The influence of structural disorder on the luminescence of niobates: Scandium niobate (ScNbO4) and magnesium niobate (MgNb2O6)

Materials

Chemistry

THE INFLUENCE NIOBATE

and Physics,

OF STRUCTURAL

(S&Oh)

485

14 (1986) 485-494

DISORDER

AND MAGNESIUM

ON THE LUMINESCENCE

NIOBATE

OF NIOBATES:

SCANDIUM

(MgNb206) _-

G. BLASSE and G.J. DIRKSEN Physical Laboratory, State University 3508 TA Utrecht (The Netherlands)

Utrecht,

P.O. Box 80.000,

L.H. BRIXNER and Company, Central Research and Development E.I. du Pont de Nemours Department, Experimental Station, Wilmington, Delaware 19898 (USA) Received

25 November

1985; accepted

20 December

1985

ABSTRACT

The luminescence of niobates is strongly influenced by structural luminescence study of ScNbOq (and disorder. We report a low-temperature ScTaOq) in order to explain the absence of efficient luminescence in this disorder seems to be one of the main reasons why compound. Crystallographic the luminescence efficiency is low. On the other hand we report the preparawith a much higher efficiency than published before, which tion of MgNb20 proves that earP ier preparations yielded imperfect samples.

INTRODUCTION

Compounds efficient

with

photoluminescent

ture, and CdNb206 rule

are,

recent

for

ficiencies ture

materials:

ScNbO4

would

of

prepare MgNb206

0254-0584/86/$3.50

MgW04

and

properties the

authors

of [3-61.

disorder

and

Also

with a high luminescence

(and we

lattice

are often

wolframite

[l]. Exceptions

MgNb206 (CdNb206

structo this

(columbite). and

CaNb206)

in the superstructures

In

a

it has

of a-Pb02

for these low luminescent

of this paper ScNbOk

ZnW04 with

structure

be responsible

[2]. It is the purpose

work

on the a-Pb02

of columbites

that crystallographic

and columbite)

luminescence

based

(wolframite)

on the luminescence

been suggested (wolframite

structures

and CaNb206 with columbite

example,

report

earlier

crystal

ef-

to report on the low-temperaScTaO4).

report

efficiency

that

This

study

extends

it is possible

by careful

to

preparation.

0 ElsevierSequoia/Printedin The Netherlands

486

EXPERIMENTAL

Samples of ScNbO4 and ScTaO4 were prepared as described before [5], viz. high temperature solid state reaction (175O'C) in Al203 crucibles or by the use of a flux (Li2SO4 or LiCl) at temperatures between 1200 and 1300°C. X-ray powder diffraction showed the samples to have wolframite structure. An analysis of these data was presented before r5,6J. Samples of MgNb206 were prepared from MgO and Nb205 using a slight excess of MgC. The starting materials were dissolved and dispersed in dilute HNO3, respectively. The solution was evaporated to dryness. Mixtures were slowly heated to 1200°C. More details are given below. Samples were checked by X-ray powder diffraction. The luminescence spectra were obtained using a Perkin Elmer NPF 3 spectrofluorometer equipped with an Oxford CF 100 liquid helium flow cryostat. Reflection spectra were recorded on a Perkin Elmer Lambda 7 spectrometer. Raman spectra were obtained using an Ar+ laser (488nm) by courtesy of Prof. J.H. van der Maas (Analytical Chemistry Department). The reflection and Raman spectra were obtained at room temperature.

RESULTS AND DISCUSSION

Scandium niobate (ScNbO4) At room temperature ScNb04 shows a blue emission under short wavelength ultraviolet excitation with low efficiency 13-51. Cooling to lower temperatures does not increase the efficiency much. The liquid helium temperature (LHeT) luminescence is of considerable complexity. Since all the spectra contain broad emission or excitation bands, it is hard to unravel the spectra with high accuracy. Our results can be summarized as follows. There are at least three luminescent centers, called i, e and e'. Table I presents their luminescence characteristics.The band shapes in the luminescence spectra are

Table I Luminescence characteristics of the several centres in ScNbO4 at LHeT

Centre

1 e e'

Maximum excitation band (nm)

Maximum emission band (nm)

Tq*

260 290 -320

410 490 540

>300 K -150 K (100 K

* Temperature at which the luminescence has practically disappeared

487 similar to those reported earlier [3-51. The luminescence efficiency at LHeT is about as low as at 300K. The diffuse reflection spectrum is given in Fig.1. Here the Kubelka-Munk function has been plotted, so that in an approximate way the vertical axis is linearly proportional to the concentration of the absorbing centers [7]. Finally Fig.2 presents a part of the Raman spectrum -viz. that of the Nb-0 stretching frequencies.

