Pyroelectric

Optical Materials 30 (2007) 101–105 www.elsevier.com/locate/optmat

Pyroelectric properties of high-resistivity rubidium titanyl phosphate crystals in the 4.2–300 K temperature range Yu.V. Shaldin a, S. Matyjasik b, M. Tseitlin c, M. Roth b

d,*

a Institute of Crystallography, Russian Academy of Sciences, 119333 Moscow, Russian Federation International Laboratory of High Magnetic Fields and Low Temperatures, 53-421 Wroclaw, Poland c The Research Institute, College of Judea and Samaria, Ariel 44837, Israel d Faculty of Science, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

Available online 12 December 2006

Abstract Pyroelectric properties of high resistivity RbTiOPO4 single crystals cut from different growth sectors ({1 0 0}, {1 1 0} and {2 0 1}) have been studied using the modified static method in a broad temperature interval, from 4.2 to 300 K. A substantial scatter of the room temperature pyroelectric coefficient values has been observed, ranging from 1.3 to 4.6 105 C/m2 K, for samples of distinct stoichiometry. Different types of low-temperature anomalies in the pyroelectric coefficient behavior have been revealed in samples related to the various growth sectors. Crystals cut from the {1 1 0} sector exhibit particularly interesting anomalies at 90 and 250 K which are attributed to oxygen deficient PO4(1) and PO4(2) coordinational tetrahedral and their relaxor contribution. This and other anomalies are discussed in terms of the growth related variation of the RTP crystals’ nonstoichiometry and point defect structure.  2006 Elsevier B.V. All rights reserved.

1. Introduction The wide family of isomorphic MTiOXO4 compounds, where M = {K, Rb, Tl} and X = {P, As} is an important class of crystals for application in laser systems utilizing frequency conversion and electro-optic modulation [1,2]. These crystals exhibit large optical nonlinearities and electro-optic coefficients, high laser damage threshold and excellent thermal stability. However, the properties of these crystals may alter due to the variation of their chemical composition and defect distribution during growth from self-fluxes [2–4]. In particular, generation of color centers under green light illumination (gray tracking) with a gradual degradation of optical transparency takes place [5], while the oxygen nonstoichiometry has been claimed to

*

Corresponding author. Tel.: +972 2 566 3878; fax: +972 2 658 6364. E-mail address: [email protected] (M. Roth).

0925-3467/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.11.005

cause a substantial decrease in specific resistivity along the z-axis, which has been interpreted also in terms of an increase of the alkali metal superionic conductivity [1]. In similarity with its potassium isomorph, the RbTiOPO4 (RTP) structure is described as a three-dimensional framework comprising PO4 tetrahedra and TiO6 octahedra [6]. The unit cell of the low-temperature ferroelectric RTP phase contains eight formula units, and it is described as belonging to the noncentrosymmetric mm2 point group (Pna21 space group) symmetry. The rubidium ions are localized in the main framework cavities, strictly around the polar axis direction. Helical structural channels have been identified in this direction [7] which, at certain temperatures, facilitate the drift of rubidium ions, or the ionic conductivity, and an abrupt resistivity decrease along the z-axis. Recently [8], we have suggested an alternative view on the structure of KTP-type crystals as composed of mesotetrahedra (Fig. 1). The A and B mesotetrahedra form two oppositely polarized sublattices with respective P(A) and –P(B) spontaneous polarizations. They form infinite

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2. Experimental

Fig. 1. Structural fragment of the RTP single crystal viewed as an infinite sequence of A and B mesotetrahedra along the polar (z) axis. Mesotetrahedra are formed by Ti(1) and two types of tetrahedral, PO4(1) and PO4(2), connected by Ti(2) ions.

chains along the polar axis direction, which determines the packing order. The mesotetrahedra are linked by Ti(2) ions thus stabilizing the chains. With such representation of the RTP structure, the role of rubidium ions localized around the 21 screw axis in forming the crystal’s spontaneous polarization is secondary. As-grown RTP crystals obviously contain point defects typical for oxide crystals. Inherently present oxygen vacancies induce the formation of compensating vacancies in the alkali metal sublattice, and alkali ions are partially moved into interstitial positions. This process can be substantially depressed by evening out the oxygen/alkali ion vacancies concentrations which, in turn, is expected to reduce the ionic conductivity. Thus induced donor–acceptor pairs, at certain concentrations, may create a defect subsystem exhibiting its own dipole moment and influencing significantly the behavior of the overall pyroelectric coefficient cS [9–14]. The latter is defined from DP S ¼ cS  DT ;

