Zeeman measurements of Pr3+ centres in CaO and CaF2

JOURNAL OF

Journal of Luminescence 51(1992) 149—156 North-Holland

Zeeman measurements of Pr3~centres in CaO and CaF2 Tosporn Boonyarith

a, John P.D. Martin a Boazhu Luo b and Neil B. Manson a Laser Physics Centre, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 2601, Australia b Changchun Institute of Physics, Academia Sinica, Changchun, PR China Received 22 July 1991 Revised 4 November 1991 Accepted 6 November 1991 3~ centres in two different crystal hosts have been studied by site selective laser spectroscopy and Zeeman spectroscopy Three Pr with attention directed at the 3P~,3P 3H 3P 1 and 116 crystal field level assignments. The 4 -~ 11 excitation line is readily identified in each case. For a tetragonal and a trigonal centre in CaF2 there is, in each case, a further weak excitation line higher 3~centre in energy in CaO which there exhibits are two a large lines Zeeman adjacent splitting. to that associated The associated with the excited 3P levels are attributed to 116 states. For the Pr 0 state and although they do not exhibit linear Zeeman splittings, there are quadratic Zeeman shifts due to large off-diagonal terms. Again the related levels are attributed to 16 states. The centre is shown to have orthorhombic symmetry and the energy levels for this centre are determined.

1. Introduction Porter and Wright [1] have reported the excita3t The crystal was tion and of CaO:up-conversion Pr shown to emission exhibit efficient and with the current interest in up-conversion for laser applications it is worthwhile to identify the nature of the active centre or centres. Porter and Wright [1] have established the centre the up-conversion is the that dominant centregiving in CaO. The spectroscopy of the centre has several interesting features. In particular there are three excitation lines in the region of the 3H 3P() wave4 length region where, if it is a single Pr3~ion centre, only one line is expected. From various considerations such as the temperature depen—‘

dence of the dependence excitation and emission the concentration of the energy and transfer between the 3P 0 and ‘D2 states, Porter and Wright [1] concluded 3~ion. that theThus centre thewas twoassociaddiated with a single Pr are associated with excited tional excitation lines state levels adjacent to the 3P 0 level. This paper 0022-2313/92/$05.OO ©

1992

—

is concerned primarily with the origin of these lines and the geometric nature of the centre. The Zeeman effect is investigated and compared with of thattheof levels two other Pr3~centres; one of tetragonal symmetry and one of trigonal symmetry in CaF 2. The analysis of the Zeeman splittings gives strong support for Porter and Wright’s [1] conclusions both3~ regarding the single centre and their ion nature the CaO : Pr model of theofcentre. The two high symmetry centres investigated in CaF 3~ion substiare Ca2~but both centres where the Pr tutes2 for charge compensation arises in two different ways. One is by an interstitial F ion in an adjacent location along the (100> direction forming the centre of C 4~symmetry. 3~centre in CaF This 3~ is the predominant Pr Pr and has been extensively discussed in the2 :literature including Zeeman measurements [2,31.When oxygen another is present in themechanism growth of for the charge CaF2 crystal common compensation is by the substitution of an O2~ion for a nearest-neighbour F

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ion giving in this

150

T Boonyarith et al.

/ Zeeman measurements of Pr3

instance a centre with C3~symmetry. Some aspects of this centre have been given before by Gustafson and Wright [41and by Hasan and Manson [5]. Masui and Ibuki [61have also 3~but theidentiexcified a trigonal centre in CaF2 : Pr tation wavelengths are different from those reported here or those given by Gustafson and Wright [41and, hence, involves a different centre. It is not clear that their crystal was grown in the presence of oxygen and, hence, may not involve O2 charge compensation.

centres in CaO and CaF

*

2

Excitation spectra were performed using a N2 pumped dye laser and the emission was dispersed by a monochromator and detected by a photomultiplier. samples were cooled by (i) a flow of The helium gas (10 K),either or (ii) a Oxford Instruments cryomagnetic dewar (4.2 K) which included a split pair superconducting magnet (0—5 T).

