Luminescence and nonlinear optical properties of perovskite-like niobates and titanates

Materials

Chemistry

and Physics,

36 (1994)

Luminescence and nonlinear niobates and titanates M. Wiegel, (Received

M. Hamoumi

February

5, 1993; accepted

289

289-293

optical properties

of per~vskite-hike

and G. Blase May 39, 1993)

Abstract The luminescence and nonlinear optical properties of perovskite-like niobates and titanates are reported and discussed in relation with other perovskite-like compounds. It is found that the nonlinear optical properties of these compounds can be predicted rather reliably by considering both the structure and the luminescence properties.

Introduction

Experimental

Considering some of the known efficient nonlinear optical materials, viz., potassium lithium niobate [l, 21, potassium niobate [3] and potassium titanyl phosphate (KTP) [4, S]> it is striking that all of these compounds are built up from corner-sharing octahedra. It has been proposed that in such systems some delocalisation of the wavefunctions of the excited state occurs. The luminescence is ascribed to self-trapped exciton emission with a low quenching temperature and a small Stokes shift [SJ. Energy band broadening or deloca~~sati~~ is known to increase the covalence of chemical bonds. As a consequence, the optical bandgap decreases. For instance, for niobates with corner-sharing niobate octahedra the bandgap is about 3.5 eV, whereas for other niobates it is about 5 eV [6, 71. According to Lines [8], the nonlinear optical response of a material will increase if the optical bandgap decreases. In fact, Philips et al. [9] have recently shown that the nonlinear optical properties of KTP are due to delocalisation of the excited state. This leads to the following rule of thumb: non~entrosymmetric transition metal (do) 0x0 compounds with a defocalised excited state and a relatively small bandgap, should give a large second harmonic generation (SHG) response. To test the generality of this rule, the luminescence and nonlinear optical properties of perovskite-like titanate and niobate com~unds are investigated and discussed.

Most samples were prepared via conventional hightemperature solid state reactions. Firing procedures were carried out in air. Corundum crucibles were used. Samples of &J&,0, [IO] and Ca,Nb,O, [ll] were prepared by firing stoich~ometric mixtures of Nb,O, (ultrapure grade, H. Starck) and SrCO, (p.a., Merck) or CaCO, (p.a., Merck) at 850 “C for 8 h. After grinding, the samples were refired for 8 h at 1200 “C. The ALaNb,O, (A = Cs, Rb and K) compounds [12] were obtained by using appropriate quantities of La@, (Highways Intemational~ 99.997%), Nb,05 (ultrapure grade, H. Starck), C&O, (Johnson Matthey, Ultrapure), Rb,CO, (Merck, 99%) and K&O, (Merck, ultrapure). A 25% excess of carbonate was used to compensate for volatilization. The mixtures were first fired at 900 “C for 10 h, ground again, and then fired at 1100 “C for 10 h (twice). Finally, the products were washed with distilled water and dried at 120 “C. LiLaN,O, and NaLaNb,O, could not be synthesized by solid state reactions, but were made via ion exchange [12, 131 by reacting KLaNbzO, powder with molten LiN& and NaNO, (both Merck, 99.5%) at 270-310 “C for 1 h. The products were filtered, washed, and dried at 120 “C. This procedure was repeated twice. Ca,Ti,O, [ 14] was prepared in the following way. First, TiO, was synthesized by firing (NH&TiO(C,O,),*H,O (Merck, Optipur) at 900 “C for 9 h. An intimate mixture of CaCO, and TiOz was fired at 900 “C for 8 h. After firing, the mixture was ground

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290

again, pressed into pellets, and then fired at 1200 “C for 10 h. All samples were checked using X-ray powder diffraction and found to be single phase. Instrumentation

For measuring the effective nonlinear optical tensor coefficient deE of the powdered samples, a Philips Second Harmonic Analyser with a pulsed Nd3+ glass laser (A = 1064 nm) was used [ 151.The samples were measured versus a potassium lithium niobate standard [2]. The luminescence measurements were performed as described elsewhere [16], using a Spex fluorolog spectrometer equipped with an Oxford liquid helium flow cryostat.

