CONDENSED MATTER
HF STUDY OF THE ELECTRICAL PERMITTIVITY FOR SOME PEROVSKITE-LIKE SYSTEMS FROM TWO DIMENSIONAL PEROVSKITE LAYER FAMILY
D. IONESCU1, I. B. CIOBANU2 1,2 “Gh. Asachi” Technical University of Iaºi, Romania 1 Faculty of Electronics and Telecommunications, Carol I Blvd., no. 67, 700506 Iaºi 2 Department of Physics, D. Mangeron Blvd., no. 67, 700050 Iaºi, Romania, E-mail: [email protected] Received October 10, 2008
The metal organic compounds consisting of organic groups covalently or ionically bound to inorganic layers present special properties for applications, starting with organic synthesis and finishing with the nonlinear optical applications, switching and memory applications. We have considered here the two dimensional
perovskite layer family [(NH3)-(CH2)n-(NH3)] MX4, with n = 2, 3,…, where M is a metal like Mn, Fe, Cu, Cd, etc. and X = Cl, Br. A simulation study was performed for characterizing the electric behavior of some compounds from the considered family, in the frequency range of 1–10 GHz, motivated by the fact that not cogent results are available yet in literature for the microwave range. Structural simulations were performed with help of the High Frequency Structural Simulator. Physical structure of the molecular composite compounds was re-constructed considering the indication from literature. For the electric anisotropic materials, electric permittivity was determined from energy density variations at field propagation through the material samples. A resonant behavior was found, depending on the internal order by constituents' nature and their spatial disposure. Resonances shift by internal parameters modifications (ionic radii, ion relative disposure, dimensions of the basic cell, etc.). Conclusions were synthesized for a structural interpretation of the permittivity peaks. A thermal analysis performed above 273 K confirms relaxation phenomena inside the material, which involves large reorientable dipoles in the total polarization. A peak around 310–390 K depending on the composition indicates that the most of the two dimensional perovskite layer family members are dipolar relaxation ferroelectrics. Microwave dielectric loss was also determined. The curves tan versus frequency are available in the range of 1–10 GHz. The obtained values indicate low to medium losses in microwave range for the considered perovskites.
Key words: metal organic compounds, perovskite layer family, structural simulation, microwave range, effective permittivity, thermal analysis, dielectric loss.
Paper presented at the National Conference of Physics, September 10–13, 2008, Bucharest – Mãgurele, Romania.
Rom. Journ. Phys., Vol. 54, Nos. 9–10, P. 899–909, Bucharest, 2009 900 D. Ionescu, I. B. Ciobanu 2
1. INTRODUCTION
The metal organic compounds, referred also as molecular composite compounds present special properties for applications, starting with organic synthesis (heterogeneous catalysis, adsorption and ion-exchange processes) and finishing with the nonlinear optical applications, switching and memory applications. These materials consist of organic groups covalently or ionically bound to inorganic layers. The metal organic crystals combine magnetic or optical properties of the inorganic solid state, like magnetism and luminescence, with properties of the organic solid state, like mesomorphism or polymerization [1, 2]. We have considered for analysis the two dimensional perovskite layer family [(NH3)-(CH2)n-(NH3)] MX4, with n = 2, 3, …, where M is a metal like Mn, Fe, Cu, Cd, etc. and X = Cl, Br. Study of this material class was performed in literature in the low frequency range and the results indicates the electrical behavior up to 1 MHz, but viable results at higher frequencies are necessary for high-tech applications. We have performed in this paper a simulation study for characterizing the electric behavior of some compounds from the considered family, in the frequency range of 1–10 GHz. For these electric anisotropic materials, electric permittivity was determined from energy density variations at field propagation through the material samples. Parallel and transverse permittivity evolution with frequency was illustrated on graphs and also the polarization maxima. Structural interpretation is available, for material exploitation optimization. In our data base, results obtained for the following metal organic compounds were included: layer perovskite halide salts [NH3-(CH2)n-NH3] MX4 with M = Cd, Mn, Co, Ni, Pb, Cu, Pd, Cr, and with X = Cl, Br, for 1 n 12; diamine complexes [NH3-(CH2)n-NH3] MCl4-xBrx (where x = 0, 2, n = 2, 3 and M = Cu, Co); [(NH3)-(CH2)6-(NH3)] FexZn1-xCl4 (or HDAFxZ1-x), where x = 1, 0.8, 0.5 and 0, etc. Material behavior is very complex, being characterized by phase transitions, ferroelasticity, or even ferromagnetism (compounds with M = Cr are ferromagnets, etc.).
