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Raman scattering study of Y1−xCaxTiO3 single crystals
Physica B 329–333 (2003) 833–834
Raman scattering study of Y1xCaxTiO3 single crystals Takao Tsuruia,*, Norio Ogitab, Masayuki Udagawab, Masami Tsubotac, Kazuyuki Uchihirac, Fumitoshi Igac a
b
Venture Business Laboratory, Hiroshima University, Higashi-Hiroshima, 739-8527, Japan Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima, 739-8521, Japan c ADSM, Hiroshima University, Higashi-Hiroshima, 739-8530, Japan
Abstract Raman scattering spectra of single crystalline Y1x Cax TiO3 ðx ¼ 0; 0:1; 0:3; 0:37; 0:38Þ have been studied in the temperature region from 298 to 8 K: The structural phase transition from orthorhombic to monoclinic one occurs for xp0:38 at around 200 K: The increase of the peak intensity, related to the metal–insulator transition, has been observed below 100 K in Y0:62 Ca0:38 TiO3 : The temperature dependence of the peak intensity shows strong polarization dependence. The phase separation from the monoclinic phase to the monoclinic + orthorhombic phase occurs below the metal–insulator transition temperature in Y0:62 Ca0:38 TiO3 : r 2003 Elsevier Science B.V. All rights reserved. Keywords: Y1x Cax TiO3 ; Metal–insulator transition; Raman scattering
1. Introduction
2. Experimental
YTiO3 is well known as a Mott–Hubbard insulator with 3d1 (Ti) configuration and has a GdFeO3 -type structure with the Pbnm symmetry. In Y1x Cax TiO3 ; the metal–insulator (M–I) transition has been observed at around x ¼ 0:40 [1]. Recent X-ray diffraction experiments at around x ¼ 0:40 have reported that the structural phase transition from orthorhombic (Pbnm) to monoclinic ðP21 =nÞ at around 230 K and also the phase separation between the low-temperature orthorhombic and monoclinic ones below the M-I transition temperature ðTMI Þ [2]. Raman scattering spectra of polycrystalline Y1x Cax TiO3 at 50 K were reported by Katsufuji and Tokura [3], but they did not discuss about the structural phase transition. In this report, we have studied the polarization and temperature dependence of Raman-scattering spectra using the single crystalline Y1x Cax TiO3 ; in order to clarify the microscopic origin of the M–I transition and the structural anomaly.
The single crystals of Y1x Cax TiO3 were synthesized by a floating-zone method. In this experiments, the specimens with x ¼ 0; 0.1, 0.3, 0.37, and 0.38 were used. TMI of x ¼ 0:38 is about 120 K: In the Raman scattering measurements, an Arþ laser light with a wavelength of 514:5 nm was employed as the incident beam. The scattered light was analyzed by a triple monochromator (JASCO, NR-1800), and the analyzed light was detected by a CCD multi-channel detector (Princeton Instruments). In the orthorhombic structure of Pbnm, the number of the Raman active phonons ðGo Þ is given as Go ¼ 7Ag þ 7B1g þ 5B2g þ B3g [4]. Phonons with total symmetric Raman active modes, Ag ; appear in the polarization geometries of ða; aÞ; ðb; bÞ and ðc; cÞ; while B1g ; B2g and B3g appear in ða; bÞ; ða; cÞ and ðb; cÞ; respectively, where a; b; and c corresponds to the [1,0,0], [0,1,0], and [0,0,1] axes. In the symbol of ki ðpi ; ps Þks ; k and p are the directions of the propagation and polarization of light, respectively, and the suffix of i and s denote the incident and scattered light, respectively.
*Corresponding author. E-mail address: [email protected] (T. Tsurui).