2

r2

T 0

I

300

250

350

400 nm

Fig.1. Diffuse reflection spectrum of ScNbO& at 300K. The diffuse reflection is plotted as the Kubelka-Munk function [7]. The arrows correspond to excitation maxima given in Table I and discussed in the text.

J

1000

I

I

6 00 cm-’

Fig.2. Raman spectrum of ScNbOq in the Nb-0 frequency region. The arrow indicates the shoulder discussed in the text.

Let us now consider and discuss those

for

spectrum and

the

band

the shows

emission

[1,8].

phosphor, ions

in

efficient a clear

In

fact

which

has

the

a-PbO2

wolframite-type absorption

band

does

Wb5+ does

pattern

leaves

not

MgW04 is been

several excitation

maxima

reflection

spectrum

We assign excitation

the

maxima,

of

in

perfect

of

to

the

overlap

different Its

from

reflection

excitation

with

an efficient

order

the

band,

excitation

wolframite-type

between

ScNbOh shows that

this

is

wolf ramite, However,

expected at

observed, not

center

around

longer

the

although the

Mg2+ and

the

to

the

with

order the

optical

260nm

[1,2].

X-ray

The

These

the

intrinsic

at

between

W6+

Sc3’

diffraction

absorption

edge

spectrum

correspond

longer-wavelength

to any emission center

i,

perfect

wavelengths.

although lead

luminescent i.e.

prototype

conclusion.

regions

does

spectral

are

HgWO4.

corresponding

any

to

They

[Z].

exist

niobate

absorption

the

related

lattice

no other

wolframite-type

have

results. phosphor

edge,

not

The reflection spectrum and

these

tail

of

a

shows to in

the the

all.

highest-energy niobate

group

emission

and

in ScNb04. By

this we indicate a niobate group which has the surroundings in the crystal structure that are prescribed by the ideal wolframite structure. The corresponding emission and excitation spectra are situated at an acceptable wavelength for such an assignment [1,2]. The emission band, with its maximum at 410 nm, extends on the shorter wavelength side to 320nm. This then, together with the reflection spectrum explains the low efficiency of the intrinsic niobate emission. Energy transfer from this niobate group to the weakly or non-luminescent centers e‘ must be very efficient in view of the favourable spectral overlap and the relatively high concentration of e' centers, resulting in a considerable amount of quenching of the intrinsic emission [9]. The conditions for energy transfer seem so favourable that the intrinsic emission observed originates probably from parts of the crystals which are more or less perfect. The true emission band may have its maximum at shorter wavelength than 410nm due to the occurrence of radiative energy transfer. This would bring the luminescence characteristics of S&b04 close to those of LaNbO4 and YNb04 with a different structure, viz. fergusonite. However, as has been shown recently, the niobate luminescence characteristicsdo not depend critically on coordination [IO]. In addition to the quenching center (probably family of centers> e', there is a center e, which must be an extrinsic niobate group. Let us now consider the possibilities to have niobate groups in ScNbO4 which are different from the intrinsic ones. (i) The preparation procedure. The high-temperature process may result in oxygen vacancies. It Is well known that this implies an additional luminescence [l]. Also a small amount of AI from the containers may be introduced.

489

The flux may yield a small amount ment

2Sc3+ + Li+ + Nb5+. All these possibilities

which

experience

a crystal

so that their energy reflection least

spectra

Crystallographic between

to have

one. In addition surrounding about

the main

structure

an energy

a whole

ions. family

reason

but

why

it

the

occurs

time.

Because

Raman

halfwidth

825cm-',

there

observed

two

maximum

are known

The is

spectral Therefore

perfect

levels of the .. Scg or Nbsc,

i.e.

groups.

This, added to

of ScNbO4

are

order is

so different

compound.

areas of wolframite-type

between

is not

ordered

to remove

of ScNbO4.

spread

domains.

to disorder

the

In view of

such imperfections

during

in crystals

The lines are rather broad

to the highest-energy

875cm-'

ScNbO4 over

(see Fig.2).

line

These facts

at

point

of disorder.

