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Phys. Status Solidi B 246, No. 7, 1686–1691 (2009) / DOI 10.1002/pssb.200844281 p s sbasic solid state physics

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Spectroscopic study of the low-dimensional (C3H7NH3)2CdCl4 crystal irradiated with X-rays

Volodymyr Kapustianyk*, 1, 2, Myron Panasyuk2, Maryan Partyka**, 1, 2, Viktor Rudyk2, and Volodymyr Tsybulskyy2

1 Department of Physics, Ivan Franko National University of Lviv, Dragomanova st. 50, 79005 Lviv, Ukraine 2 Scientific-Technical and Educational Center of Low Temperature Studies, Ivan Franko National University of Lviv,

Dragomanova st. 50, 79005 Lviv, Ukraine

Received 9 July 2008, revised 26 March 2009, accepted 3 April 2009

Published online 12 May 2009

PACS 61.44.Fw, 64.70.K–, 78.40.Me, 78.70.Dm

** Corresponding author: e-mail kapustianyk@yahoo.co.uk, Phone: +38 032 239 4679 ** e-mail maryan.partyka@gmail.com, Phone: +380 050 431 5935

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Alkylammonium metal halides are layered perovskite-type compounds, (CnH2n+1NH3)2MX4: n = 1, 2, . . . 18; M = Cd, Co, Cu, Fe, Mn, Pb, Pd, Zn; X = Cl, Br, I. They show a large variety of physical proper-ties depending on the kind of metal cation. They are ferro-magnetic with M = Fe and Cu, antiferromagnetic for Mn, nonmagnetic semiconductors for Pb and insulators for Cd and Zn [1]. The crystals of such a type belong to the large class of layered compounds that can be used as substances for scintillators and other sensors of high-energy (ionizing) electromagnetic or charged particle radiation. However, the optical properties and electronic structures of these compounds are not well studied yet. The alkylammonium metal halides are two-dimen-sional materials and exhibit strong structural anisotropy. They consist of inorganic layers of corner-sharing MX6 oc-tahedra and organic layers of alkylammonium ions. The NH3

+ polar heads of the alkylammonium occupy cavities among the metal–halogen octahedra, forming NH . . . X hydrogen bonds [2–5]. Some of propylammonium com-

pounds of this family have been reported to undergo a transition into the incommensurate (IC) phase [6]. The cation dynamics and interionic interactions through hydro-gen bonds are expected to be closely related to the physical properties and mechanisms of structural phase transitions in these materials. Bis(n-propylammonium) tetrachlorocadmate n-(C3H7NH3)2 CdCl4 (abbreviately n-PA-CdCl4) possesses a complicated sequence of structural phases including two rather unusual incommensurate phases γ and ε [6–8]. The high-temperature incommensurate (IC) γ phase of a “re-enter” type is located between β and δ phases with the same orthorhombic space groups of symmetry. The γ phase is limited by the second-order phase transitions at Ti1 and Tc1 = 374 K, whereas some authors reported that the phase transition at Ti1 cannot be investigated by the optical meth-ods due to the thermal decomposition of the sample in vi-cinity of its surface on heating above this temperature [7]. In the low-temperature region n-PA-CdCl4 crystal un-dergoes phase transitions at 178.7 K, 156.8 K and 105.5 K

The optical absorption spectra of the two-dimensional crys-

tals of n-(C3H7NH3)2CdCl4 induced by X-ray irradiation at

20 K were studied and compared with those for the as-

received sample. It has been found that X-irradiation causes

effects similar to those observed in the related crystals of

(CnH2n+1NH3)2CdCl4 (n = 1, 2). New optical absorption bands

(IR bands) were found in the region 10–17 kcm–1, besides the

absorption band of the Cl2– molecular center characteristic of

this compound. The absorption intensities of these bands de-

crease at temperatures higher than 40 K accompanying the

decrease of the Cl2– concentration in n-(C3H7NH3)2CdCl4 crys-

tals. These IR bands are assigned to the electron-trapped cen-

ter where an electron is trapped by an alkylammonium head

in the neighborhood of a Cl– vacancy. Such a model was

found to be in good agreement with the results of the X-ray

luminescence and thermoluminescence study.

