Optical properties of Mn2+ in KCaF3 single crystal

Spectrochimica Acta Part A 55 (1999) 375 – 380

Optical properties of Mn2 + in KCaF3 single crystal Z. Mazurak a,*, A. Ratuszna b, Ph. Daniel c a

Department of Solid State Physics, Polish Academy of Sciences, ul. Wandy 3, 41 -800 Zabrze, Poland b Institute of Physics, Silesian Uni6ersity, ul. Uniwersytecka 4, 40 -007 Katowice, Poland c Laboratoire de Physique de l’Etat Condense-URA CNRS No 807, Uni6ersite´ de Maine, A6enue Oli6ier Messien, 72017 Le Mans, France Received 30 October 1997; accepted 28 May 1998

Abstract It is known that the spectroscopic properties of 3d impurities in crystals are very sensitive to the environment of the ion and can be changed considerably by using different matrices. The crystal structure of KCaF3 has been previously determined by the Rietveld profile method. At room temperature, KCa1 − x Mnx F3 (x B0.1) crystallizes in monoclinic C2h (B21/m) symmetry. The local geometries around Mn2 + in this crystals, in their ground and excited states, are the primary properties that govern the spectroscopic behavior of these systems, which enjoy of fundamental and technological interest. The present work reports the absorption and luminescence spectra of the Mn2 + -doped KCaF3 (fluoroperovskite). The luminescence spectra recorded over a range of temperatures are dominated by wide bands, corresponding to the 4T1(G)“ 6A1(G), Mn2 + transition. The lifetime (t = f(T)) of the first excited state 4T1(G) was measured as a function of temperature. The lifetime of the Mn2 + emission, in this crystal have been found to be temperature independent (tB7 ms). The absorption and emission spectra of Mn2 + (3d5) in KCaF3 are analyzed using a C4 crystal-field hamiltonian. The calculated energy levels are in good agreement with those obtained experimentally. The resulting crystal-field parameters Bnm are a good representation of the crystal-field interactions of Mn2 + in KCaF3. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Manganese; Fluoroperovskites; Luminescence spectra; Crystal-field parameters

1. Introduction Currently most operating vibronic lasers are based on 3d ions such as Cr3 + , Cr4 + , Co2 + or Ni2 + [1]. To increase the spectral range covered by tunable solid-state lasers, there must be a search for doped crystal systems based on differ* Corresponding author. Fax: +48-3-2710658; e-mail: [email protected].

ent ions or ions on different ionization states. Recently, a new laser material LiBaF3:Ni2 + was discovered [2]. Because of the novelity of laser action in this fluoroperovskite crystal and our interest in the new tunable laser materials, we decided to characterize Mn2 + -doped in KCaF3 matrix. In anhydrous fluorides the environments of a Mn2 + ion are most commonly tetrahedral or octahedral. When tetrahedral coordination is present the materials may be regarded as contain-

1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 8 ) 0 0 1 9 6 - 6

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ing MnF24 − ions, whilst those materials where the coordination is octahedral generally consist of MnF46 − complexes. Materials which contain discrete MnF24 − ions luminescence strongly, even at room temperature. In contrast, the room temperature luminescence from crystals which contain extended arrays of octahedral MnF46 − complexes is usually very weak or even undetectable. The relatively strong magnetic coupling associated with extended arrays leads to rapid exciton translation. For manganese compounds exciton migration requires thermal activation but is usually very fast at room temperature. The quenching of the Mn2 + luminescence undoubtedly results from exciton trapping at defect centers in the crystal. In the present work, we have undertaken a systematic analysis of the optical spectra (absorption and luminescence) of the Mn2 + ion in KCaF3, using a crystal-field hamiltonian of C4 point group symmetry. The present data together with those available for KMgF3:Mn2 + and KZnF3:Mn2 + [3] complete the whole series of fluoroperovskites and allow us to establish important correlations between the optical properties and the local structure of the MnF46 − complex.