(ScTaO4)_ we performed

some measurements

centers,

is about

200 K. At increasing

transfer

these

intrinsic

data

in

is situated

in the same

of trivial

the

group

is expected

reflection

is not observed

as

26Onm

and

Their quench-

the e' emission

intensity,

at higher

[5]. We

indicating spectrum

in-

thermal-

is similar

energies.

in the case

of ScNbO4,

but

origin.

range of the spectrofluorometer, the i center

maximum

temperature

way

ScTa04

respectively.

from e to e'. The reflection

tantalate only

excitation

275 and 460nm,

at the cost of the e emission

energy

interpret

on isomorphous

e with

viz.

390nm and e' with

observed

from the intrinsic

that imperfect

to be sensitive

at about

of

ion on a Sc3+

the energy

niobate

In addition

30cm-l).

there are a few differences

This

the of at

[2], the degree

defect,

that the disorder

at boundaries

to scNbo4, but the whole spectrum We

to influence

properties

the Raman spectrum 20 -

amount

increases

ly activated

be excluded

luminescent

ing temperature

groups

A Nb5+

different

for a wolframite-type

spectra

tantalate

In addition

emission

groups groups,

However,

niobate

before

antistructure

luminescence

is a shoulder

also to a certain

tensity

Every

of other extrinsic

mainly

recorded

(typical

Scandium

niobate

different.

be high.

level diagram

rate it may be difficult

the reaction

we

be

As suggested ions might

seem to contain

cannot

the low reaction

t11,121,

also

of extrinsic

121, leads us to the conclusion

arguments

Since our samples

crystal,

the intrinsic

in such high amounts.

disorder.

from what is to be expected

(see above),

by the replace-

will result in niobate

from

will

concentrations

such an ion is expected

intrinsic

the previous

different

the Sc3+ and Nb5+

is expected

brings

level

indicate

above can be present

(ii) disorder

field

for example

It does not seem realistic to assume that the type of centers

10 mole %.

mentioned

site

of Li in ScNbO4,

to absorb

spectrum, which

but

is equipped

in luminescence.

around

it lies

230nm

[l].

outside

the

with a Xenon lamp.

None of the e centers

490

in ScTaO4 can be a niobate group (due to a possible Nb impurity in the starting material Ta205), because ScTa0.P5NbU.0504 yields still another emission with slightly different spectral characteristics, as well as a much higher quenching temperature. This must be

the niobate emission in tantalate,

described before f4,5,6]. In passing it may be mentioned that the efficient emission of the nfobate group in ScTaO4 is well understood and an indication that well-ordered ScNbO4 should emit efficiently. If the niobate groups are isolated, the loss due to energy transfer to killer centers with low-energy absorption bands vanishes for two reasons. In the first place the distance between the niobate groups has become large, due to the low niobate concentration. In the second place the tantalate group (i or e) has its absorption band at higher energy than the corresponding niobate group, so that transfer from niobate to tantalate is very

improbable. The e and k' tantalate centers in ScTaO4 are, therefore, ascribed to

centers similar to the e and e' niobate centers in ScNbO4a It is interesting that we were able to observe energy transfer from e to e' in the case of ScTsO4. This implies that the two types of centers are close to each other and confirms our argument that a disordered niobate group is surrounded by niobate groups which can no longer be considered as intrinsic. The Raman spectrum of the ScTa04 sample is similar to that of ScNbO4.

The highest peak is now at

835cm-', its halfwidth is 35cm-' and the high-energy shoulder more pronounced. This would indicate an even higher degree of disorder than in ScNbO4. We also measured the indium analogues, reported in [6]. There is no doubt that these samples are more efficient than the scandium ones. This would indicate a lower degree of disorder in the indium compounds. It is interesting to note that the size difference between Sc3+ and Nb5' is smaller (0.10 A) than that between In3+ and Nb5+ (17.16A). In the Raman spectrum the half width of the highest peak has decreased to 25cm-1 and the high-energy shoulder has disappeared. All this fits in our model described above.

