The Double Pervoskite NaTbMnWO6 : A Likely

Multiferroic Material†Alison Pawlicki,

‡A. S. Sefat,

‡David Mandrus

†Florida State University,

‡Oak Ridge National Laboratory

IntroductionDouble perovskites, of the chemical formula AA’BB’O6, are known to show multiferroic behavior, exhibiting coupled ferroelectricity and magnetism. Materials displaying both of these properties are unique because they tend to be mutually exclusive since magnetism arises in transition ions with partially filled d- or f-orbitals and all conventional ferroelectricity arises in transition ions with empty d-orbitals. However, some perovskites have been found to display both behaviors, where the origin of ferroelectricty is unconventional. The study of such materials is motivated by the possibility of controlling electric charges by applied magnetic fields and controlling magnetic fields by applied voltages. In this study, we focus on one particular double perovskite, NaTbMnWO6.

Experimental ProcedureWe synthesized polycrystalline samples of NaTbMnWO6 by using two slightly different solid state routes. First we reacted stoichiometric amounts of MnO and WO3 at 950° C to yield MnWO4.

MnO + WO3 → MnWO4

Then we reacted stoichiometric amounts of MnWO4, Tb4O7, and an approximate 8% excess of NaCO3.

MnWO4 + ¼ Tb4O7 + ½ Na2CO3 → NaTbMnWO6

One sample was heated at 950° C in a dynamic flow of Ar/H2 gas twice and another sample was heated in air at 800° C then heated at 950° C in a dynamic flow of Ar/H2 gas. We also attempted single crystal growth by the use of floating zone image furnace pictured above. The phase purity of the samples was analyzed by using Philips Powder Diffractometer at room temperature. Magnetic susceptibility data was collected using a Superconducting Quantum Interference Device (SQUID) in an applied field of 1000 Oe. For the first time, heat capacity measurements were done via Physical Property Measurement System (PPMS).

Results and Discussion

Heat capacity of the initial Ar/H2 heated sample is only measured because it is found more pure. Two sharp anomalies were also found at approximately the same temperatures as in the magnetic susceptibility, suggesting two long range ordering temperatures.

ConclusionsPolycrystalline samples of NaTbMnWO6 of true orthorhombic phase were synthesized by solid state methods. Two long-range magnetic ordering transition temperatures were found in magnetic susceptibility and heat capacity results at 15 K and at ~11 K. Because of the interesting magnetic behavior, we would like to pursue further study of magnetoelectric coupling and phase transitions in single crystals. Powder Neutron diffraction measurements are currently underway.

The magnetic susceptibility for samples produced by the two different heating processes is displayed below. Both samples show anomalies at 15 and ~11 K. Using a high temperature fit to the Curie-Weiss Law, the Curie-Weiss temperature, Curie constant, and effective magnetic moment were found and listed in the table below. As noted, μeff differs for both samples and the theoretical value is closer to the value obtained for the initial Ar/H2 heated sample.

AcknowledgmentsWe would like to thank Brain Sales and Michael McGuire for laboratory assistance. This research was carried out at Oak Ridge National Laboratory and was supported by the Division of Materials Sciences and Engineering, The Office of Basic Energy Science, and the U.S. Department of Energy.

ReferencesKing, Graham; Thimmaiah, Srinvasa; Dwivedi, Akansha; Woodward, P. M. Chem. Mater. 2007, 19, 6451-6458

Pictured above is one unit cell of the crystal structure of NaTbMnWO6. The Na and Tb ions order simultaneously in layers. Powder x-ray diffraction confirmed the samples to be of the true orthorhombic phase.

Sample TN (K) C (emu K/ mol Oe) θ (K) μeff/μB

Initial Heat in Air 15 10.730(3) −21.90(5) 9.265(6)

Initial Heat in Ar/H2 15 17.274(4) −25.43(7) 11.755(4)

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Powder X-Ray diffraction was used in every step of the reaction process. The diffraction pattern in purple is from the sample initially heated in air and it shows excess Bragg peaks about 8°, 9°, 20°, and 21°. These peaks are due to impurities and were reduced in slightly throughout the heating. The diffraction pattern in orange is from the sample initially heated in a flow of Ar/H2 gas and shows excess Bragg peaks about 16° and 28°. These peaks are reduced in magnitude in the second heating and are thought to be from impurities. Because there are less significant excess peaks in the second synthesis process, we conclude that this method produces less impurities.

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