2. THEORETICAL CONSIDERATIONS
The considered perovskite-like systems are electric anisotropic materials, characterized by two different values for the permittivity, parallel, respectively transverse on the long molecular axis. For the electric permittivity determination we have estimated variations of the volumetric densities of energy at field propagation through the material samples [3, 4]: 3 Electrical permittivity for some perovskite-like systems 901
dwee E dP E d 0 E (1) which can be written like: dwee0 E dE (2) We have denoted: E – electric intensity vector; P – the polarization vector;
e – the material electric susceptibility 33 tensor; 0 – vacuum electric permittivity. Considering the tensor representation of the electric relative permittivity, one obtains:
dwex dE11 1 E x21 E y 31 E z
dEyx12 E 22 1 E y32 E z (3)
dEzx13 E 23 E y 33 1 E z An exciting plane wave with linear variations of the E field components was considered to propagate through the material samples. The permittivity tensor components were calculated by vanishing the corresponding field components and its variations, e.g.:
dwe dwe 21 ; 22 1; … (4) EdEyx EExz,0 EdEyy EExz,0 dEyz,0 dE dExz,0 dE Due to the multicomponents internal structure of the molecular composite compounds, the permittivity evolution with frequency presents peaks and valleys, corresponding to the field interaction with the structure. Consequently, polarization maxima can be identified and localized on frequency scale, indicated by the permittivity extremes: PE00 er 1 E (5) The method can give results also for the materials with magnetic properties, at which the magnetic permeability tensor components can be determined from magnetic energy density variations at field propagation through the material samples.
3. SIMULATION RESULTS
Structural simulations were performed with help of the HFSS 9.2 (High Frequency Structural Simulator – Ansoft Technologies), a 3D full-wave electro- 902 D. Ionescu, I. B. Ciobanu 4 magnetic field simulator that utilizes a 3D full-wave Finite Element Method to compute the electrical behavior of high-frequency and high-speed components. Determinations are available in the microwave frequency domain of 1–10 GHz. Physical structure of the molecular composite compounds was re-constructed considering the indication from literature [5–7], where the structure and phase transition were studied by X-ray diffraction and differential scanning calorimetry, IR and DSC for the complexes, optical microscopy [8–10]. For the two dimensional perovskite layer family the structure consists of infinite sheets made of corner-sharing octahedral MX4. The cavities between the octahedra are occupied by the terminal NH3 ends of the alkyl-chains, [(NH3)-(CH2)n-NH3]. The internal structure for the 1,4-Butanediammonium Tetrabromopalladate (II) is illustrated in Fig. 1, for exemplification.
Fig. 1 – Internal structure of the 1,4-Butanediammonium Tetrabromo- 2+ 2– palladate (II), with the formula [NH3-(CH2)4-NH3] [PdBr4] , from the two dimensional perovskite layer family [(NH3)-(CH2)n-(NH3)] MX4 (after R. Zouari, 1998).
Parallel and transverse permittivity evolution with frequency was determined for the considered materials. Analysis was performed at room temperature (thermal evolution below 273 K was not considered here). Simulation data were given by the HFSS program for one hundred points per GHz. Data were processed in Mathcad for obtaining the corresponding values of the permittivity components [11]. The eigenmode solver of the HFSS program 5 Electrical permittivity for some perovskite-like systems 903 was set to determine the electrical resonances corresponding to the polarization maxima, in the frequency range of 1–10 GHz. A resonant behavior was found (see Fig. 2 for exemplification, in the case of the [NH3-(CH2)4-NH3]PdBr4 perovskite-like system), depending on the internal order by constituents' nature, dimensions and spatial disposure.
Fig. 2 – Parallel and transverse relative permittivities versus frequency, for the metal organic compound [NH3-(CH2)4-NH3] PdBr4, in the frequency range of 1–10 GHz. The short continue curves terminated with dots were extrapolated after literature data for comparison.
