Formation, Structure, and Frequency-Doubling Effect of a Modification

Article pubs.acs.org/IC

Formation, Structure, and Frequency-Doubling Effect of a Modification of Strontium Cyanurate (α-SCY) Markus Johannes Kalmutzki,†,‡,∥ Konstantin Dolabdjian,†,∥ Nadja Wichtner,§ Markus Ströbele,† Christoph Berthold,§ and Hans-Jürgen Meyer*,† †

Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany Department of Chemistry, University of California, Berkeley, California 94720, United States § FB Geowissenschaften, AG Angewandte Mineralogie, Eberhard Karls Universität Tübingen, Wilhelmstraße 56, 72074 Tübingen, Germany ‡

S Supporting Information *

ABSTRACT: The low-temperature modification of Sr3(O3C3N3)2 was prepared and assigned as α-SCY after the high-temperature phase (now called β-SCY) and its frequencydoubling properties were reported recently. The crystal structure of α-SCY was solved and refined by single-crystal X-ray diffraction. Both modifications of SCY crystallize in noncentrosymmetric space groups, with the low-temperature phase (α-SCY) adopting the lower symmetry structure (Cc). Atomic positions in α-SCY (Cc) reveal only small deviations in comparison to those in the structure of β-SCY (R3c). The reversible phase transition between both modifications of SCY was studied by means of temperature-dependent powder X-ray diffraction. NLO measurements of both SCY modifications are reported in comparison to the commercial frequency-doubling material KTiOPO4 (KTP).

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INTRODUCTION Nonlinear optical (NLO) materials exhibiting second-order NLO effects are of interest for a variety of applications such as variable-wavelength lasers, optical signal transfer, and processing or optical data storage. β-Barium borate (β-Ba(B2O4) or β-BBO) represents one of the most important NLO materials for frequency conversion and is used in the development of laser sources in a wide spectral range from near-IR to UV. BBO is known to exist in two crystalline modifications: the high-temperature modification (α-BBO) crystallizes in a centrosymmetric space group (R3̅c) and the low-temperature modification in a noncentrosymmetric space group (R3c).1,2 Only the low-temperature modification of BBO (β-BBO) is of interest for frequency-doubling applications due to its noncentrosymmetric structure. However, upon cooling of a melt of BBO the high-temperature phase (α-BBO) is formed (mp 1095 °C) and crystals thereof tend to crack upon further cooling, when the phase transition into the low-temperature phase (β-BBO) occurs at 925 °C. Therefore, single crystals of β-BBO are grown from flux assisted melt media which are capable of depressing the melting point below the phase transition temperature. This preparation procedure is one of the main disadvantages of BBO, another drawback being their sensitivity to hydrolysis.3 Recently, alkaline-earth cyanurates Ca3(O3C3N3)2 (CCY), Sr 3 (O 3 C 3 N 3 ) 2 (SCY), Eu 3 (O 3 C 3 N 3 ) 2 (ECY), and © 2017 American Chemical Society

Ba3(O3C3N3)2 (BCY) have been reported. These binary metal cyanurates are prepared by exothermic solid-state metathesis reactions between alkaline earth (or europium) dichloride and potassium cyanate (K(OCN)) at temperatures of around 550 °C.4−7 The crystal structures of CCY, SCY, and ECY are reported to be isostructural with that of β-BBO (R3c), whereas BCY crystallizes in a form isostructural with α-BBO (R3c̅ ). The structure of the cyanurate ion (O3C3N3)3− shows close resemblance to the oxoborate ion (O3B3O3)3− in BBO. In crystal structures, both cyanurate and oxoborate ions tend to arrange in a slightly offset, coplanar fashion, often leading to columns of stacked rings. The crystal structures of β-BBO, CCY, SCY, and ECY involve such columns of stacked rings, whereby adjacent rings are rotated relative to each other around the (3-fold) stacking axis. The spatial arrangement of M2+ cations and (O3B3O3)3− ions (and (O3C3N3)3−) as well as the highly polarizable π electrons within these ions can be regarded to be the reason for the high nonlinear susceptibilities measured for these materials. These high nonlinear susceptibilities combined with the noncentrosymmetric structures of these compounds give rise to their pronounced second-order nonlinear optical properties.8 Received: November 29, 2016 Published: March 7, 2017 3357

