A divalent manganese garnet with red luminescence

Maferials

Chemistry

and Physics,

30 (1992)

A divalent manganese 0. Debye

garnet with red luminescence

Nijs Research

~~tit~te,

J. 141. W. Verweij Debye

199

199-203

Research

(Received

fnstitute,

~n~~ersi~

of

Utrecht,

Dept.

of Geo~hem~t~,

Dept.

of Sofid State

P.O. Bux 80.021,

3508

TA Utrecht

(The ~et~erla~ds~

and G. Blasse ~niversi~

July 11, 1991; accepted

of Utrecht,

September

Chemistry,

P.O. 30.x 8O.~UO, 3508

TA Utrecht

(The ~etherta~ds~

12, 1991)

Abstract The luminescence of the garnet Mn(II)3AI,Si,0,, is reported. Below 150 K the Mn(II) ions show a red emission. At higher temperatures energy migration over the Mn(I1) ions starts. This energy is trapped by several impurity centres, viz. defect Mn(l1) ions, Cr(III) ions on aluminium sites, and Mn(II1) ions on aluminium sites. -

Introduction The garnet Y3A15012 has extensively been used as a host lattice for luminescent ions [l]. Other compositions with a garnet structure have been optically studied in a restricted amount. One of in the these, Ca3A12Si3012, occurs extensively earth’s crust. In this paper we report on the hydrothermal synthesis of Mn,A1,Si,OIZ. This garnet is known in the literature [2]. Our synthesis procedure yields weakly rosa-coloured samples, which suggests that the manganese is more or less completely present in the divalent state. The Mn(II) ion has a long history as a luminescent species and its spectroscopy has been studied extensively 13, 41. For this reason the Mn3A1&012 samples were investigated spectroscopically. They show a red luminescence which is quenched at room temperature due to energy migration to quenching centres.

Experimental Samples of Mn3A12Si3012 were prepared hydrothermally according to the following procedure. For the syntheses of the garnet a Tuttle vessel conditions were was used. The experimental 2 Bar, 550 “C, and a duration of 14 days. The starting material consisted of a gel with a composition of A1203.3Si02 prepared according to the gelling method described by Hamilton and Henderson [5]. Grounded metallic manganese is added to the gel. After this the gel is grounded once

0254-0584/92/$5.00

again and mixed thoroughly. The final result is a fine powder with composition A1,03.3Si02.3Mn. The starting material plus excess water (0.12 g Mn/Al/Si mixture +0.34 g H20) is put in a gold capsule (outer diameter = 5.8 mm, length = 30 mm). The Au capsule is sealed by welding its ends. The pressure vessel is first brought to pressure and subsequently heated. After the temperature has reached the desired value, the pressure is adjusted to the exact value. Quenching is achieved by removing the furnace from the pressure vessel and is accelerated by blowing cold air along the vessel. The result of the experiments are checked by Xray powder diffraction (XRD), scanning electron microscopy (SEM) and infrared spectroscopy. For infrared spectroscopic measurements the material is grounded and mixed with poly-(chloro-triAuoroethene). A Galaxy series FTIR 5000 infrared spectrometer was used. The starting materials are checked by atomic absorption spectroscopy (AAS). Optical measurements were performed using a Perkin-Elmer Lambda 7 spectrometer and a Perkin-Elmer MPF-44B spectrofluorometer. The latter is equipped with a liquid-helium flow cryostat.

Results Both XRD and SEM reveal that the Mn3A12Si30f2 garnet is formed. Under the optical polarizing microscope (magnification 200 X ) the garnets are transparent and show a slightly pink body colour. Infrared spectroscopy reveals that no

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200

hydroxyl groups are present. AAS reveals that the manganese contained about 5 ppm chromium and the aluminium nitrate about 1.4 ppm chromium. Below room temperature they show a red luminescence. Figure 1 shows the diffuse reflection spectrum of Mn3A12Si30,2. The spectral features observed can be assigned to the well-known crystal-field transitions of Mn(II), viz. 1: 6A1 + 4T1; 2: 6A1 --j 4T2; 3: 6A, + 4A1, 4E; 4: 6A, + 4T2, 4E(4D). This indicates that the greater part of the manganese ions are divalent. The spectrum shows also an intense absorption band with a maximum at 240 nm and a tail extending far into the visible region. Figure 2 shows some emission spectra of the luminescence of Mn,Al,Si,O,, for different excitation wavelengths and temperatures. At 4.2 K the dominant emission band has a maximum at 610 nm. The onset of this band is at 580 nm. There is also another emission band present which starts at 555 nm. Finally we notice a weak sharp peak at 690 nm, which is probably due to C?‘. However, by using a different excitation wavelength it is also possible to excite an emission band with a maximum at 650 nm. Upon increasing the temperature the emission band with an onset at 555 nm is readily quenched and a tail appears on the long wavelength side of the 610 nm emission band. This is shown in Fig. 2 for 65 K. At still higher temperatures the 610 nm emission band disappears in favour of the tail and the Cr3+ peak. Figure 2 gives as an example the situation at 150 K. Above 200 K only the Cr3+ peak is still present. To make direct comparison possible, these spectra were not corrected for photomultiplyer response. This falls off rapidly for A > 700 nm. The emission maximum of the ‘tail’ is estimated to be at about 725 nm from a corrected version of the 150 K emission spectrum. Figure 3 shows the 4.2 K excitation spectrum of the 610 nm emission band. It is clear that excitation is only possible in the crystal-field tran-