Results Luminescence

properties

At low temperatures the samples show a broad structureless emission band under UV excitation; at room temperature (RT) this luminescence is quenched. The luminescence data of the samples and some other perovskite-like compounds are summarized in Table 1. The luminescence of Sr,Nb,O,, Ca,Nb,O,, CsLaNb,O?, RbLaNb,O, and Ca,Ti,O, is strong, whereas the other compounds emit only weakly. As a representative example, the emission and excitation spectra of the luminescence of CsLaNb,O, are given in Fig. 1. Only for SrJ?b,O, and Ca,Nb,O, does the optical absorption edge in the diffuse reflection spectrum at RT agree with the rn~~urn of the excitation band at liquid helium temperature (LHeT), apart from a small TABLE

1. Luminescence

data of perovskite-like

niobates

temperature dependence. The excitation spectrum of Sr,NbzO, reveals a shoulder. Excitation in the maximum of this shoulder at 320 nm gives a weak, broad emission band with a maximum at 505 nm. The most intense luminescence of Sr,Nb,O, lies at 475 nm (A,,,= 295 nm; see Table 1). This does not agree with the literature. Zeinally et al. [17] measured the luminescence of Sr,Nb,O, on crystals, and found a broad, intense emission band at 505 nm (A,, =330 nm 2181). Obviously, the luminescence at 505 nm dominates in their crystals, whereas for powdered Sr2MnZ07 the most intense luminescence lies at 475 nm. For LiNbO, a similar behaviour has been observed [19]. The luminescence properties of Lib&O, are very sensitive to deviations from stoichiometry: in single crystals, known to be off stoichiometric, the intrinsic NbO, emission is completely quenched, and only a weak emission from defect niobate groups is observed. The intrinsic niobate emission is found to occur only in the stoichiometric powder materials. Since there are two very similar Nb’+ sites present in Sr,Nb,O, [lo], the luminescence at 475 nm is ascribed to these intrinsic NbO, groups. The weak emission at 505 nm is ascribed to extrinsic niobate groups. The absorption edge of Ca,Ti,O, in the ditfuse reflection spectrum ( ~350 nm) does not agree with the maximum of the excitation band (see Table 1). At 4.2 K two emission bands (with maxima at 445 and 465 nm) and two excitation bands (with maxima at 310 and 330 nm) are found. From luminescence measurements that we performed on CaTiO, it becomes clear that the Ca,Ti,O, samples contains some CaTiO,, which has an emission m~imum at 445 nm and an excitation ma~um at 330 nm. The absorption edge is ~mparable to the excitation rn~~urn of CaTiO,.

and titanates

Compound

Emission max, 4.2 K (nm)

Excitation max, 4.2 K (nm)

Absorption edge”, 300 K (nm)

Stokes shift 4.2 K (103 cm-‘)

T1/2” (K)

La2Tiz07’ CazNbzO, Sr,Nb,O, Sr,Ta,O,d CaaTizO, LiLaNbzO, NaLaNb*O,

465 465 475 485 465 605 605 605 585 585

270 280 295 =310 310 340 340 340 330 340

280 290 300 =310 3.50’ 330 330 330 330 340

16.0 14.8 13.2 11.5 10.8 13.9 13.9 13.9 13.8 12.8

120 loo 65 ==lOO 20 130 110 130 120 130

-NM37

RbLaNbzO, CsLaNbzO, “The “The cSee dSee *See

absorption edge is taken from the diffuse reflection spectrum temperature at which the luminescence intensity has dropped ref. 22. ref. 18. text.

at RT. to 50% of its intensity

at 4.2 K.