3. Excitation of CaO: Pr3~ The low temperature 4.2 K excitation of CaO Pr3~ in the 3H 3P() spectral region 4 —* three lines at 486.2, shows, as mentioned earlier, 484.9 and 484.5 nm. The excitation spectrum obtamed by monitoring the emission at 486.2 nm which coincides with the lowest energy excitation line, is shown in fig. 1(a). Non-selective excitation gives the same spectrum with only weak additional features indicating that the excitation spectrum is associated with the dominant centre in

2. Experimental The crystals were all grown in the Laser Physics Centre. The CaO was grown by the arc fusion process and pieces 2 mm3 were cleaved from the crystalline mass. The CaF 3~was grown 2 : Pr by the Bridgman technique with the oxygen centres formed subsequently by heating crystals in moist air at 800°C.

(a) EXCITATION 486.2

452.4

484.5 448

450

452

454

/ /

484

486

488

.0

>%

486.2 ~

(b) EMISSION : 3P

3H 0

->

4 492.2

~16.4 490

500

510

wavelength 3H 3P (nni)

3H 3P 3~ obtained by Fig.monitoring 1. (a) Lowfluorescence temperatureat(10 486.2 K) and excitation 619.0 nm, spectrum respectively. of the (b) 3P 4 —~ 03H and 4 —~ 2 transitions of CaO:Pr 0 —~ 4fluorescence spectrum when exciting at 486.2 nm.

T. Boonyarith et al.

/ Zeeman

measurements of Pr~+ centres in CaO and CaF

CaO:Pr~ 22500— 22233

_________________

22101

151

3H The 4 —~ ‘D~excitation and emission has also been recorded and is shown in fig. 3. In this case there were no coincidences in the low temperature spectrum between excitation and emission lines. However, by plotting the temperature dependence of the emission from two emitting levels the energy separation between the levels

2200020634

was determined and hence the relative position

20562 ~2o616

3H also been established. of the levels of ‘D2 and 4 states (fig. 2) has In3Paddition, no excitation lines consistent with the 1 state could be detected whereas the two lines at 22101 and 22233 cm~ are assigned to 3P the 2 state (fig. 1(a)).

20750

205001

16750 ~.—16539

1D -

2

2

~ 16417*

16250W

2

3~

(a) EXCITATION

.

CaO;Pr

3H 4 —‘.102

EMISSION —

607 0

~1217 -1201

(b) EMISSION

1000—

-

652 500—

_

508 370 ___________

-

267

~0

w I—

ilL

2 EMISSION

0-

634.2

619.0 1D

0

a-

604.5

498

4.2 K

252

0

3~centre in Fig. 2.The CaO. Energy energy levelofdiagram nine 3Hfor the principal Pr 4 levels have3Pbeen established some from ‘D2 emission and some from 02 emission and 1D these are shown separately. The position of the lowest 2 level indicated by an asterisk has been determined from the

temperature dependence of the ‘D2 emission (fig. 3).

the crystal. Exciting any of the three lines gives the same emission as shown in fig. 1(b) with four sharp lines and some broad features. The energy levels associated with these transitions are summarised in fig. 2. The strongest transition at 486.2 nm, observed in both excitation and emission, is 3P considered to be associated with the 0 excited state.

~

607.0

~

~ I

600

610

6263

I

620

630

640

wavetenglh (nm) Fig. 3. ‘D2 —~ 4 emission spectra 3H at 4.2, 19.8, 79.2 K, during excitation at 607 nm and the —~ ‘D2 excitation spectrum at 4.2 K observed by monitoring 4the emission at 619 nm. No extra lines are seen in the excitation spectra recorded at temperatures up to room temperature. 3H

152

T Boonyarith et a!.

3+ :F

—

/ Zeeman measurements of Pr~+

and

4. CaF Excitation3~:O2 of CaF2 : Pr 2:Pr As the origin of the three lines in the CaO : Pr3~excitation spectrum in the neighbourhood of the 3H 3P() transition is of some con4 cern, it is advantageous to investigate the analogous excitation of other centres. The excitation of two such centres is shown in fig. 4. It is noteworthy that in both cases there are lines to the high energy side of the 3H 3P() excitation line. The 4 lines in the three centres presumably have a common physical origin and this can be substantiated by the Zeeman data presented below. In axial symmetry the 3P 1 state is split into a doublet and a singlet and from a knowledge of the free ion energies it can be anticipated that 3P there will be two levels 500 cm I above the 0 state. The flourine compensated centre is the only one that gives such lines in the appropriate spectralofregion Reeves ments these and lines based[7]onhasa made crystalassignfield analysis. The oxygen compensated centre has spectral features to both lower and higher energy but none that can be readily attributed to 3P 1 —,

—~

centres in (]aO and CaF

2

states. As mentioned earlier there are no lines in this spectral region in CaO:Pr3~.