300

600 nm

Fig. 1. Diffuse reflection spectrum of Mn&si,OIZ Figures are explained in text. R gives the reflection

at 295 K. coefficient.

sitions of the Mn(I1) ion, and not in the broadband transitions shown in the diffuse reflection spectrum (Fig. 1). The excitation spectrum is rather complicated, except for the broad 6A, -+4T, transition. There are more components in the individual transitions than allowed by theory. This is ascribed to the well-known effect that excitation spectra of the emission of concentrated systems show dips at the corresponding absorption maxima which is due to the smaller penetration depth. For this reason the excitation spectrum was not analysed further. The excitation spectrum of the longer wavelength emission (e.g. 665 nm, see Fig. 3) differs from that of the 610 nm emission. The 6A, +4T1 transition is stronger and the splittings are different (Fig. 3). The excitation spectrum of the Cr3+ emission peak at 200 K is also given in Fig. 3. It agrees completely with the diffuse reflection spectrum as far as the Mn(I1) features are concerned. This indicates energy transfer from Mn(I1) to Cr(II1).

Discussion The spectral data show clearly what the spectral characteristics of the intrinsic Mn(I1) ions in the garnet Mn,Al,Si,O,, are. Simultaneously they indicate that the luminescence properties of this garnet are not only determined by the presence of the intrinsic Mn(I1) ions. Impurities or other manganese ions must be present as well. Let us first consider the spectra of the intrinsic, dodecahedrally coordinated Mn(I1) ions. The diffuse reflection spectrum and the excitation spectra show the crystal-field transitions. The emission is due to the 4T, +6A, transition and peaks at 610 nm. The Stokes shift is estimated to be 2500 cm-‘. The red emission points to a strong crystal field. In many cases (e.g. [6]) the Mn(I1) ion emits in the green in eight coordination. It is interesting to note that Petermann and Huber [7] have reported for Mn(I1) in the garnet Y,Al,O,, an emission which is practically the same as the intrinsic one reported here for Mn3A12Si3012. Obviously the Mn(I1) ion experiences a relatively strong crystal field in the dodecahedral garnet site. Perhaps the edge sharing with the octahedral sites is responsible for this effect. It is interesting to note that Ce3+ in the garnet Y,Al,O,, experiences also a strong crystal field in the excited 5d configuration [8]. One might argue that this emission is not intrinsic at all but due to Mn(I1) trap emission as observed

201

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620 nm

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Fig. 2. (a) Emission spectrum under excitation at 405 nm of Mn,AlzSi,Otz at 4.2 K.The arrows indicate the Mn(I1) emission with an onset at 5.55 nm and the Cr(II1) emission peak at 690 nm. See also text. fb) Emission spectrum under excitation at 555 nm of Mn,Al,Si30iZ at 4.2 K. (c) Emission spectrum under excitation at 405 nm of Mn3Al,Si,0u at 65 K. Compare to Fig. 2a. (d) Emission at 150 K. The peaks are due to Cr(III). spectrum under excitation 405 nm of Mn,Al&Otz

for manganese fluorides with MnF, as the most well-known example [3]. The key question is whether the Mn(II) excitation energy is mobile or not. There are several reasons why it is improbable that the excitation energy in Mn,Al,Si,O,, is mobile at low temperatures. In the first place the Stokes shift in the garnet (2.500 cm-‘) is much larger than in MnF, (5 2000 cm-‘). Secondly, our spectra do not reveal the slightest indication for a zero-phonon line like they do in MnF, [3]. The absence of a zero-phonon line forbids energy transfer at low temperatures [9]. Finally, the Mn(I1) subsystem in Mn3A12Si3012 is less concentrated than in MnF,. Powell ef at. [lo] have studied the luminescence and energy transfer in Mn2Si04. In this compound the Mn(II) emission has the same Stokes shift as in Mn3A12Si30,2, and lacks also a zero-phonon line. Energy migration in the Mn(I1) subsystem of Mn,SiO, occurs only above some 50 K [lo]. The situation in Mn3A12Si30r2 appears

to be similar (see below). Even at 4.2 K there is already evidence for other luminescent centres. Most clear, due to its line emission, is the Cr(II1) ion. Since Petermann and Huber [7] report for the Mn(IV) emission in Y,Al,Or,? line emission at 648 and 673 nm, we reject the possibility that the 690 nm line in our samples is due to Mn(IV). As indicated above the starting materials manganese and aluminium nitrate contain a few ppm chromium. It is obvious to ascribe the two other 4.2 K emissions (the one with the onset at 555 nm and the other with the 650 nm maximum) to Mn(I1) ions in different sites, i.e. to extrinsic Mn(II) ions. Proposals for the nature of these centres will be made below. First we consider what happens upon increasing the temperature. The emission spectrum changes drastically at higher temperatures. This indicates that energy migration over the intrinsic Mn(I1) ions starts and