Results obtained by simulation for considered materials from the two dimensional perovskite layer family are presented in Table 1. If we compare the results corresponding to different molecular composite compounds, the following conclusions can be formulated. Longer the [NH3-(CH2)n-NH3] chain, equivalent with grater the n coefficient, higher the permittivity values, longer molecules being harder to be orientated by the field. The resonances number is low for high values of n (n > 10). For the compounds with ferromagnetic properties the electric permittivity is grater in comparison with the other considered perovskite-like systems with the same n coefficient, molecular dipoles being more difficult to be aligned due to the local order (dipoles are “captured” in the domains). The electric anisotropy is lower is these cases, due to the random orientation of the domains, which increase the homogeneity of the material. In the same time, the resonances number for these compounds is lower. Compounds with the same number of (CH2) groups (with the same n) present approximately the same resonances, due to the fact that their molecular dimensions are almost the same. 904 D. Ionescu, I. B. Ciobanu 6
Table 1 Parallel and transverse relative permittivity for some of the considered perovskite-like systems and the frequencies corresponding to the polarization maxima
II / II / Resonant Resonant No. Metal organic compound at 1 GHz at 1 GHz frequencies frequencies
(simulated) (literature) of II [GHz] of [GHz] 1.22, 2.74, 1.86, 2.14, 26.3/ 27.9/ 3.08, 5.15, 3.25, 3.76,
1. [NH3-(CH2)3-NH3] MnCl4 22.1 22.8 6.98, 7.36, 4.43, 6.12, 8.07, 8.35, 7.18, 8.05, 9.19, 9.78 9.18 2.04, 3.82, 2.87, 3.69,
2. [NH3-(CH2)4-NH3] CdCl4 36.4/ 37.6/ 5.30, 6.76, 4.60, 6.86, 24.7 26.6 8.36, 8.58, 7.45, 8.06, 9.16, 10.18 9.52 1.67, 3.82, 2.62, 3.68,
3. [NH3-(CH2)4-NH3] PdBr4 31.2/ 31.9/ 5.28, 7.22, 4.58, 7.39, 25.3 25.8 8.32, 8.51, 8.23, 9.21 9.27, 9.54 3.15, 4.28, 3.28, 4.57,
[NH3-(CH2)6-NH3] FeCl2Br2 64.6/ 65.8/ 4.49, 6.25, 6.45, 7.83, 4. (antiferromagnetic) 61.1 62.4 7.56, 8.24, 9.19 8.95, 9.14, 10.12
5. [NH3-(CH2)12-NH3] PbBr4 128.6/ 131.1/ 2.14, 3.75, 2.83, 4.11, 114.3 118.3 5.26, 7.16 5.73
[NH3-(CH2)3-NH3] CrCl4 55.8/ 57.0/ 3.96, 5.45, 3.95, 5.17, 6. (ferromagnetic) 52.6 52.9 7.66, 8.72, 7.84, 9.62 9.36, 10.85 2.12, 3.26, 2.91, 3.74, 48.3/ 49.9/ 3.88, 5.09, 3.92, 4.85,
7. [NH3-(CH2)3-NH3] CuCl2Br2 39.5 42.1 5.41, 6.90, 5.15, 6.94, 8.25, 8.72, 7.32, 7.62, 9.31 8.25, 9.81 1.98, 3.75, 2.61, 3.47,
8. [NH3-(CH2)3-NH3] CoCl4 42.6/ 44.3/ 5.22, 6.65, 4.42, 6.68, 35.9 37.0 8.24, 8.49, 7.28, 7.89, 9.12, 10.14 9.38 3.21, 4.32, 3.31, 4.64,
9. [(NH3)-(CH2)6-(NH3)] Fe0.8Zn0.2Cl4 82.4/ 84.6/ 4.56, 6.33, 6.58, 7.96, (antiferromagnetic) 78.3 79.1 7.68, 8.35, 9.27 9.05, 9.29 3.18, 4.26, 3.31, 4.64,
10. [(NH3)-(CH2)6-(NH3)] Fe0.5Zn0.5Cl4 74.5/ 78.4/ 4.38, 6.19, 6.58, 7.96, (antiferromagnetic) 68.2 69.3 7.49, 8.11, 9.27 8.92, 9.12 7 Electrical permittivity for some perovskite-like systems 905
Compounds with double halogen (with X = Cl and Br, e.g. X = Cl2Br2) present a few resonances in plus, due to the fact that the metal ion in the molecule is covalently bound with halogen ions having different radii, consequently the molecular geometrical parameters are different to the parameters of the single halogen compounds. Permittivity results were compared with the data given in literature at 1 GHz [1, 12–16]. The medium determination accuracy is of ca. 4% relative error, lower for the compounds with shorter molecules (with lower n) and higher for the compounds with magnetic properties whose structure is more difficult to be reproduced by structural simulation. A systematical positive error obtains due to the method, which can be partially corrected. Resonances shift by internal parameters modifications (ionic radii, ion relative disposure, dimensions of the basic