0921-4526/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4526(02)02533-4
834
T. Tsurui et al. / Physica B 329–333 (2003) 833–834
3. Results and discussion The polarization dependence at 8 K and the temperature dependence of cða; aÞc% for YTiO3 are shown in Fig. 1(a) and (b), respectively. The peaks below 600 cm1 have been assigned as phonons. The assigned phonons by the symmetry of Pbnm are marked by closed circles in the figure, and their observed number satisfies this symmetry. However, new additional phonons, marked by arrows in the figure, have been observed below 200 K: The additional phonons appear in ða; aÞ; ðc; cÞ; ða; cÞ; and ðb; cÞ: The crystal symmetry is not orthorhombic below 200 K; since the observed number of Ag at 8 K exceeds the estimated number for Pbnm. For the monoclinic symmetry of P21 =n; the number of the Raman active phonons ðGm Þ is estimated as Gm ¼ 12Ag þ 12Bg ; where the Ag modes appear in the polarization geometries of ða; aÞ; ðb; bÞ; ðc; cÞ and ðb; cÞ; and Bg in ða; bÞ and ða; cÞ: The results at 8 K satisfy the case of P21 =n: Thus, this result shows that the structural phase transition from orthorhombic to monoclinic one occurs at about 200 K in YTiO3 : Fig. 2 shows the polarization dependence of the aðb; bÞa; % bðc; cÞb% and bða; aÞb% at the representative temperatures for x ¼ 0:38: The additional phonon at about 500 cm1 ; marked by the arrow in the figure, has been observed in aðb; bÞa% below 200 K (Fig. 2(a)). This is the similar phonon observed in x ¼ 0: The structural phase transition from orthorhombic to monoclinic one occurs in xp0:38: The intensity of the peaks, marked by P1 or P2 in the figure, drastically increases at the temperature between 130 and 100 K in bðc; cÞb% (Fig. 2(b)). These peaks correspond to the peaks observed by Katsufuji and Tokura [3]. The temperature dependence of the peak intensity shows strong polarization dependence. Since this remarkable change of the peak intensity has not
Fig. 2. Polarization dependence of Y0:62 Ca0:38 TiO3 : (a) aðb; bÞa% at 50 and 12 K; (b) bðc; cÞb% at 130 and 100 K and (c) % Note that TMI of this temperature dependence of bða; aÞb: sample is B120 K: Closed circles denote the assigned phonons by Pbnm and the arrow is the additional phonon observed below 200 K: Remarkable peaks are marked by P1 or P2 :
been observed in aðb; bÞa% down to 50 K: The appearance of the peak at about 500 cm1 ; marked by the arrow in aðb; bÞa; % shows that the monoclinic phase exist below TMI : On the other hand, as shown in Fig. 2(c), P1 and P2 exist even at 297 K and their intensities increase gradually with decreasing temperature. Thus, P1 and P2 are assigned phonons by the symmetry of Pbnm. Furthermore, the peak at about 500 cm1 ; observed below TMI ; disappears with the increase of the intensity of P1 and P2 at 12 K in aðb; bÞa: % These results indicate that the phase separation from the monoclinic phase to the monoclinic+orthorhombic phase occurs below TMI at x ¼ 0:38: In summary, the phase separation has been observed below TMI in Y0:62 Ca0:38 TiO3 : The polarization dependence is influenced by the anisotropic properties caused by the phase separation. The details of the polarization dependence will be discussed elsewhere.
Acknowledgements This work is supported in part by a Grant-in-Aid for COE Research (No. 13CE2002) of the Ministry of Education, Culture, Sports, Science and Technology of Japan. The Raman scattering experiment was also supported by the cryogenic center of Hiroshima University.
References Fig. 1. (a) Polarization dependence at 8 K and (b) temperature dependence of the cða; aÞc% spectra for YTiO3 : Closed circles denote the assigned phonons by Pbnm and the arrows are the additional phonons observed below 200 K:
[1] [2] [3] [4]
Y. Taguchi, et al., Phys. Rev. B 48 (1993) 511. K. Kato, et al., cond-mat/0204227. T. Katsufuji, Y. Tokura, Phys. Rev. B 50 (1994) 2704. M. Udagawa, et al., Solid State Commun. 16 (1975) 779.