2. Experimental A good optical quality monocrystal KCaF3:Mn2 + was grown by a modified Bridgman–Stockbarger method. Large crystals of several cm3, were obtained; they were transparent and homogenous. The crystal structure of KCaF3 has been previously determined by us, using the Rietveld profile method. At room temperature, KCa1 − x Mnx F3 (x B 0.1) crystallizes in monoclinic B21/m symmetry, with the lattice parameters: a= 0.8754(2) nm, b =0.8765(4) nm, c= 0.8760(5) nm, b = 90.48(3)°, Z = 8. The structure contains a tilting of the CaF46 − octahedra of the a − b + c − type, and which are responsible for the monoclinic distorsion in perovskite crystals [4]. For optical measurements we used a polished 6×6× 6 mm cube of KCaF3:Mn2 + with a measured Mn2 + concentration of 4 mol%. The absorption spectrum was recorded with a Cary-Varian Model 2400 spectrophotometer at

room temperature. The luminescence spectra were excited by the 514 nm line of a cw Ar + ion laser and analyzed by Carl-Zeiss Model GDM-1000 grating monochromator equipped with a phasesensitive detection system. In emission measurements, the sample was mounted in continuousflow helium cryostat equipped with temperature controller. In the luminescence decay investigation a Lambda-Physik FL2000 pulsed dye laser (Exciton, Rhodamine-520) was used as an excitation source. The sample luminescence was analyzed by a GDM-1000 monochromator. The output from the RCA-C31034A photomultiplier tube was processed by a boxcar integrator and data were stored by an IBM-AT microcomputer. All emission spectra were corrected for the spectral response of the system using a standardised lamp.

3. Results

3.1. Absorption and luminescence spectra In this section we present experimental results including absorption and luminescence spectra. Fig. 1 shows the room temperature absorption spectrum of fluoroperovskite KCaF3:Mn2 + . The spectrum clearly reveals several broad bands typical of octahedrally coordinated Mn2 + . The band centered at 530 nm corresponds to the 6A1g(S)“ 4 T1(G) transition. It is important to note that the peak found for the 4T1(G) transition coincides with the value reported for other fluoroperovskites doped with Mn2 + [3]. The luminescence spectra of this crystal were investigated in the temperature range from 5 to 100 K. Throughout the range the Mn2 + emission consisted of a single broad band, devoid of sharp features. The luminescence spectra of KCaF3:Mn2 + crystal at several temperatures are shown in Fig. 2. In all cases, the bands are asymmetric. There is a rather big shift (700 cm − 1) of the emission band to higher energy as the KCaF3:Mn2 + crystal is warmed from 5 to 100 K. The integrated intensity of the emission at 100 K is about one tenth of that at 5 K. Fig. 3 shows the

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377

Fig. 1. The room temperature absorption spectrum of KCaF3:Mn2 + single crystal. The actual dopant concentrantion was 4 mol%; The crystal thickness was 6 mm.

temperature dependencies of the emission intensities from KCaF3:Mn2 + crystal. In our crystal the luminescence completely disappears before 150 K. In addition to recording the spectra, the luminescence lifetimes of the 4T1(G) excited state were measured from 5 to 100 K using a pulsed dye laser as an excitation source.

Fig. 2. The emission spectrum as a function of temperature of a KCaF3:Mn2 + crystal. Each trace was recorded at the some spectrofluorometer sensitivity. The crystal was excited at 520 nm (19 250 cm − 1).

Changes in the lifetime of the Mn2 + emission were found to be temperature independent (tB 7 ms). The luminescence spectra of the other fluoroperovskites (KMgF3 and KZnF3) doped with Mn2 + are quite similar to that KCaF3 except for slight variations in the emission wavelength [3].

Fig. 3. Variation of the luminescence peak maxima Eem at different temperatures for a KCaF3:Mn2 + crystal. The variation of the energy maximum is given for excitation at 520 nm.

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3.2. Crystal-field analysis of Mn 2 + (3d 5) in KCaF3 Crystal-field theory predicts that the energy level sequence for a 3d5 ion in an octahedral environment should be 6A1(G) B 4T1(G)B 4 E(G)B 4A1 B 4T2(D) B 4E(D) B 4T1(D). The order is the one that would be expected from Tanabe–Sugano diagram with Dq/B $ 1.0.

