Selective Pure-Phase Synthesis of the Multiferroic BaMF 4 (M ) Mg, Mn, Co, Ni, and Zn) Family Sun Woo Kim, Hong Young Chang, and P. Shiv Halasyamani* Department of Chemistry, UniVersity of Houston, 136 Fleming Building, Houston, Texas 77204-5003, United States Received October 5, 2010; E-mail: psh@uh.edu Abstract: A low temperature, high yield hydrothermal route has been discovered for the phase-pure synthesis of the multiferroic BaMF 4 (M ) Mg, Mn, Co, Ni, and Zn) family. The synthesis involves the use of CF 3 COOH instead of HF in an aqueous medium and, under the correct conditions, produces pure and polycrystalline BaMF 4 . In addition to the synthetic description, second-harmonic generation, thermogravimetric, and differential scanning calorimetry data are presented. Multiferroic materials, compounds where at least two primary ferroic s ferroelectric, ferromagnetic, ferroelastic, etc. s properties occur in the same materials, are of topical interest attributable to their applications in advanced devices. 1-3 With such materials, the magnetic properties may be controlled by an electric field and vice versa. A great deal of effort is dedicated to to not only discovering new multiferroic materials, e.g., MnWO 4 4-6 and Ca 3 CoMnO 6 , 7,8 but also investigating multiferroic behavior in known materials, e.g., BiFeO 3 . 9-16 A family of materials that has received renewed interest are the mixed-metal fluorides BaMF 4 (M ) Mg, Mn, Fe, Co, Ni, and Zn). 17-21 These iso-structural and crystallographically polar materials s all of the materials crystallize in the polar space group Cmc2 1 s are ferroelectric at room temperature for M ) Mg, Co, Ni, and Zn 22 and exhibit antiferromagnetic ordering with T N ≈ 20-80 K for M ) Mn, Fe, Co, and Ni. 23 Thus simultaneous ferroelectric and antiferromagnetic behavior, at low temperatures, may only be observed in BaCoF 4 and BaNiF 4 . Originally, the BaMF 4 family of materials were grown as crystals from the melt of BaF 2 and MF 2 in an HF atmosphere at elevated temperatures s above 800 °C. 24 Large, centimeter size, single crystals of BaMgF 4 have been grown by Czochralski and Bridgeman methods. In these reports either CF 4 or anhydrous HF gas was used at ∼920 °C. 25-27 Sol-gel methods to synthesize polycrystalline BaMgF 4 have also been reported; however, in these reports BaF 2 and/or MgF 2 impurities were found. 28,29 To date, there are no published reports on the bulk, polycrystalline, pure-phase syntheses of the BaMF 4 family of materials. Such syntheses would aid in growing high quality films of these materials, as the pure bulk materials could be used as an ablation target. In this communication, we report on the synthesis of pure and polycrystalline BaMF 4 (M ) Mg, Mn, Co, Ni, and Zn) through a convenient low temperature hydrothermal route. All of the materials were synthesized by combining the reagents (see Table 1) into separate 23 mL autoclaves. The autoclaves were closed, heated to 230 °C, held for 24 h, and cooled slowly to room temperature at 6 °Ch -1 . The mother liquor was decanted from the products, and the products were recovered by filtration and washed with distilled water. Reagent ratios used were as follows (see Supporting Information, S1, for complete details regarding the synthetic procedure). All of the reported materials, BaMF 4 (M ) Mg, Mn, Co, Ni, and Zn), were synthesized as phase-pure polycrystalline powders. Experimental and calculated X-ray diffraction (XRD) data for BaNiF 4 are shown below (see Figure 1). Powder XRD data for the other reported phases have been deposited in the Supporting Information (see Figure S2). Additionally, magnetic measurements were performed on BaNiF 4 to confirm the magnetic ordering. As seen in Figure S3, an antiferromagnetic transition at ∼60 K is observed, consistent with previous reports. 23 With our hydrothermal method, we were also able to grow crystallographic quality crystals of BaMnF 4 , BaCoF 4 , and BiNiF 4 . Crystallographic information for these three materials have been included in the Supporting Information. The structures are consistent with previous reports (see Table S4). Briefly, BaMF 4 (M ) Mg, Mn, Co, Ni, and Zn) represent an iso-structural family of materials that contain layers of corner-shared MF 6 octahedra that are separated by Ba 2+ cations (see Figure 2). The M-F bond distances, for BaMnF 4 , BaCoF 4 , and BiNiF 4 , range from 1.981(9) to 2.159(5) Å, with Ba-F distances of 2.611(5)-3.355(5) Å. Bond valence calculations 30,31 resulted in values ranging from 1.85 to 2.02 for the Ba 2+ cation and 1.95, 1.99, and 2.00 for Mn 2+ , Co 2+ , and Ni 2+ , respectively. In connectivity terms, the structures may Table 1. Reagent Amounts and Yields for the Synthesis of BaMF 4 (M ) Mg, Mn, Co, Ni, and Zn) BaMgF 4 BaMnF 4 BaCoF 4 BaNiF 4 BaZnF 4 BaF 2 0.187 g 0.200 g 0.150 g 0.100 g 0.180 g MF 2 0.245 g a 0.106 g 0.110 g 0.385 g b 0.351 g b CF 3 COOH 4 mL 2 mL 3 mL 5 mL 4 mL H 2 O 5 mL 5 mL 5 mL 5 mL 5 mL Yield c 70% 80% 70% 80% 80% a For BaMgF 4 , Mg(ac) 2 · 4H 2 O was used. b For BaNiF 4 and BaZnF 4 , MF 2 · 4H 2 O was used. c Yields based on BaF 2 . Figure 1. Calculated (black) and experimental (red) powder X-ray diffraction data for BaNiF 4 . Published on Web 11/29/2010 10.1021/ja108965s  2010 American Chemical Society 17684 9 J. AM. CHEM. SOC. 2010, 132, 17684–17685