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and shows a normal – incommensurate – commensurate – commensurate phase transition sequence with decreasing temperature [6]. In other studies these three transitions have been detected at 180 K, 158 K and 110 K, respec-tively [9]. In our previous work, the temperature evolution of the optical absorption edge of n-PA-CdCl4 crystals appearing in the vicinity of 3.7 eV was investigated [10]. It has been shown that the absorption edge follows the empirical Ur-bach rule in all low-temperature phases except the incom-mensurate one. Analysis of the temperature dependences of the Urbach rule parameters confirmed the existence of the phase transitions at Ti2 = 180 K, Tc2 = 158 K and Tc3 = 106 K. The crystals of (CnH2n+1NH3)2CdCl4 (where n = 1, 2, 3) irradiated with X- or γ-rays at liquid-nitrogen temperature (LNT) [11–13] were investigated by means of absorption spectroscopy. It was found that under irradiation a molecu-lar ion Cl2

– is created. From the detailed studies, it was con-cluded that under irradiation the hydrogen bond between a Cl– ion and a hydrogen of NH3 is broken and two Cl– ions form a Cl2

– molecular ion, releasing an electron. The elec-trons released in such processes are trapped primarily in shallow potential defects, and at higher temperatures they are released again from them and finally are captured by metal cations or impurities. On the other hand, under the influence of X-irradiation at 15 K additional absorption bands in the near-infrared region were observed in the related crystals of (CnH2n+1NH3)2CdCl4 (where n = 1, 2) [14]. These bands (according to [14] – “IR bands”) were assigned to an elec-tron center where an electron is trapped by an alkylammo-nium head in the neighborhood of a Cl– vacancy. In order to extend information about the radiation effects in the crystals with longer organic chains detailed investigations of the absorption spectra of the irradiated and as-received samples of n-(C3H7NH3)2CdCl4 in the near-IR–visible–near-UV region were performed within the temperature range of 20–300 K. The obtained experimen-tal results were complemented by the data of the X-ray lu-minescence and thermoluminescence study. Special atten-tion was devoted to the study of the electron-trapped cen-ters created in a sample by irradiation at low temperatures close to that of liquid helium. 2 Experimental The n-PA-CdCl4 crystals were grown from aqueous solution of CdCl2 ⋅2H2O and [C3H7NH3]Cl salts, taken in stoichiometric ratio, by the slow evaporation method at room temperature. The obtained optically transparent samples of good quality, very similar in shape to the crystals of (CnH2n+1NH3)2CdCl4 (n = 1, 2), were freshly cleaved parallel to the layers in the shape of thin (∼1.5 mm) crystalline platelets and put into the special helium cryostat, containing quartz windows for light detection and a beryllium window for X-ray excita-tion. The direction of the X-ray beam as well as that of the light beam was normal to the ab-plane of the crystal. The

X-ray tube URS-60 with a Mo-anticathode operating at 55 kV and 10 mA was used as an excitation source (~300 mR/min). The contribution of the low-energy con-tinuum was reduced by means of a 0.5 mm aluminum filter. A photomultiplier FEU-79 operating in the single-photon counting regime was used as a photodetector. A tempera-ture controller “UTREX K43” provided necessary preci-sion of the temperature measurements and stabilization (ΔT = ±0.1 K). The optical absorption spectra and lumines-cence spectra were measured using a MDR-12 monochro-mator with an optical resolution of 2.5 nm. The thermolu-minescence was recorded at a 0.1 K/s linear heating rate, registering total luminescence in the spectral region of 350–950 nm (10.5–28.5 kcm–1). The temperature was measured in a helium cryostat using a silicon thermodiode as the temperature sensor. 3 Experimental results and discussion The n-PA-CdCl4 crystals are transparent in the visible region (see in-sert to Fig. 1). The sharp growth of absorption in the vicin-ity of 30 kcm–1 corresponds to the low-energy “tail” of the intense band formed by the self-localized exciton [10, 15]. The virgin sample is also characterised by two weak absorption bands in the vicinity of 155 kcm–1 and 25 kcm–1. According to [10] they are related to the uncontrolled im-purities, first to Cu2+. Taking into account the very high pu-rity of the salts used for the crystal growth, the band at 25 kcm–1, more precisely, is thought to be due to localiza-tion of the exciton on the above-mentioned impurity of Cu2+. This explains its appearance even in the case of a very low concentration of the impurity [10]. Figure 1 presents the temperature evolution of the X-ray induced absorption spectra of a n-PA-CdCl4 crystal after irradiation at 20 K for 45 min. We did not find any considerable changes of the spectra in the vicinity of the