cell, etc.). The simulation method offer us the possibility to determine the resonance shifting corresponding to each parameter virtual modification. Resonances are shifting selective, more or less when one or more internal geometrical parameters modify, confirming that field interaction with structure at molecular level can be described with help of these parameters and depends strongly on internal geometry and on the propagation direction in respect with the molecular axes. Permittivity evolution versus temperature was determined by simulation in the temperature range of 300–450 K, for illustrating the thermal peaks [12–16]. (Transitions to long-range order occurring at tens of Kelvin are not making the object of this paper.) The thermal analysis performed above 273 K confirms relaxation phenomena inside the material, which involves large reorientable dipoles determining the total polarization [3]. A peak around Tc = 310–390 K depending on the composition indicates that the most of the two dimensional perovskite layer family members are dipolar relaxation ferroelectrics. The thermal peak, obtained at similar temperatures for permittivity II and , is caused by the variation in the spacing between the adjacent metallic layers arising from the increase in the number of carbon atoms and/or introducing the Br- ions in the out of plane position at double-halogen compounds. A secondary thermal peak was obtained above Tc , corresponding to the temperature at which the thermal agitation is strong enough to permit the molecules rotation when the field is applied. The effect is associated with a ferroelectric phase transition [1, 3]. The results for the considered perovskite-like systems are given in Table 2, for an operating frequency of 1 GHz. The permittivity versus temperature curve are given in Fig. 3, in the case of the [NH3-(CH2)4-NH3] PdBr4 compound, for exemplification. Microwave dielectric loss was also determined. The linearized curves tan versus frequency obtained by simulation in the range of 1–10 GHz are given in Fig. 4, for the considered perovskite-like systems (at room temperature). The 906 D. Ionescu, I. B. Ciobanu 8
Table 2 Temperatures of the permittivity thermal peaks for the considered perovskite-like systems Relaxational Secondary Simulational No. Metal organic compound thermal peak thermal peak error [%] temperature [K] temperature [K]
1. [NH3-(CH2)3-NH3] MnCl4 308 332 3.18
2. [NH3-(CH2)4-NH3] CdCl4 353 428 2.11
3. [NH3-(CH2)4-NH3] PdBr4 366 416 2.16
4. [NH3-(CH2)6-NH3] FeCl2Br2 326 – 4.15 (antiferromagnetic)
5. [NH3-(CH2)12-NH3] PbBr4 315 372 2.48
6. [NH3-(CH2)3-NH3] CrCl4 328 – 4.18 (ferromagnetic)
7. [NH3-(CH2)3-NH3] CuCl2Br2 314 432 5.62
8. [NH3-(CH2)3-NH3] CoCl4 310 386 1.86
[(NH3)-(CH2)6-(NH3)] Fe0.8Zn0.2Cl4 9. (antiferromagnetic) 332 – 4.21
[(NH3)-(CH2)6-(NH3)] Fe0.5Zn0.5Cl4 10. (antiferromagnetic) 335 – 4.23
Fig. 3 – Thermal evolution of the electric permittivity for the metal organic compound [NH3-(CH2)4-NH3] PdBr4, in the temperature range of 300–450 K. The thermal peaks are illustrated on graph. Theoretical peaks indicated in literature are given for comparison. obtained values indicate low to medium losses in microwave range for the considered metal organic compounds. The compounds with ferromagnetic properties present relative high loss in comparison with the other considered compounds and also a higher increasing slope. The simulations have shown us that losses are linked rather by the ionic radii in the corner-sharing octahedral MX4, then by the alkyl-chain length. 9 Electrical permittivity for some perovskite-like systems 907
Fig. 4 – Microwave dielectric loss tan versus frequency (linearized curves), for the considered perovskite-like systems from the two dimensional perovskite layer family, in the frequency range of 1–10 GHz. Numbers 1 to 10 denote the compound in Table 1 and 2.