3.2.1. Free-ion Hamiltonian The free-ion Hamiltonian HFI used in the analysis of the spectra of Mn2 + (3d5) is given by HFI = F (2) · g2 +F (4) · g4 +a · L(L +1) + zd · %li · si

(1)

i

where gk = %

% C*kq (i) · Ckq (j)

(2)

the same manner as the F (k) are treated as parameters in fitting the free ion data. In the point charge model, these parameters are related to the Anm by Bnm = Žr nAnm

(6)

Fluoroperovskite KCaF3 has the space group B21/m. (C2h ) and the Mn2 + ion may substitute at the Ca2 + site with a C2 coordination symmetry. For an ion in C2 symmetry Eq. (5) (see Morrison [8]), we have 14Bnm. With the free ion parameters F (k) and a, we have a total of 17 parameters. This large number of parameters requires an extensive number of experimental levels for a unique fit. Because of the limited number of crystal-field levels experimentally available, we decided to use C4 symmetry in the crystal-field analysis of the data. In the C4 symmetry, Eq. (5) becomes HCEF = B20 % C20(i)+ B40 % C40(i) i

i\j q= −k

i

+ B44 % [C44(i)− C4, − 4(i)]

and Ckq (i)=[4P/(2k +1)]

C*kq = (−1)q · Ck, − q

(3)

in Eq. (1), zd is the spin-orbit parameter, the term involving a is the Trees interaction [5] and the F (k) are the Slater parameters. Frequently, the Racah parameters B and C are used in place of the Slater parameters, F (k); they are connected by the relations [6]: F (2) = 7(7B + C), F (4) =(63/5)C

(4)

4. Analysis of the experimental data The crystal-field Hamiltonian, HCEF, for the 3d5 configuration which we shall use in calculation, can be written as [7]: HCEF = = %

n

%

n m= −n m

B*nm % Cnm (i), i

B*nm =(− 1) · Bn, − m

(7)

i

· Ykq (Ui, qi ),

1/2

(5)

where the Bnm are the crystal-field parameters. The Bnm of Eq. (5) are treated as parameters in fitting the energy levels of the ion in a crystal, in

with all Bnm real. The cubic approximation to Eq. (7) is: HCEF = B cubic · % {C40(i) 40 i

+ 5/14 [C44(i)− C4, − 4(i)]} cubic 40

(8)

= − 14Dq. with B We start the fitting by using the cubic approximation given in Eq. (8). Using these results, we proceed to fit the data in C4 symmetry using Eq. (7). In both of these calculations we ignore spin– orbit coupling of Eq. (1). Finally, we introduce all the experimental data with the spin–orbit coupling included. A satisfactory fit to the experimental data set has been obtained for the 3Bnm crystal-field parameters for one Mn2 + site in KCaF3. To start the analysis of Mn2 + in KCaF3, we used the Tanabe–Sugano plot [9] to obtain a value for Dq/B$1 using values for three levels consistent with the C4 levels obtained from the absorption spectrum. These are 4T1(G) (18890 cm − 1), 4E(G) (23350 cm − 1) and 4A1(G) (26950 cm − 1). With Dq/B= 1 from this diagram, we

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Table 1 The energy levels and crystal-field parameters Bnm of Mn2+ at C4 site, in KCaF3a Item Crystal-field parameters

Mn2+ (cubic)

Mn2+ (tetrahedral)

(a) The crystal-field parameters Bnm B20 – −8250 B40 B44 – a 30 z – Level

Energy observed (cm−1)

(b) Calculated and experimental energy le6els A1 0 0 4 T1(G) 18 890 19 650 – – 4 E(G) 23 350 – – – 4 A1(G) 26 950 – 4 T1(D) – – – 29 500 6

a

4100 −10 940 −7050 30 300 Energy calculated (cm−1)