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Cl-2

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62 K

40 K

20 KIR

Abs

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Energy, kcm-1

293 K

Figure 1 Temperature evolution of the optical absorption spectra

of n-PA-CdCl4 crystal irradiated at 20 K with X-rays for 45 min.

The spectrum of the virgin sample obtained at T = 293 K is

shown in the inset.

1688 V. Kapustianyk et al.: Spectroscopic study of low-dimensional (C3H7NH3)2CdCl4

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absorption edge. On the other hand, irradiation of the sam-ple is followed by the appearance of an intense broad absorption band spreading from the near-infrared region out to the visible region. It is necessary to note that the obtained spectra are smeared and poorly structured in comparison with the case of (CnH2n+1NH3)2CdCl4 crystals (n = 1, 2) where clear absorption bands were observed [14]. Taking into account the good quality of the used sample, the only reason for such a behavior could be related to the more complicated structure of the organic cation and cor-responding broadening of the bands due to the influence of the phonon subsystem with a broader energy spectrum. At the same time, the intensity of the obtained spectra after heating manifests behavior similar to the case of (CnH2n+1NH3)2CdCl4 crystals, where n = 1, 2 [14]. Thus, one can conclude that X-irradiation should cause similar effects. Such a conclusion is confirmed by the fact that the obtained spectra are satisfactorily approximated with four Gaussian contours (see Fig. 2) that correlate with data for (CnH2n+1NH3)2CdCl4, n = 1, 2 [14]. The approximation of the observed broad band by Gaussian contours performed using the procedure de-scribed in [16] allowed identification of the corresponding electron transitions. Figure 2 presents the optical absorption spectrum of n-PA-CdCl4 irradiated with X-rays for 30 min at 20 K. An absorption band centered at 22 kcm–1 (half-width 6.5 kcm–1) remains stable at high enough temperatures (at least to LNT) (see the inset to Fig. 2) and its position and shape are very similar to those for the band observed in these crystals under X-ray irradiation at LNT [14]. There- fore, this band is thought to originate from the Cl2

– molecu-lar ion. In the energy region of 10–20 kcm–1 other absorp-

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AC

Cl-2

IR

Cl-2

Figure 2 Optical absorption spectrum of n-PA-CdCl4

X-irradiated at 20 K for 30 min (presented by symbols). The solid

line shows the fitting curve resulting from the Gaussian line-

shapes plotted by the dotted lines. The inset presents the spectrum

of n-PA-CdCl4 X-irradiated at 20 K for 45 min and heated to

76 K.

Table 1 Parameters of Cl2– and IR bands (A, B and C) for the

samples irradiated at 20 K.

time of

irradiation

band peak energy

(kcm–1)

oscillator strengths

(kcm–2)

30 min Cl2–

A*

B

C

22.0 ± 0.4

10.3 ± 1.6

14.0 ± 2.2

17.0 ± 4.0

81.0 ± 13.0

41.0 ± 40.6

32.0 ± 3.6

25.0 ± 8.3

45 min Cl2–

A*

B

C

22.0 ± 0.4

8.0 ± 1.2

13.0 ± 1.6

17.0 ± 1.9

74.0 ± 2.4

45.0 ± 31.3

36.0 ± 4.7

32.0 ± 5.0

(* roughly estimated parameters)

tion bands A, B and C (according to [14] – IR bands) could be separated. Similarly to the case of the related crystals [14], appearance of the absorption bands in the region un-der investigation should be related to the electron-trapped centers, created in the crystal at low temperatures by the ir-radiation. It is necessary to note that all bands are shifted toward lower energies in comparison to those obtained in [14] for (CnH2n+1NH3)2CdCl4, n = 1, 2. Increasing the time of irradiation to 45 min at the same temperature, 20 K, leads to growth of absorption in the en-tire spectral region of investigation (see Fig. 3). In addition, the band A shifts toward lower energies beyond the work-ing spectral region. As a result its parameters could be only roughly estimated. Introducing this band we simply indi-cate the fact that the induced absorption is shifted toward lower energies with increased irradiation time. It is worth noting that similar behavior of the band A was also ob-served for the (CH3NH3)2CdCl4 crystals [14]. Peak energies and oscillator strengths of A, B, C and Cl2