291

La -

I(Nb06-

Fig. 2. The crystal structure

r~ ~---lb 300 460

~~~

r-.---

500

Wavelenyth

I

600

700

800

(nm)+

Fig. 1. The excitation and emission spectra of the luminescence of CsLaNbz07 at 4.2 K. Q, is the spectral radiant output per constant wavelength interval, and qr is the quantum output; both are given in arbitrary units. TABLE 2. The absolute effective nonlinear ficient (&) and the optical bandgap

optical tensor coef-

after Sato er al. [20].

(= 1.7 A) is directed toward the interlayer; the opposite Nb-0 bond is considerably longer (=: 2.3 A) [20]. The structures of Sr,Nb,O, [lo], Ca,Nb,O, [ll] and Ca,Ti,O, [14] also consist of distorted perovskite-like slabs arranged with a notable lack of Nb-0-Nb or Ti-0-Ti links between successive slabs. The TiO, octahedra in the perovskite slabs of Ca,Ti,O, are very regular, whereas the NbO, octahedra of Sr,Nb,O, and Ca,Nb,O, have a much larger degree of distortion.

Luminescence properties of the perovskite-like compounds

Compound

IM (pm V-7

Optical bandgap” (ev)

Ca,Ti,O, Sr,Nb,O, Ca,Nb,O,

0.3 14 5

3.9b 4.1 4.3

“Estimated ‘Estimated

of KLaNb,O,;

from the diffuse reflection spectrum from the absorption band maximum

at RT. at 4.2 K.

Nonlinear optical properties

The ALaNb,O, compounds (with A = Li, Na, Rb and Cs) do not show a SHG signal. It is very likely that all these compounds have a centrosymmetric space group [ 121.KLaNb,O, does not show frequency doubling either; the probable space group of this compound is P2aa (noncentrosymmetric) 0rPmma (centrosymmetric) [12]. The deff values of the other compounds can be found in Table 2. Sr,Nb,O, in particular has a large effective nonlinear optical tensor coefficient.

Discussion The structure of the perovskite-like compounds involved

Before starting the discussion, it is useful to look at the structure of the perovskite-like compounds. As a representative example of the structure of the ALaNb,O, (A=Li, Na, K, Rb, Cs) compounds, the structure of KLaNb,O, is shown in Fig. 2. The alkali interlayer is disordered, since only 50% of the alkali sites are occupied statistically. Furthermore, the octahedra are strongly distorted: one short Nb-0 bond

The luminescence properties of the perovskite-like niobate and titanate compounds under study are characteristic of compounds with a delocalised excited state. The absorption maxima are at relatively long wavelengths, and a shift of the first absorption band to higher energies and/or a decrease in the Stokes shift does not result in a higher quenching temperature (see Table 1). This excludes an explanation in terms of a localised excitation [7]. By comparing Sr,Nb,O, with Ca,Nb,O, it is observed that the absorption and emission bands of Sr,Nb,O, are shifted to longer wavelengths. Moreover, the Stokes shift and the quenching temperature of the emission have decreased. Obviously, the substitution of Ca2+ for Sr2+ results in a stronger delocalisation of the excited state. This can, at least partly, be explained as follows. From the literature it is known [6] that the amount of delocalisation increases as the M-O-M angle (M=Nb’+, Ta5+, Ti4’...) between the cornersharing octahedra approaches 180” (more favourable rr-bonding). The Nb-0-Nb angle is closer to 180” in Sr,Nb,O, [lo] than in Ca,Nb,O, [ll]. So the delocalisation is expected to be stronger in Sr,Mn,O, than in Ca,Nb,O,, and indeed the luminescence data in Table 1 agree with this. The difference in luminescence behaviour between Sr,Nb,O, and the centrosymmetric perovskite-like tantalate Sr,Ta,O, can be explained in a similar way. The spectra of a tantalate are usually at shorter wavelengths than those of the corresponding niobate [7], but for Sr,Ta,O, and Sr,Nb,O, this is different, the tantalate spectra being at longer wavelengths. The tantalate

292