5. Magnetic field measurements and discussion 5.1. CaF~: Pr3 + : F

-

The CaF 3~: F centre has tetragonal 2 Pr symmetry and, thus, an external magnetic field along a (001) axis will be aligned along the principal axis for one crystallographic orientation and at right angles for the other two orientations. Now, since a non-Kramers ion in a crystal field of axial symmetry exhibits no transverse Zeeman splitting (g1 = 0), the transitions associated with the centres whose axes are perpendicular to the field will be unaffected by the field. The shifted lines in fig. 5, therefore, are all associated with the axial centre. 3H 3P~transiThe of the tion at Zeeman 477.1 nmsplitting in a field of 5 T4 is consistent with transitions from a ground state split by 8.8 cm to a nondegenerate excited state. At 4.2 K —*

one of the Zeeman components of the ground

3P 3~:F (04v)

477.1

0

(a) CaF2: Pr 466.3

~

(b) CaF

Pr3~:O2(C

3v 465

470 wavelength (rim)

475

Fig. 4. Excitation spectra of the 3H 3P 3~: F obtained by monitoring the fluorescence at 642.3 nm. (b) CaF4 —~ 3~ 0 transition :02 obtained for: (a)byCaF2 monitoring : Pr the fluorescence at 656.1 nm. 2 :Pr

T. Boonyarith et a!. (a)

H

/ Zeeman measurements of Pr3

UT

centres in CaO and CaF

+

2

153

aligned centres and the pattern corresponds to that for an E E transition with an excited state g-value of 11.9 ±0.3 and the ground state g-value of 3.8 ±0.2. These values and the Zeeman pat3~centrewith [2,31. terns are in agreement previous measurements on this Pr —*

47~ °

~

__~J

I

)b)

I

~~_~s~__) H 5T 8.8 I

=

I

cm-1

I I

1~2cm~85cm1

*

~

~—,

I

I

476.5

477.0

wavelength (nm)

—

00

90°

T

.~

27

—

_____

—

_________

0

I

3H 4—~i

—I-—

~—~-——-

8.8cm

I

* *

*

1

/1 * *

TOTAL

I

-

1

I

476.0

116 ~

: 02

along the (100> direction the various trigonal axes all subtend the same angle (54.70°)with the external magnetic field. The 475.9 nm line splits into two components consistent with a transition from an E ground state to a non-degenerate excited state. The 471.8 nm line gives an entirely

I

475.5

__________

±

1cm~

.

/

3 2 : Pr

The excitation spectrum of the 02_ compensated site shows a strong sharp line at 475.9 nm which is considered to be associated with the 3P level. In addition, there are other lines 183, 257 i and 407 cm higher in energy (fig. 4) and the Zeeman splitting or shifts of all four lines have been measured. The site symmetry of the CaF 3~:02 2 : Pr centre is C3~and when the magnetic field is

*

(c)

5.2. CaF

// //

* ,~,

3~:F (C Fig. 5. Zeeman splittings of CaF2:Pr 4 ) at 4.2 K in (a) zero magnetic field, (b) a magnetic field of 5 T, H II (001), and (c) a schematic representation of the assignments 3H 3H 3p of the Zeeman splittings for the 4 ~~~hI6 and 4 —f 0 transitions. The angle between the principal symmetry axis of a site and the magnetic field direction is given above the corresponding Zeeman energy level diagram. Asterisks are . . . . used to distinguish lines from different site orientations.

state is thermally depopulated causing loss in the intensity of the Zeeman components displaced to lower energy. The 476.0 nm line likewise has undisplaced components associated with centres whose symmetry axis is transverse to the field. The magnetic field in this case causes large splittings of the

different splitting pattern, indicating that the associated excited state must also be split. The lines at 470.2 and 466.9 nm exhibit displacements to higher energy in a field of 5 T consistent with transitions to excited states of A symmetry. The Zeeman shifts are less than the line widths (—~4 cm_i) and, hence these assignments are not conclusive. In C symmetry the g-values of the ground and excited states for an E E transition can be determined directly by applying the magnetic field along the (111) crystal direction. The observed splittings shown in fig. 6(b) were obtained in a field of 5 T. The field is along the axis of one centre and it is this centre that gives the largest splittings. The other centres all have their axis at an angle of 70.5°to the field and the associated Zeeman splittings will be subsequently ~ smaller than those for the axial case. The Zeeman spectrum is then readily assigned as shown in fig. 6(c). The ground state g-value is determined to be 5.9 ±0.2 whereas that of the excited state giving rise to the 471.8 nm line is larger at 9.4 ±0.3. —

.