Rare earth ions in ScTa04_ For reasons which will become clear below, we investigated also Eu3+ and Dy3+ activated ScTaO4. As argued above, excitation does not occur in the intrinsic tantalate groups, because of instrumental restrictions. Upon e-center excitation at LHeT we observed 90% e-center tantalate and 10% Dy3+ emission for a composition ScU,PPDyO.UlTaO4.For ScU.gS5eUO.Ol5TaO4these

VdUeS

are 45%

and 55X, respectively. In passing we note that 1.5 mol % Xu3+ is the maximum amount of Xu3+ which can be introduced into the lattice. For higher E&

con-

centrations we observed, in the optical spectra as well as in the X-ray dif-

491

I

620

600

I

580nm

2

Fig.3. Emission spectrum of ScNbO4-Eu at LHeT under e excitation. The arrow indicates a shoulder mentioned in the text. The figures give the value of J in the transitions 5DQ-7~J

fraction patterns, lines which are most probably due to a phase with the EuTa309 crystal structure [13]. The

Eu3+ emission spectrum of ScTaO4-Eu3+ at LHeT is shown in Fig.3.

The lines are slightly broadened, so that they reflect a more or less disordered structure. In ScTaO4 the site symmetry of the SC3+ site is Cg, so that we expect a complete splitting of all the lines:

5Do-7Fo

1, 5DO-7F1 3,

5Dg-yF2 5. This is actually observed. However, the lower-energy line of the 5DQ-7FL transition has a shoulder. So this emission spectrum pofnts to an fnhomogeneously broadened type of Eu3+ ions which dominates, and a minor concentration of Eu3+ ions with a different site symmetry. Dnder 395nm emission directly into the Eu3+ ion, the emission spectrum is the same. This shows that the emission spectrum of Fig.3 is due to the representative Eu3+ ions in ScTaO4, and that e excitation does not result in emission from Eu3+ ions in specific surroundings. The critical distance for transfer from the e-tantalate center to the Eu3+ ion amounts to 8 A. This follows from R, = 0.6 x 1028.Q.E'4.S0 [i4] with SO = 1.6 eV-l (the experimentally determined spectral overlap), E = 3.3 eV (the energy of maximum spectral overlap) and Q = 3 x 10Wzl cm2eV (absorption cross section [15]). For a random Eu3+ distribution over the Sc3+ sites this

492

yields, following a method described in 121, 55% tantalate and 45% Eu3+ emission, in good agreement with experiment. For

Dy3+ we

calculated Rc = 6.5 A (SO = 1.6 eV-l,

E = 3.3 eV and

Q

=

10s21 cm2 eV). This yields for e center excitation 90% tantalate and 10% Dy3+ emission, in agreement with experiment. From these results we conclude that the e centers see the normal distribution of Sc3+ and Eu3+(Dy3+) ions, as is to be expected for intrinsic centers. So the e centers might well be intrinsic centers, which are slightly influenced by the presence of antistructure defects. The low quenching temperature of the e-center emission and its low efficiency, even at LHeT, are then due to the relatively low energy of the excited state [I]. In summary the model of ScNbO4 (ScTaO4) is as follows. There is a defect concentration (probably disorder) of a few mol X (from reflection spectra), the so-called e' centers. These induce a considerably higher concentration of disturbed intrinsic centers, the so-called e centers. The luminescence of the intrinsic centers is quenched by the e' centers, the luminescence of the e centers is inefficient by itself. It is not clear whether the e' centers are distributed at random or restricted to phase boundaries. Magnesium niobate (MgNb206)_ Wachtel [16] described the luminescence of the columbite MgNb206.

The

300K efficiency was poor and increased a factor of five at lower temperatures. The diffuse reflection spectrum showed a long and intense tail. Qualitatively this is similar to ScNbO4. On the other hand, the columbites CdNb206 and CaNb206 show efficient luminescence. Therefore, the Mg2+ and Nb5+ ions were assumed to be partially disordered [2]. The size difference between the M2+ and Nb5+ ions decreases from 0.368,(M = Ca) to 0.08A (M = Mg). Here we wish to report efficient luminescence for MgNb206 in which the defect concentration is probably low. The samples were prepared with a slight excess of MgO (to prevent the presence of Nb2O5 in the final product). The final firing temperature was 1200°C. However, heating was performed slowly and interrupted for measurements. Samples which were columbite according to X-ray powder diffraction patterns, showed a pronounced tail in the reflection spectrum and 420nm emission upon 255 nm excitation. The former may point to the presence of Nb205, the latter to Mg4Nb209 [16,17]. This suggests that Mg4Nb2Og is formed during the reaction, so that this compound together with Nb205 remains as second phases during the first firing cyclus. After firing for a long time at the same temperature, the Mg4Nb2Og emission disappeared and the tail in the reflection spectrum decreased, but never disappeared completely. At the same time the luminescence intensity of MgNb206 increased drastically and became temperature independent up to 300K. Heating at higher temperatures

493 resulted in a decrease of the luminescence intensity. By comparison with standard phosphors the observed quantum efficiency is about 602. The emission maximum is at 450nm, the excitation maximum at 275nm (LHeT values, see Fig.4). These values agree with Wachtel's [16].