octahedra in SrzTa,O, are more regular than the niobate octahedra in Sr,Nb,O, [21], so the Ta-0-Ta angle in SrzTa,O, is closer to 180” than the Nb-0-Nb angle in Sr,Nb,O,. Owing to this, the delocalisation is larger in Sr,Ta,O,. As a result, the maximum of the excitation band of the luminescence of Sr,Ta,O, shifts to lower energy and the Stokes shift decreases (see also Table 1). As in Sr,Ta,O,, the octahedra in the perovskite layer of Ca,Ti,O, are very regular. As a result, the maximum of the excitation band and the Stokes shift of Ca,Ti,O, are comparable to those of SrzTa,07. In the perovskitelike titanate La,Ti,O, (centrosymmetric) 1221the excited state is more localised than in Ca,Ti,O,: the maximum of the excitation band is shifted to higher energies (270 nm versus 310 nm) and the Stokes shift is larger (16 000 cm-l versus 11000 cm-‘) (see also Table 1). This was conf%med by photoelectrochemical measurements [22]. The Ti-0-Ti angle in La,Ti,O, [23] is much smaller than that in Ca,Ti,O,. Compared with the other perovskite-like compounds, the absorption and emission bands of the ALaNb,O, (A= Li, Na, K, Rb, Cs) compounds are shifted to even longer wavelengths. This suggests that the delocalisation in these compounds is relatively pronounced. However, the quenching temperature is high and the Stokes shift is large. Moreover, the maximum of the excitation band at 4.2 K is not at a shorter wavelength than the absorption edge at room temperature (derived from the diffuse reflection spectrum). This behaviour may be related to the absence of crystallographic order in the alkali interlayer. As a consequence, the energy levels of the niobate groups will vary from site to site, and especially important - some of the niobate octahedra will have a slightly lower excited level. These groups will act as traps for the migrating exciton. Consequently, the emission will originate from a more localised level than one would predict from the low value of the absorption edge. This model, tentative as it may be, is able to explain the observations. The relatively large Stokes shift and high quenching temperature are due to the localised character of the traps. Excitation into the traps, which takes place below the absorption edge, is very efficient; this makes a comparison of the position of the absorption edge and excitation maximum less significant. Going from LiLaNb,O-I to CsLaNbzO, the absorption and emission maxima do not shift (see Table 1). Sato et al. [20] have shown that the stacking of the perovskite layers depends on the kind of interlayer ions, i.e., the layers are displaced along the unit cell axis or the diagonal if the size of the alkali ion increases. But the luminescence of these compounds is determined by the niobate layers, and not by their mutual orientation.

Therefore the independence of the spectral data on the nature of the alkali ion is not unexpected. The nonlinear optical properties of the perovskite-like compounds

The ALaNb,O, compounds do not show frequency doubling. This may have two reasons. The space groups may be centrosymmetric. However, even if the space groups are noncentrosymmetric, the nonlinear response will be low, because the perovskite layers of the ALaNb,O, compounds contain mirror planes perpendicular to the c-axis (see Fig. 2). Owing to these planes, the individual nonlinear optical contributions of the Nb-0 bonds will cancel, and the total nonlinear optical response will be low, since only the more covalent bonds contribute to the nonlinear optical effect [24, 251. Ca,Ti,O, belongs to a noncentros~et~c crystal class (space group Ccm2, [14]). The amount of delocalisation is large: the Ti-0-Ti angle approaches 180”. The optical bandgap cannot be taken from the diffuse reflection spectrum, because the sample contains some CaTiO, (see also the preceding section). An optical bandgap of about 3.9 eV is estimated from the excitation band maximum at 4.2 K. For comparison, the very covalent ~centro~mmetric) rutile TiO, has a bandgap of about 3.1 eV [22]. Since the bandgap of Ca,Ti,07 is still relatively small and the delocalisation, therefore, relatively large, a considerably larger effective nonlinear optical tensor coefficient d,, is expected than the measured d,,value of 0.3 pm V-l. This disagreement can be explained as follows. The titanate octahedra of CasTizOT are very regular. Since a slightly