.

.

.

154

T Boonyarith ci a!. (a)

I

H

=

/ Zeeman

3 + centres in CaO and CaF,

measurements of Pr

case the ground state is not split by a magnetic

UT

471 8

475.9

field. This behavior is consistent with optical transitions of non-Kramers ions in low symmetry sites where all degeneracy has been lifted. Supporting this claim is the observation that with the exception of the two excitation lines at 484.5 and 484.9 nm none of the other excitation or emission lines shown in fig. I give splittings or shifts in a magnetic field of 5 T.

I

S I

I

II I

(b)

I

H

2.7 cm~

I

I

5T

C

*

_____________________________________________________________

I

~j ~J~

Ia) 18 cm 14849

__________________________________________________ II I I I

471.5

472.0

475.5

476.0

wavelength (rim)

I

00

(C)

116 ~

H

=

UT

486I 2

E H

0

70.5

~

~

1

1

____________

21,9cm1

484.5 I

cc

I 7.3

I

// I

(b)

am’1

I

H=5T

C

*

It

1

3P

___________________________

22cm

________________

0

*1 __ ____________

LI

_____________

1

~ 484.5 ~ I

4.6cm’1 )c)

I

*

~

,~

~ I 485.5 486.0 wavelength (nm)

____

1

-

I *l

I

I

T —r I

18cm I

6

I

*

___

-

1

I

3~:02 (C 2:Pr 7~)at 4.2 K in (a) zero magnetic field, (b) a magnetic field of 5 T, H II (iii) and (c) a schematic representation of3H the assignments 3H 3P of Zeeman splittings for the 4 ‘J~and 4 ~ 0 transi-

P0

—

I t.

I

I

IIt

1

6cm’

3

_______ ___________

2cm —

1

Fig. 6. Zeeman splittings of CaF

tions. Notation is the same as for fig. 5.

487.0

60°

00 ____

1*

TOTAL

F

13.8cm’

*

I

__________

______________

_________

____________

_

II I

5.3. GaO. Pr~±

IT_

—

____

________

~

Unlike the3~former twonot centres the nature of centre has been reliably estabthe CaO : Pr lished. As discussed earlier there are three excitation lines in the blue spectral region. The lowest en-

Fig. 7. Zeeman splittings of Ca0:Pr3~(C

ergy line 3H 3P at 486.2 nm has been assigned to the 4 0 transition and it exhibits no splitting or shift in magnetic fields up to 5 T. Thus in this

2~) at 4.2 K(a) zero magnetic field, (b) a magnetic field of 5 T, HIl and (c) 3H a schematic representation of the assignments of(110), the3Pquadratic Zeeman shifts for the ~H4 —~ 1~and the 4 —. 0 transitions. Notation is the same as for fig. 5.

—*

*

TOTAL

~

/F

T. Boonyarith ci a!.

/ Zeeman measurements

Table 1 Magnetic 3Hsplitting (g) factors for the lowest crystal field levels of the 4 ground multiplet and the ~J6 multiplet, as determined from low temperature Zeeman measurements in a magnetic field of 5 T. 3~:F CaF2:Pr CaF

3~:02

CaO:Pr3’1 2:Pr a) b)

3.8±0.2 4 3.89 al 3.71 b) 5.9±0.2 0

‘I 11.9±0.3 6 11.87 9.4±0.3 10.3±0.3 (effective)

3 + centres in CaO and CaF

of Pr

2

155

along such axes that gives rise to the interaction between levels. The Zeeman patterns can, therefore be predicted and the strength of the interac.

tion determined from the experiment (fig. 7). The magnitude of the Zeeman interaction is found to two levels beentodegenerate. be equivalent a geff~c,jve= 10.3 ±0.3 had the

6. Discussion

Macfarlane et al. [2]. Tissue and Wright [3].