0 91 19 020 19 740 21 646 22 330 23 410 23 512 25 956 27 057 27 944 28 130 28 511 28 880 29 078 29 656

D

0.24 75 1 4 69 8 1 390 387 121 – 92 93 – 110 –

Label

A A E A A E E A E E A A A E A E

F (2) =63 120 cm−1, F (4) = 34 750 cm−1.

have E/B = 22 for the 4T1(G) level. Substituting the above value of the energy for E gives a value of B= 858 cm − 1; for free ion values, we have C/B$ 3.8B. Thus, we have F (2) =64862 cm − 1, F (4) = 41076 cm − 1 and B40 = −12010 cm − 1 as starting values in Eq. (1) (zd =0) and Eq. (8) for the first iterative fit of the experimental data. In the initial phase the parameter a was arbitrary set at 30 cm − 1. With all the parameters the same as the cubic case, the experimental data for the 4 T1(G) and 4E(G) were changed, and the parameters were varied to obtain best fit. After the fitting of the experimental data in C4 symmetry as given above, a single run was made with the spin–orbit interaction included as given in Eq. (1). The spin– orbit constant zd was taken to be 300 cm − 1, which is slightly less than the free ion value. In the double group the singlet states (with zd =0), 2A, with spin S =1/2 becomes a doublet and the

ground 6A state splits into four levels. That is, all the levels are Kramers doublets, and the splitting induced by the spin–orbit is listed under D. A summary of the parameters that give least-rms-deviations between the theoretical levels and the experimental levels for Mn2 + in C4 symmetry sites is given in Table 1(a).

5. Discussion The photoluminescence measurements were performed under excitation with 514 nm light. The lifetime (t) of the first excited state 4T1(G) was measured in KCaF3:Mn2 + crystal, with Mn2 + concentration 4 mol%, at different temperatures. The temperature independent and very short lifetime is due to nonradiative exciton processes. It is interesting to observe that the center

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of the structured band maxima Eem at low temperatures are very sensitive to the host lattice. We associate this with the fact that the equilibrium Mn–F distance for the MnF46 − complex embedded in fluoroperovskite depends on the host lattice. Due to the rich vibrational structure shown by the luminescence spectra of KCaF3:Mn2 + , KMgF3 and KZnF3, the corresponding centers of band may be determined as (Eem1 =16600 cm − 1, Fig. 3), (Eem2 =16600 cm − 1 [3]) and (Eem3 = 16870 cm − 1 [3]), respectively. The accuracy estimated for all of these series is DEem =50 cm − 1. In the observed Mn – F distances in the Mn2 + –F − ions (206B R B216 pm) [3,4], it can be seen that the Eem parameter increases with R. This behaviour of Eem has been explained within a ligand-field scheme and in a first approximation it can be written Eem =6B + 5C − 10Dq, where B and C are the effective Racah parameters. Experimentally, it has been demonstrated that B and C for MnF46 − are almost independent of R while 10Dq =KR − n. A value n=4.7 has been obtained experimentally for MnF46 − [10]. The results have the consequence for octahedral MnF46 − complexes formed in different crystals; The local Mn– F distance can be derived by measuring the excitation or emission spectra. An analysis of the reported optical spectra of Mn2 + in KCaF3 taken at low temperatures indicate that the Mn ions substitute for Ca ions at Ca(2) sites where the point group symmetry is C4. The analysis in C4 symmetry again assumes that the observed by us spectra arise predominately from Mn2 + ions that occupy the site with that symmetry. The calculated energies of the excited states of Mn2 + in KCaF3 fluoroperovskite are presented in Table 1(b), in the column headed

.

calculated. Our results clearly show that the broad band for the 4T1(G)“ 6A1(G) transition may be partly ascribed to the large non-cubic component of the crystal-field (and to the weak spin–orbit interaction to a smaller extent). The analysis reported here should be a good starting point in the interpretation of such experimental data.

Acknowledgements This work was supported by the grant: Spectroscopic Investigations of Transition Metal Ions in Crystals, Contract No.6-P04D-043-12 from the Committee for Scientific Research, Warsaw, Poland.

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