raised. The rather complicated splitting of the emitting triplet state [12] may account for this behaviour. Other explanations, for example, emission from the singlet state as well as the triplet state(s) due to a relatively slow filling of the latter, cannot be ruled out either. The temperature dependence of the decay time of the luminescence of bariumfluoromolybdate is shown in Figure 2. It resembles that of Ba2WO3F 4. Such behaviour has been observed also for other molybdates with MoO2(cis)-groups [9,10]. Moreover the decay times have about the same order of magnitude (some ms) [9,10]. In view of the results mentioned above and of the other luminescence data, the luminescence of bariumfluoromolybdate is ascribed to the MoO2(cis)-grou p. 3.2. Nonlinear optical properties.
2200
Ba2MoO3F 4 has an effective nonlinear optical tensor coefficient, which is 20 % of that of a potassium lithium niobate standard [7]. This is eight times larger than that of ~-quartz and thus larger than the signal measured for Ba2WO3F 4 [2], which can be explained by the fact, that the optical bandgap of BazMoO3F 4 is smaller (- 3.5 eV) than that of Ba2WO3F 4 (- 4.0 eV). Since the dominating (phase-matchable) tensor coefficient d33 of the potassium lithium niobate powder standard is (27 ± 4) pm/V [13], td.f~l of Ba2MoO3F 4 is estimated to be 5 pm/V. With the help of a geometric bond parameter model [14], which calculates bulk tensor coefficients by means of geometric factors and microscopic nonlinear bond polarizabilities, some nonlinear optical tensor coefficients of BazMoO3F 4 were calculated. The following estimated average values of the nonlinear bond
1800
I
1400
Q
lO00 0 0
600
polarizabilities 200 I
20
I
o
i
40 Temperature
?
60 (K)
)
Figure 2: The temperature dependence of the decay time of the luminescence of BazMoO3F 4 (~ext~ 337 nm and ~.m= 655 run).
E II and fll of
the Mo-O bonds
were used: (72.0 x 10 -28) and (12.0 × 10 -28) m 3 (relative to d36 of KH2PO4), respectively [15]. The contributions of the ionic Mo-F bonds to the nonlinear optical effect were neglected, since only more covalent bonds contribute to the nonlinear optical effect [13,14]. In this way we obtained values for d31 and d33 of; (-3.4 ± 50%) and (-2.2 i 50%) pm/V, respectively. These values agree with the estimated Ideffl value of 5 pm/V. Furthermore the calculations show that the major contributions to the d-coefficients (> 70%) come from the Mo-O bonds of the MoO2(cis )
Vol. 86, No. 4
BARIUMFLUOROMOLYBDATE (Ba2MoO3F4)
groups. This agrees nicely with the luminescence properties, of Ba2MoO3F 4, since the luminescence is probably also mainly due to the MoO2~cis)-group. In conclusion BazMoO3F 4 shows interesting
nonlinear optical properties, only a t low temperatures.
241 but
luminesces
ACKNOWLEDGEMENT / This work was supported Philips Research Laboratories, Eindhoven.
by
REFERENCES
I. G. Wingefeld and R. Hoppe, Z. anorg, allg. Chem., 518, 149 (1984). 2. C.C. Torardi and L.H. Brixner, Mat. Res. Bul., 20, 137 (1985). 3. G. Blasse, H.C.G. Verhaar, M.J.J. Lammers, G. Wingefeld, R. Hoppe, P. De Maayer, J. Lumin., 29, 497 (1984). 4. M.Wlegel and G. Blasse, J. Sol. State Chem., 99, 388 (1992). 5. M.E. Lines, J. Appl. Phys., 69, 6876 (1991). 6. J.P. Dougherty and S.K. Kurtz, J. Appl. Cryst., 9, 145 (1976). 7. M. Ouwerkerk, Adv. Materials, 3, 399 (1992). 8. F.A. Kr6ger, Phillps Res. Rep., 2, 177
(1948). 9. M.F. Hazenkamp and G. Blasse, Ber. Bunsengesell. Phys. Chem., in press. i0. M.F. Hazenkamp, E.H. Voogt and G. Blasse, J. Sol. State Chem., i01, 26 (1992). ii. G. Blasse, Struct. Bonding, 42, i (1980). 12. W. Barendswaard and J.H. van der Waals, Mol. Phys., 59, 337 (1986). 13. C.R. Jeggo and G.D. Boyd, J. AppI. Phys., 41, 2741 (1970). 14. J.G. Bergman and G.R. Crane, J. Solid State Chem., 12, 172 (1975). 15. C.R. Jeggo, J. Phys. C: Solid State Phys., 5, L133 (1972).