– bands are sum-marized in Table 1. It is clearly seen that the oscillator

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IR

Figure 3 Optical absorption spectrum of n-PA-CdCl4

X-irradiated at 20 K for 45 min (presented by symbols). The solid

line shows the fitting curve resulting from the Gaussian line-

shapes plotted by the dotted lines.

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strengths of the A, B and C bands increase with the irradia-tion time. This fact reflects the increase in the correspond-ing radiation defect concentration. As we noted previously, when the temperature of n-PA-CdCl4 is raised above 40 K the intensity of the IR bands decreases and at 76 K they almost completely disap-pear as is clearly displayed in Fig. 1. Another consequence of heating is decreasing of the intensity corresponding to the Cl2

– band. Taking into account that the Cl2– becomes un-

stable only at temperatures higher than 100 K [11–13] one can conclude that the mechanism of Cl2

– band bleaching at temperatures higher than 40 K is not connected with the thermal instability of Cl2

–. Therefore, one can suppose (as was assumed in [14]) that the defect centers responsible for IR bands, which could be assigned rather to electron-trapped center, are involved in such a process. These cen-ters become unstable at temperatures higher than 40 K. The electron released in this process recombines with the Cl2

– center causing a decrease of the intensity of corre-sponding band. The activation energy of the electron-trapped center could be determined from the temperature dependence of the optical absorption at constant wavelength in the vicin-ity of the absorption maxima of the corresponding band us-ing the formula [14]:

{ }01 exp ,

Ef

kTα α

Ê ˆ= - ¥ -Ë ¯ (1)

where k is the Boltzmann constant, α0 corresponds to the optical absorption initially induced at 20 K by X-irradiation and f, E are the frequency factor and activa-tion energy, respectively. From the temperature dependences measured at 14 kcm–1 (Fig. 4) we obtain f = 102 and E = 32 meV. The obtained value of activation energy is lower compared to related crystals of (C

nH2n+1NH3)2CdCl4 (where n = 1, 2)

[14]. The observed decreasing absorption can be explained in the framework of the model description of the electron-trapped center proposed in [14], where the IR bands are as-signed to originate from an electron center where an elec-tron is trapped by the alkylammonium head in the vicinity of a Cl– vacancy. At low temperatures under X-ray irradia-tion an electron is removed from Cl– forming a molecular ion Cl2

–. Therefore, the electron-trapped center is formed simultaneously with the Cl2

– center. For better clarity of the proposed model the schematic illustration of the electron trapping center formation under the influence of X-ray ir-radiation is presented in Fig. 5. The scheme was built by analogy with those proposed in [14]. Figure 5a illustrates the relative configuration of an alkylammonium head to a MCl6 octahedron. A hydrogen bond between NH3

+ and chlorine is depicted by the dashed line. Figure 5b shows schematically a Cl2

– molecular ion formed in the ac-plane by the X-irradiation. In this case, the chlorine anion site located in the c-direction of the octahedra becomes vacant.

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Temperature, K

Figure 4 Temperature dependence of the absorption coefficient

at the constant wavelength 14 kcm–1 for the n-PA-CdCl4 crystal.

The bold solid line corresponds to the fitting based on formula (1).

As the hydrogen bond is broken, the alkylammonium head becomes able to acquire an electron. In contrast to an anion vacancy in alkali halide crystals, the alkylammonium head in the vicinity of the Cl– vacancy becomes an electron trapping center. Therefore, the trapped electron is expected to have a molecular orbital perturbed by the Cl– vacancy. By analogy with the related crystals manifesting the simi-lar temperature dependences of the IR-bands intensities [14] it was assumed that the A, B and C bands originate from the same defect center and correspond to transitions from the same ground state to different excited states. Thus, bands A and B would be assigned to the transitions from the molecular ground state of Σg to the excited states of Π

u

and Σu, respectively. The C band may be assigned to the

transition to a higher excited state than Πu and Σ

u [14].