As shown in fig. 7, however, the lines at 484.9 and 484.5 nm split in the presence of a magnetic field. This can be readily understood as a consequence of a shifting of the levels, rather than a splitting of levels. It is observed that only the lower energy transition has components displaced to lower energy and the higher energy line components to higher energy. In addition, the magnitude of the Zeeman shifts depended on the square of the field strength. The pseudo-Zeeman splitting is a consequence of field mixing between the two excited state levels separated by 18 cm For a field along the (100) direction, each line splits into two components and, for a field along the (110) direction, into three components (two of which are not fully resolved). In addition for both lines a component remains unshifted. The patterns are what would be expected for two interacting levels of a centre with its axis along a (110) direction. In C2.~,symmetry all irreducible representations are non-degenerate and, hence, two levels can only interact via a perturbation of appropriate symmetry. along the separate axesMagnetic of the Cfields aligned 2.,, site each transform according to a different irreducible representation (A2, B1 or B2) and, hence, only one giveoccur rise to the interaction. maximumcan shifts when the externalThe magnetic field is along a <110> direction and this indicates it to be the major axis of one of the C 2~centres, The equivalent axis of the other C2,,, centres will correspond to the other <110) crystallographic directions and it is the component of the field ~.

In each of the three Pr3~centres investigated, the transition from the ground state to the 3P 0 level has been identified and the magnetic field measurements confirm that the excited state level in all cases is, indeed, non-degenerate. To higher energy there are additional levels and in all three cases it has been shown that the first of these levels has associated with it a large g-value. In the axial centres the g-value is that associated with a linear splitting of a two-fold degenerate state. In CaO the states are non-degenerate and the effective g-value was determined from the repulsion of two levels by an off-diagonal Zeeman interaction. These two levels would be degenerate in higher symmetry (cubic or axial) and the distortion which produces the C2~symmetry is what causes the splitting of 18 cm Once the magnetic field is large enough to make the Zeeman interaction comparable with the zero field splitting the levels are repelled from one another and the magnitude of the interaction can be expressed in terms of an effective g-value that would appropriate if there was no zero field splitting. The magnitude of the excited state g-value in all three centres (9.4, 11.9 and 10.3)3P~ islevel much (3 larger than that which is possible for a in first order) but compatible with that expected for a ~16state. Spectroscopic g-values of similar magnitude3~[8]. have been states in The established extra levels for can,~j6therefore, LaCl3 : Pr be clearly assigned to crystal field 116 states. For the CaF 3~: F centre this is consistent with 2 : Pr the assignment of Tissue and Wright [3]. Charge compensation for the trivalent ions in the alkaline-earth oxides is generally achieved through the incorporation of cation vacancies in ~.

156

T. Boonyarith ci a!.

/

1 + centres in CaO and CaF,

Zeeman measurements of Pr’

the ratio one vacancy for every two trivalent cations. There is, therefore, a binding energy between the ion substituting for the Ca2~ions in the °h site and the vacancy. This encourages an association between the species and results in lower symmetry sites. With a nearest neighbour vacancy the Pr3~centre will have C 2~symmetry and the Zeeman measurements have established that this is the symmetry of the dominant centre. It is clear that this is the major centre in CaO but there also exists a 33 significant ions [1].number of non-compensated cubic Pr

single Pr3 site ion levels associated with the II~ state. Porter and Wright [1] have proposed that the predominant Pr3~ion centre in CaO is formed by a substitutional Pr3 ion with an adjacent charge compensating Ca2~vacancy giving an or-

7. Conclusions

[1] L.C. Porter and J.C. Wright, J. Chem. Phys. 77 (1982) 2322. [2] R.M. Macfarlane, D.P. Burum and R.M. Shelby, Phys. Rev. B 29 (1984) 2390. [31B.M. Tissue and J.C. Wright, Phys. Rev. B 36 (1987) 9781.

~±

In these studies it has been found that Pr doped crystals have crystal field energy level patterns which frequently exhibit ~ 16 states lying 3P close in energy to the 0 state. This behaviour is found for both six-fold and eight-fold co-ordinated systems. In particular, from the Zeeman measurements, itt isabove shownthethat3P()thelevel two in levels 54 CaOat are and 72 cm

thorhombic site of C2~symmetry. An analysis of the quadratic Zeeman effect reported here gives convincing support for this model of the centre.

References

[4] F.G. Gustafson and J.C. Wright, Anal. Chem. 51 (1979) 1762. [5] Z. Hasan and NB. Manson, J. Lumin. 38 (1987) 40. [6] H. Masui and S. lbuki, J. Phys. Soc. Jpn. 22 (1967) 1387. [71R.J. Reeves, Ph.D. Thesis, University of Canterbury, [8] Christchurch, R. Sarup and New M.H.Zealand Crozier, (1987). J. Chem. Phys. 42 (1965) 371.