DOI: 10.1021/acs.inorgchem.6b02893 Inorg. Chem. 2017, 56, 3357−3362

Article

Inorganic Chemistry

HOPG-primary monochromator, a 500 μm monocapillary optic, and a two-dimensional VÅNTEC-500 detector covering 40° in 2θ in one measurement frame. Diffraction patterns were collected sequentially with a fixed detector position in a range of 22° < 2θ < 59° and integration times of 20 s per frame. Simultaneously the sample was heated/cooled with heating/cooling rates of 10 K/min under a N2 atmosphere (flow rate 300 sccm) in a cyclic heating−cooling procedure. All measurements were performed on a X-ray-pure sample of β-SCY that was ground in a ball mill (ZrO2 balls) with isopropyl alcohol prior to use. NLO Measurements. The nonlinear optical spectra of both the α and β modifications of SCY as well as KTP were collected on a homebuilt inverted confocal microscope.9,10 The apparatus and measuring conditions were recently reported in a comparative study on CCY, ECY, and β-SCY as well as solid solutions of SCY-CCY.4,5,7 KTiOPO4 (KTP) used as a reference material was provided by Crystal Laser Company, Messein, France.

All cyanurate compounds crystallizing isostructurally with βBBO exhibit second-order NLO properties such as second harmonic generation (SHG, frequency doubling), with efficiencies comparable to those measured for β-BBO, a material widely used in frequency doubling applications.4,5 In the course of the preparation of SCY powders a new noncentrosymmetric crystalline modification was discovered and characterized. On the basis of this finding, we rename the already known strontium cyanurate phase as β-SCY and introduce α-SCY as a low-temperature modification, which is presented with its crystal structure, thermal behavior, phase transition, phase relationship, and SHG properties.

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EXPERIMENTAL SECTION

Preparation. Sr3(O3C3N3)2 (β-SCY) was obtained from a reaction of SrCl2 with K(OCN) at 525 °C following eq 1.5

3SrCl 2 + 6K(OCN) → Sr3(O3C3N3)2 + 6KCl

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RESULTS AND DISCUSSION Phase Formation. The previously reported high-temperature modification of Sr3(O3C3N3)2 (β-SCY) was prepared from stoichiometric amounts of SrCl2 and K(OCN) at 525 °C, whereupon coproduced KCl could be washed out with water to obtain the single-phase product. The melting point (628 °C) has been determined in previous DSC measurements, exhibiting no signs of any other thermal event taking place during the heating and cooling procedures.5,7 Heating a washed sample of SCY in an evacuated silica tube near its melting point leads to the formation of a new phase that was identified as the low-temperature phase of SCY (named α-SCY) by means of single-crystal diffraction methods. The reproducibility of this reaction is only given for washed samples of β-SCY. As the relation between both modifications of SCY was unclear, phase transition studies were undertaken by means of thermal analysis and temperature-dependent in situ XRD studies. Phase Transition Studies. Temperature-dependent in situ XRD studies were performed to analyze the phase relationship between α-SCY and β-SCY. A sample of β-SCY was heated and cooled in consecutive (heating/cooling) cycles between 50 and 300, 400, 500, 550, and 600 °C. The cyclic heating procedure up to 550 °C, shown in Figure 1, reveals line shifts which can be attributed to thermal expansion without any indication of a phase transition. Additional reflections due to the formation of Sr(CO3) appeared in the XRD pattern during the isothermal segment and remained present during the following cooling step. When the β-SCY sample was further heated to 620 °C, additional reflections were observed, which were assigned to Sr(CO3) and β-Sr(CN2), the low-temperature modification of Sr(CN2) which is reported to exist up to 647 ± 20 °C.11 The cooling cycle reveals the phase transition from β-SCY into αSCY near 150 °C, as indicated by the assignment of the corresponding (hkl) indices for α-SCY in Figure 2. The reversibility of the phase transition was investigated by starting from the previously formed α-SCY. When α-SCY was heated in a cyclic procedure between 50 and 200 °C, the reversible phase transition was observed near 150 °C (Figure 3). Our studies have shown that the known phase β-SCY does not show a phase transition in the absence of impurities. The conditions for the formation of α-SCY could not be determined precisely, since the phase transition from β-SCY into α-SCY

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The white crystalline powder of β-SCY was rinsed several times with water to remove KCl and afterward dried at 80 °C in air. The purity of the synthesized β-SCY was confirmed by powder X-ray diffraction (PXRD). The phase transformation from β-SCY to α-SCY was accomplished in a fused silica tube by annealing a washed sample of β-SCY near its melting point (mp 628 °C) for a short period (approximately 3 min). After the silica tube was cooled to room temperature, the PXRD pattern of the colorless crystal powder confirmed the conversion of βSCY into α-SCY, with β-Sr(CN2) and SrCO3 as side phases (