RTP crystals were grown by the top-seeded solution growth (TSSG) method with pulling on X- or Z-oriented seeds from self-fluxes containing initial Rb to P atomic ratios from 1.5 to 2. Solutions of RTP in self-fluxes were prepared by reacting the Aldrich 3N purity TiO2, Rb2CO3 and Merck (Suprapur) NH4H2PO4 and (NH4)2HPO4 in appropriate proportions. The charges were loaded into 200–1000 ml Pt crucibles and subjected to 24 h soaking through homogenization aided by a Pt stirrer. Crystal growth proceeded in a custom resistance furnace with a long hot zone allowing to obtain a uniform temperature distribution with a gradient 62 C in the solution. The seed rotation and pulling rates varied from 70 rpm to 20 rpm and from 0.02 to 1 mm/day respectively. The general temperature range for growth of RTP crystals was 980–880 C, while the temperature lowering rates changed from 0.5 to 3 C/day. As a result, RTP crystals of up to 50 · 60 · 50 mm3 dimensions along the X, Y and Z axis could be grown. Z-cut plane parallel crystal samples with thicknesses varying from 0.5 to 1.5 mm were used for determination of the Curie temperatures and pyroelectric measurements. They were cut from different growth sectors (volumes terminated by specific facets) shown schematically in Fig. 2. The Tc was measured using a standard dielectric technique (with an accuracy of ±0.5 K) by thermal ramping and recording the capacitance anomaly at the ferroelectric transition temperature. The samples coated with Pt electrodes on both sides were linked into an ac impedance bridge circuit based on a Hewlett-Packard model 4276A LCZ meter operating at a 20 kHz frequency. The substantial difference in Tc values of samples cut from the top and bottom of the {1 0 0} growth sector (Fig. 3) manifest the apparent variation of the RTP crystal stoichiometry in course of the growth process. This implies that the gradual variation of the defect structure, practically not studied in RTP [1], must be reflected in the corresponding pyroelectric effect measurements. Samples for pyroelectric measurements were prepared from three growth sectors: {2 0 1}, {1 0 0} and {1 1 0}. The respective Tc and electrical resistivities values and the

ð1Þ

where DPS is the change in spontaneous polarization under a small temperature change DT. According to an empirical rule [15], P 2S  T c , where Tc is the ferroelectric transition (Curie) temperature depending on the composition and defect structure of KTP-family crystals [2,5]. In the present work, an investigation of the pyroelectric properties of RTP crystals in the 4.2–300 K temperature range is carried out, and it is primarlily aimed at revealing the influence of the melt composition and crystal nonstoichiometry on the temperature dependence of the pyroelectric coefficient.

Fig. 2. Schematic vertical cut of RTP crystal grown on X- and Z-oriented seeds showing the various growth sectors.

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Capacitance, nF

6

4

2

0 1020

1040

1060

1080

1100

Temperature, K Fig. 3. Curie temperatures (measured by capacitance method) of two RTP samples cut from the top (circles) and bottom (squares) of the {1 0 0} growth sector.

sample areas were: 1049.6, 1066.6 and 1058.0 K; 2 · 1012, 1011 and 1011 X cm; 34.0, 47.0 and 30.0 mm2. One {2 0 1} sample of a 24.7 mm2 area was annealed at T = 800 K in vacuum for 7 h. Silver paste was used as an electrode and current guiding material in pyroelectric measurements. Investigation of the pyroelectric properties of RTP crystals cut from the various growth sectors has been carried out using the modified static method. A helium open-flow cryostat was used, and the samples were mounted on a cold finger equipped with a Cemox-1050CD thermocouple. Temperature control at a level of better than ±0.001 K was performed using a resistive coil connected to a stabilized power supply. The electric charge generated by the samples in course of temperature change was measured by a Keithly-17F universal digital electrometer. Each sample was shortened at the start of the experiment and then immersed into liquid helium. When the 4.2 K temperature was reached, the sample was connected to the electrometer in order to check for the zero drift of the entire measurement circuit. The exposure time at liquid helium temperature was defined by the possibility of linear approximation of the zero drift as a function of time. The typical proportionality coefficient in our experiments was 1014 C/s. The sample extraction from the liquid helium was a delicate procedure which could be accompanied by generation of extra charge due to temperature gradients. This, in turn, could cause a systematic error effecting primarily the initial set of measurements.

Fig. 4. Temperature dependences of spontaneous polarization change for z-plates cut from the {2 0 1} sector (a), {2 0 1} sector after annealing at T = 800 K in vacuum (b), {1 0 0} sector (c) and {1 1 0} sector (d); d+ and d curves taken after electric field (±103 V/cm) removal at 4.2 K.