1

L

2 ir

0

I

2

T

0

0

Fig.4. Diffuse reflection spectrum (R) of MgNb206 at 300K. See also Fig.1. The curve R' is magnified by a factor of 10. The curve E gives the emission spectrum of @Nb206 at LHeT. @ denotes the spectral radiant power per constant wavelength interval in arbitrary units.

However,

a careful examination shows that at LHeT there is an additional,

weak emission with a maximum at about 53Onm and an excitation maximum at about 300nm. Such an emission has also been observed for CaNb206 and CdNb206 [2]. It has been assigned to niobate groups occupying a site in the Ca2+(Cd2+) chain by a comparison with

LiNb308

[2].

If we compare these results with the diffuse reflection spectrum of ~~206,

we note: (i)

that the intrinsic emission at 450nm cannot be ef-

ficiently quenched by killer sites like in ScNbO4, because the spectral overlap nearly vanishes and the contentration of the killer sites is low in the case of MgNb206; (ii) that in spite of all efforts these killer sites are still present (it seems improbable that the tail is due to unreacted Nb2O5, since it does not diminish upon another firing with a further excess of MgO, and m205

absorbs at slightly longer wavelengths [161).

Finally, the Raman spectrum of MgN'b206 shows much sharper lines than that of ScNbO4.

The highest energy peak at 900cm-1 has a halfwidth of about

15cm-l. This indicates a relatively high degree of order too.

494

In summary, we have shown that the luminescence efficiency of MgNb206 can be high as is to be expected from theoretical arguments. The requirement is a preparation method which yields well-ordered products. It remains a tempting task for preparative inorganic chemistry to find a method to prepare ordered ScNbO4 and ScTaO4, because these are also expected to show a high luminescence efficiency.

REFERENCES 1

G. Blasse and A. Bril, 2. Physik. Chemie N.F., 57 (1968) 187; G. Blasse, Structure and Bonding, Springer Verlag, Berlin, -42 (1980) 1. G. Blasse and M.G.J. van Leur, Mat. l&s. Bull., 20 (1985) 1037. G. Blasse and A. Bril, J. Chem. Phys., 50 (1969) 2924. G. Blasse and A. Bril, J. Solid State Chem., 3 (1971) 69. L.H. Brixner, J. Chem. Educ., 57 (1980) 588. L.H. Brixner and H.Y. Chen, Mat. l&s. Bull., 15 (1980) 607 G. Kortiim,Reflectance Spectroscopy, Springer Verlag, Berlin, 1969. F.A. Krgger, Some Aspects of the Luminescence of Solids, Elsevier Publ. Comp., Amsterdam 1948.

9

B.C. Powell and G. Blasse, Structure and Bonding, Springer Verlag, Berlin, 42, (1980) 43.

10

G. Blasse, M.J.J. Lammers, H.C.G. Verhaar, L.H. Brixner and C.C. Torardi, J. Solid State Chem., 60 (1985) 258.

11

W.B. White and V.G. Keramidas in R.S. Both and S.J. Schneider Jr.(eds), Solid State Chemistry, Nat. Bureau of Standards, Washington, 1972, p.113.

12

G. Blasse and A.F. Corsmit, J. Solid State Chem., 10

13

P.N. Iyer and A.J. Smith, Acta Cryst., 23 (1967) 740.

14 G. Blasse, Philips Res. Bepts., 24 15 W.T. Carnall, Ch. 24 in &mpg, Amsterdam, 1979.

(1974) 39.

(1969) 131.

Wandbook of the Rare Earths, North Holland Publ.

16

A. Wachtel, J. Electrochem. Sot., 111

17

A.J.H. Macke, J. Solid State Chem., 19

(1964) 534 (1976) 221.