4. CONCLUSIONS
Some perovskite-like systems from the two dimensional perovskite layer family were analyzed by a simulational method in microwave range, from the point of view of their electrical properties. We have considered layer perovskite halide salts and diamine complexes, mono and double halogenated. The electric permittivity evolution with frequency (1–10 GHz) and temperature (300–450 K) was obtained and also the dielectric loss for the considered microwave range. Permittivity behavior presenting evolutional extremes has a structural interpretation and corresponds to the interaction mechanism between the exciting field and the microstructure of the molecular composite compounds. The following conclusions are available concerning this matter. The structure consists of infinite sheets made of corner-sharing octahedral MX4 (M = metal, X = halogen), with the cavities between the octahedra occupied by the terminal NH3 ends of the alkyl-chains, [(NH3)-(CH2)n-NH3]. . Electrical resonant behavior depends on the internal order by consti- tuents' nature, dimensions and spatial disposure. . Longer the alkyl-chain, higher the permittivity values, longer molecules being harder to be orientated by the field. The resonances number is low for high values of n (n > 10). 908 D. Ionescu, I. B. Ciobanu 10
. Compounds with ferromagnetic properties have grater permittivity than the other compounds with the same alkyl-chain length, molecular dipoles being more difficult to be aligned due to the domain local order. The electric anisotropy is lower, due to the random orientation of the domains, which increase the homogeneity of the material; also the resonances number is lower. . Compounds with the same number of (CH2) groups present approximately the same resonances, due to the fact that their molecular dimensions are almost the same. . Compounds with double halogenated present a few resonances in plus, having molecular geometrical parameters different to that of the single halogen compounds (halogen ions have different radii). . Resonance shifting by internal parameter modifications (ionic radii, ion relative disposure, dimensions of the basic cell, etc.) is selective and can be determined by simulation. This fact confirms that field interaction with structure at molecular level can be described with help of these parameters and depends strongly on internal geometry and on the field propagation direction. . The thermal analysis performed above 273 K point out two thermal peaks in the considered domain. First is a relaxational peak at 310–390 K depending on the composition and caused by the variation in the spacing between the adjacent metallic layers arising from the increase in the number of carbon atoms and/or introducing the Br- ions in the out of plane position at double-halogen compounds. Second is a ferroelectric peak, corresponding to the temperature at which the thermal agitation is strong enough to permit the molecules rotation when the field is applied. . Low to medium losses in microwave range were obtained for the considered metal organic compounds. The compounds with ferro- magnetic properties present relative high loss and also a higher increasing slope. The simulations have shown us that losses are linked rather by the ionic radii in the corner-sharing octahedral MX4, then by the alkyl-chain length. For a considered application, the previous conclusions are to be selected for material exploitation optimization.
REFERENCES
1. M. F. Mostafa, M. M. AbdelKader, S. S. Arafat, Z. Naturforsch. 57a, 897–908 (2002). 2. Ahmed A. A. Youssef, Z. Naturforsch. 57a, 263–269 (2002). 3. M. S. Young-Il Kim, Syntheses, Crystal Structures, and Dielectric Property of Oxynitride Perovskites, dissertation, The Ohio State University 2005, on-line: http://www.ohiolink.edu/ etd/send-pdf.cgi?osu1124291783 11 Electrical permittivity for some perovskite-like systems 909
4. T. Tsurumi, J. of the Ceramic Society of Japan 115(1), 17–22 (2007). 5. R. Zouari, J. M. Leger, T. Maris, N. B. Chanh, Acta Crystallographica C54 (9), 1253–1255 (1998). 6. M. M. Abd-el.Aal, M. A. Ahmed, Physica B: Condensed Matter 217 (1–2), 133–142 (1996). 7. R. Kind, S. Plesko, P. Günter, J. Roos, J. Fousek, Phys. Rev. B 23, 5301–5315 (1981). 8. G. Udubasa, S. Constantinescu, N. Popescu-Pogrion, P. Hirtopanu, S. S. Udubasa, Romanian Reports in Phys. 59 (3), 819–824 (2007). 9. T.S. Low, W. Guo, J. of Microelectromechanical Systems 4 (4), 230–237 (1995). 10. N. Tigau, Rom. Journ. Phys. 53, 203–208 (2008). 11. D. Ionescu, B. Ciobanu, I. Radinschi, J. of Optoelectronics and Adv. M. 9 (8), 2608–2616 (2007). 12. M. Khechoubi, J. of Physics and Chemistry of Solids 55 (11), 1277–1288 (1994). 13. C. Sourisseau, G. Lucazeau, J. of Raman Spectroscopy 8 (6), 311–319 (2005). 14. M. F. Mostafa, M. A. Semary, M. Abdel-Kader, Phys. Lett. A 82 (7), 350–352 (1981). 15. M. F. Mostafa, A. A. Youssef , S. S. Montasser, S. S. Khyami, Zeitschrift für Naturforschung. A 60 (11–12), 837–847 (2005). 16. M. A. Ahmed, A. M. Ibrahim, F. A. Radwan, S. M. A. Wahab, Physica Scripta 55 (6), 00746– 00749 (1997).