(c), exhibits some anomalies at 90 and 230 K, while the room temperature DPS is half the value of curve (a). In the case of the {1 1 0} sample, control measurements of the DPS dependence under conditions of a fixed surface charge (that may stem from a possible drift of Rb-ions in the ±103 V/cm field) show that the main difference between the curves is at temperatures above 280 K. The apparent scatter of the DPS values and the existence of anomalies points at the variable stoichiometry of the different samples. The DPS(T) dependencies described above have been used to calculate the unclamped pyroelectric coefficients, from Eq. (1), for all RTP samples cut from different crystal growth sectors, and the results are presented in Figs. 5–7. Apparently, cS exhibits no anomalies in the lower temperature range as compared to low-resistivity KTP crystals [9–11]. Moreover, the cS/T2 versus temperature dependencies, shown for the {2 0 1}- and {1 0 0}-sector samples in Fig. 8, are linear at least in the 4.2–30 K range. The latter result is suffiucient to confirm the universal c  T3 dependence characteristic for all pyroelectrics [16]. At higher temperatures, certain kinds of anomalies emerge, like shown for the {1 1 0}-sector crystal in Fig. 5, which are presumably associated with the oxygen sublattice

3. Results and discussion All measurements of the electrical charge variation as a function of temperature were carried out on temperature increase. The experimental data were used to calculate the temperature dependencies of the polarization change, DPS, as shown in Fig. 4 for all four types of samples. The results obtained for the two {2 0 1}-sector samples, curves (a) and (b), are identical within the experimental error, and at T = 300 K the polarization change reaches a value of 102 C/m2, which is comparable with the typical data for lithium niobate. The {1 0 0}-sector sample, curve

Fig. 5. Temperature dependence of the unclamped pyroelectric coefficient of RTP crystal cut from the {1 1 0} growth sector.

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Fig. 6. Temperature dependences of the unclamped pyroelectric coefficients of RTP crystals cut from the {2 0 1} growth sector; a – as-grown crystal, b – after vacuum annealing at T = 800 K.

Fig. 7. Temperature dependence of the unclamped pyroelectric coefficient of RTP crystal cut from the {1 0 0} growth sector.

tor crystal in Fig. 7, is difficult due to the onset rubidium ions’ drift along the helical channels (Fig. 1) within the RTP crystal lattice. It is noteworthy that high resistivity KTP crystals exhibit a similar behavior to the {1 0 0}sector RTP sample, namely showing no anomalies at low temperature but rather above 250 K [8]. The cation drift in both KTP and RTP is presumed to be assisted by the oxygen vacancies present, and an indepth study of this process is currently underway. Since no direct experimental data on the RTP spontaneous polarization are yet available, it can be evaluated based on the optical and pyroelectric measurements according to the methodology suggested earlier [19]. With the existing approximate data on the RTP birefringence and its temperature derivative [1], one can only roughly estimate the value of spontaneous polarization to be as large as P  0.5 C/m2. This does not contradict the empirical rule suggested by Abrahams et al. [15], namely the higher is the ferroelectric phase transition temperature the larger is the spontaneous polarization of ferroelectrics containing oxygen octahedrals, according to: T c  P 2S . In view of the large magnitude of spontaneous polarization in high resistivity RTP crystals, even the observed significant scatter of the room temperature values of pyroelectric coefficients (between 1.3 and 4.6 · 105 C/m2 K for samples cut from different growth sectors) is not apt to affect the nonlinear optical susceptibility. However, the issues of optical uniformity and stability do relate to the variable stoichiometry in the various growth sectors. 4. Conclusions

Fig. 8. 4–30 K segments of temperature dependences of unclamped pyroelectric coefficients (cS) of RTP crystals cut from (a) {2 0 1} and (b) {1 0 0} growth sectors and their representation in terms of cS/T2 temperature dependences.

nonstoichiometry. The dipole moments of the coordinational PO4(1) and PO4(2) tetrahedra change on loss of oxygen, and the latter may be assumed as relaxors [17,18] which are altering the pyroelectric coefficient temperature dependence significantly around 90 and 250 K. Similar, but weaker, anomalies are observed also in as-grown {2 0 1}-sector crystals (Fig. 6, curve (a)). Annealing in vacuum at T = 800 K results in a tangible transformation of the anomaly (Fig. 6, curve (b)) providing an evidence for the important role of color centers in formation of the crystal’s pyroelectric properties. A detailed analysis of cS(T) anomalies above 280 K, such as shown for the {1 0 0}-sec-

Since the choice flux composition and pulling direction determine the crystal morphology and composition, our present results show that growth conditions of RTP crystals have a tangible impact on their pyroelectric properties. Samples cut from different growth sectors, such as {1 0 0}, {1 1 0} and {2 0 1}, known to be of different stoichiometry, also exhibit essentially distinct low-temperature anomalies in the pyroelectric coefficient behavior. A substantial scatter in the room temperature values of the pyroelectric coefficients related to samples of different stoichiometry is observed, from 1.3 to 4.6 · 105 C/m2 K, indicating that a problem of optical uniformity and stability exists, and it should be taken into account in manufacturing of RTP nonlinear and linear optical elements. References [1] M.N. Satyanarayan, A. Deepthy, H.L. Bhat, Crit. Rev. Solid State 24 (1999) 103. [2] M. Roth, N. Angert, M. Tseitlin, G. Schwarzman, A. Zharov, Opt. Mater. 26 (2004) 465. [3] N. Angert, M. Tseitlin, E. Yashchin, M. Roth, Appl. Phys. Lett. 67 (1995) 1941. [4] M. Roth, N. Angert, M. Tseitlin, J. Mater. Sci. Mater. Electron. 12 (2001) 429.

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