Raman scattering study of Y1xCaxTiO3 single crystals Takao Tsuruia,*, Norio Ogitab, Masayuki Udagawab, Masami Tsubotac, Kazuyuki Uchihirac, Fumitoshi Igac a
b
Venture Business Laboratory, Hiroshima University, Higashi-Hiroshima, 739-8527, Japan Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima, 739-8521, Japan c ADSM, Hiroshima University, Higashi-Hiroshima, 739-8530, Japan
Abstract Raman scattering spectra of single crystalline Y1x Cax TiO3 ðx ¼ 0; 0:1; 0:3; 0:37; 0:38Þ have been studied in the temperature region from 298 to 8 K: The structural phase transition from orthorhombic to monoclinic one occurs for xp0:38 at around 200 K: The increase of the peak intensity, related to the metal–insulator transition, has been observed below 100 K in Y0:62 Ca0:38 TiO3 : The temperature dependence of the peak intensity shows strong polarization dependence. The phase separation from the monoclinic phase to the monoclinic + orthorhombic phase occurs below the metal–insulator transition temperature in Y0:62 Ca0:38 TiO3 : r 2003 Elsevier Science B.V. All rights reserved. Keywords: Y1x Cax TiO3 ; Metal–insulator transition; Raman scattering
1. Introduction
2. Experimental
YTiO3 is well known as a Mott–Hubbard insulator with 3d1 (Ti) configuration and has a GdFeO3 -type structure with the Pbnm symmetry. In Y1x Cax TiO3 ; the metal–insulator (M–I) transition has been observed at around x ¼ 0:40 [1]. Recent X-ray diffraction experiments at around x ¼ 0:40 have reported that the structural phase transition from orthorhombic (Pbnm) to monoclinic ðP21 =nÞ at around 230 K and also the phase separation between the low-temperature orthorhombic and monoclinic ones below the M-I transition temperature ðTMI Þ [2]. Raman scattering spectra of polycrystalline Y1x Cax TiO3 at 50 K were reported by Katsufuji and Tokura [3], but they did not discuss about the structural phase transition. In this report, we have studied the polarization and temperature dependence of Raman-scattering spectra using the single crystalline Y1x Cax TiO3 ; in order to clarify the microscopic origin of the M–I transition and the structural anomaly.
The single crystals of Y1x Cax TiO3 were synthesized by a floating-zone method. In this experiments, the specimens with x ¼ 0; 0.1, 0.3, 0.37, and 0.38 were used. TMI of x ¼ 0:38 is about 120 K: In the Raman scattering measurements, an Arþ laser light with a wavelength of 514:5 nm was employed as the incident beam. The scattered light was analyzed by a triple monochromator (JASCO, NR-1800), and the analyzed light was detected by a CCD multi-channel detector (Princeton Instruments). In the orthorhombic structure of Pbnm, the number of the Raman active phonons ðGo Þ is given as Go ¼ 7Ag þ 7B1g þ 5B2g þ B3g [4]. Phonons with total symmetric Raman active modes, Ag ; appear in the polarization geometries of ða; aÞ; ðb; bÞ and ðc; cÞ; while B1g ; B2g and B3g appear in ða; bÞ; ða; cÞ and ðb; cÞ; respectively, where a; b; and c corresponds to the [1,0,0], [0,1,0], and [0,0,1] axes. In the symbol of ki ðpi ; ps Þks ; k and p are the directions of the propagation and polarization of light, respectively, and the suffix of i and s denote the incident and scattered light, respectively.
*Corresponding author. E-mail address: [email protected] (T. Tsurui).