distorted octahedron is avery symmetrical unit, the total nonlinear response will be small, because the individual nonlinear optical contributions of the Ti-0 bonds will almost cancel. The niobates Sr2Nb207 and Ca,Nb,O, also have noncentrosymmetric space groups: Cmc2, and Pn2,a, respectively. They show a significant deR. The perovskite layers in these compounds do not contain s~rnet~ elements that cancel the individual nonlinear optical contributions of the more covalent Nb-0 bonds. Furthermore, the niobate octahedra have a large distortion. The main difference between these two niobates appears to be the amount of delocalisation. The optical bandgap is smaller for Sr,NbzO, than for CazNb,O, (see also Table 2). The effective nonlinear optical tensor coefficient will increase if the optical bandgap decreases. Thus a larger d,, is expected for Sr,NbzO,. This is in agreement with the powder SHG measurements (see also Table 2). The d,, values of Sr,Nb,O, and Ca,Nb,O, are smaller than that of the potassium lithium niobate standard, since the phase-matchable tensor coefficient d,, of the

293

powdered standard is (27 Lf:4) pm V-l [2]. The latter has a larger delocalisation, expressed, among other ways, in a smaller optical bandgap (3.5 eV ]26]). This agrees with the rule of thumb mentioned in the introduction: compounds with a larger delocalisation (a smaller optical bandgap) give a larger d,@ KNbO, [6] and KTiOPO, [26] also obey this rule, in view of their optical properties reported before.

10

Conclusions

11 12

The luminescence properties of the perovskite-like compounds are determined mainly by the amount of delocalisation of the excited state. This effect depends on the structure, i.e., on the M-O-M angle between the corner-shearing octahedra. Only if the luminescence properties and the structure of the perovskite-like compounds are considered, can the nonlinear optical properties be predicted reliably.

13 14

15 16 17 18 19

Acknowledgement

20

This work was supported oratories, Eindhoven.

by Philips Research

Lab21 22

References S.C. Abrahams, P.B. Jamieson and J.L. Bernstein, J. Chem. P!zyir, 54 (1971) 2355. M. Ouwerkerk, A& Mater., 3 (1991) 399. M.J. Weber (ed.), CRC Handbook of Laser Science and Technology, Vol. 3, CRC Press, Boca Raton, FL, 1988. I. Tordjman, R. Masse and J.C. Guitel, Z. kWtalZogr., 139 (1974) 103.

23 24 25 26 27

F.C. Zumsteg, J.D. Bierlein and T.E. Gier, J. Appl. Phys., 47 (1976) 4980. G. Blame and L.G.J. de Haart, Mater. Chem. Phys., 24 (1986) 481. G. Blasse, Strucr. Bonding (Berlin), 42 (1980) 1. M.E. Lines, J. Appl. Phys., 69 (1991) 6876. M.L.F. Philips, W.T.A. Harrison, G.D. Stucky, E.M. McCarron III, J.C. Calabrese and T.E. Gier, Chem. Mater., 4 (1992) 222. N. Ishizawa, F. Marumo, T. Kawamura and M. Kimura, Acta Crystaliogr. Sect. B, 31 (1975) 1921. J.K. Brandon and H.D. Megaw, Philos. Mug., 21 (1970) 189. J. Gopalakrishnan, V. Bhat and B. Raveau, Mater. Res. Bull., 22 (1987) 413. R. Jones and W.R. McKinnon, Solid State Zonics, 4.5 (1991) 173. M.M. Elcombe, E.H. Kisi, K.D. Hawkins, T.J. White, P. Goodman and S. Matheson, Acta Crystallogr., Sect. B, 47 (1991) 305. J.P. Dougherty and S.K. Kurtz, J. Appl. Crystallogr., 9 (1976) 145. M. Wiegel and G. Blasse, J. Solid State Chem., 99 (1992) 388. A.K. Zeinaliy, N.N. Lebedeva, A.R. Mordukhaev and MA. Osman, Sov. Phys. - Solid State, 24 (1982) 432. G. Blasse and L.H. Brixner, Mater. Res. Bull., 24 (1989) 363. D.M. Krol, G. Blasse and R.C. Powell, J. Chem. Phys., 73 (1980) 163. M. Sato, J. Abo, T. Jin and M. Ohta, J. Alloys Compounds, in press. N. Ishizawa, F. Marumo, T. Kawamura and M. Kimura, Acta Crystallogr.., Sect. B, 32 (1976) 2564. L.G.J. de Haart, A.J. de Vries and G. Blasse, J. Solid State Chem., 59 (1985) 291. K. Scheunemann and H. Mtiller-Buschbaum, J. Inorg. Nucl. Chem., 37 (1975) 1879. CR. Jeggo and G.D. Boyd, .f. Appl. Phys., 41 (1970) 2741. J.G. Bergman and G.R. Crane,J. Solid State Chem., f.? (1975) 172. M. Wiegel, G. Blasse and M. Guwerkerk, Mater. Res. Bull., 27 (1992) 617. G. Blasse, G.J. Dirksen and L.H. Brixner, Mater. Res. Bull., 20 (1985) 989.