Figure 5 (a) Schematic illustration (built by analogy with [14])

of an ammonium head and a CdCl6 octahedron. (b) Model of the

electron-trapped center.

1690 V. Kapustianyk et al.: Spectroscopic study of low-dimensional (C3H7NH3)2CdCl4

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800

850

900

950

1000

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Inte

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, a.u

.

Energy, kcm-1

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Figure 6 X-ray luminescence spectrum of the n-PA-CdCl4 crys-

tal obtained at 7 K in the process of irradiation.

Under such circumstances one can interpret the men-tioned decreasing Cl2

– band intensity upon heating up to 100 K as follows. When the electron-trapped center be-comes thermally unstable, the released electron starts to migrate through the conduction band and could be trapped by the nearest Cl2

–, impurity and/or a metal cation. In the work [15] a conduction band of new type composed of NH3

+ heads about 1 eV below the conduction band com-posed of Cd2+ cations was revealed. Thus, the ground state of the electron-trapped center is just below this band com-posed of NH3

+ heads and the trapped electron can be re-leased to this band by a small amount of thermal energy. It is interesting to note that the n-PA-CdCl4 crystal emits phosphorescence during and for several minutes after the X-irradiation. The corresponding X-ray luminescence spectrum of the n-PA-CdCl4 crystal obtained at 7 K is pre-sented in Fig. 6. Such a process is in good agreement with the model description of the electron-trapped center [14]. Thus, in the process of X-ray irradiation an electron trapped by a NH3

+ center is excited to one of the excited states and recombines through the conduction band, proba-bly with the nearest Cl2

–. As a result, the luminescence is observed. The thermoluminescence curve of a n-PA-CdCl4 crys-tal irradiated at 5 K for 10 min is presented in Fig. 6. Its spectral distribution is characterized by a wide, asymmetric band with a maximum at 550 nm mainly coinciding with that of the X-ray luminescence observed for n-PA-CdCl4 at temperatures below 45 K. The band of thermolumines-cence at 62 K would be related to the thermal delocaliza-tion of the electrons localized in the traps. The activation energy of this thermal process calculated using the Gar-lick–Gibson method is approximately 26 meV [17, 18]. On the other hand, the characteristic low-temperature wing of thermoluminescence observed in the vicinity of the liq-uid-helium temperature could be related to the tunnel

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5000

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, a.u

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Temperature, K

Figure 7 Thermoluminescence curve of n-PA-CdCl4 crystal.

mechanism of recombination of the trapped electrons with

the nearest Cl2– centers [19].

4 Conclusion On the basis of the investigation per-formed it has been found that the nature of radiation effects in n-PA-CdCl4 is very similar to those observed in the re-lated crystals of (C

nH2n+1NH3)2CdCl4 (where n = 1, 2) [14].

We clarify the nature of visible and IR bands arising in the absorption and luminescence spectra of n-PA-CdCl4 due to the X-ray irradiation. They originate, respectively, from Cl2

– molecular centers and the electron-trapped centers where an electron is trapped by an alkylammonium head in the neighborhood of a Cl– vacancy. The thermal activa-tion energy of the electron-trapped centers was found to be equal to 32 meV. This value is lower in comparison with those in the case of the related crystals of (C

nH2n+1NH3)2CdCl4 (where n = 1, 2) [14]. Such a differ-

ence is connected with the more complicated structural or-ganization of the organic cation. Due to the much longer [C3H7NH3]

+ chains the affinity of the ammonium head to the electron should be considerably different in comparison with the more simple organic cations. The intense luminescence observed under the influence of X-irradiation and the peak of thermoluminescence at 62 K connected with thermal delocalization of the trapped electrons should be considered as an experimental verifica-tion of the proposed model description.

Acknowledgement This work was supported by the Min-

istry for Science and Education of Ukraine.

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