202

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(c) Fig. 3. (a) Excitation spectrum of the 610 nm emission of Mn&12SiS0,2 at 4.2 K. (b) Excitation spectrum of the 665 nm emission of Mn,Al$i,O,, at 4.2 K. (c) Excitation spectrum of the 690 nm emission (Cr(II1) peak) of Mn&Si,0,2 at 200 K.

feeds extrinsic luminescent centres and quenching centres [9]. The same phenomenon has been observed for several manganese fluorides [3] and Mn,SiO, [lo]. The first consequence of this energy migration is a decrease of the intrinsic emission intensity. Above 50 K the total emission intensity starts to drop, so that part of the excitation energy reaches quenching centres. The other centres which are fed are the Cr(III) impurity, the centre with the 720 nm emission band and the Mn(I1) ion with the 650 nm emission. Since Petermann and Huber [7] have also reported Mn(II1) emission in Y3Al5O12 which consists of a broad band with a maximum at 722 nm, the centre with the 720 nm emission is ascribed to Mn(II1) in Mn,Al,Si,O,,. This band

was reported to be broader than the Mn(II) emission [7], which observation is also made here for Mn,Al,Si,O,,. If Mn(II1) is present in the garnet Mn,Al,Si,O,,, the most obvious site is the octahedral one in view of its ionic radius and its site preference. This implies that a small amount of the Al(II1) ions (ionic radius 0.535 A) is replaced by the larger Mn(II1) ions (ionic radius 0.645 A). These Mn(II1) ions are surrounded by Mn(I1) ions on dodecahedral sites. The latter sites will be slightly suppressed due to the large ion on the octahedral site. Therefore, these Mn(I1) ions probably feel a stronger crystal field, so that we can assign the 650 nm emission to these ions. Due to exchange interaction between the two types of manganese ions, the transition probabilities are influenced [3]. This agrees with the intensity changes which we observed in the excitation spectrum of these Mn(I1) ions (Fig. 3). The presence of Mn(I1) and Mn(II1) ions close together, requires that an inter-valence charge transfer band should be observed in the absorption spectrum [ll]. This is probably the strong tail in the diffuse reflection spectrum. Excitation into such a band is not expected to yield luminescence [ 111. However, if one of the ions is excited selectively in the crystal-field levels, luminescence appears as is clear from the experimental results. At not too low temperatures the emission originates Mn(II1) which suggests mainly from Mn(I1) -+ Mn(II1) energy transfer. Obviously this transfer needs some thermal activation. In the absence of data on the optical properties of Mn(II1) in this host, the results cannot be analyzed further. The Mn(I1) ion responsible for the emission with an onset at 555 nm has a smaller crystal field than the intrinsic Mn(I1) ions. The rapid quenching is ascribed to thermally activated energy transfer to the intrinsic Mn(I1) ions. The Mn(I1) ions with the small crystal field can be considered as antitraps [ 121. They might be Mn(I1) ions on octahedral sites. In order to study these phenomena further the growth of crystals from a melt was tried. However, only black products were obtained.

Conclusion The garnet Mn3A12Si30i2 shows red luminescence from the Mn(I1) ions on dodecahedral sites, but due to thermally activated energy migration to several types of defect centres thermal quenching of the emission occurs below room temperature.

203

References See e.g. C. P. Khattak and F. F. Y. Wang, in K. A. Gschneidner Jr. and L. Eyring (eds.), Handbook on the Physics and Chemistry of Rare Earfhs, North Holland, Amsterdam, 1979, Vol. 3, Chapt. 29. R.W. G. Wyckoff, Crystal Structures, 2nd edn.. Vol. 3, Intersclence, Wiley, New York, 1965, pp. 223-225; L. C. Hsu, 1. Petrology, 9 (1968) 40. B. Henderson and G. F. Imbusch, Optical Specfroscopy of Inorganic solids, Clarendon Press, Oxford, 1989. A. P. B. Lever, lnorgnnic Electronic Spectroscopy, 2nd edn., Elsevier Science, Amsterdam, 1984.

and C. M. B. Henderson, Min. Msg., 36 5 C. L. Hamilton (1968) 832. and G. Blasse, Phys. Sfar. Sol. (b), 127 6 M. J. J. Lammers (1985) 663. and G. Huber, J. Luminescence, 31/32 (1984) 7 K. Petermann 71. 8 G. Blasse and A. Bril, J. Chem. Phys., 47 (1967) 5139. 9 R.C. Powell and G. Blasse, Structure and Bonding, 42 (1980) 43. Lin Xi, G. M. Loiacono and R. 10 R. C. Powell, B. Elouadi, S. Feigelson, J. Chem. Phys., 84 (1986) 657. and Bonding, 76 (1991) 153: 11 G. Blasse, Structure R. G. Denning, T. J. Barker and D. I. 12 J. R. R. Thorne, Grimley, J. Physique, 46 Coil. C7, (1985) 125.