0921-4526/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4526(02)02533-4
834
T. Tsurui et al. / Physica B 329–333 (2003) 833–834
3. Results and discussion The polarization dependence at 8 K and the temperature dependence of cða; aÞc% for YTiO3 are shown in Fig. 1(a) and (b), respectively. The peaks below 600 cm1 have been assigned as phonons. The assigned phonons by the symmetry of Pbnm are marked by closed circles in the figure, and their observed number satisfies this symmetry. However, new additional phonons, marked by arrows in the figure, have been observed below 200 K: The additional phonons appear in ða; aÞ; ðc; cÞ; ða; cÞ; and ðb; cÞ: The crystal symmetry is not orthorhombic below 200 K; since the observed number of Ag at 8 K exceeds the estimated number for Pbnm. For the monoclinic symmetry of P21 =n; the number of the Raman active phonons ðGm Þ is estimated as Gm ¼ 12Ag þ 12Bg ; where the Ag modes appear in the polarization geometries of ða; aÞ; ðb; bÞ; ðc; cÞ and ðb; cÞ; and Bg in ða; bÞ and ða; cÞ: The results at 8 K satisfy the case of P21 =n: Thus, this result shows that the structural phase transition from orthorhombic to monoclinic one occurs at about 200 K in YTiO3 : Fig. 2 shows the polarization dependence of the aðb; bÞa; % bðc; cÞb% and bða; aÞb% at the representative temperatures for x ¼ 0:38: The additional phonon at about 500 cm1 ; marked by the arrow in the figure, has been observed in aðb; bÞa% below 200 K (Fig. 2(a)). This is the similar phonon observed in x ¼ 0: The structural phase transition from orthorhombic to monoclinic one occurs in xp0:38: The intensity of the peaks, marked by P1 or P2 in the figure, drastically increases at the temperature between 130 and 100 K in bðc; cÞb% (Fig. 2(b)). These peaks correspond to the peaks observed by Katsufuji and Tokura [3]. The temperature dependence of the peak intensity shows strong polarization dependence. Since this remarkable change of the peak intensity has not
Fig. 2. Polarization dependence of Y0:62 Ca0:38 TiO3 : (a) aðb; bÞa% at 50 and 12 K; (b) bðc; cÞb% at 130 and 100 K and (c) % Note that TMI of this temperature dependence of bða; aÞb: sample is B120 K: Closed circles denote the assigned phonons by Pbnm and the arrow is the additional phonon observed below 200 K: Remarkable peaks are marked by P1 or P2 :
been observed in aðb; bÞa% down to 50 K: The appearance of the peak at about 500 cm1 ; marked by the arrow in aðb; bÞa; % shows that the monoclinic phase exist below TMI : On the other hand, as shown in Fig. 2(c), P1 and P2 exist even at 297 K and their intensities increase gradually with decreasing temperature. Thus, P1 and P2 are assigned phonons by the symmetry of Pbnm. Furthermore, the peak at about 500 cm1 ; observed below TMI ; disappears with the increase of the intensity of P1 and P2 at 12 K in aðb; bÞa: % These results indicate that the phase separation from the monoclinic phase to the monoclinic+orthorhombic phase occurs below TMI at x ¼ 0:38: In summary, the phase separation has been observed below TMI in Y0:62 Ca0:38 TiO3 : The polarization dependence is influenced by the anisotropic properties caused by the phase separation. The details of the polarization dependence will be discussed elsewhere.
Acknowledgements This work is supported in part by a Grant-in-Aid for COE Research (No. 13CE2002) of the Ministry of Education, Culture, Sports, Science and Technology of Japan. The Raman scattering experiment was also supported by the cryogenic center of Hiroshima University.
References Fig. 1. (a) Polarization dependence at 8 K and (b) temperature dependence of the cða; aÞc% spectra for YTiO3 : Closed circles denote the assigned phonons by Pbnm and the arrows are the additional phonons observed below 200 K:
[1] [2] [3] [4]
Y. Taguchi, et al., Phys. Rev. B 48 (1993) 511. K. Kato, et al., cond-mat/0204227. T. Katsufuji, Y. Tokura, Phys. Rev. B 50 (1994) 2704. M. Udagawa, et al., Solid State Commun. 16 (1975) 779.